jgofs U.S. JGOFS Synthesis & Modeling Project

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U.S. JGOFS Synthesis & Modeling Project
Principal Investigators' Workshop
July 21-24, 2003
Woods Hold Oceanographic Institution
Quissett Campus, Clark 507

REGISTER Logistics Agenda Abstracts Attendees

Abbott* Armstrong* Bates* Cassar et al. Chai & Barber*
Chai et al. #1 Chai et al. #2 Christian & Letelier Daniels et al. Dinniman et al.
Dunne* Dutkiewicz et al. Fach et al. Follows et al.* Friedrichs #1
Friedrichs #2 Gardner #1 Gardner #2 Gnanadesikan* Hiscock
Hofmann et al. Hood et al. Jiang et al. Keigwin* Key*
Kim et al. Klinck et al. Landry et al. Lehman & Cullen Letelier et al.
Li & Peng Lima & Doney Lomas et al. Marchal* Matsumoto et al.
McGillicuddy et al. #1 McGillicuddy et al. #2 Mishonov & Gardner Mongin & Nelson Moore et al.
Murtugudde et al. Najjar et al. Richardson* Salihoglu & Hofmann Sarma & Saino*
Sarmiento et al.* Shi & Chai Spitz Sweeney et al. Thompson*
Toggweiler* Toole et al. Wang et al. #1 Wang et al. #2 Westberry et al.
Wiggert et al. #1 Wiggert et al. #2

* Plenary Speaker

Mark Abbott1

Ocean Remote Sensing in 2020

1 College of Oceanic and Atmospheric Sciences Oregon State Univ. 104 Ocean Admin. Bldg. Corvallis, Or 97331

The last two decades have seen significant advances in our understanding of global and mesoscale ocean processes, made possible in part by the availability of research-quality satellite data sets. I will review some of these results as they apply to carbon cycle science research. The next 20 years will experience a fundamental shift as we move from research-driven missions to application-driven missions. There are both benefits and risks to this strategy, and the research community must prepare for a remote sensing environment where science is not the only requirement.

Rob Armstrong1

The Paradox of Uncertainty

1 Marine Science Res. Center, Stony Brook Univ., Stony Brook, NY


... the goal of developing an improved model of biogeochemical cycling cannot be met simply by deriving from JGOFS observations better estimates of parameter values for use in standard models (e.g., the model of Fasham et al. ...). Instead, achieving [this goal] ...will likely require major structural changes in the modeling of a range of critical ocean processes. The development of such models will require close collaboration among those who make observations, modelers of detailed processes, and modelers who will attempt to capture the essence of carbon cycle processes compactly enough to be usable in large-scale models
- U.S. JGOFS Implementation Plan, Section II.D. "The challenge".

There is a little-known law of human existence, the "Law of Diminishing Certainty", which states that although knowledge ("what you know") increases with age at an increasing rate, "what you know you don't know" eventually increases much faster. The intersection of these curves is a "point of maximal confidence" (for individuals) or a "point of maximal certainty " (for scientific disciplines). Physics went through its "point of maximal certainty" in the years preceding quantum mechanics, when it was asserted that all that was left for physics was to add a few more decimal points. Discoveries during the JGOFS era of new biological taxa and processes may have pushed biogeochemical oceanography past its point of maximal certainty into a period where "what we know we don't know" is increasing faster than "what we know." In such an environment, increasing knowledge may paradoxically not "reduce uncertainty" sensu early JGOFS planning documents; it may even (temporarily) increase uncertainty. During this period of rapid advance, the ability to assess the potential large-scale effects of novel mechanisms is exceedingly important. As part of this capability, we must be able to cast new biogeochemical mechanisms into forms that can be incorporated directly into large-scale models; this translation must be done in a timely manner and with maximal scientific fidelity, desiderata that are ever in competition. Technical and sociological vignettes arising from interactions with collaborators and European colleagues are offered to initiate discussion of how we might plan for the modeling enterprise of the future.

Nick Bates1

- title pending -

1 author address

- abstract pending -

Nicolas Cassar1, Edward A. Laws1, Robert R. Bidigare1 and Brian N. Popp1

Bicarbonate uptake by Southern Ocean phytoplankton

1 Department of Oceanography, School of Ocean and Earth Science and Technology, Univ. Hawaii, Honolulu, Hawaii 96822, USA

Marine phytoplankton may represent a significant sink for anthropogenic carbon dioxide (CO2). Algal growth could potentially be limited by CO2 availability (Riebesell et al. 1993). In order for marine primary production to be regulated by CO2, marine algae must be limited to CO2 uptake and not be able to assimilate bicarbonate (about 2 mM in seawater). Estimation of the extent of bicarbonate (HCO3-) uptake in the oceans is therefore required to determine whether the anthropogenic carbon sources will enhance carbon export to the deep ocean. We performed isotopic disequilibrium experiments during the SOFeX cruise to better understand the importance of carbon-concentrating mechanisms (CCMs) in the Southern Ocean. We found that approximately half of the photosynthetic inorganic carbon uptake was direct HCO3- uptake, the other half being direct CO2 uptake (passive and/or active uptake). A low-CO2 treatment induced an increase in uptake of CO2 through increased enzymatically mediated extracellular dehydration of HCO3- (carbonic anhydrase activity), which was at the expense of direct HCO3- transport across the plasmalemma. Biological productivity in the Southern Ocean is therefore unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of CCMs.

Fei Chai1 and Dick Barber2

Modeling the Decadal Variability of Ecosystem and Carbon Cycle in the Pacific Ocean

1 School of Marine Sciences, Univ. Maine, Orono, ME 04469-5741
2 Duke Univ., NSOE Marine Laboratory, 135 Duke Marine Lab Road, Beaufort, NC 28516

The decadal climate variability affects marine ecosystems and carbon cycle in the Pacific Ocean. To improve our understanding of physical variability and the marine ecosystem and biogeochemical response in the Pacific Ocean, especially on decadal time scale, we have developed a physical-biogeochemical model for the Pacific Ocean. The lower trophic level ecosystem processes are linked with upper ocean carbon chemistry and embedded into a three-dimensional circulation model that is forced with observed the air-sea fluxes between 1950 and 2000. The improved physical-biogeochemical model produces a 50-year (1950-2000) retrospective analysis for the Pacific Ocean. Analyses of the modeled results are focused on two regions, the equatorial Pacific and the central North Pacific. The physical-biogeochemical model captures the slowdown of the meridional overturning and decrease of the equatorial upwelling transport, which cause primary production and phytoplankton biomass decrease by about 10% after the 1976-77 climate shift in the equatorial Pacific. The sea-to-air CO2 flux from the equatorial Pacific reduces 20% after 1976-77. In the central North Pacific, the modeled primary productivity and phytoplankton biomass in the transition zone (30°N-45°N) increase after the 1976-77 climatic shift. Elevated chlorophyll in the central North Pacific expands the higher chlorophyll region and pushes the transition zone chlorophyll front (defined as surface chlorophyll = 0.2 mg/m3) equatorward. Overall, the physical-biogeochemical model responds to the abrupt climate shift reasonably well, and the modeled results are consistent with the limited observations in the Pacific Ocean. We also use the physical-biogeochemical model to investigate the impacts of decadal variability on carbon cycle, and separate the natural variability of carbon cycle from the anthropogenic CO2 accumulation in the North Pacific.

Fei Chai1, M.-S. Jiang2, T.-H. Peng3 and R.T. Barber4

Modeling Decadal Variability of Carbon Cycle in the Pacific Ocean

1 Univ. Maine
2 Univ. Mass. Boston
4 Duke Univ.

To improve our understanding of physical variability and the carbon cycle response in the Pacific Ocean, especially on seasonal to decadal time scales, we have developed a physical-biogeochemical model for the Pacific Ocean. The lower trophic level ecosystem processes are linked with upper ocean carbon chemistry and embedded into a three-dimensional circulation model that is forced with observed the air-sea fluxes between 1950 and 2000. The improved physical-biogeochemical model produces a 50-year (1950-2000) retrospective analysis for the Pacific Ocean. The physical-biogeochemical model is capable of reproducing many observed features and their variability in the Pacific Ocean. Analyses of the modeled results are focused on the North Pacific, a sink region for both natural and anthropogenic carbon. The abrupt shift in the North Pacific climate system that occurred during the mid 1970s, the modeled air-sea flux of CO2 and the response of the upper ocean carbon cycle to this climate shift are discussed. Using the physical-biogeochemical model, we estimate how much anthropogenic CO2 has entered into the North Pacific Ocean during the past several decades. The model estimated anthropogenic CO2 invasion rate for various regions compare favorably with observational (Sabine et al., 2002, GBC) and other modeling (Xu et al., 2000, Mar. Chem.) estimates.

Fei Chai1, M.-S. Jiang2, F. Chavez3 and R.T. Barber4

Modeling Iron Enrichment in the Equatorial Pacific Ocean

1 Univ. Maine
2 Univ. Mass. Boston
3 Monterey Bay Aquarium Res. Inst.
4 Duke Univ.

In situ iron-enrichment experiments in the Southern Ocean and the equatorial Pacific Ocean have shown that transient addition of very low concentrations of iron to high-nitrate, low-chlorophyll (HNLC) waters sets in motion changes in the productivity and growth of picoplankton, larger phytoplankton and the grazers of both of these groups. The logistic constraints on the length of observation have prevented these otherwise successful efforts from resolving the full temporal pattern of responses. These experiments necessarily have been limited to 20 days or less. To overcome the temporal (and spatial) constraints we have used a three-dimensional physical-biogeochemical model developed for the equatorial Pacific Ocean could contribute substantially to the design and evaluation process. The model consists of ten compartments describing two size classes of phytoplankton and zooplankton, detritus nitrogen and detritus silicon, silicate, total CO2 and two forms of dissolved inorganic nitrogen: nitrate (NO3) and ammonium (NH4), which are treated separately, thus enabling division of primary production into new production and regenerated production. This ten-component biological model is coupled with a three-dimensional ocean circulation model based on the Modular Ocean Model and forced with COADS monthly wind and heat flux. In the eastern equatorial Pacific, an iron-enrichment experiment in an area of 500,000 km2 is simulated by changing the photosynthetic efficiency and nutrient uptake kinetics in a given spatial domain (or patch). With this ecosystem model it is possible to investigate the physical, biological and geochemical consequences of varying the size of the enriched patch and frequency of enrichment.

James Christian1 and Ricardo Letelier2

Phytoplankton photoacclimation and spectral attenuation of solar irradiance in a blue-water ocean column model

1 Fisheries and Oceans Canada, Canadian Centre for Climate Modelling and Analysis
2 College of Ocean and Atmospheric Sciences, Oregon State University

The spectral quality of light in water changes rapidly with depth, with red wavelengths most rapidly attenuated. In blue-water regions photosynthesis can occur even at depths > 100 m. At such depths, solar irradiance is entirely in the blue wavelengths, and the attenuation coefficient is significantly less than the spectrally averaged values usually assumed in ocean models. The depth of the nutricline and the deep chlorophyll maximum layer (DCML) are sensitive to this assumption and tend to shoal unrealistically in a nonspectral model.

Spectrally averaged irradiance (e.g., total photosynthetically available radiation, or PAR) has historically been used in most ocean models for simplicity and computational efficiency. The cost of a full spectral model is 300 or 150 times that of a PAR model for 1 or 2 nm resolution respectively. We show here that a model with only 6 bands, with variable width based on a priori knowledge of the absorption spectrum of phytoplankton, gives negligible error relative to a full spectral (1 nm) model, and the error is independent of depth. In a PAR model the error increases monotonically with increasing depth and exceeds 100% at depths > 100 m. We have implemented the 6-band model in both Redfield (fixed elemental ratios) and non-Redfield (variable elemental ratios) versions of a one-dimensional ocean ecosystem model at JGOFS Station ALOHA, which prevents the unrealistic shoaling of the nutricline and the DCML that occurs in the PAR model.

In the fixed-ratio model, simulation of the DCML has also been poor because the available parameterizations of the light-dependence of the chlorophyll:carbon ratio (CHL2C) overestimate the light level at which most photoacclimation occurs. Using the non-Redfield model (in which chlorophyll is a prognostic variable), we show that at least in this environment, adaptation of CHL2C to ambient light appears to occur mostly at very low irradiance (<10 W/m2). Incorporation of this finding into a new CHL2C parameterization significantly improves simulation of the DCML in the fixed-ratio model.

Robert Daniels1, Hugh Ducklow1, George Jackson2, Tammi Richardson2, Mike Roman3, Robin Ross4, Langdon Quetin4, Raymond Smith4 and William Fraser5

Reconstruction of Plankton Food Web Structure from the North Atlantic Bloom Experiment (May, 1989) and the Western Antarctic Peninsula (Jan, 1996) Using an Inverse Method

1 Virginia Institute of Marine Science, 2 Texas A&M Univ. 3 Univ. Maryland 4 Univ. California Santa Barbara 5 Montana State Univ.

We are investigating relationships between food web structure and function across different oceanic biomes using an inverse method to recover estimates of material flows in food webs from sparse data. Specifically, we focus on how food web structure, as defined by the relative magnitude of C and N flows, influences particle export, nutrient regeneration, and dissolved organic carbon (DOC) cycling. Our model food web for NABE includes large and small phytoplankton, meso-and microzooplankton, bacteria, dissolved and particulate detritus, ammonium and nitrate. For the West Antarctic Peninsula (WAP), the model food web contains the same groupings as above as well as krill, salps, myctophids, and penguins. The initial nitrogen solution for NABE was unobtainable given the measured data. A solution was obtained when the observed new and regenerated production were allowed to vary, but the f ratio was much higher than measured. When the inverse method was altered to allow the solution to reproduce the measurements within a range of constraints rather than exactly, a solution was reached with an f ratio agreeing with the measured value. The carbon solution was also recalculated using observations as constraints. After analyzing the C:N relationships between the two solutions and finding a few of the flows were unreasonable, further constraints (C:N of 3-20) were put on the Nitrogen model. Analysis of the NABE solution shows that DOC and DON flows were two to three times the detrital flows and an active microbial loop along with microzooplankton grazing dominated the processing of carbon and nitrogen.

Michael S. Dinniman1, John M. Klinck1, Walker O. Smith2 and Eileen E. Hofmnn1

Cross shelf exchange in the Ross Sea and west Antarctic Peninsula from models of the circulation and biogeochemistry

1 Center for Coastal Physical Oceanography, Old Dominion Univ.
1 Virginia Institute of Marine Science, College of William and Mary

Exchange of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes. This water mass moderates the ice cover through heat flux, provides a relatively warm subsurface environment for some animals and provides nutrients to stimulate primary production. CDW exchange is known to be episodic, but persistent, and is thought to occur at specific locations due to bottom topography. The present study uses a high-resolution 3D numerical model to investigate locations and dynamics of this exchange on the Ross Sea and west Antarctic Peninsula continental shelves.

Calculations of the advective transport across the shelf break and budgets on the shelf of heat, salt and nutrients show that the cross shelf break transport of CDW is very important to the total budgets on both shelves. Horizontal curvature of the shelf break and transport across the shelf break are significantly correlated. A momentum term balance shows that momentum advection forces flow across the shelf break in locations where the isobaths curve in front of the flow. For the model to create a strong intrusion of CDW onto the shelf, two mechanisms are necessary. First, CDW is driven onto the shelf at least partially by momentum advection and the curvature of the shelf break; then, the general circulation on the shelf pulls CDW into the interior.

John Dunne1

The next generation of coupled ocean biogeochemical general circulation models

1 NOAA/Geophysical Fluid Dynamics Lab., PO Box 308, Forrestal Campus B Site, Princeton, NJ 08542-0308, 609-452-6596 jpd@gfdl.noaa.gov

A robust description of biogeochemical cycling in the ocean requires an understanding of the interplay of factors controlling ocean circulation, chemistry and ecology. Ocean Biogeochemical General Circulation Models (OBGCMS) attempt to synthesize our understanding of these processes within a single, detailed, mathematically consistent framework to diagnose biogeochemical fluxes and predict sensitivity to physical and biogeochemical change. Specifically, OBGCMs can be used to address such issues as: the relative impact of oceans (versus land) on atmospheric pCO2 variability; ecological responses to climate change with respect to the cycling of carbon, nitrogen, phosphorus, oxygen, silicon, iron and calcium carbonate; ecological and biogeochemical responses to purposeful iron fertilization; ecological feedback onto physics through short wave radiation penetration; relationship between biogeochemistry and fisheries; mechanisms of glacial-interglacial biogeochemical variability. While much progress has been made in combining these physical, chemical and biological processes into OBGCMS, there remain challenging frontiers.

The next generation in OBGCMs will need to include improvements in ocean physics, biogeochemistry, and operational community availability of model code and output. The physical/dynamical challenges include: spatially resolving eddies, river influence, continental shelves and bottom topography; coupling of the ocean to the atmosphere to reproduce inter-annual and decadal modes of climate variability while freeing models of "flux adjustment"; developing a realistic description of sea ice; improving descriptions of thermocline ventilation and bottom water formation pathways; deciding among the various coordinate and numerical schemes. Generally, the biogeochemical challenges relate to describing the role of ecosystem structure on interplay between elemental cycles through efficiency and stoichiometry of export. Specifically, these processes include: establishing mechanistic alternatives to "The Martin Curve" for interior particle remineralization; description of the calcium carbonate cycle; nutrient co-limitation of phytoplankton growth; iron uptake and scavenging; role of eddies; nitrogen fixation/denitrification; coastal/continental shelf processes. Operational challenges include: community availability of model output such as the pre-industrial steady state scenarios, a 50-year re-analysis and IPCC centennial predictions; community availability of models for adaptation to particular purposes (FMS/ESMF); establishment of a single framework to readily implement and compare ecological parameterizations (Regional Testbeds). Significant progress being made on each of these topics will be discussed.

Bettina A. Fach1, David M. Glover1 and Maureen H. Conte1

A coupled epipelagic-mesopelagic particle flux model for the Bermuda Atlantic Time-series Station (BATS)/Oceanic Flux Program (OFP) Site: Phase 2, the one-dimensional framework to 4000 meters

1 Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, bfach@whoi.edu dglover@whoi.edu mconte@whoi.edu

The overarching goal of this project is to mechanistically connect euphotic zone processes with mesopelagic zone processes. We plan to accomplish this by means of a prognostic model that can be used to further our understanding of unparalleled time-series of deep-water sediment traps (25+ years) at the Oceanic Flux Program (OFP) and euphotic zone measurements (10+ years) at the Bermuda Atlantic Time-series Site (BATS). In order to realize this goal, we have derived a mesopelagic ecosystem structure. Furthermore, we have coupled this ecosystem structure with an epipelagic ecosystem and now model the flux of biogeochemically active constituents (carbon, nitrogen, phosphorus, silica and iron) from the surface to 4000 m. In this second phase, we present the joined ecosystem models competing over the entire water column in an one-dimensional framework. Schematics and initial results of this modeling effort as well as a discussion of the interplay between epi- and mesopelagic ecosystems are presented.

Stephanie Dutkiewicz1, Payal Parekh1 and Mick Follows1

Sensitivity of an ecosystem model to aeolian fluxes of iron

1 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, stephd@plume.mit.edu

We present a global ocean biogeochemistry model with an explicit parameterization of the deep water iron cycle and an ecosystem model with two functional groups of phytoplankton, limited by light, phosphate, dissolved silica and iron. This model is coupled to the MIT three dimensional global ocean circulation model. HNLC regions emerge naturally as a result of explicit iron limitation and the decoupling of phosphorus and iron in the ocean's interior.

We investigate the sensitivity of this coupled model to changes in aeolian dust fluxes. First we consider two experiments where the dust flux is double and half that of the control run. Increasing the dust increases biomass and export in the HNLC regions, but productivity decreases in the oligotrophic subtropical gyres of the Pacific. Opposite results are found for the half dust experiment. We also consider the changes to the ecosystem when we use a different dataset of dust deposition. We find shifts in the limiting nutrient of the phytoplankton classes.

Mick Follows1, Galan McKinley1,2 and John Marshall1

Interannual Variability in a Global Model of the Air-Sea Fluxes of CO2 and O2

1 Dept Earth, Atmospheric and Planetary Sciences, Massachusetts Inst. Tech.
2 present affiliation: Instituto Nacional de Ecologia, Mexico City, Mexico.

We examine the interannual variability of air-sea carbon and oxygen fluxes between 1980 and 1999 in the MIT ocean circulation and biogeochemistry model configured at moderate resolution and forced with daily surface fluxes and twice daily wind stress fields from meteorological reanalyses. The model applies a simplified parameterization of nutrient export with a regionally varying characteristic export rate adjusted such that the annual mean phosphate distribution does not drift from observed climatology.

Key inferences from the model include:

  1. There is a significant global, interannual flux of oxygen across the sea surface driven by physical variability. The magnitude of variability is important for interpretations of carbon sinks from atmospheric CO2 and O2 variations on the annual timescale.
  2. Two regions of the ocean, the Tropical Pacific and the North Atlantic dominate this global variability in oxygen fluxes. These regional variations are modulated by ENSO in the Tropical Pacific and variations of deep winter mixing (related to the NAO) in the North Atlantic.
  3. The interannual variability of air-sea carbon fluxes is relatively small compared to the variation in fluxes to and from the terrestrial biosphere.
  4. The global variability of air-sea carbon fluxes is dominated by the Tropical Pacific. The magnitude of this regional variation is consistent with various independent estimates.
  5. In strong contrast with oxygen, the North Atlantic does not play a significant role. While the model appears to capture the local variations at the subtropical time-series sites, these sites are not strongly representative of the gyre and basin scale fluxes, in contrast to the inference from atmospheric inverse models of CO2.

Marjorie Friedrichs1, Larry Anderson2, Robert Armstrong3, Fei Chai4, James Christian5, John Dunne6, Scott Doney2, Jeff Dusenberry2, Katja Fennel7, David Glover2, Raleigh Hood8, John Klinck1, Dennis McGillicuddy2, J. Keith Moore9, Ragu Murtugudde10, Yvette Spitz11 and Jerry Wiggert1

The Regional Ecosystem Modeling Testbed Project

1. Old Dominion Univ.
3. SUNY Stony Brook
4. Univ. of Maine
5. CCCMA, Victoria
7. Rutgers Univ.
8. HPL, Univ. Maryland
9. Univ. California, Irvine
10. ESSIC, Univ. Maryland
11. Oregon State Univ.

This poster presents results from the first Regional Testbed Workshop, which was held last March at Old Dominion University. Ten ecosystem models of varying complexity participated in this initial comparison effort. All models were run within the same testbed framework, which included computations of mixing, diffusion, advection and attenuation as well as initial and boundary conditions. Each model was also required to use the same physical forcing time series of temperature, PAR, vertical velocity and mixed-layer depth, which were obtained either from circulation model output or data. Ecosystem model output was compared to five types of data from each site: chlorophyll, zooplankton biomass, nitrate concentration, sediment trap flux, and primary production. Model-data misfit was computed in terms of a weighted cost function. For the primary comparison participants were required to use identical ecosystem parameters at both locations, but were free to optimize their model to any extent they desired. An optional second comparison involved optimizing models for each site individually.

Not surprisingly, model performance depended on the degree to which each participant tuned his or her model; thus for a more formal and insightful model comparison, an objective method for optimizing each of the models is required. (See Friedrichs #2 poster.) Model-data fit for nitrate was found to be relatively high and similar for all ten models, largely because of deficiencies in the physical forcing fields. On the contrary, model-data misfit for chlorophyll varied significantly between ecosystem models, and was reduced for those models that were tuned for the testbed implementation. Finally, when optimizing for individual sites, very simple (4 component) ecosystem models performed nearly as well as the most complex ecosystem models. However, when models were simultaneously optimized for two very different ecosystems simultaneously, the complex models had a greater advantage. This is presumably because they are better able to simultaneously represent a greater diversity of ecosystem processes with a fixed set of parameter values.

Marjorie Friedrichs1, Raleigh Hood2 and Jerry Wiggert1

Ecosystem Model Comparison in the Arabian Sea: A Prototype Regional Modeling Testbed

1 Center for Coastal Physical Oceanography, Old Dominion Univ., Norfolk, VA 23529
2 Univ. Maryland Center for Environmental Science, Cambridge, MD 21613

As part of the Joint Global Ocean Flux Study, many models have been developed to simulate biogeochemical cycling in various oceanographic regions; however, few quantitative comparisons of these models have been made. In order to critically assess which ecosystem structures and model formulations are best able to simulate observed biogeochemical cycling in the Arabian Sea, we apply three fundamentally different ecosystem models within a consistent one-dimensional framework (i.e. regional testbed) at the site of the WHOI mooring (15.5°N, 61.5°E). The testbed contains one-dimensional physical forcing fields from two different 3D physical models, and biogeochemical data that are used for assimilation and evaluation. The data include primary production, chlorophyll-a, zooplankton, nutrient and sediment trap observations and are assimilated using the variational adjoint method. The three ecosystem models that we examine consist of: a four-component model with diatom-like phytoplankton growth, a five component model emphasizing the microbial loop, and an eight-component model containing multiple plankton size classes. After objectively optimizing each model, we quantitatively compare the performance of the different models to assess which model structure best represents the fundamental underlying biogeochemical processes. Results suggest that after optimization, all three models behave very similarly, implying that the additional complexity of the multiple size-class model may not be justified. Furthermore, a change in physical model (mixed-layer depth and vertical velocity fields) typically produces a far greater change in plankton distributions than does a change in ecosystem model complexity, highlighting the fact that biological distributions are largely a result of the physical environment.

W.D. Gardner1, A.V. Mishonov1 and M.J. Richardson1

Global POC synthesis using ocean color measurement calibrated with JGOFS and WOCE data on beam attenuation and POC

1 Department of Oceanography, Texas A&M Univ., College Station, TX, 77843-3146, wgardner@ocean.tamu.edu

Discrete samples of particulate organic carbon (POC) concentration obtained in the Atlantic, Pacific, Indian and Southern oceans during the entire JGOFS program were used to calibrate synchronously measured beam attenuation profiles for continuous POC determination. The resulting regressions were applied to a global set of WOCE, SAVE, and other beam attenuation data collected over 15 years time-span and processed in our lab in order to assess horizontal and vertical distribution of the POC concentration. Beam attenuation and POC concentration averaged over one attenuation depth of the upper ocean in South Atlantic and South Pacific were related to the SeaWiFS-derived ocean-color products. A good correlation was obtained for a log regression of POC and normalized water-leaving radiance at 555 nm. Based on these regressions, global seasonal maps of the surface POC concentration were created for two different seasons. However, the POC below one optical depth is not sensed by the satellite, thus the ability to assess the total standing stock of POC needs further research.

W.D. Gardner1, M.J. Richardson1 and A.V. Mishonov1

POC Distribution in the Pacific: Estimates using in-situ Optical Data and SeaWiFS Products

1 Department of Oceanography, Texas A&M Univ., College Station, TX, 77843-3146, wgardner@ocean.tamu.edu

Beam attenuation data collected on 7 WOCE lines during 12 cruises to the Pacific were used for assessment of the concentration of particulate organic carbon. A regression of beam attenuation due to particles (beam cp) vs POC calculated using Equatorial Pacific (EqPac), Hawaii Oceanographic Time-series (HOT) and Antarctic Polar Front Zone (APFZ) field data was applied in order to obtain spatial and vertical distribution of POC. Four SeaWiFS data products were compared with integrated oceanic POC estimated to determine the best bio-optical product to use. A comparison between integrated POC within one attenuation depth and POC standing stock from surface to the depth of background POC was made to evaluate POC determined by remote sensing with the measured total standing stock of POC in the upper ocean. Estimates of the summer/winter POC in the upper layer over the entire Pacific was mapped based on monthly SeaWiFS data.

Anand Gnanadesikan1

Upwelling pathways and the carbon cycle

1 Geophysical Fluid Dynamics Lab, P.O. Box 308, Princeton, NJ 08542-0308 609.987.5062, a1g@gfdl.noaa.gov, gnana@splash.princeton.edu

Pathways of vertical exchange within the ocean are controlled by subgridscale parameterizations and surface winds. In recent years the outline of a theory for understanding these pathways has emerged. This talk presents results from a suite of models with differing upwelling pathways, discussing the imprint of the differing circulations on carbon cycling. In particular, we consider the following questions

  1. How do upwelling pathways affect the pattern of anthropogenic carbon uptake?
  2. How do upwelling pathways affect the expression of the biological and solubility pumps?
  3. How do upwelling pathways affect the impact of nutrient depletion on atmospheric carbon dioxide?

Michael R. Hiscock1

Size-class dependent amplification of photosynthetic quantum yield in an iron enriched Southern Ocean

1 Duke Univ., Nicholas School of the Environment

We tested the iron limitation of three size classes of phytoplankton (<5m, >20m, and the fraction between 5 and 20 m) within two different high nutrient, low chlorophyll regimes in the Pacific Sector of the Southern Ocean. Repeated in situ iron addition resulted in an increase in the maximum quantum yield of photosynthesis, however the response within size fractions differed between the two sites.

Eileen E. Hofmann1, Marjorie A. Friedrichs1, James Christian2, Wendy Wang3, Raghu Murtugudde3 and Antonio J. Busalacchi3

Comparison of Model-derived Phytoplankton Distributions with SeaWiFS data at Two Sites in the Equatorial Pacific Ocean

1 Center for Coastal Physical Oceanography, Old Dominion University, Norfolk VA 23529, hofmann@ccpo.odu.edu
2 Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 1700 STN CSC, Victoria, BC V8W 2Y2 Canada, jim.christian@ec.gc.ca
3 Earth system Science Interdisciplinary Center, 2207 Computer and Space Sciences Building, University of Maryland, College Park, MD 20742, wwang@essic.umd.edu ragu@essic.umd.edu tonyb@essic.umd.edu

SeaWiFS observations for 2 degree by 2 degree regions, centered around 140W and 165E, were obtained for 1998 to 2002, which includes a strong El Nio-La Nia period. The SeaWiFS data were compared with simulated plankton and nutrient distributions for the same period obtained from a multi-component lower trophic level ecosystem model developed for the equatorial Pacific. The results of the comparisons indicate that the model accurately simulates the dynamics governing chlorophyll distributions at 140W, but underestimates chlorophyll variability and concentrations at 165E by more than 50%. This difference at 165E reflects strong grazer control at this location in the model-derived fields. Such a strong top-down control is not suggested by the available in situ data sets. These results indicate research directions associated with model structure and parameterizations that need adressing for application of ecosystem models to basin-scale systems. Suggestions are made for alternative grazing parameterizations that relate biological and physical dynamics over large areas. These results also indicate the usefulness of ocean color data sets for validating model dynamics, as well as model-derived distributions.

Raleigh R. Hood1, Kevin E. Kohler2, Julian P. McCreary, Jr.3 and Sharon L. Smith4

A 4-Dimensional Validation of a Coupled Physical-Biological Model of the Arabian Sea

1 Univ. Maryland Center for Environmental Science, Cambridge, Maryland
2 Oceanographic Center, Nova Southeastern Univ., Dania, Florida
3 International Pacific Research Center, Univ. Hawaii, Honolulu, Hawaii
4 Rosenstiel School of Marine and Atmospheric Science, Univ. Miami, Miami, Florida

In this paper, we use a coupled biological/physical model to synthesize and understand observations taken during the US JGOFS Arabian Sea Process Study (ASPS). Its physical component is a variable-density, 4.5-layer model; its biological component consists of a set of advective-diffusive equations in each layer that determine nitrogen concentrations in four compartments, namely, nutrients, phytoplankton, zooplankton, and detritus. Solutions are compared to time series and cruise sections from the ASPS data set, including observations of mixed-layer thickness, chlorophyll concentrations, inorganic nitrogen concentrations, particulate nitrogen export flux, zooplankton biomass, and primary production. Through these comparisons, we adjust model parameters to obtain a "best-fit" main-run solution, identify key biological and physical processes, and assess model strengths and weaknesses.

Substantial improvements in the model/data comparison are obtained by: 1) adjusting the turbulence-production coefficients in the mixed-layer model to thin the mixed layer; 2) increasing the detrital sinking and remineralization rates to improve the timing and amplitude of the model's export flux; and 3) introducing a parameterization of particle aggregation to lower phytoplankton concentrations in coastal upwelling regions.

With these adjustments, the model captures many key aspects of the observed physical and biogeochemical variability in offshore waters, including the near-surface DIN and phytoplankton P concentrations, mesozooplankton biomass, and primary production. Nevertheless, there are still significant model/data discrepancies of P for most of the cruises. Most of them can be attributed to forcing or process errors in the physical model: inaccurate mixed-layer thicknesses, lack of mesoscale eddies and filaments, and differences in the timing and spatial extent of coastal upwelling. Relatively few are clearly related to the simplicity of the biological model, the model's overestimation of coastal P being the most obvious example. Overall, we conclude that future efforts to improve biogeochemical models of the Arabian Sea should focus on improving their physical component, ensuring that it represents the ocean's physical state as closely as possible. We believe that this conclusion applies to coupled biogeochemical modeling efforts in other regions as well.

M.-S. Jiang1, F. Chai2, R.C. Dugdale3 and R.T. Barber4

The regulation of iron and silicate on the new and export production in the equatorial Pacific: A coupled physical-biological model study

1 UMass Boston
2 University of Maine
4 Duke University

Based on a coupled physical-biological model with monthly mean forcings, numerical experiments were conducted to examine the effects of iron availability on phytoplankton growth, in terms of changing phytoplankton photosynthesis efficiency (alpha) and molar ratio Si/N of diatom uptake. The increase of alpha (decrease of Si/N) would release the iron stress in the eastern equatorial Pacific. More importantly, this would increase significantly the utilization efficiency of silicon in the process of producing organic matter in the whole area due to reduced silicate consumption by diatoms. With enhanced photosynthesis and reduced Si/N, the new and export organic production in the equatorial Pacific increased dramatically, while opal production and its export remain largely unchanged. As a result, the opal production is lower than new production and silicate is higher than nitrate, which represents a situation consistent with glacial periods but opposite to the modern oceans. The high organic production also tends to drive the system into nitrate depletion without changing other factors. The experiments suggest that diatoms in glacial periods may not have advantage over small phytoplankton as previously thought.

Lloyd D. Keigwin1

Abrupt climate change about 8200 yrs ago and during the Younger Dryas (~12ka)

1 McLean Lab., m/s 8, Woods Hole Oceanographic Inst., 360 Woods Hole Rd., Woods Hole, MA 02543

The "8.2 ka" event and the Younger Dryas (YD) are examples of the kinds of rapid climate cooling that may be possible in global warming scenarios. Each is widely cited in the literature as having been caused by fresh water and iceberg discharge from eastern Canada, yet oxygen isotope (d18O) evidence for these discharges is either meager or lacking entirely. In addition to the well-known cooling, ocean records of the YD show that the age and nutrient content of the deep North Altantic increased at that time. This has led authors to suggest the connection between surface ocean freshening, stable stratification, and reduced convection. Our work on cores from the Laurentian Fan, close to the postulated source of YD meltwater discharge, clearly records the expected cooling and the ice rafting, but not the lowered d18O. Perhaps the meltwater was actually diverted northward to the Arctic, but with no record of it anywhere it is difficult to use d18O as a tracer.

In contrast, about 8200 yrs ago we do see evidence for lowered d18O (and salinity) on the Laurentian Fan, but there is no good evidence for meridional overturning circulation change at the same time. We have tried to follow the low d18O to its supposed source in Hudson Strait, but it has not been identified anywhere in the Labrador Sea. Surprisingly, low d18O is present as far south as 36.5N in the slope waters off Norfolk, VA. Because the prevailing surface boundary currents flow equatorward, we speculate that somehow the proglacial meltwater associated with the final deglaciation of Hudson Bay may have been routed southward to the St. Lawrence drainage system.

Bob Key1

A data-based global ocean carbon climatology

1 Atmospheric & Ocean Sciences Program, Sayre Hall, Princeton Univ., Princeton, NJ 08544-1003

During the 1990s the combined JGOFS/WOCE/OACES field programs provided the first oceanic data set of sufficient quantity and quality to determine the three-dimensional distribution of the biogeochemically important carbon system parameters. These data were assembled into a common format then fully calibrated by a team of U.S. Scientists. The combined data set includes measurements from a total of 277,103 samples. The WOCE-era data were supplemented with an additional 77731 samples from high quality historical expeditions. The WOCE era data were assembled and the carbon data subjected to accepted quality control procedures on a cruise by cruise basis. For all cruises having at least two of the four carbon system parameters (total CO2, alkalinity, pH and/or pCO2) alkalinity and total inorganic carbon were calculated when not measured. The alkalinity and TCO2 were then calibrated using a variety of techniques to assure that the combined data set was as globally uniform as possible. The fully calibrated data set was used to make global estimates of anthropogenic CO2 using a modification of the C* method devised by Gruber. This assembly additionally included radiocarbon data. The calibrated data have been used to produce global-objectively mapped properties on a 1x1 degree grid at 33 surfaces in the vertical. Once mapped, the surfaces were integrated to yield global inventories. The mapped properties include: alkalinity, potential alkalinity, total CO2, anthropogenic CO2, 14C and bomb-produced 14C. By difference we also have an estimate of the preindustrial/natural distribution of total inorganic CO2 and 14C. Both the data sets and the mapped quantities are available for viewing and download via a live access server supported by CDIAC.

Hae-Cheol Kim,1, Eileen E. Hofmann,1, Barbara B. Prezelin2 and Walker O. Smith, Jr.3

Estimation of primary production and carbon flux in Antarctic Coastal Waters: A modeling study

1 Center for Coastal Physical Oceanography, Old Dominion Univ.

A bio-optical production model that is forced with simulated surface and underwater light fields is used to estimate primary production and subsequent carbon flux at a range of sites along the western Antarctic Peninsula and in the Ross Sea that were chosen to represent advectically-controlled regions, diatom-dominated and phytoflagellate-dominated phytoplankton communities, and regions with high grazer impact. The primary production simulations show that diatom-dominated communities have higher production potential compared to phytoflagellate-dominated phytoplankton communities, which is not found for biomass comparisons. Sensitivity studies show a 1-to-3 fold increase in simulated primary production estimates obtained with photosynthetic parameters that have a diel periodicity versus simulations in which the parameters are constant. A similar difference is obtained using spectrally-resolved versus spectrally-neutral photosynthetic parameters. The fate of newly-produced phytoplankton carbon was investigated by estimating the magnitude of grazing, advection, and sinking from available data. This scaling analysis showed that the across-shelf components of advection are the dominant processes that remove phytoplankton carbon from the shelf waters. In particular, across-shelf advection is always high at the outer-shelf, which is typically dominated by diatoms, due to the presence of the Antarctic Circumpolar Current. The effect of grazing tends to be highest in inner-shelf waters, where Antarctic krill are abundant. This analysis allows a comparison of the physical and biological processes that control primary production and carbon flux in the west Antarctic Peninsula and Ross Sea regions and provides a first attempt at developing generalized models for estimating primary production in Antarctic coastal waters.

John M. Klinck1, Michael S. Dinniman1, Walker O. Smith, Jr.2 and Eileen E. Hofmann1

Biogeochemical climatologies for the Ross Sea, Antarctica: Temporal patterns of primary production

1 Center for Coastal Physical Oceanography, Old Dominion Univ.
2 Virginia Institute of Marine Science, College of William and Mary

The temporal pattern of nutrient (nitrate and silicic acid) and chlorophyll distributions in the Ross Sea is formulated in two independent ways. The first procedure compiles all available data from cruises from 1970 to the present and gene rates a three-dimensional grid for November through February using an iterative difference-correction scheme. The second method uses a three-dimensional circulation model that includes the off-shelf Ross Sea gyre and phytoplankton standing stocks to investigate the effects of currents and phytoplankton uptake on nutrient distributions. The two approaches produced similar results, although the circulation model results were more variable in space due to its finer resolution. The nutrient distributions were characterized by elevated concentrations in early spring and gradual reductions to ca. 15 and 40 M (nitrate and silicic acid, respectively) in summer. Nutrient depletion did not occur despite the favorable growth conditions in summer, suggesting that an alternative limitation occurs. Chlorophyll concentrations reached ca. 5 g/L in December and declined thereafter. Seasonal primary production calculated from the nitrate deficits and the circulation model suggested that production was ca. 120 g C/sq m, similar to other estimates using independent methods. Both the nutrient/pigment climatologies and circulation model results confirm that the Ross Sea continental shelf is among the most productive regimes of the entire Southern Ocean.

M.R. Landry1, S.L. Brown2 and A. Calbet3

Carbon cycling through the microbial community

1 Scripps Institution of Oceanography 2 Dept. Oceanography, Univ. Hawaii at Manoa 3 Institut de Cincies del Mar, CMIMA (CSIC), Barcelona

A literature synthesis of phytoplankton growth () and grazing (m) rate estimates from dilution experiments reveals that microzooplankton account for most phytoplankton mortality in the oceans, averaging 60-75% of daily phytoplankton production (PP) across a spectrum of open-ocean and coastal systems. Given reasonable assumptions for transfer efficiencies within the microbial community, the amount of carbon remineralized by protistan consumers is comparable to that of heterotrophic bacteria, but protists substantially exceed bacteria as secondary producers and as a regulatory node for fluxes to higher trophic levels and/or export. In this poster, we consider the derivations of these estimates and the principles by which they can be applied to broadening our understanding of ocean community dynamics and carbon cycling through remote sensing and modeling.

Moritz Lehmann1 and John J. Cullen

Model- and observation based analysis of phytoplankton size distribution

1 Dept. Oceanogr., Dalhousie Univ.

This poster presents an approach that is currently being developed as a PhD research topic to investigate the geographic and temporal variations in size structure of oceanic phytoplankton communities. Variability in the biomass of smallest phytoplankton size class, the picophytoplankton (< 2 m), and in the ratio of the picophytoplankton biomass to the biomass of the large phytoplankton is examined in data from three cruises along a North-South transect in the Atlantic Ocean. In an attempt to explain some of the variability shown, the data is correlated with derived variables that describe characteristics of the environment. To aid understanding and build intuition on size distribution, the output of a mathematical ecosystem model is presented which shows the dependence of steady state biomass of phytoplankton on temperature and nutrient supply. Presenting a thesis research project in its early stages, the poster aims to spark discussion and suggestions. Any offers of datasets to add to the analysis are welcomed.

Ricardo Letelier1, Angelicque White1, Yvette Spitz1 and Jim Christian2

Using laboratory and in situ derived physiological parameters to model Trichodesmium spp. vertical migration capabilities in the NPSG

1 Oregon State Univ.
2 Univ. Victoria

Buoyancy-driven vertical migration by photoautotrophs has strong relevance to oceanic biogeochemistry, for the reason that it represents a uniquely biological source of nutrient injection into the euphotic zone. In the northern subtropical gyre of the Pacific Ocean, Trichodesmium spp. by virtue of their diazotrophic capacity, are recognized as a significant source of new production. However, their potential ability to exploit nutrient sources that are relatively unavailable to other community members via buoyancy-driven migrations to the depth of the nutricline and the ensuing implications of this uptake on elemental cycling, community structure, and the NPSG carbon cycle has not been constrained. Past research with Trichodesmium spp. has shown that vertical migration is both a function of light-mediated carbohydrate ballasting and physiological nutrient status such that the relatively short-term buoyancy response to light is nested in a longer- term response to nutrient climate. Thus, via field and laboratory experimentation, we aim to assess the dynamic range of photosynthetic parameters and elemental stoichiometry (C:P) exhibited by this organism. This information will serve to parameterize a flexible model of Trichodesmium physiology that can adapt to environmental conditions thus assessing the capacity of biologically driven vertical migration as a source of phosphorus to NPSG surface waters. The knowledge of Trichodesmium bioenergetics gained from this research will serve to further constrain the potential role of Trichodesmium spp. in biogeochemical cycling.

Telu Li1 and Tsung-Hung Peng1

Re-evaluation of preformed alkalinity in the oceans for estimating the anthropogenic CO2 inventory.

1 Univ. of Hawaii


Preformed alkalinity is a key parameter in the estimates of anthropogenic CO2 inventory in the ocean. Currently, a single empirical equation has been derived from available surface water data and applied to all three major oceans. However, regional variations of preformed alkalinity within each ocean could result in significant changes in the estimates of anthropogenic CO2 inventory. In this poster, we present results of re- evaluation of preformed alkalinity. Empirical equations for preformed total titration alkalinity (Alk*) as functions of salinity and potential temperature in source regions of the deep ocean waters (North Atlantic, Southern Ocean, and North Pacific) in winter season are estimated. The results from the three regions are all different. In order to estimate anthropogenic CO2 inventory in the oceans, it is necessary to sort out the Alk* contributions from different sources. The empirical equations that were obtained by using surface water data alone appear to be in doubt.

Ivan Lima1 and Scott Doney1

The role of silica limitation and community structure in a marine ecosystem model for the North Atlantic

1 Woods Hole Oceanographic Institution, Woods Hole, MA

Ocean biology plays a fundamental role in the global carbon cycle and climate system as a major sink for anthropogenic carbon. A better understanding of the factors controlling the magnitude of carbon fixation and export from the upper ocean by marine biological processes is fundamental for projecting the ocean response to and feedbacks on anthropogenic perturbations and future climate change. In this study, we incorporate a relatively complex ecosystem model into a three-dimensional, general ocean circulation model to investigate the effect of phytoplankton community structure and silica limitation on the temporal and spatial distribution of chlorophyll and primary production in the North Atlantic. In addition to multi- element nutrient limitation, the ecosystem model includes distinct phytoplankton functional groups as well as size structure (picophytoplankton and diatoms) and incorporates a more realistic, mechanistic based phytoplankton growth and photoadaptation model. Model performance is evaluated against field data from time series stations, process oriented studies sites, and SeaWiFS imagery. The model reproduces the magnitude and the general spatial and temporal patterns in nutrients, chlorophyll and primary production seen in in situ and satellite data and shows substantial improvements over prior basin-scale simulations of the North Atlantic. Discrepancies between model results and observations are relatively few and mostly related to deficiencies in the circulation model. Sensitivity experiments and comparison with previous similar modeling studies show that the explicit inclusion of size structure in the phytoplankton and detritus compartments and silica limitation (for diatoms) result in a significant improvement in model skill.

M.W. Lomas1, A.H. Knap1, N.R. Bates1 and R.J. Johnson1

The Bermuda Atlantic Time-series Study (BATS): A time-series Window on Climate forcing of ocean variability.

1 Bermuda Biological Station for Research, Inc., 17 Biological Lane, St. George's GE01, BERMUDA, Contact: mlomas@bbsr.edu, ph (441) 297-1880 x303, fax (441) 297-8143

The Bermuda Atlantic Time-series Study (BATS) was started over 14 years ago as part of the Joint Global Ocean Flux Study. The BATS sampling region lies ~82km southeast of Bermuda in about 4600m of water near the Ocean Flux Program site and the Bermuda Testbed Mooring. Over this 14-year period, a suite of core measurements has been made monthly or bi-weekly during the winter/spring bloom period (January to April). These measurements cover a wide range of physical, chemical and biological stock measurements. In conjunction with these stock measurements, a number of BATS core rate process measurements are made such as primary and bacterial production, and particle mass flux. Over the record of this program, numerous ancillary projects have greatly enhanced the context of these core measurements.

This 14-year time-series data set affords us the opportunity to re-examine some of the dominant paradigms in biological oceanography. The past decade has seen a shift in fate of the carbon fixed during primary production that is correlated with variability in phytoplankton community structure and climatic forcing. The interannual anomalies of hydrography and ocean biology and biogeochemistry are partially linked to large-scale climate variability such as North Atlantic Oscillation (NAO) and El Nio Southern Oscillation (ENSO). Temperature, mixed layer depth, primary production, phytoplankton community structure and TCO2 anomalies are correlated with NAO variability, with cold anomalies at BATS coinciding with NAO positive states. Salinity, alkalinity and nTCO2 anomalies were correlated with the Southern Oscillation Index (SOI), lagging ENSO events by 6-12 months.

Olivier Marchal1

Abrupt Climate Change: A Pause

1 Dept. Geol. and Geophys., Woods Hole Oceanographic Institution, Woods Hole, MA 02543

We will review some of the recent fascinating and provocative ideas about abrupt climate change and the role of the meridional overturning circulation of the ocean (MOC). The possibility that past changes occurred with a fundamental pacing cycle of 1.5 kyr is the subject of on-going debate. Indeed, the discovery of a spectral line at (1.5 kyr)-1 in the climate system would be "near revolutionary", as the line is located in a gap in the spectrum of external (orbital) forcing. The notions that the energy present at low and high frequencies in the forcing could excite the millennial band or that a complex system like the climate system could keep a memory of 1.5 kyr without forcing at a similar period, are far form clear and rigorous. A leading "theory" to explain the apparent climate variability at the millennial time scale involves an important role of the MOC. Physical oceanography studies, however, point out the at the MOC is not driven by surface fluxes of buoyancy, as implied by the theory. A major requirement for the establishment of a MOC would be a source of mechanical energy to raise deep heavy water across the stable gradient of the large-scale density field. The only possibility for deep heavy water to raise across the gradient is a net gain of buoyancy, which can be achieved only through mixing across density surfaces (diapycnal mixing) The power required to mix the global abyssal ocean was estimated to ~ 2.1 x 1012 W. It was argued that the mixing power is supplied, not by surface buoyancy fluxes, but mostly by the work down by the wind stresses and the tidal forces. With a mixing power that is only one thousandth the magnitude of the poleward heat flux (on order of 1015 W), the MOC should be viewed as a "remarkably effective transporter of heat energy". The possible role of ocean mixing in climate change is basically unexplored.

Katsumi Matsumoto1, Jorge Sarmiento2, Anand Gnanadesikan3 and Robert Key2

How good are ocean carbon cycle models

1 Geological Survey of Japan
2 Princeton Univ.

Today ocean carbon cycle models are used frequently to characterize the modern ocean's response to increasing atmospheric CO2 concentration, as presented, for example, in the influential science reports of the Intergovernmental Panel on Climate Change (IPCC). In such reports, a simple ensemble mean of multiple models is often presented, without accounting for how well individual models perform. Here we compare simulations of anthropogenic CO2, natural radiocarbon, and CFC11 from a suite of Princeton models and those participating in the Ocean Carbon Cycle Intercomparison Project with appropriate observations and show that model performance varies considerably. Some models are clearly better than others. We therefore suggest more caution in the practice of simply taking the mean of available models and somehow imply that it represents the state-of-the-art modeling without considering the credibility of each model. Also, our study suggests that models can be tuned to match the "observed" oceanic inventories of anthropogenic CO2 or CFC11 but not both. Since CFC11 is directly measured, whereas anthropogenic CO2 is actually estimated, this implies that the latter is not entirely accurate.

D.J. McGillicuddy, Jr.1, L.A. Anderson2, S.C. Doney3 and M.E. Maltrud4

Eddy-driven sources and sinks of nutrients in the upper ocean: results from a 0.1 degree resolution model of the North Atlantic

1 Bigelow 209b - MS 11, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-2683 tel., 508-457-2194 fax., dmcgillicuddy@whoi.edu
2 Department of Applied Ocean Physics and Engineering, Bigelow 404 - MS 9, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-3742, landerson@whoi.edu
3 Clark 4 - MS 25, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-3776, sdoney@whoi.edu
4 Los Alamos National Laboratory, Fluid Dynamics Group - MS B216, Los Alamos, NM 87545, 505-667-9097, maltrud@lanl.gov

A nitrate-based model of new production is incorporated into eddy-resolving (0.1 degree) simulations of the North Atlantic. The biological model consists of light and nutrient limited production within the euphotic zone and relaxation of the nitrate field to climatology below. Sensitivity of the solutions to the parameters of the biological model is assessed in a series of simulations. Model skill is quantitatively evaluated with observations using an objective error metric; simulated new production falls within the range of observed values at several sites throughout the basin. Results from the "best" fit model are diagnosed in detail. Mean and eddying components of the nutrient fluxes are separated via Reynolds decomposition. In the subtropical gyre, eddy-driven vertical advection of nutrients is sufficient to overcome the mean wind-driven downwelling in the region and fuels a significant fraction of the annual new production in that area. In contrast, eddies constitute a net sink of nutrients in the subpolar gyre. Geostrophic adjustment to deep winter convection through mesoscale processes causes a net flux of nutrients out of the euphotic zone; the magnitude of this sink is sufficient to counterbalance the mean wind-driven upwelling of nutrients over much of the region. Based on these simulations, it appears that the oceanic mesoscale has major impacts on nutrient supply to, and removal from, the euphotic zone.

D.J. McGillicuddy, Jr.1, E.N. Sweeney2, K.O. Buesseler2 and V.K. Kosnyrev3

Biogeochemical impacts due to mesoscale eddy activity in the Sargasso Sea as measured at the Bermuda Atlantic Time Series (BATS) site

1 Bigelow 209b - MS 11, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-2683 tel., 508-457-2194 fax.,
2 Clark 4 - MS 25, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-3776, erins@mit.edu
3 Dept Applied Ocean Physics and Engineering, Bigelow 404 - MS 9, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, 508-289-3742, vkosnyrev@whoi.edu

A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to assess the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the three years of this study, four out of six high thorium-234 flux events occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two mode-water eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage of diatoms and dinoflagellates, and in cyclones, which had an increased percentage of Synechococcus. These variations in species composition within mode-water eddies and cyclones may be associated with the ages of the sampled eddies, and/or differences in physical, chemical, and biological factors in these two distinct eddy types. In general, eddies that were one to two months old elicited a large biological response; eddies that were three months old may show a biological response and were accompanied by high thorium flux; eddies that were four months old or older did not show a biological response or high thorium flux. A conceptual model depicting temporal changes during eddy upwelling, maturation, and decay can explain the observations in all seven upwelling eddies present in the time-series investigated herein.

A.V. Mishonov1 and W.D. Gardner1

Assessment and correction of the historical beam attenuation data from HOT - ALOHA & BATS sites

1 Department of Oceanography, Texas A&M Univ., College Station, TX, 77843-3146, avm@tamu.edu wgardner@ocean.tamu.edu

Comprehensive analysis, quality control, and adjustment of the beam attenuation data from the Hawaii Oceanographic Time-Series (HOT) and the Bermuda Atlantic Time Series (BATS) data sets collected during the last decade has been performed. These data were collected at fixed locations in the Pacific and Atlantic Oceans and provide an important opportunity to examine seasonal and interannual variations. Some of the raw data exhibit characteristic problems produced by the type of transmissometers used, making those data unreliable for modeling and analysis in their present condition. We have analyzed and corrected the data, making it possible to evaluate Beam Attenuation - Particulate Organic Carbon (POC) relationships, which can then be expanded to study seasonal and interannual variability in POC. These data will be available on HOT, BATS, and TAMU web-servers for the purpose of modeling ocean carbon cycles or evaluating the output from existing models.

Mathieu Mongin1,2 and David M. Nelson1

Flexible-composition phytoplankton modeling: Why and how?

1 College of Oceanic and Atmospheric Sciences, Oregon State Univ., Corvallis, Oregon 97331, U.S.A.
2 Institut Universitaire Europen de la Mer - UMR-CNRS 6539, Technopole Brest-Iroise, Place Nicolas Copernic F-29280, Plouzan, France

We have developed an upper-ocean biogeochemical model in which the elemental composition of the phytoplankton is flexible, and responds to changes in light and nutrient availability. It also allows the elemental composition of particulate detritus and dissolved organic matter to vary in response to changing source and sink terms. The present version of the model includes two phytoplankton groups, diatoms and nonsiliceous picoplankton. Both fix C in accordance with photosynthesis-irradiance relationships used in other models and take up NO3-, NH4+, Fe and (for diatoms) Si(OH)4 following Michaelis-Menten kinetics. The model allows for light dependence of NO3- uptake, and for the observed near-total light independence of NH4+ uptake and Si(OH)4 uptake. It tracks the resulting C/N and Fe/C ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N, Fe and Si to proceed independently when those ratios are close to those of nutrient-replete phytoplankton. When the C/N, Fe/C or Si/N ratio indicates that growth is limited by N, Fe, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (1974). This model structure enables us to determine both the growth-limiting nutrient and the degree of nutrient limitation from the elemental composition of the phytoplankton.

We have applied a first version of this model (without Fe cycling or Fe limitation) to the western Sargasso Sea, and the full model to the Indian Ocean sector of the Southern Ocean. The model reproduces the observed seasonal cycles of nutrients, chlorophyll, primary productivity, f-ratios, biogenic silica production, export of particulate organic carbon (POC) and biogenic silica BSiO2) in both systems with little tuning of parameters. One potentially significant result of these simulations is that the ratio of annual BSiO2 production to annual POC production is quite low (< 0.02) in the Sargasso Sea and extraordinarily high (> 0.30) in the Southern Ocean. This result has been observed in data, and in the model it is a consequence of the flexible elemental composition of the phytoplankton, not arising from model tuning or from an a priori assumption that it would be so. In the model this difference results from the combined effects of severe N limitation of diatom growth in the Sargasso Sea, greater contribution of diatoms to primary productivity in the Southern Ocean and high Si/C and Si/N ratios in Southern Ocean diatoms, resulting from Fe limitation of their photosynthetic efficiency and nitrate uptake.

J. Keith Moore1 and Scott Doney2 Keith Lindsay3

Upper ocean ecosystem dynamics and iron cycling in a global 3D model

1 Univ. California at Irvine
2 Woods Hole Oceanographic Inst., Woods Hole, MA
3 National Center for Atmospheric Research, Boulder CO

A marine ecosystem model with multiple phytoplankton functional groups and explicit iron cycling is linked with an ocean biogeochemistry module in the context of a global circulation model. The coupled biogeochemistry/ecosystem/circulation (BEC) model is used to simulate ecosystem dynamics and carbon fluxes at the global scale. The model reproduces the known basin-scale patterns of primary production, biogenic silica production, calcification, chlorophyll concentrations, macronutrient and dissolved iron distributions. The High Nitrate, Low Chlorophyll (HNLC) region in the equatorial Pacific is larger than observations due to excessive upwelling in the circulation model. Spatial patterns of nitrogen fixation are in good agreement with observations. The diatoms account for 35% of primary production and more than 50% of sinking particulate organic carbon (POC) export from surface waters. CaCO3 from the coccolithophores is an important driver of POC flux to the deep ocean.

Atmospheric mineral dust deposition and shallow ocean sediments are sources of iron to the ocean BEC model. We examine the relative importance of these iron inputs and the sensitivity of ocean biogeochemistry to variations in these sources. The sedimentary iron source is critical in preventing widespread iron limitation in the Arctic and parts of the North Atlantic. However, at the global scale, the ocean is relatively insensitive to variations in this localized iron source. In contrast, primary production, export production, nitrogen fixation, and air-sea CO2 flux are all sensitive to variations in atmospheric iron inputs. The global residence time for iron in surface waters is estimated at ~2 years for the upper 111m and ~6.5 years for the upper 466m. In areas with low dust deposition and no sedimentary source for iron (most of the world ocean), these residence times increase to 6 and 35 years, respectively.

Ragu Murtugudde1, Prasanna Kumar, Jim Christian and Wendy Wang

Mechanisms of subsurface Chl-a maximum in the Bay of Bengal

1 ESSIC, Univ. Maryland

The Bay of Bengal is known for the freshwater input from the rivers and the associated sediment loading. A tropical biogeochemical model is employed along with in situ data for model/data intercomparisons to investigate the mechanisms of subsurface chl-a maximum in the open Bay. Without the incorporation of the impact of riverine discharges on the optical properties of the upper ocean, the model produces a deeper and a stronger subsurface chl-a maximum. Ship-based observations for 2001 and 2002 are analyzed to derive the attenuation of radiation by the sediment loading. The model results clearly indicate that the observed strength of the subsurface chl-a maximum is shallower and weaker when incident radiation is attenuated in the Morel (1988) formula to simulate the sediment effects. The details of the model/data intercomparison are presented.

R. G. Najjar1, X. Jin, F. Louanchi, O. Aumont, K. Caldeira, S. C. Doney, J.-C. Dutay, M. Follows, G. M. Kay, E. Maier-Reimer, R. J. Matear, A. Mouchet, J. C. Orr, G. K. Plattner, J. L. Sarmiento, M. F. Weirig, Y. Yamanaka and A. Yool

Export production simulated by the OCMIP-2 models

1 Dept Meteorology, 503 Walker Bldg, The Pennsylvania State Univ., University Park, PA 16802-5013

Results are presented of export production simulated by twelve global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). A common, simple biological model is utilized in which surface phosphate concentrations are restored towards an observed climatology in the upper 75 m. Organic matter formed as a result is partitioned into dissolved and particulate forms. Oxygen is included by linking to phosphate with a fixed Redfield ratio. Physical transport is the only difference among the models. The downward flux of organic matter across 75 m depth varies from 8 to 20 Gt C yr-1 among the models, with a mean of 14 Gt C yr-1 and a standard deviation of 4 Gt C yr-1. This is in reasonable agreement with satellite-based estimates (121 Gt C yr-1). The range in model export production (40%) is considerably larger than the range in chlorofluorocarbon uptake (30%) and anthropogenic CO2 uptake (20%). Similar to the results for other tracers, the most pronounced differences among the models occurs in the Southern Ocean. The fraction of organic matter exported across 75 m in dissolved form varies from 14 to 34% with a mean of 23% and a standard deviation of 6%, in reasonable agreement with observational estimates (17%). Seasonal net outgassing of oxygen in the models with seasonal circulation varies from 5.0 to 101014 mol, with a mean of 6.61014 mol and a standard deviation of 1.61014 mol, significantly smaller than observational estimates (101014 mol).

Tammi L. Richardson1

Food webs of the equatorial Pacific at 140°W: Synthesizing EqPac carbon flux data into more complete pictures of planktonic ecosystem structure and function

1 Dept. of Oceanography, Texas A&M Univ., MS 3146, College Station, TX 77843-3146, 979-845-2977 (ph), 979-845-8219 (fax), tammi@ocean.tamu.edu

The JGOFS program has provided a wealth of information on the structure and function of planktonic food webs in the equatorial Pacific Ocean. Such information is critical to characterizing interactions between various components of planktonic food web, which will ultimately determine export fluxes of carbon. While the data sets are extensive, the real power of the individual measurements lies in the synthesis of these components into ecosystem-scale models of elemental cycling and ecosystem dynamics. In this talk I will first briefly review EqPac data synthesis efforts to date. Second, I will present new work I've done in collaboration with George Jackson, Hugh Ducklow, and Michael Roman that used an inverse analysis approach to synthesize EqPac carbon flux data from 140°W into food webs that included multiple phytoplankton and zooplankton compartments, as well as compartments for bacteria, DOC, and detritus. This approach allowed us not only to synthesize information, but also to estimate carbon fluxes for which we have no data. As with any data synthesis exercise there were assumptions and decisions that were made with respect to the synthesized data. I will discuss some of the difficulties encountered during the course of the study and will identify measurements that, if made during future large scale programs, would further aid in the characterization of food web processes and their associated export dynamics.

Baris Salihoglu1 and Eileen E. Hofmann1

A one-dimensional Model of Lower Trophic Level Interactions in the Equatorial Pacific.

1 Center for Coastal Physical Oceanography, Old Dominion University, Norfolk VA 23529, baris@ccpo.odu.edu

A complex ecosystem model is developed to determine the underlying dynamics of the phytoplankton community response to the physical processes in the equatorial Pacific Ocean at 140W. In particular the model is used to investigate the mechanisms that lead to temporal and spatial shifts in phytoplankton species composition. The model is forced by time series of spectral light, temperature, and water column mixing. Autotrophic growth is represented by five algal groups (AG) of phytoplankton. The groups have light and nutrient utilization characteristics that reflect those of Prochlorococcus, Synechococcus, autotrophic eukaryotes and large diatoms. The model can also account for accumulation and mobilization of energy reserves (i.e. variability of N:C and N:Fe), photoacclimation (i.e. effect of spectral irradiance on chlorophyll a) and also variability of chlorophyll a with respect to nitrogen and carbon concentrations in the phytoplankton cell. An extension of this model which includes silicate limitation is currently under development. The model results for 1992, suggest that shifts in the species composition of phytoplankton occur seasonally but also may take place on shorter time scales (e.g. 10 days) which are correlated with changes in upwelling/downwelling. Autotrophic eukaryotes and Prochlorococcus (AG1 + AG2) are alternately the most dominant groups in the surface waters. Below the mixed layer a shift towards a system dominated by three groups, Prochlorococcus, autotrophic eukaryotes and large diatoms is observed.

V.V.S.S. Sarma1 and T. Saino1

Impact of sinking carbon flux on accumulation of deep-ocean carbon

1 Hydrospheric-Atmospheric Research Center, Nagoya Univ. Furo-cho, Chikusa-ku, Nagoya 464 8601, Japan

The export of carbon by biological pump from the surface to the deep ocean has a direct influence on the removal of CO2 from the atmosphere. It is because the carbon that is absorbed in the surface waters would sequester in few days to months whereas carbon removed from the surface waters to deep takes several tens to hundreds of years to reenter to atmosphere. Deep water of North Pacific should have the highest DIC among the World Oceans due to aging of waters by long thermohaline path. On the contrary to this, the higher deep water DIC is found in the Northern Indian Ocean than North Pacific. In addition to this, higher DIC were also found in the Subarctic Pacific and Panama Basin compared to elsewhere in the World Oceans. The sinking fluxes of POC and CaCO3 in these regions are found to be the highest among observed in the World Oceans. The organic carbon regeneration rates and dissolution rates are higher in the Northern Indian Ocean than in the North Pacific. Efficient biological pump in the northern Indian Ocean seems to transport surface driven organic carbon to deeper layers thus it can sequester on long time scale.

J. L. Sarmiento1, N. Gruber2, M. A. Brzezinski3 and J. P. Dunne4

Control of diatom production by thermocline nutrient re-supply from the deep ocean

1 Atmospheric and Oceanic Sciences Program, Princeton Univ., Princeton, NJ 08540-0710; 609-258-6585; jls@princeton.edu
2 IGPP and Department of Atmospheric Sciences Univ. California at Los Angeles, Los Angeles, California, 90095; 310-825-4772; ngruber@igpp.ucla.edu
3 Department of Ecology, Evolution and Marine Biology and the Marine Science Institute, Univ. of California, Santa Barbara, California, 93016; 805-893-8605; brzezins@lifesci.ucsb.edu
4 NOAA/Geophysical Fluid Dynamics Laboratory, P.O. Box 308, Forrestal Campus B Site, Princeton, New Jersey, 08542; 609-452-6596; jdunne@splash.Princeton.EDU

Biological export across the base of the main thermocline would deplete the thermocline of nutrients within 60 years if there were no re-supply from the deep ocean. We show that the main return path for nutrients is by deep overturning in the Southern Ocean coupled with lateral flow at the base of the main thermocline as Subantarctic Mode Water (SAMW). There is also a large input of nutrients by high vertical mixing in the northeast Pacific (possibly driven by tidal forcing), which is coupled with lateral flow of North Pacific Intermediate Water (NPIW). Because of processes that occur in the SAMW and NPIW formation regions, the former water type is rich in nitrate but poor in silicate, whereas the latter is rich in both nitrate and silicate. We show that these properties reach all the way to the surface ocean, leading to silica limitation throughout most of the southern hemisphere and North Atlantic (where SAMW is drawn northward as part of the formation of North Atlantic Deep Water), and to silica rich conditions in the surface waters of the North and Equatorial Pacific.

Lei Shi1 and Fei Chai1

Nitrogen, Dissolved Oxygen and Carbon Budget for the Central California Coastal Upwelling System

1 School of Marine Sciences, Univ. Maine

A coupled three-dimensional biogeochemical model is used to simulate the seasonal variability of the nutrients, phytoplankton, zooplankton, O2, and CO2 in central California coastal upwelling region. The material flow (nutrients, dissolved oxygen, and TCO2) in the euphotic zone, with a horizontal area of 10,000 km2 centering the Monterey Bay, has been estimated. By calculating the elemental fluxes among the different compartments of the ecosystem model and physical fluxes in and out of euphotic zone, the model has illustrated how the 'biological pump' functions in this upwelling system. The main conclusions are: the ecosystem is highly regulated by the strength of the upwelling and the nutrient supply, and shows a very strong seasonal variation; the upwelling is the main supply of the inorganic nitrogen to the euphotic zone; more than half of the upwelled inorganic nitrogen is advected horizontally out of the upwelling area; 51% of the primary production is new production (the f-ratio 0.51), 49% of primary production is the recycled production; the modeled air-sea flux of O2 in the domain is a small ingassing (0.08 106 mol O2 d-1), and the region is a weak sink of atmospheric CO2 (11.84 106 mol C d-1).

Yvette Spitz1

Changes in the North Pacific circulation and ecosystem during the last decade

1 ESSIC, Univ. Maryland

- abstract pending -

Colm Sweeney1, Anand Gnanadesikan2, Anthony Rosati2 and Jorge Sarmiento1

The impact of two parameterizations for penetrating shortwave radiation using ocean color observations on GCMs

1 Atmospheric and Oceanic Sciences Program, Princeton Univ., Princeton, NJ
2 Geophysical Fluid Dynamics Lab, Princeton, NJ

The impact of two different parameterizations for penetrating shortwave radiation using ocean color data (Morel and Antoine, 1994 and Ohlmann 2003) on the ocean circulation is studied using the GFDL Modular Ocean Model (MOM4). We find that with increases in the depth of the 1% solar irradiance penetration depth of between 10 and 18% for solar irradiance in the water column there is a consequential increase in mixed layer depths of 3-26 m. The change in mixed layer depth is dependant not only on the increase in solar irradiance penetration depth of but wind speed, density gradient below the mixed layer and existing mixed layer depth. The increase in mixed layer depths results in a decrease in the export of heat (2%) from the tropics with a slow down in meridional transport of surface waters away from the equator and a decrease in the net return flow at the base of the mixed layer. In MOM4 a slow down in overturning circulation results in a 10% decrease in the required restoring heat flux needed to maintain sea surface temperatures over a 10 day period in the Eastern Equatorial Atlantic and Pacific Oceans. At higher latitudes (5°-40°), higher restoring heat fluxes are needed to maintain sea surface temperatures due to deeper mixed layers and an increase in storage of heat below the mixed layer. This study offers a way to evaluate the effect of new irradiance parameterizations on couple ocean-atmosphere GCMs and the potential effect of changes in chlorophyll a concentrations will have on ocean circulation.

LuAnne Thompson1, Ivan Lima2 and Steve Emerson1

Thermocline Ventilation and Apparent Oxygen Utilization in the North Pacific: Physical or Biological Changes?

1 School of Oceanography, Univ. of Washington, Seattle WA, 98195
2 Woods Hole Oceanographic Inst., Woods Hole, MA

There is strong observational evidence that the apparent oxygen utilization (AOU) in the subtropical North Pacific has increased from 1981 to 1990. This could be the result of either an increase in biological producitivity or a decrease in the ventilation of the thermocline. To investigate these possibilities, biogeochemical tracers have been incorporated into an ocean general circulation model to investigate how changing atmospheric forcing over the last five decades has changed ocean ventilation and oxygen utilization. The isopycnal model includes CFCs, dissolved oxygen and an idea age tracer. The depth dependent oxygen utilization rate is taken from Jenkins is applied below the euphotic zone. Numerical experiments suggest that most of the changes can be explained by physical changes in the circulation as opposed to changes in the biological productivity. This is consistent with studies in other parts of the world where decrease dissolved oxygen concentrations at the base of the thermocline have been observed. This results in a need to revise estimates of CO2 uptake by the oceans.

Robbie Toggweiler1, Joellen Russell2 and Steve Carson3

CO2 Amplifier in the Physical Climate System

2 AOS Program, Princeton Univ.
3 Princeton Regional Schools

Atmospheric CO2 has varied through four major cycles over the last 400,000 years yet the cause remains unknown (Petit et al., 1999). The standard view is that these CO2 changes are caused by biogeochemical changes in the ocean (Broecker, 1982). Here it is argued that changes in ocean biogeochemistry are an important but fairly small piece of the puzzle. The main event is a physical amplifier that boosts a slow ocean biogeochemistry cycle into the full set of 100-ppm cycles seen in Antarctic ice cores. The proposed amplifier is a feedback loop that ties together changes in tropospheric temperatures, the strength and position of the mid-latitude westerlies, the overturning of the deep ocean, and atmospheric CO2. Aspects of the amplifier seem to be at work today in response to the CO2-induced warming of the troposphere. The key to the amplifier is a link between ocean overturning and atmospheric winds. We suggest that warm high-CO2 climates are characterized by "high-index" westerlies, now often described as the annular mode, that draw more deep ocean water to the surface around Antarctica. Cold glacial climates are characterized by weak "low-index" westerlies that draw less deep water to the surface and induce less deep water to form.

Dierdre Toole1 et al.

Light and the dimethylsulfide (DMS) summer paradox in the Sargasso Sea

1 Univ. California, Santa Barbara

- abstract pending -

Wendy Wang1 Jim R. Christian2 Ragu Murtugudde1 Antonio J. Busalacchi1

Simulating pCO2 and air-sea CO2 flux in the equatorial Pacific for the last decade: impacts of biological and physical forcing

1 Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742 USA, Tel: (301) 405 1532, Fax: (301) 405 8468, wwang@essic.umd.edu
2 Canadian Centre for Climate Modelling and Analysis, University of Victoria, Victoria, BC, V8W 2Y2 Canada

A carbon model is implemented into a coupled basin-scale physical-ecosystem model to simulate the carbon cycle in the Equatorial Pacific. The model is forced by climatological monthly data of solar radiation, cloudiness, and precipitation. Air temperature and humidity are computed by an advective atmospheric mixed layer model coupled to the OGCM-biogeochemical model. The model is forced with 6-day mean surface wind-stresses and wind-speeds such that only wind-forcing and latent/sensible heat fluxes are interannually varying. Biological parameters were selected to capture the surface variability observed in the SeaWiFS derived chlorophyll a concentration. While the model is capable of reproducing seasonal to interannual variability in pCO2 and air-sea CO2 flux, spatial variations in the carbon field are sensitive to the wind stress product. The model forced by the NCEP wind stresses simulates higher primary production in the west, but lower primary production in the central and eastern Equatorial Pacific compared to the model simulations forced by the ECMWF wind stresses. Overall, the ECMWF wind produces lower pCO2 and out-gassing along the equator than the NCEP reanalyses winds.

Wendy Wang1 Jim R. Christian2 Ragu Murtugudde1 Antonio J. Busalacchi1

Ecosystem dynamics and primary production in the Equatorial Pacific: a model and ocean color synthesis study

1 Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742 USA, Tel: (301) 405 1532, Fax: (301) 405 8468, wwang@essic.umd.edu
2 Canadian Centre for Climate Modelling and Analysis, University of Victoria, Victoria, BC, V8W 2Y2 Canada

Accurate simulations of carbon cycle cannot be achieved without a realistic ecosystem model in terms of its ability to simulate the ecosystem structure and to estimate primary production. Our modeling effort includes testing the sensitivities of an existing ecosystem model that has nine components with nitrogen and iron limitations. A series of sensitivity studies are carried out to identify the range of biological parameters. We vary the biological parameters so that the model can reproduce the spatial and seasonal to interannual variability in the SeaWiFS derived chlorophyll a. We further validate model solution of primary production and ecosystem structure by comparing the simulations with all available observations. This allows us to select appropriate constants for parameterizations of grazing, light penetration, Fe/N uptake ratios, and interaction of nutrient limitation and light limitation. These sensitivity studies also indicate that including an aggregation term can significantly improve model simulation. While our studies indicate the usefulness of ocean color data for estimating model parameters, additional constraints such as primary production and pCO2 are needed for validating model structure and parameterization. The new parameter set for the biological model not only improves the simulated primary production, but also better reproduces many features in observed chlorophyll a in the equatorial Pacific. Our results from the model indicate that the background surface value of chlorophyll a (0.2 mg/m3) in this region is representative of small phytoplankton. Deep chlorophyll maximum, and spatial variability and the seasonal to interannual signal in chlorophyll a are largely associated with the large phytoplankton that contribute, on average, less than 20% of the total community biomass in the surface.

Toby Westberry1, David Siegel1 and Ajit Subramaniam2

A new technique for the remote Sensing of Trichodesmium

1 Inst. for Computational Earth System Sci., Dept Geogr., Univ. California, Santa Barbara, CA 93106
2 Earth System Sci. Interdisc. Ctr, Univ. of Maryland, College Park, MD 20742-2465

Ocean color remote sensing of Trichodesmiumspp. provides a method to estimate the importance of N2 fixation in global ocean biogeochemical cycling. This requires a globally applicable bio-optical model that relates Trichodesmiumbiomass to its water leaving radiance signal. Previous empirical models do not perform well compared with a global dataset containing concurrent measurements of Trichodesmiumabundances and available radiometric measurements. Hence, alternative approaches must be developed. Here, we develop and present a new Trichodesmium-specific inverse reflectance model to determine the presence of Trichodesmiumblooms. Model coefficients were optimized using the in situ global dataset and >75% of Trichodesmiumblooms are correctly identified while the number of false positive retrievals is minimized. An example application of the model to SeaWiFS imagery is shown. Preliminary results show spatial distributions consistent with published syntheses of Trichodesmiumbloom occurrences suggesting the validity of this approach. Further work will be focused on understanding the oceanographic and atmospheric conditions which lead to Trichodesmiumblooms and an estimate of global N2 fixation due to these blooms.

J.D. Wiggert1, R.G. Murtugudde1 and J.R. Christian1

The role of iron in the Indian Ocean ecosystem: Results of a 3-D bio-physical model

1 CCPO, Old Dominion Univ., Norfolk, VA
2 ESSIC, Univ. Maryland, College Park, MD
3 CCCMA, Univ. Victoria, Victoria, BC, Canada

A fully coupled, 3-D bio-physical ocean general circulation model has been applied to the Indian Ocean. The biological portion of the model is a 9-component oceanic ecosystem, consisting of a large and small size class for phytoplankton, zooplankton and detritus, as well as three phytoplankton nutrients (nitrate, ammonium and iron). In our standard solution, the iron flux boundary condition derived from the GOCART model (Ginoux et al., 2001) was applied. An alternative oceanic solution was obtained by using the flux condition derived from the GISS atmospheric transport model (Tegen and Fung, 1994). Over the whole basin, the standard climatological solution has been validated with the NODC seasonal nitrate climatology and a monthly SeaWiFS climatology of surface chlorophyll a. A more comprehensive validation of the ecosystem was possible in the Arabian Sea through the use of in-situ measurements obtained during the US JGOFS Arabian Sea Process Study. The model's overall success at simulating the considerable temporal and spatial biogeochemical variability characteristic of the Arabian Sea engenders confidence in the model solution in regions of the IO that are observation deficient. Here we present seasonal, basinwide distributions, suggested by the standard solution, of surface waters in which phytoplankton growth tends toward either N- or Fe- limitation. Because of the inherent differences between the two mineral dust transport schemes utilized, we infer from distributions of surface chlorophyll a and maps of limiting nutrient tendency that aeolian deposition over the Arabian Sea helps define the region's summertime distribution of offshore phytoplankton blooms.

J.D. Wiggert1, R.R. Hood2, K. Banse3, J.C. Kindle4, J.P. McCreary5, R.G. Murtugudde6 and C.R. McClain7

Monsoon-driven biogeochemical processes in the Arabian Sea

1 CCPO, Old Dominion Univ., Norfolk, VA
2 UMCES, Cambridge MD
3 Univ. Washington, Seattle, WA
4 Oceanography Division, NRL, Stennis Space Center, MS
5 IPRC, Univ. Hawaii, Honolulu, HI
6 ESSIC, Univ. Maryland, College Park, MD
7 NASA-Goddard, Greenbelt, MD

The comprehensive observational data sets of the Arabian Sea obtained during the 1990s by the various JGOFS survey expeditions and automated measurement platforms represent a significant addition to the in situ observational database for this region. In addition, the SeaWiFS ocean color measurements of the last five years have served to both reinforce and revise our CZCS-based view of the region's seasonal to interannual biogeochemical variability. Within the context of the JGOFS-SMP, both the in situ and remotely sensed data have provided a significant impetus to, and are an integral component of, ongoing research employing coupled physical-biogeochemical models that cover the full range of spatial domains (i.e., local to regional to basin scale). These modeling studies have already significantly advanced our understanding of the physical mechanisms behind the complex spatial and temporal biogeochemical variability that characterizes the Arabian Sea.

Although it is in a nominally tropical locale, the semiannual wind reversals associated with the Monsoon result in two distinct periods of elevated biological activity over much of the Arabian Sea. While in both cases monsoonal forcing drives the surface layer nutrient enrichment that supports the increased rates of primary productivity, different entrainment mechanisms are in force for the winter (convective mixing) and summer (vigorous coastal upwelling and Ekman pumping) monsoons. Here we revisit the pre-JGOFS paradigms related to these two monsoon periods, the seasonal cycle and interannual variability, and contrast them with the fresh insights garnered from the recent JGOFS-SMP modeling and data synthesis efforts.