16 Planning the next JGOFS North Atlantic study
Jorge L. Sarmiento
Princeton University
Princeton, NJ 08544-0710
I identify three major goals and at least one secondary but very interesting goal that we should attempt to address in the next North Atlantic Study. I will discuss each in turn:
(1) The first major goal is to improve our understanding of the air-sea flux of CO2. The following table gives my view of our present understanding based on Taro Takahashi's observations as reported in Tans et al. (1990), results from a GCM study done by Rick Murnane here at Princeton, and results from the Keeling et al. (1989) study:
Table 16.1
Air-sea CO2 pre-industrial fluxes.
(GtC/yr)
Tans et al. Princeton Keeling et
(1990) model al. (1989)
(a)* (b)** (a)-(b)
Total Perturbation Pre-industri Pre-industrial Pre-industri
air- al Flux Flux al Flux
sea flux
15°N-90°N -0.59 -0.44 -0.16 -0.64 -1.68
15°S-15°N +1.62 -0.54 2.16 1.04 1.83
90°S-15°S -2.59 -0.93 -1.66 -0.64 -0.14
Total -1.56 -1.90 0.34 -0.24 0
*Combined pre-industrial and perturbation air-sea fluxes observed by Tans et al. (1990)
**Air-sea flux due to the anthropogenic perturbation as calculated by Sarmiento et al. (1992).
The observational constraints of Takahashi show a large equatorial efflux balanced by a southern hemisphere uptake. The atmospheric transport model study of Keeling et al. (1989) suggests a large equatorial efflux balanced by a northern hemisphere uptake. Our simulations show a more modest equatorial efflux balanced by an equal uptake in both hemispheres. We have subsequently shown that our ocean flux can be made consistent with atmospheric transport by reducing slightly the equatorial efflux and southern hemisphere uptake, and increasing slightly the northern hemisphere uptake.
If one adds the anthropogenic perturbation to the above results one obtains the following estimates of the sea-air CO2 gradients required to drive the fluxes using a radiocarbon based gas exchange coefficient:
The difference between the extreme scenarios of Tans et al. (1990) and Keeling et al. (1989) is of order 15 ppm to 37 ppm in the North Atlantic. If we could measure the pCO2 to of order a few ppm, we would be able to make a major advance in constraining the spatial distribution of air-sea CO2 fluxes.
Table 16.2
Northern Hemisphere Sea-Air pCO2 Gradients
Sea-Air pCO2 (ppm)
North Pacific North Atlantic
latitude band
Tans et al. (1990) 2 -37 >50°N
observations
-15 15°N to 50°N
Keeling et al. (1989) -19 -52 >23.5°N
estimate
Can this be done? I am not sure it can be as yet. One problem is that the scientific community is still not agreed on what the gas exchange coefficient is within a factor of 2. The second is the sampling problem. My view of this is summarized in the following table:
Table 16.3
Detecting Oceanic Uptake of Anthropogenic CO2
Property Anthropogenic Seasonal Measurement
Signal Variability Precision
(BATS & HOTS)
DIC 40 mmol since 1860 30 mmol 2 mmol
1 mmol /yr today
pCO2 -7 to -15 ppm today 80 ppm 0.5 ppm
The range in the pCO2 anthropogenic signal reflects the uncertainty in the gas exchange coefficient. The enormous size of the seasonal variability relative to the anthropogenic signal is what makes our sampling task so difficult. At the present time I believe that our best hope is continuous monitoring by moored instruments such as those that Peter Brewer is developing at MBARI. If these instruments can be developed by the time of the North Atlantic Study, we should give their deployment a very high priority. The gas exchange coefficient problem continues to be addressed by a variety of approaches using multiple tracers (e.g., the work of Spitzer and Jenkins, as well as Watson and crew, and Wanninkhof and crew using purposeful tracers). In addition the eddy correlation technique may have matured to the point where one might be able to set up some experiments combining atmospheric eddy correlation measurements with oceanic measurements to try and constrain the system.
(2) The second major goal is to improve our understanding of the oceanic processes that control surface pCO2. I have addressed the gas exchange problem above. The other processes that are important are the effect of heating and salinity changes, and the effect of biology. I like the basic concept of a control volume, but I am not at all enthusiastic about an experiment that would not also properly address the temporal variability problem noted above. I would not support the notion of steaming out somewhere in the middle of the North Atlantic for a few months to do a control volume. I would therefore recommend that the control volumes should be centered on time-series stations. The purpose of the control volumes would be to contribute to our understanding of the role of horizontal processes in the processes observed at the time series stations. We have Bermuda, where there are several mysteries that may be difficult to resolve until the role of horizontal processes is clarified (e.g., the big drop in carbon apparently unaccompanied by an adequate nutrient supply). We should attempt to divert as much of our resources as possible towards setting up additional time-series stations in critical (hopefully non-oligotrophic) regions that would last for a few years (hopefully of order 3) centered on the period of the study. The time series stations have to include carbon system measurements. If chemical moorings are feasible, they should be a central part of the monitoring strategy.
(3) A third major goal is to provide observations that can be used to validate models for translating the satellite OCI observations into useful information on the role of biology in the ocean carbon cycle. This means not only chlorophyll, but also estimates of important quantities, primarily the new production. We have been trying to reproduce the Balch et al. type analysis comparing predicted with observed PP using the results of a new North Atlantic GCM simulation we have run in which we assimilate CZCS observations by correcting the phytoplankton concentrations to match those inferred from satellite chlorophyll. After using Balch's filter to remove stations that he had doubts about, there is very little left in the open Atlantic (several hundred of his useful stations--the great majority--are in the New York Bight). The only "trustworthy" data that we have ready access to are the BATS and NABE results. These are insufficient for a serious effort to develop techniques for analyzing satellite observations.
(4) An ancilliary goal to all the above is based on A. Michael's suggestion that Bermuda may have a substantial amount of nitrogen fixation going on. Michael's and I have made an estimate based on BATS data and tritium ventilation time scales that the excess nitrogen evident in the North Atlantic gyre may require fixation of order 60 Tg/yr of nitrogen. R. Key has subsequently produced a set of global sections based on an analysis scheme that I suggested for detecting excess nitrate as well as deficient nitrate waters due to denitrification. These sections suggest that the North Atlantic is a major source of excess nitrogen to the rest of the world. One possibility that has arisen is that the fixation, dependent as it is on high iron supply at just the right time, may be temporally quite variable. The carbon uptake resulting could be as much as of order 1 GtC/yr. Our GCM calculations suggest that on a short time scale perhaps half or more of this carbon could come from the atmosphere.