Synthesis & Modeling Project
Impacts of mesoscale processes on biogeochemical fluxes
in the North Atlantic: Basins-scale eddy-resolving simulations.
Coupled physical--biogeochemical models provide a useful framework for
testing our ideas about the underlying physical, biological and chemical
controls on elemental cycling in the ocean. Up to now, efforts to do so
on the scale of ocean basins have produced mixed results. The seminal work
of Sarmiento et al. (1993) and Fasham et al. (1993) demonstrated that the
large scale distributions of chlorophyll in the surface waters of the North
Atlantic could be simulated with a reasonable degree of accuracy using
a simple planktonic ecosystem embedded in a coarse resolution general circulation
model. While the general characteristics of these large scale patterns
were consistent with observations, detailed comparisons with time series
data at specific sites revealed that the underlying flux balances maintaining
these distributions were not as satisfactory.
During the last several years, there has been significant progress in the development of more realistic general circulation models. Areas of improvement particularly relevant to biogeochemical processes include (1) aspects of the time-mean flow, (2) treatment of the surface boundary layer, and (3) the resolution of mesoscale eddies. This next generation of GCMs represents a substantial step forward in the physical bases on which coupled biogeochemical simulations can be built. It is proposed herein to conduct basin scale eddy-resolving biogeochemical simulations of the North Atlantic to examine the impact of mesoscale processes on large scale elemental budgets.
Our prior results have demonstrated a dramatic shift in the phenomenology of nutrient transport as resolution is increased from coarse scales (1.6 degree) to the eddy resolving case (0.1 degree). Whereas coarse resolution models suggest the dominance of convective and diffusive nutrient supply, eddy-induced upwelling plays an important role in the high resolution calculations. In 1999 we were very fortunate to have the opportunity to participate in a pilot high-resolution (0.1 degree) simulation at LANL, carried out by M. Maltrud. Diagnosis of the model solutions suggests that, except for the wintertime period of deep convection, the dominant mechanism of nutrient input to the euphotic zone is vertical advection by eddies. Annual fluxes in the Sargasso Sea are of the same order as those derived from regional eddy-resolving models (McGillicuddy, et al., 1998; McGillicuddy and Robinson, 1997) and satellite-based estimates of new production (Siegel et al., 1999).
A suite of experiments is planned with a five-component biogeochemical model (the OCMIP-2 formulation) run at resolutions of 1.6 degree (coarse), 0.4 degree (eddy-permitting) and 0.1 degree (eddy-resolving). The coarse resolution runs will be used as a benchmark for comparison with other such models, and serve as a control run for the higher resolution calculations. Although the eddy-permitting case will most likely not be entirely sufficient for a comprehensive study of mesoscale processes, it does represent a meaningful intermediate step that will provide useful information without incurring the substantial computational cost of the 0.1 degree runs. This will set the stage for a relatively small number of fully eddy-resolving runs which have only recently become feasible from a computational point of view. Understanding gained from these experiments will be used to guide the development of parameterizations for mesoscale biogeochemical processes to permit their inclusion in larger scale (global) models.
|DATA:||- no data submitted -|
|PUBLICATIONS:||McGillicuddy, D.J., Jr., L.A. Anderson, S.C. Doney and M.E. Maltrud. 2003. Eddy-driven sources and sinks of
nutrients in the upper ocean: Results from a 0.1-degree resolution model of the North Atlantic, Global Biogeochem. Cycles,
17(2), 1035, doi:10.1029/2002GB001987.
McGillicuddy "The role of
eddies in basin-scale biogeochemical budgets of the North Atlantic"
Department of Applied Ocean Physics and Engineering
Woods Hole Oceanographic Institution
98 Water Street, MS#12
Woods Hole, MA 02543
tel: (508) 289-2683
fax: (508) 457-2194