DAVID M. HOLLAND and DOUGLAS G. MARTINSON, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964
The relatively large heat capacity of the global ocean makes it likely to be an important player in determining both the general nature and variability of our climate system. In this regard, an important aspect of the ocean that requires better understanding is the rate at which it forms and ventilates its deep waters. In this study, we focus on the contribution of antarctic waters, and in particular those waters forming in the Weddell Sea, to the ventilation of the global ocean. At present, a large international program is underway to study this process under the name of the Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) experiment (see overview by Muench, Antarctic Journal , in this issue). The geographical setting for the study, shown in figure 1, is the vicinity of the Weddell-Scotia Confluence (WSC), which is a key region for the exchange of water masses between the Weddell Sea and global oceans.
The specific objectives of the DOVETAIL study are fourfold:
These objectives are to be met by carrying out an integrated field and modeling program. This article is a preliminary report on one of the DOVETAIL modeling activities.
The WSC is a region of extreme characteristics, dominated by strong currents, intense storms, and complex bottom topography. Therefore, our modeling activity focuses on assessing statistically robust features of the ventilation and flow and general sensitivities of the rate, paths, and variability of the flow to the forcing and lateral boundary conditions. Specifically, we are addressing these statistical aspects by experimenting with two different models and performing a series of experiments under a variety of statistically reasonable forcings and boundary conditions.
Our initial modeling activity has involved the configuration of the Miami Isopycnal Coordinate Ocean Model (MICOM); a layered model that uses density as its vertical coordinate (Bleck et al. 1992). This model discretizes the ocean into a series of horizontally stacked isopycnal layers with the exception of the mixed layer, which is nonisopycnal because it is driven by surface buoyancy fluxes. For each layer, we solve for temperature, salinity, thickness, and velocity using the appropriate shallow-water equations. Our motivation for choosing an isopycnal coordinate model is that we believe that the subsurface waters in the real ocean flow and mix primarily along isopycnal surfaces. The present modeling framework allows us to mimic that behavior. An additional benefit is that the only vertical mixing processes that occur between layers are the diapycnal fluxes that we explicitly choose to include in our model—there is no hidden or otherwise implicit numerical vertical diffusive contribution. We have also coupled a dynamic-thermodynamic sea-ice model to our ocean model for the purpose of more accurately estimating the exchange fluxes of heat, mass, and momentum at the ocean surface in the WSC region. The coupled ice-ocean model is driven by standard atmospheric forcing products available from the National Center for Atmospheric Research and the European Center for Medium Range Weather Forecasting archives.
To test the suitability of the MICOM model to our study region, we set up the model with a vertical resolution of 10 isopycnal layers ranging from a surface density of 1,027.4 kilograms per cubic meter (kg/m3) to a bottom density of 1,028.2 kg/m3. The horizontal resolution is approximately 20 kilometers. The model domain spans from 65 west to 35 west and from 70 south to 55 south. We initialize the model temperature and salinity fields using the Levitus (1982) climatology. Along the open boundaries of our domain, we restore the model hydrography in a relatively narrow band to that of the Levitus climatology throughout the model run. We are presently analyzing the model output from our first completed runs to determine how the model simulates water mass exchange and transformation between the Weddell Sea and Scotia Sea via the channels and passageways of the WSC.
As an example of the simulated exchange, we show the modeled temperature and flow field (figure 2) as well as salinity and flow field (figure 3) for the intermediate 1,027.7 potential density surface. This surface occurs at an intermediate depth in the confluence region and is thus representative of how water masses flow and mix there. We clearly see the southward flowing warm and salty water from the Scotia Sea as it travels through the confluence; likewise, the northward flowing cold and fresh Weddell waters are also evident. Even at this modest horizontal resolution, the strong impact of topographic steering is quite evident by comparing the correlation between topographic features in figure 1 and the flow fields in figures 2 and 3.
Our next modeling activities are to increase both the horizontal and vertical resolutions to increase, we hope, the accuracy of the simulation. The increased horizontal resolution to approximately 3-kilometer size will mean that we can begin to resolve mesoscale eddies. The output from such higher resolution model runs will be used in diagnostic studies to quantify the amount of Weddell Sea deep water that is flowing through the confluence zone and thereby contributing to the deep waters of the world ocean. In that context, we will be comparing the amount of heat and freshwater that is transported laterally by the ensemble mean circulation, the stationary eddies, a nd the transient eddies in an effort to determine the dominan t mechanism for lateral transport and ventilation. These runs and their robust aspects will ultimately be compared to the observational data once those become available (the moorings have been deployed for a year, although the conductivity-temperature-depth data acquired during the deployments will provide some initial constraints and diagnostics).
Computing resources were kindly provided by the Arctic Region Supercomputing Center of the University of Alaska, Fairbanks. This work was supported by National Science Foundation grant OPP 95-27752.
Bleck, R., C. Rooth, D. Hu, and L.T. Smith. 1992. Salinity-driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the North Atlantic. Journal of Physical Oceanography , 22, 1486-1505.
Levitus, S. 1982. Climatological atlas of the world ocean (National Oceanic and Atmospheric Administration publication 13). Washington, D.C.: U.S. Department of Commerce.
Muench, R.D. 1997. Deep Ocean Ventilation Through Antarctic Intermediate Layers: The DOVETAIL program. Antarctic Journal of the U.S., 32(5).