Summary of achievements

As part of the ocean thermohaline circulation, deep water forms in polar regions. While the convective formation processes for these cold, dense waters can be represented reasonably well in global ocean models, in general z-coordinate ocean models are unable to represent the deep penetration of these waters due to coarseness of the horizontal grid, along with the stair-step bottom topography which promotes excessive entrainment of ambient waters. To address this deficiency, I have been working to implement a new ocean parameterization, termed the overflow parameterization, into the ocean component of Community Climate System Model. This new parameterization attempts to represent the subgrid-scale, bottom topographic dense waters that are thought to be the main source of the deep waters. These dense deep waters hug the bottom topography and overflow sills or flow through narrow canyons from polar shelves or shallower polar basins into the abyssal ocean. We have found a workable method to put this parameterization into the POP2 ocean model, and are presently testing the parameterization for a realistically forced ocean. If the results look promising, we will attempt to include this parameterization into CCSM4.

This work was done under the direction of Dr. Gokhan Danabasoglu and Dr. Bill Large of NCAR.

Publications

Legg, S., B. Briegleb, Y. Chang, E.P. Chassignet, G. Danabasoglu, T. Ezer, A.L. Gordon, S. Griffies, R. Hallberg, L. Jackson, W. Large, T.M. Özgökmen, H. Peters, J. Price, U. Riemenschneider, W. Wu, X. Xu and J. Yang. 2009: Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team. Bulletin of the American Meteorological Society, 90, 657-670, doi:doi:10.1175/2008BAMS2667.1.

Figure 1: High resolution figure

Abstract: Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.

Figure Caption: Salinity in the Mediterranean outflow plume shown as a function of latitude and depth along a section at 8.5°W from (a) the World Ocean Circulation Experiment (WOCE) observations and (b) HY COM regional simulation at 0.08° horizontal resolution with 28 layers in the vertical. The regional model was integrated for six months, and salinity is shown for the end of the integration.