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Circulation and Climate Variability in the Atlantic Driven by Spatially Non-Uniform Mixing

OCCI Project Funded: 2001

Proposed Research

The overturning circulation of the ocean plays an important role in modulating the Earth's climate. Amongst many competing physical processes, diapycnal mixing has been identified as one of the most important factors governing the size of the meridional overturning cell. For spatially uniform mixing, it is well known that the meridional overturning rate increases as the mixing rate is enhanced. While almost universally portrayed in terms of a turbulent diffusion coefficient, diapycnal mixing is ultimately driven by sources of mechanical energy. The sources of this mechanical energy can vary widely in space and time, and they have not hitherto been portrayed realistically. As a result, our understanding of the meridional overturning cell remains rudimentary. Since the diapycnal mixing rate directly affects water mass transformations in the world oceans, we anticipate that non-uniform diapycnal mixing will have major impact on the oceanic general circulation. We propose here to implement an improved representation of diapycnal mixing in an Ocean General Circulation Model (OGCM) study of the Atlantic Ocean. Our intent is: (1) to document the differences resulting from this new scheme relative to the spatially uniform case for the modern ocean, (2) to assess the sensitivity of the model with the new diapycnal mixing scheme to small amplitude changes in surface forcing, and (3) to modify the mixing scheme to represent likely conditions at the last glacial maximum and examine the model's response.

Final Report

With financial support from the Ocean & Climate Change Institute, we have successfully completed this project.

First, we have developed a new scheme of turbulent mixing appropriate for use in numerical models. Previous field measurements suggested mixing is spatially heterogeneous, being typically larger and bottom enhanced above rough topography associated with mid-ocean ridges. Our new scheme combines both theory of internal waves and in-situ observations in a concise way, and it predicts a mixing rate that is fairly realistic compared with previous in-situ observations. We are currently working on a manuscript describing the resulting parameterization that will be ready for submission soon.

This scheme has been used in a series of numerical experiments on the circulation of the Atlantic Ocean with realistic forcing and bottom topography. Compared to a model with spatially uniform mixing, the model with our new scheme of mixing produces deep circulation that is much more realistic. The results obtained from these studies are presently being written into a manuscript that will be submitted to the Journal of Physical Oceanography.

In addition, our studies of a hierarchy of models under the energy constraint indicate that thermohaline circulation under the energy constraint behave dramatically different from existing models based on a fixed rate of mixing. Most importantly, when the hydrological cycle is enhanced, models under the energy constraint predict stronger meridional overturning circulation and poleward heat flux. However, most existing models predict an opposite trend. This study has recently been submitted to Nature.

Originally published: January 1, 2001