Thermohaline Circulations and Variability in a Two-Hemisphere Sector Model of the Atlantic

DAVID MCDERMOTT AND EDWARD S. SARACHIK1

ABSTRACT

A coarse-resolution, simplified-geometry (sector) model of the Atlantic is described. Important features of this model include a Northern Hemisphere sinking region, a Southern Hemisphere sinking region, and a re-entrant channel to simulate the effects of the Antarctic Circumpolar Current. The circulation of the model ocean is described and diagnosed under restoring boundary conditions for both temperature and salinity. The model proves capable of simulating the grosser aspects of the deep-water Atlantic circulation. The variability of the thermohaline circulation under a switch to mixed boundary conditions (restoring on temperature and flux condition for salinity) is described.

INTRODUCTION

The thermohaline circulation (THC) of the Atlantic has a sinking branch, mostly in the Nordic seas and partly in the Labrador Sea, that puts cold water beneath the warm surface water. It also has a southern sinking branch, mostly in the Weddell Sea but partly in the Ross Sea. Cold, dense waters from these sinking regions spread out through the world ocean and are generally responsible for the maintenance of the stratification of the world oceans against the effects of heat diffusion from the surface. The export of cold water from high latitudes and its replacement by warmer surface waters implies a net heat transport toward the sinking regions: The THC is generally recognized as an important determinant of the mean climatic state of the earth's surface and overlying atmosphere.

The water from the Antarctic sinking region is extremely cold and relatively fresh, so when it sinks it is more compressible and therefore denser than the more saline deep water formed in the North Atlantic. The deep water formed in the Antarctic sinks to the bottom and becomes Antarctic Bottom Water (AABW, see Figure 1). The sinking from the Nordic seas becomes North Atlantic Deep Water (NADW), which overlies the AABW. Sector models with only one deep-water source cannot reproduce the interleaving of water masses seen in the deep ocean. Intermediate waters invade the ocean through wintertime convection and through the strong saline outflow of the Mediterranean. Intermediate water is much harder to simulate in a numerical model than deep water; this paper will concentrate on the deeper waters.

The water masses in the ocean can be identified by standard hydrographic measurements and by following the inputs of transient tracers at the surface through specialized

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Department of Atmospheric Sciences, University of Washington, Seattle, Washington



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