Equilibrium Response of Ocean Deep-Water Circulation to Variations in Ekman Pumping and Deep-Water Sources

Abstract

A multilayer ocean model that is physically simple and computationally efficient is developed for studies of competition and interaction among deep-water sources in determining ocean circulation. The model is essentially geostrophic and hydrostatic in the ocean interior with Rayleigh friction added in boundary-layer and equatorial regions. A stably stratified density structure is specified at static equilibrium, and cross-isopycnal mixing is parameterized as a diffusive flux. The model is forced by latitudinally varying Ekman pumping velocities at the base of the ocean surface Ekman layer and localized deep-water sources. A four-layer version of the model has been run in a rectangular basin with 5000-m depth, extending from 65°S to 65°N latitude and covering 70 degrees of longitude. The four layers mimic the major water masses observed in the Atlantic Ocean: thermocline water, intermediate water, North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). For forcing corresponding to the current climate, warm water and cold water circulation routes produced in the model agree with those inferred from observations, for example, southward-flowing NADW overriding northward-flowing AABW in the western boundary. The model shows that subtropical gyres intensify, and thermocline depths become shallow, when deep-water formation rates increase, or when vertical diffusivity kv decreases, or when more NADW is formed from the thermocline layer than that from the intermediate layer. Consistent with the advective thermocline depth scaling, distributions of the Ekman pumping contribute little to deep-water circulations. The interaction between NADW and AABW sources is demonstrated. Changes in the formation rate of a deep-water source alter cross-isopycnal flows, especially along the related circulation route, thus altering the extent that the other sources can travel before they detrain significantly. These changes feed back onto the thermocline circulation and cross-equatorial transports. The model suggests that reduction in deep-water formation rate may increase the transient response time of the atmosphere to perturbations, because the thermocline depth becomes deeper. Also, poleward heat transport may decrease, thus acting to self-regulate the temperatures in polar regions.