North Atlantic Deep Water and Antarctic Bottom Water: Their Interaction and Influence on Modes of the Global Ocean Circulation

Abstract

Interhemispheric signal transmission in the Atlantic Ocean connects the deep water production regions of both hemispheres. The nature of these interactions and large scale responses to perturbations on time scales of years to millenia have been investigated using a global general circulation model based on the primitive equations coupled to a dynamic-thermodynamic sea ice model with a viscous-plastic rheology. The coupled model reproduces many aspects of today´s oceanic circulation. Testing the model´s sensitivity revealed a strong dependence of the model results from eddy diffusivities, filtering and topographic effects.:p:The internal variability in the ocean-sea ice system has been addressed by analyzing the model results with statistical techniques. A decadal oscillation could be identified in the Southern Ocean. A sequence of Kelvin and Rossby waves carries anomalies in this frequency band northward across the equator.:p:Longer-term variability in the ocean is mainly determined by advective processes. A set of experiments in which the surface boundary conditions were changed showed the necessity to continue integrations over thousands of years until new equilibria are established. Buoyancy changes in the Weddell and Labrador Seas exert a direct effect on the overturning cells of the respective hemisphere. They influence the density structure of the deep ocean and thereby lead to alterations in the strength of the ACC. The model results suggest an influence of the ACC on convective activities in the Southern Ocean. Changing the wind stress south of 30S influences the magnitude of the deep water production of both hemispheres. The interhemispheric effect in these experiments cannot be explained solely by advective mechanisms (contradicting previous studies). Switching off the wind stress over the latitude band of the Drake Passage leads to a slow gradual decrease of the water mass transport in the ACC resulting in an almost complete cessation.