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

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
three-dimensional general circulation model based on the primitive
equations (GFDL's MOM-2) 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 with regard to changes in the model configuration and
parameterizations revealed a strong dependence of the model results
from eddy diffusivities, filtering and topographic effects. Restoring
time scales for surface salinity were found to be of minor importance.

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.

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 30oS 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 by
Toggweiler and Samuels, 1993a, 1995). 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.

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