The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage
is found to influence the Atlantic overturning circulation in a low-resolution
Ocean General Circulation Model. The model used is the Hamburg Large-Scale
Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas
Leakage is parameterized by sources of heat and salt in the upper South
Atlantic Ocean, that extend well into the intermediate layers.
It is shown that the models overturning circulation is sensitive to the applied
sources of heat and salt. The response of the overturning strength to changes in
the source amplitudes is mainly linear, interrupted once by a stepwise change.
The South Atlantic buoyancy sources influence the Atlantic overturning strength
by modifying the basin-scale meridional density and pressure gradients. The nonlinear,
stepwise response is caused by abrupt changes in the convective activity
in the northern North Atlantic.
Two additional experiments illustrate the adjustment of the overturning circulation
upon sudden introduction of heat and salt sources in the South Atlantic.
The North Atlantic overturning circulation responds within a few years after the
sources are switched on. This is the time it takes for barotropic and baroclinic
Kelvin waves to reach the northern North Atlantic. The advection of the anomalies
takes 3 decades to reach the northern North Atlantic.
The model results give support to the hypothesis that the re-opening of the
Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological
data, may have stimulated the coincident strengthening of the Atlantic overturning
circulation