| Abstract: The standard version of the coupled ocean-atmosphere
model developed at the Geophysical Fluid Dynamics Laboratory (GFDL) of
NOAA has at least two stable equilibria. One has a realistic and active
thermohaline circulation (THC) with sinking regions in the northern North
Atlantic Ocean. The other has a reverse THC with extremely weak upwelling
in the North Atlantic and sinking in the Circumpolar Ocean of the Southern
Hemisphere. Although the model has the seasonal variation of insolation,
the structure of these two stable equilibria are very similar to those
of a previous GFDL model without the seasonal variation. It is noted that
the inactive mode of the reverse THC mentioned above is not a stable equilibrium
for another version of the same coupled model which has a large coefficient
of vertical subgrid scale diffusion. Although the reverse THC cell was
produced in the Atlantic Ocean by a massive discharge of freshwater, it
began to transform back to the original direct THC as soon as the freshwater
discharge was terminated. It appears that there is a critical value of
diffusivity, above which two stable equilibria do not exist. Based upon
paleoceanographic evidence, we suggest that the stable state of the reverse
THC mentioned above did not prevail during the cold periods of Younger
Dryas event which occurred during the last deglacial period. Instead, it
is likely that the THC weakened temporarily, but reintensified before it
reached the state of the reverse THC with no deep water formation in the
North Atlantic Ocean. |