Are two modes of thermohaline circulation stable?

Abstract

The standard version of the coupled ocean–atmosphere model developed at the GeophysicalFluid Dynamics Laboratory (GFDL) of NOAA has at least two stable equilibria. One has arealistic and active thermohaline circulation (THC) with sinking regions in the northern NorthAtlantic Ocean. The other has a reverse THC with extremely weak upwelling in the NorthAtlantic and sinking in the Circumpolar Ocean of the Southern Hemisphere. Although themodel has the seasonal variation of insolation, the structure of these two stable equilibria arevery similar to those of a previous GFDL model without the seasonal variation. It is notedthat the inactive mode of the reverse THC mentioned above is not a stable equilibrium foranother version of the same coupled model which has a large coefficient of vertical subgridscale diffusion. Although the reverse THC cell was produced in the Atlantic Ocean by a massivedischarge of freshwater, it began to transform back to the original direct THC as soon as thefreshwater discharge was terminated. It appears that there is a critical value of diffusivity, abovewhich two stable equilibria do not exist. Based upon paleoceanographic evidence, we suggestthat the stable state of the reverse THC mentioned above did not prevail during the cold periodsof Younger Dryas event which occurred during the last deglacial period. Instead, it is likelythat the THC weakened temporarily, but reintensified before it reached the state of the reverseTHC with no deep water formation in the North Atlantic Ocean.