WIND CHANGES, ATMOSPHERE-OCEAN INTERACTIONS, AND THE TROPICAL THERMOCLINE AT THE LAST GLACIAL MAXIMUM AND DURING THE MID-HOLOCENE

Numerical simulations with a coupled atmosphere-ocean general circulation model configured for the LGM and for the mid-Holocene indicate that the trade easterlies over the tropical Pacific Ocean were stronger than they are today during both time periods (more so at the LGM) and that these changes increased the tilt of the mean tropical thermocline.

Diagnostics of the general circulation indicate that, compared to today, there is enhanced equatorward flux of easterly momentum in the northern hemisphere during both time periods, but for different reasons. During the mid-Holocene, a majority of the increase is associated with transient eddy activity in the upper troposphere at northern midlatitudes; increased eddy activity is related to the increased seasonality associated with mid-Holocene insolation. At the LGM, on the other hand, the majority of anomalous momentum flux arises from stationary eddies that are also induced by the presence of the ice sheets themselves.

These enhanced momentum fluxes increase the strength of the surface equatorial easterlies through intensification of subtropical subsidence and modification of the lower troposphere's meridional pressure gradient. Through atmosphere-ocean interactions, this increases the spatial extent of the tropical Pacific cold tongue in both simulations. Results imply that the mean state of the tropical thermocline may be changed in a similar way either by increasing seasonal radiative forcing or by introducing strong topographic forcing.