Pliocene equatorial temperature: Lessons from atmospheric superrotation

Abstract

There is proxy evidence that the pronounced east-west temperature difference observed today across the equatorial Pacific Ocean may not have existed in the early Pliocene (4–5 Ma BP) and that the east Pacific cold tongue developed gradually toward the end of the Pliocene (2 Ma BP). The east Pacific temperature influences weather and climate worldwide, and the Pliocene climate may be an instructive analogue to a future warm climate arising from anthropogenic elevation of CO2, making understanding the Pliocene equatorial SST gradient especially relevant. A mechanism for maintaining a weaker Pliocene equatorial temperature gradient is proposed that borrows from theories of atmospheric superrotation. The mechanism is based on enhanced or rearranged tropical convective activity during the warmer Pliocene climate exciting atmospheric Rossby waves that propagated poleward from the equator. These waves produced an equatorward flux of westerly momentum that weakened the surface easterlies and therefore the east-west thermocline slope and SST gradient.