Impacts of Interhemispheric Asymmetric Thermal Forcing on Tropical Pacific Climate: Surface Air–Sea Coupling and Subduction

Abstract

aleoclimate observations and modeling studies suggest that extratropical climate change affects the tropical Pacific. A global coupled general circulation model is used to investigate the equatorial Pacific response to extratropical surface heat flux forcing that is downward (upward) poleward of 40°N (S). The equatorial response consists of two distinct stages: the zonal sea surface temperature (SST) gradient strengthens for the first two to three decades and then weakens afterward. In the first stage, fast surface air?sea coupling feedback mechanism communicates the extratropical warming (cooling) from the North (South) Pacific toward the equator. The second stage is characterized by a basinwide shoaling of the tropical Pacific thermocline as the subtropical cell (STC) advects cold water from the South Pacific along the thermocline. This preference of Southern Hemisphere anomalies is due to the meridional asymmetry in the mean circulation: the interior pathway for STC is open south but partially blocked north of the equator. Paleoclimate implications are discussed.