The Effect of Tropical Pacific Air-Sea Coupling on the Rainfall Response to Quadrupled CO2 Forcing

We perform quadrupled CO2 climate simulations with the Community Earth System Model version 1 (CESM1) to study how air-sea coupling affects the response of tropical rainfall under global warming. We use a hierarchy of ocean models to separate the effects of seasonal mixed-layer entrainment, wind-driven Ekman flows directed perpendicular to the wind, and the near-equator frictional flows directed in the same direction as the wind. We show that the Pacific Ocean's enhanced equatorial warming pattern (EEW) and equatorward ITCZ contraction observed in previous climate simulations emerge when the ocean model includes wind-driven Ekman and frictional flows. Furthermore, the near-equator frictional flow contributes more than half of the heat convergence in the equatorial Pacific Ocean. Finally, we show that although Ekman flow and near-equator frictional flow can both result in EEW, their coupled interactions with the Hadley circulation lead to opposite feedbacks on EEW's strength.

Automated summary

We conducted quadrupled CO2 climate simulations using CESM1 to investigate the impact of air-sea coupling on the response of tropical rainfall under global warming. By employing a range of ocean models, we were able to identify the specific contributions of seasonal mixed-layer entrainment, wind-driven Ekman flows, and near-equator frictional flows. Our findings demonstrate that the enhanced equatorial warming pattern and the contraction of the intertropical convergence zone in the Pacific Ocean, observed in previous climate simulations, only emerge when the ocean model includes wind-driven Ekman and frictional flows. Additionally, we discovered that the near-equator frictional flow accounts for more than half of the heat convergence in the equatorial Pacific Ocean. Moreover, the interactions between the Ekman flow, near-equator frictional flow, and the Hadley circulation result in opposite feedbacks on the strength of the enhanced equatorial warming pattern.