Mechanisms of Tropical Indian Ocean circulation biases in CESM historical experiments

Abstract The study examines the mechanisms of Tropical Indian Ocean (TIO) circulation biases in Phase 6 of the Coupled Model Intercomparison Project (CMIP6) historical simulations across four variants of the Community Earth System Model (CESM): CESM2, CESM2-FV2, CESM2-WACCM, and CESM2-WACCM-FV2. The dominant equatorial flow, known as the Wyrtki Jets (WJ), is considerably underestimated due to the predominance of easterly wind bias, with the fall WJs showing the least skill, particularly in CESM2-WACCM-FV2, due to the underestimated westerlies. However, the eastward Equatorial Undercurrent (EUC) is strongly overestimated in all models, with maximum amplitudes observed in CESM2-WACCM-FV2, followed by CESM2-FV2, due to wind-induced westward thermocline tilt through the positive Bjerknes feedback mechanism. The northward intensification and deepening of south equatorial currents are attributed to strong easterly wind stress in the southern TIO in CESM models. The equatorial easterly wind bias in CESM models originates from a southeasterly wind bias in their Atmospheric Model Intercomparison Project (AMIP) counterparts during June-August, with air-sea coupling driving the westerly wind bias over the equatorial region. Overly strong easterlies and the intensification of midlatitude westerlies in AMIP models contribute to the intensification and poleward shift of the subtropical gyre in CESM models. This, in turn, weakens the Agulhas leakage (AL) transport from the south Indian Ocean to the Atlantic, which is partially due to the underestimated Indonesian Throughflow (ITF) in CESM models.