Diagnosing CESM2 Biases in Northwest Indian Ocean Warming and Its Implications for India Summer Monsoon Precipitation

Abstract Declining monsoon rainfall over agriculture‐dependent Eastern Central India (ECI) during 1901–2014 threatens regional food and water security. Observations link ECI drying to rapid Northwest Indian Ocean (NWIO) warming, which weakens monsoon circulation. However, the widely used Community Earth System Model version 2 (CESM2) Large Ensemble fails to reproduce this drying, instead simulating increasing ECI precipitation while underestimating NWIO warming. Using the fully coupled CESM2 Large Ensemble and Pacific pacemaker experiments where realistic tropical Pacific SST anomalies are specified, we identify ocean biases limiting NWIO warming: (a) a lack of El Niño intensification diminishes Pacific‐to‐Indian Ocean heat transmission; (b) an overly deep southwestern Indian Ocean thermocline weakens the thermocline–SST feedback, constraining NWIO warming. The drying effect of NWIO warming‐induced monsoon weakening could be masked by thermodynamic forcing from global warming in the CESM2 Large Ensemble, leading to ECI wetting. Addressing these model biases is critical for monsoon rainfall projections.