Simulation of Central Indian Ocean Mode in S2S Models

Abstract

AbstractSimulation of monsoon intraseasonal oscillations (MISO) during the Indian summer monsoon (ISM) is a grand scientific challenge. The Subseasonal‐to‐Seasonal (S2S) prediction project provides a unique way to examine the key dynamics of MISO. Recently, a Central Indian Ocean (CIO) mode was proposed as an intrinsic climate mode over the Indian Ocean, and it has a close relation with MISO during the ISM. In this study, the simulations of the CIO mode events in S2S models are examined. The results confirm that a better rendition of the CIO mode in S2S models tends to result in a better simulation of northward propagating MISO and leads to stronger subseasonal rainfall during the ISM. The positive CIO mode cases are classified into well‐simulated and poorly simulated groups. It is shown that the barotropic energy conversion due to the meridional shear of background zonal winds ( ) enhances the kinetic energy at subseasonal timescales during the CIO mode in the well‐simulated group, which also agrees with observations. As a result, the enhanced meridional wind anomalies transport moisture from the tropics to the subtropics and reinforce midtropospheric moisture loading in the subtropics, which nourishes the northward propagating MISO. Therefore, a better simulation of and moisture transport by subseasonal meridional wind anomalies are required to improve the CIO mode simulation, which is expected to benefit the simulation and prediction of MISO during the ISM.