Simulation of Central Indian Ocean Mode in S2S Models

Simulation 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 ( partial differential u over bar partial differential y) 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 partial differential u over bar / partial differential y 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.