The Central Indian Ocean Mode Simulation in the S2S Models

Recently, a central Indian Ocean (CIO) mode was found to be a natural air-sea coupled mode and shown to have a close relationship with the Indian summer monsoon precipitation and northward propagation of monsoon intraseasonal oscillation (MISO). However, such relationship is not well captured in some models. In this study, the simulation of CIO mode is examined in Subseasonal to Seasonal (S2S) database due to its good performance on intraseasonal variabilities (ISVs) signals of the sub-seasonal to seasonal time range. Results show well simulation in all air-sea coupled models on day 0, but the skill deteriorates with time. Then, cases during CIO index peaks are classified to well and bad simulated groups by pattern correlation with CIO mode in nature. Compared with observations, northward propagation of intraseasonal precipitation cannot be clear seen in bad simulated cases. The force and response relationship between atmosphere and ocean associated with the CIO mode in bad simulated cases is weaker than that in nature. Furthermore, the reasons of bad CIO simulation are investigated. Kinetic energy (KE) and moist state energy (MSE) show obvious difference, controlled by meridional gradient of zonal winds on large-scale (∂u/∂y) and specific humidity (q) on intraseasonal timescale. In bad simulated cases, weak ∂u/∂y from tropical central-eastern Indian Ocean to north reduces kinetic energy transfer from the mean winds to the ISVs and cuts off the energy source to MISO. Meanwhile, weak positive q around 20N in bad simulated cases is not conducive to maintaining the strong deep convection, which is important on barotropic instability there. Therefore, better simulations of ∂u/∂y and q are expected to advance CIO mode simulation skill and our understanding in the mechanisms of the monsoon system.