Barotropic energy conversion during Indian summer monsoon: implication of Central Indian Ocean Mode Simulation in CMIP6

The simulation and prediction of the Indian summer monsoon (ISM) and its intraseasonal component in climate models remain a grand scientific challenge for numerical simulations. Recently, an intraseasonal mode was proposed over the tropical Indian Ocean, named central Indian Ocean (CIO) mode. The CIO mode index and the monsoon intraseasonal oscillations (MISO) have a high correlation. In this study, the simulations of the CIO mode in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) models are examined. Although the coupled ocean-atmosphere feedbacks associated with the CIO mode are not fully reproduced, the results show that a better depiction of the CIO mode in CMIP6 models is favorable for a better simulation of northward-propagating MISO and heavy rainfall during the ISM. Dynamic diagnostics unveil that the rendition of the CIO mode is dominated by kinetic energy conversion from the background to the intraseasonal variability. Furthermore, kinetic energy conversion is controlled by the meridional shear of background zonal winds (partial derivative(u) over bar/partial derivative y), which is underestimated in most CMIP6 models, leading to a weak barotropic instability. As a result, a better simulation of partial derivative(u) over bar/partial derivative y is required for improving the CIO mode simulation in climate models, which helps to improve the simulation and prediction skill of northward-propagating MISO and monsoonal precipitation.