Influence of northward-propagating intraseasonal oscillations and ENSO background states on summer monsoon onset over the Arabian Sea and India

Monsoon onset marks an abrupt seasonal transition from a dry to a moist atmosphere, but physical processes associated with the monsoon onset over India and the Arabian Sea (AS) are not fully understood. In this study, a northward propagating convective phase of intraseasonal oscillations (ISOs), associated with low-level cyclonic circulation, is identified as a crucial factor in initiating the monsoon onset. The northward propagation is sustained by a positive moist static energy (MSE) tendency to the north and a simultaneous negative tendency to the south of the convective center. Results from the MSE budget diagnosis indicate that the MSE tendency dipole is attributed to horizontal moisture advection. Under a wetter (dryer) background environment over southeastern (northwestern) AS, the intraseasonal cyclonic circulation enhances (reduces) the MSE to its north (south). In addition, the northward propagation is controlled by a meridional asymmetry of background convective instability (BCI). During the pre-onset stage, the accumulation of background low-level moisture over the northern AS due to meridional moisture transport by cross-equatorial flow enhances local BCI. A more unstable background environment over the AS, compared to the equatorial western Indian Ocean (EWIO), facilitates the northward propagation of ISOs. ENSO exerts a marked impact on the monsoon onset through the modulation of the meridional asymmetry of BCI. During post-La Nina Springs, both the enhanced meridional SST gradient over EWIO and the stronger cross-equatorial low-level flow over the AS help trigger the northward-propagating ISOs and thus lead to an earlier monsoon onset.