Contrasting controls on convection at latitude zones near and away from the equator for the Indian summer monsoon

Abstract

Understanding controls on convection on various timescales is crucial for improved monsoon rainfall forecasting. Although the literature points to vertically homogeneous vorticity signatures preceding rainfall during the Indian summer monsoon, we show using reanalysis data that, for rainfall associated with northward propagating intraseasonal oscillations (ISOs), different controls are present at different latitude zones. For the latitude zone close to the equator (5 ^∘ N–14 ^∘ N) and including the southern Indian region, a conventional dynamical control on rainfall exists with barotropic vorticity leading ISO rainfall by about five days. In contrast, for the latitude zone away from the equator (15 ^∘ N–24 ^∘ N; covering the central Indian region), thermodynamic fields control ISO rainfall, with barotropic vorticity following rainfall by two days on average. Over central India, the pre-moistening of the boundary layer (BL) yields maximum moist static energy (MSE) about four days prior to ISO rainfall. Analyzing the statistics of individual events verifies these observations. Similar thermodynamic control is also present for the large-scale extreme rainfall events (LEREs) occurring over central India. These high rainfall events are preceded by positive MSE anomalies arising from the moisture preconditioning of the BL. The resulting convection then leads to a maximum in barotropic vorticity 12 h after the rainfall maximum. Characterizing these influences on convection occurring over various timescales can help identify the dominant mechanisms that govern monsoon convection. This can help reduce climate model biases in simulating Indian monsoon rainfall.

Keywords

• monsoon intraseasonal oscillations
• convection–circulation coupling
• extreme rainfall
• vorticity evolution
• thermodynamic control