Atmospheric dynamics and internal processes during organization and intensification of Boreal Summer Intraseasonal Oscillation (BSISO) based on TRMM and reanalyses data

Abstract

ABSTRACTBoreal summer intraseasonal oscillations (BSISOs) show a meridional propagation from the Indian Ocean (IO) region to the continental India. The BSISOs are the main source of rainfall variability over the Indian Summer Monsoon (ISM) region. The recent research has revealed a basic understanding of the scale selection and northward propagation of the BSISOs. But the complete understanding about the organization and intensification of BSISOs remains elusive. This study uses observation and reanalysis data that covers summers (June–September) of 1998–2013 to investigate the dynamical and atmospheric internal processes during the organization and intensification of BSISOs. Based on rainfall analyses, two types of BSISOs are defined, one with weaker and the other with stronger intensity. Stronger BSISOs show persisting lower level moisture convergence to the north of the convection centre (CC) and a strong vertical velocity collocated with CC. The analysis of mean kinetic energy (MKE) budget suggests that MKE significantly increases during the organization of the strong BSISOs but the strength is relatively weaker during the evolution of weak BSISOs. The enhanced MKE during the strong events is supported by the increase of baroclinic energy conversion from mean available potential energy (MAPE) to MKE but the MAPE to MKE conversion is significantly weak during the evolution of weak BSISOs. There is conversion from MKE to eddy kinetic energy (EKE) during organization of strong BSISOs and the conversion increases as the events approach towards the organized and intense phase. The eddy momentum‐vertical wind shear interaction is the significant contributor for conversion of MKE to EKE during the organization of strong BSISOs. The eddy momentum‐mean flow divergence interaction and eddy momentum–vorticity interaction at the lower level support to enhance EKE as the strong BSISOs approach the organized and intense phase. But the above interactions are weak during evolutions of the weak BSISOs.