Role of equatorial convection and moisture in the structure and propagation of Boreal Summer Intraseasonal Oscillations

Abstract

Boreal Summer Intraseasonal Oscillation (BSISO) is the dominant mode of tropical variability during Boreal summers. BSISO is manifested by northward propagation of the Maximum Cloud Zone (MCZ) from the warm equatorial Indian Ocean up until 20oN latitude across a large longitudinal extent. BSISO modulates Indian summer monsoon rainfall (ISMR) by modulating the dry and wet spells over India. Past studies have identified BSISO as the unstable mode of monsoon flow, and the zonal wind shear mainly controls this instability. In this study, we investigate the role of equatorial convection in the structure and propagation of BSISO, emphasizing the moisture dynamics. We also try to understand how the equatorial convection controls the processes that are responsible for the northward propagation of BSISO. To understand the role of equatorial convection, we examine how the Indian Ocean Dipole (IOD) influences the BSISO. We find that northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) events, with only 28% of events propagating northward. At the same time, the number stands at 81% during the negative Indian Ocean Dipole (nIOD) events. The “moisture mode” framework is used to understand the processes responsible for the weakening of northward propagations during pIOD years. Our analyses show that moistening caused by horizontal advection is the major contributor to northward propagations during negative IOD (nIOD) years, but the amplitude of horizontal advection is much smaller during pIOD years. Further analyses revealed that the reduction in the advection of the background entropy/moisture by zonal wind perturbations during pIOD is primarily responsible for the reduction in the horizontal advection. The mean structure of entropy between 925 and 500 hpa levels remained similar over most of the Asian monsoon region across the contrasting IOD years, and the reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. These weaker zonal wind perturbations during ISO events in pIOD years result from weak Rossby Vortex lobes. The weakening of Rossby wave response owing to cooler than average sea surface temperatures in the South-East Equatorial Indian Ocean and warmer than average West Equatorial Indian Ocean is proposed to be the possible reason for the weakening of northward propagations during pIOD years. Past studies indicate that the Coupled Model Intercomparison projects (CMIP) models do not faithfully capture the northward propagation of BSISO. Thus, we examine if the failure to capture the structure of BSISO is related to the equatorial convection in the CMIP6 models. We use a counting algorithm to distinguish the CMIP6 model’s propagation faithfully (above-averge-performing-model, AAPM) from those that do not (below-average-performing-model, BAPM). A moisture budget shows that AAPMs have a robust horizontal advection that helps moisten the atmosphere ahead of the BSISO convection centre, thus enhancing the BSISO’s northward propagation. We further break down the horizontal advection to illuminate the processes responsible for robust northward propagation in AAPMs. We find that the ISO wind perturbations and meridional gradient of moisture perturbations mainly control the horizontal advection to the north of the convection centre, suggesting that difference in the observed northward propagations between AAPMs and BAPMS can be attributed to how these models represent equatorial convection, supporting our hypothesis that strength/structure of equatorial convection is central to BSISO’s northward propagation. The structure of BSISO rainfall is relatively less studied. This study shows that the ISO rainfall anomalies weaken across the south Bay of Bengal (SBoB) before they re-strengthen over the north Bay of Bengal (NBoB).We use the moisture budget to understand the reason for these weakening-strengthening cycles. The convergence of background moisture by the ISO wind perturbations decides the ISO rainfall structure. Past literature suggests that the Planetary Boundary Layer (PBL) convergence is caused by barotropic vorticity, so we will conduct a vorticity budget to understand the rainfall structure. We find that though vorticity tilting helps generate the positive tendency of the ISO vorticity, the vorticity stretching enhances it. Further splitting of the stretching term helps us conclude that the convergence due to wind perturbations is the predominant term, representing feedback between dynamics and thermodynamics. We hypothesize that the weaker rainfall anomalies in the SBoB result from the weaker background column relative humidity and moisture, which do not allow the initial dynamic perturbations to grow as fast as they do in an environment with stronger background relative humidity and moisture (NBoB). Finally, we examine the future of BSISO by examining the projections of Shared Socioeconomic Pathways 370 (SSP370) from CMIP6. The moisture increases more or less uniformly in the future projections, keeping the spatial gradients nearly constant. The equatorial convection broadens and enhances, leading to no significant change in the wind perturbations. This increases the BSISO rainfall by 42% in the Arabian Sea and 63% in the Bay of Bengal compared to the historical runs.