Multiscale Convection During Monsoon Intraseasonal Oscillations in the Bay of Bengal

Abstract:

Monsoon intraseasonal oscillations (MISOs) are an essential component of the South Asian monsoon, as they modulate the seasonal mean precipitation characteristics as well as the spatial structure, frequency and duration of active (convective) and break (dry) spells. During the 30–60-day MISO, an area of enhanced precipitation first develops over the equatorial Indian Ocean just south of Bay of Bengal (BOB), thereafter propagating northward and eastward. In a more granular view, mesoscale convective systems (MCSs) are found embedded in these MISO signals and they are associated with stratiform precipitation over BoB. Each individual MCS propagates south and west while their initiation locations gradually shift northward as monsoon season initiates. These bands of enhanced clouds and precipitation as well as associated circulation anomalies are oriented from northwest to southeast, and they interact closely with BoB internal dynamics via air-sea exchange of both sensible and latent heat fluxes. Moreover, convection-generated SST gradients have also shown to play an important role in cloud cluster development and propagation. MISO in BoB plays a significant role in the variation and distribution of meso to synoptic scale systems, as well as in the intraseasonal scheme in which these are confined. Here we analyze the evolution, propagation and air-sea interactions of MCSs during MISO using data collected from MISO-BoB field campaigns. Instruments were deployed aboard a research vessel and over land in Seychelles, Sri Lanka and Maldives. In addition, a comprehensive set of radiosonde profiles provided by the Indian Meteorological Department (IMD), satellite products and reanalysis data sets were used for the study. Atmospheric measurement results show that equatorial specific humidity signals share similar frequencies as northward propagating MCSs derived from TRMM data. Meridional synoptic scale pressure gradient is shown to exist as MCSs propagate through BoB. Local SST gradient and increase in surface heat fluxes indicate strong convective heating and moisture feedback associated with strong vertical motions.