The Origin and Structure of the Monsoon Onset Vortex

Two events over the Arabian Sea are clear precursors to the Indian summer monsoon: the formation of a "mini warm pool" of heightened sea surface temperatures (SSTs), and the extension of the low-level Somali jet across the Sea from the East African coast to the west coast of India. The jet brings moisture evaporated from the warm Arabian SSTs to coastal India triggering the monsoon, and in many years monsoon onset is accompanied by a cyclonic vortex known as the Monsoon Onset Vortex (MOV). The MOV is of interest not only for its association with monsoon onset but also because it can evolve into a typhoon and cause extensive damage on landfall. Work under this award seeks to understand the formation of the MOV, taking into account the influence of the mini warm pool and the influence of the Somali jet. The former encourages deep convection which in turn can promote vortex formation at mid-tropospheric levels, while the latter can generate a vortex through barotropic instability at the lower-tropospheric level where the jet is found. The goal of the work is to understand how these two mechanisms combine to create an MOV. The work involves analysis of data from satellite missions and reanalysis datasets along with simulations generated with both simple idealized models and the Weather Research and Forecasting (WRF) model. The project also includes a stability analysis of the Somali jet, taking into account both its latitudinal shear and its time variation. The work is of societal as well as scientific interest given the connection of the MOV with monsoon onset and its tendency to generate powerful landfalling typhoons. The project also provides support and training to two graduate students and two undergraduates. In addition, the PI serves as a mentor to high school students and contributes to the summer research program of the NC School of Science and Math. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.