The Structure of a Developing Arabian Sea Monsoon Onset Vortex: Role of Moist Convection

Abstract The monsoon onset vortex (MOV) forms in the Arabian Sea around the beginning of the Indian summer monsoon. Many MOVs intensify into tropical cyclones and can potentially impact the onset and advance of the monsoon. The prevailing theory for the genesis of the MOV invokes the adiabatic hydrodynamic instability of the lower-tropospheric Somali jet. However, the role of precipitating moist convection in MOV formation remains unexplored. This study examines the structure of a canonical MOV, defined as a composite of 23 past cases, using reanalysis data. The MOV structure indicates lower-tropospheric maxima (from ∼700 to 850 hPa) and midtropospheric maxima (∼550 hPa) in the potential vorticity. A potential vorticity budget shows that this observed structure of the MOV likely results from adjustment to diabatic heating and vertical advection to a leading order. Both the low- and midlevel vortices in the developing MOV are primarily attributed to moist convection. Similarities and differences between the composite structures of the MOV and Bay of Bengal monsoon depressions are discussed. Additional work is needed to clarify the role of the traditional hydrodynamic instability in MOV formation. Significance Statement The monsoon onset vortex (MOV) is an important weather system that forms in about 60% of the years in the Arabian Sea during the onset of the Indian summer monsoon. The MOV can affect the northward advance of the monsoon and be a coastal and maritime hazard. Compared to monsoon depressions, much less is known about the role of moist convection in MOV formation and growth. In this paper, we examine a composite MOV based on 23 past cases. The MOV structure shows two potential vorticity maxima—one around 700–850 hPa and the other around 550 hPa. Through a potential vorticity budget, we show that the genesis of both these vortices is predominantly attributed to moist convection. Diabatic heating and vertical advection are the main contributors to the MOV growth in its early stages.