Monsoon onset vortex

The monsoon onset vortex (MOV) for the Arabian Sea is a synoptic-scale vortex that features near-surface wind speeds ≥ 17 kt (similar to those associated with a tropical depression). It forms within ±10 days of the onset of the Indian summer monsoon over the state of Kerala or prior to the time at which the northern extent of the monsoon reaches 20°N latitude over the Arabian Sea.

The low-level Somali jet is an important feature associated with MOV development, although it is not the only factor. Diabatic heating and vertical advection of potential vorticity are also important processes that govern MOV formation and intensification. The MOV can impact the timing of the onset and northward advance of the summer monsoon. It can potentially lead to monsoon-induced rain and flooding in western and southern India, or result in an anomalously drier start to the summer monsoon. It can also induce coastal and maritime hazards for countries bordering the Arabian Sea.

Dhavale, S., and A. Aiyyer, 2025: The structure of a developing Arabian Sea monsoon onset vortex. J. Atmos. Sci., 82, 2451–2467, https://doi.org/10.1175/JAS-D-24-0212.1.

Dhavale, S., and A. Aiyyer, 2023: The impact of the Madden-Julian Oscillation on the formation of the Arabian Sea monsoon onset vortex. Geophys. Res. Lett., 50, e2023GL104156, https://doi.org/10.1029/2023GL104156.

Term Edited on 22 January 2026.