A Study on the Structural Transformation of the Monsoon Low-Level Jet Stream on Its Passage Over the South Asian Region

A comprehensive study of the cross-sectional structure of the cross-equatorial low-level jet stream (LLJ) at various locations over the Asian summer monsoon region has been carried out for the period 1979–2014 using a reanalysis dataset. Structural transformation of the LLJ along its path over the south Indian Ocean (SIO), cross-equatorial region, Arabian Sea, Indian land region and Bay of Bengal (BoB), has been investigated by taking the vertical cross section of the jet stream perpendicular to the direction of its flow. During the passage of the LLJ over the monsoon area, the core of the LLJ is lower over the oceanic region, whereas the core height increases over land. The LLJ has its maximum wind speed at a lower height (925 hPa) over the Indian Ocean region, whereas the core height raises to around 850 hPa over the Indian sub-continent. The strength of the monsoon LLJ is found to be varying in different temporal scales. The LLJ is much stronger in the active monsoon spells than in break periods, except in the SIO sector and BoB. During monsoon breaks, the LLJ axis shifts poleward over the Arabian Sea, and the intensity of wind over the SIO and BoB increases. The strength of the LLJ is greater during wet monsoon years than in dry monsoon years, except in the BoB zone. The changes noted in the monsoon LLJ during dry monsoon years is possibly due to the increased number of break days observed in the recent decades.