A Study on the Structural Transformation of the Monsoon Low-Level Jet Stream on Its Passage Over the South Asian Region SHINU SHEELA WILSON, 1,2 K. MOHANAKUMAR, 1,3 and SHINTO ROOSE 1 Abstract—A comprehensive study of the cross-sectional structure of the cross-equatorial low-level jet stream (LLJ) at var- ious 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. Key words: Low-level jet stream, cross-sectional structure, core height, Asian summer monsoon. 1. Introduction According to the World Meteorological Organi- sation, a jet stream is a strong narrow current, concentrated along a quasi-horizontal axis charac- terised by strong vertical and lateral wind shears. The existence of such a jet stream in the lower tro- posphere with less intensity over the southwest Arabian Sea during the summer monsoon season was found by Bunker (1965) during the International Indian Ocean Expedition of 1962–1965. These winds showed strong vertical wind shear both below and above the maximum speed located about 1 km above sea level. Appearance of strong wind fields in the lower levels over peninsular India, during the south- west monsoon season, was reported by Joseph and Raman (1966). Later, Findlater (1969a) found a cross-equatorial flow embedded in the low-level cir- culation of the Asian summer monsoon, which included the wind maxima over the southwest Ara- bian Sea. The cross-equatorial flow of the low-level jet stream (LLJ) had its origin in the easterly trade winds of the south Indian Ocean (SIO). It crosses the equator in a narrow belt close to the east African coast. It further turns anti-cyclonically over the Arabian Sea near the Somali coast and passes through peninsular India and the Bay of Bengal (BoB) (Joseph and Sijikumar 2004; Wilson et al. 2018). The emergence of the LLJ coincides with the onset of the monsoon over Kerala, the extreme southwest part of India (Joseph et al. 2006; Boos and Emanuel 2009). After the monsoon onset, the LLJ exists during the entire monsoon season. The LLJ is an essential semi-permanent compo- nent of the Asian summer monsoon. The LLJ accounted for nearly half of the global inter-hemi- spheric transport of air in the lower troposphere during the monsoon season extending from June to September (Findlater 1969b). It carries the moisture generated by the winds over the SIO and the Arabian Sea to the areas of monsoon rainfall over south Asia. Joseph and Raman (1966) studied the features of the 1 Cochin University of Science and Technology, Ernakulam, India. E-mail: shinusheelawilson@gmail.com; kmkcusat@gmail.- com; thejusshinto@gmail.com 2 Nansen Environmental and Research Centre India, Er- nakulam, India. 3 Present Address: Advanced Centre for Atmospheric Radar Research, Ernakulam, India. Pure Appl. Geophys. Ó 2019 Springer Nature Switzerland AG https://doi.org/10.1007/s00024-019-02162-y Pure and Applied Geophysics