The Mid-latitudinal influence on the formation of the monsoon gyre in August 1991

This study investigates the formation of the monsoon gyre (MG) in August 1991. The results suggest the mid-latitudinal processes play an important role in the formation of the MG. The repeating upper-tropospheric Rossby wave breaking events took place in the exit region of mid-latitude jet during the period of interest. The wave energy dispersed southeastward and downward from the jet exit region, exciting a low-level subtropical low. Furthermore, the Rossby wave breaking induced salient ageostrophic forcing by disturbing the upper-level PV fields. The Q-vector analysis shows that the ageostrophic motion forced by the upper PV streamer favors the early development of the subtropical low. Thereafter, this system moved southwestward, and interacted with the tropical monsoon flows to induce a strong convective band to the southern and southeastern periphery. The enhanced convective heating further strengthened the cyclonic low through the Gill-type response. Furthermore, the asymmetric water vapor transport induced asymmetric convective structure. This pattern suppressed the scale contraction and maintained a large size, leading to the formation of a large-scale monsoon gyre eventually.