Offshore Migration of Summer Monsoon Low-Level Jet on a Diurnal Scale

This study investigates the diurnal migration of the low-level jet (LLJ) over the Bay of Bengal in the summer monsoon, which remains understudied despite the well-documented diurnal offshore propagation of rainfall worldwide. The southwesterly LLJ exhibits maximum strength during the night and morning. The LLJ undergoes a substantial eastward migration of approximately 600 km from the eastern offshore of India at midnight to the center of the Bay of Bengal by afternoon. A linear land-sea breeze model is effective in capturing the diurnal migration with similar speed and phase. Including background wind offshore extends the migration, rather than confining it to the coast. This migration is driven by inertia-gravity waves arising from the diurnal land-sea thermal contrast and is influenced by the westerly upstream wind from the Indian subcontinent. Our results have important implications for understanding diurnal monsoon circulations and their associated precipitation. The low-level jet (LLJ) in the summer monsoon plays a crucial role in affecting nocturnal downstream precipitation in South Asia. Previous studies have primarily concentrated on changes in the jet's strength during the day, neglecting its varying location. The present study addresses this gap by providing a comprehensive examination of the previously neglected aspect of the diurnal offshore migration of the LLJ over the Bay of Bengal. The diurnal migration is driven by the difference in temperature between the land and sea and is also aided by upstream wind, in a manner similar to the diurnal offshore propagation of rainfall. Overall, this study offers a more complete understanding of the LLJ' behaviors in the summer monsoon, which can help explain how it affects rainfall in the region. The monsoon low-level jet migrates eastward from the eastern offshore of India at midnight to the center of the Bay of Bengal by afternoonThe offshore diurnal migration is driven by inertia-gravity waves resulting from the diurnal thermal contrast between land and seaThe westerly upstream wind from the Indian subcontinent extends the offshore migration, rather than confining it to the coast