The core Indian monsoon region receives more than half of the rainfall extremes from low-pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time-mean low-level westerlies. Yet, the relationship between the trends of LPSs and rainfall extremes remains uncertain. Using two tracking algorithms and reanalyses-derived LPSs, we find that LPS activity and extreme rainfall exhibit coherent trends during the post-1979 satellite era. Over time, the LPSs propagate preferentially into south-central India rather than north-central India, imparting a corresponding dipole footprint in rainfall extremes. Consistent with existing theories that the diabatic heating is instrumental in shifting the LPSs west-southwestward, the LPSs traveling through south-central India have stronger updrafts on their west-southwestern flank than those passing through north-central India. The increased frequency of LPSs propagating into south-central India is likely due to a strengthened cross-equatorial moisture transport, which favors stronger storm ascents. Plain Language Summary The South Asian synoptic-scale low-pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time-mean low-level westerlies, produce more than half of the summer rainfall extremes over the densely populated central India. Given the vulnerability of societies in this region to rainfall extremes, investigating the connection between LPSs and extreme rainfall regarding their long-term trends has important implications for climate adaptation. Using two different tracking algorithms and reanalyses-derived LPS tracks, we find that the trends of extreme rainfall and LPS activity exhibit a strong coherence during the post-1979 satellite era. Specifically, the LPSs prefer to propagate into south-central India than north-central India over time, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward-propagating component due to horizontal nonlinear adiabatic advection and the southwestward-propagating component due to diabatic heating, we notice that the LPSs migrating through the south-central India have stronger updrafts on their west-southwestern flank compared to those passing through north-central India. Our results indicate that the increasing number of LPSs propagating into south-central India is likely due to a strengthened cross-equatorial moisture transport, which provides a wetter environment and favors stronger storm ascents. Key Points An extreme rainfall dipole with positive trends over south-central India and negative trends over north-central India is observed since 1979 The extreme rainfall dipole aligns with the trends in the number of Indian monsoon low-pressure systems passing through the two regions The changing LPS translation is likely associated with a wetter environment owing to a strengthened cross-equatorial moisture transport
You, YJ; Ting, MF
Observed Trends in the South Asian Monsoon Low-Pressure Systems and Rainfall Extremes Since the Late 1970s
Geophysical Research Letters