Scale interaction during an extreme rain event over southeast India

The major rains and floods over southeast India (during October through to December 2015) are addressed in the context of atmospheric scale interactions in the frequency domain. Some of the salient observational features of this period include: (i) the major El Nino of 2015, (ii) the stretch of lower tropospheric easterlies from the region of the warm sea-surface temperature anomalies westwards to the east coast of India, (iii) presence of shear flow instability, in the presence of convection, along a long stretch of the easterly wind belt from the eastern Pacific Ocean to the eastern Bay of Bengal, (iv) large conversions of horizontal shear vorticity to curvature vorticity along this stretch of easterly trades, where these rain-producing storms were forming, (v) an active IntraSeasonal Oscillation (ISO) time-scale oscillation in the wind field that alternated between cyclonic and anticyclonic phases over southeast India during this period of heavy rains, and (vi) an active quasi-biweekly oscillation that provides alternating onshore and offshore winds during this same period. The ISO and the quasi-biweekly components contribute to the enhancement of the moisture supply from the Bay of Bengal during the extreme rain events. The synoptic scale receives its energy largely from organized convection within these disturbances on horizontal scales of the order of 2500 km. Other aspects such as the role of the sea-surface temperatures of the Bay of Bengal and the role of Gill's antisymmetric heat source of the El Nino are also examined in this study.