Monsoon breaks and subseasonal sea surface temperature variability in the Bay of Bengal

The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with "active" periods of heavy rain interrupted by drier "break" periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°-2°C and local changes can exceed 3°C over 2 weeks; these changes are as large as those seen in the local climatological seasonal cycle. This raises the possibility that air-sea interaction may be a significant factor in monsoon variability: the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation. A schematic coupled air-sea oscillator mechanism is offered for further study, in which oceanic changes play a dynamical role in monsoon variability.