Impact of the Upper Tropospheric Cooling Trend over Central Asia on the Indian Summer Monsoon Rainfall and the Bay of Bengal Cyclone Tracks

The Indian summer monsoon rainfall had three decade long alternate dry and wet epochs during the 150 years from 1840 to 1989. The dry epochs had frequent drought monsoons affecting agriculture, power generation and the overall economy of the country. A high percentage of severe cyclones in the Bay of Bengal moved northwards during the dry epochs causing disasters in Bangladesh, Myanmar and the Indian states of Odisha and West Bengal. These dry epochs have been shown to be associated with the cold phase of the Atlantic multi decadal oscillation in sea-surface temperature. Using the available tropospheric temperature (re-analysis) data since 1948, the recent dry epoch during 1960-89 which had 10 monsoon drought years was found to have cold upper tropospheric temperature anomaly over Central Asia. This cold anomaly region has also experienced a long-term cooling trend. Extrapolating the naturally occurring epochal nature of the ocean-atmosphere system into the future, we fear that the epoch 2020-49 is likely to be another dry one, and the cooling trend over the Asian continent is likely to make it even more severe in its impact than 1960-89. This article presents details of an ocean-atmosphere instability that generates frequent drought monsoons during dry epochs which needs urgent research.