The tropical Atlantic influences the number of monsoon depressions in the Bay of Bengal

What is it about?

Once in every few years the sea surface temperature in the areas around the equator in the Atlantic ocean in the months of June-July-August are colder or warmer than usual. Such an anomalous sea surface temperature is also accompanied by anomalous changes in winds and temperature of waters below the ocean surface. This phenomenon is called the Atlantic Zonal Mode. In this study, we have shown for the first time that the Atlantic Zonal Mode, whenever it occurs, influences the number of monsoon depressions formed in the remotely located Bay of Bengal in the months of June-July-August. The monsoon depressions are low pressure systems of certain magnitude which usually form in the ocean, in our case the Bay of Bengal, and move to the west or northwest onto India and produce rainfall over that region. Our study specifically shows that when the sea surface temperature associated with the Atlantic Zonal Mode is cold, there is a tendency to form more monsoon depressions than usual and hence enhanced rainfall over India, and vice versa. We have also proposed a mechanism as to how these depressions are affected by the Atlantic Zonal Mode which is located in a distant Atlantic ocean.

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Why is it important?

The monsoon depressions in the Bay of Bengal contribute significantly to the seasonal (June-July-August-September) total rainfall over India. The understanding of what factors influences their number can be useful to improve our numerical models used to forecast the monsoons. This study also addresses the need to look for factors affecting the Indian summer monsoon, other than a similar but stronger phenomenon of El Nino Southern Oscillation of the tropical Pacific, as it cannot always be relied on.

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