Has the number of Indian summer monsoon depressions decreased over the last thirty years?

Thursday, 18 December 2014
Naftali Y Cohen and William R Boos, Yale University, New Haven, CT, United States
Precipitation in the summer monsoon season accounts for more than 80% of India’s annual rainfall and is crucial for the region’s agriculture and economy. A large fraction of this summer precipitation is produced by vortices with outer radii of about 1000 km that typically form over Bay of Bengal. Intense occurrences of these synoptic-scale storms are commonly classified as monsoon depressions.

Records of monsoon depression tracks maintained by the India Meteorological Department extend from the late 19th century to the current day, and show an average of about 6 monsoon depressions forming each summer season in the Indian region. Recent analyses of these records have revealed a statistically significant decreasing trend in the number of depressions but an increase in the number of weaker storms since the mid-20th century. In particular, the years 2002, 2010, and 2012 were noted for being the first summers, in over a century, in which no depressions formed.

Here satellite and reanalysis datasets are used to document the existence of multiple storms in the summers of 2002, 2010, and 2012 that meet traditional criteria for classification as monsoon depressions. This indicates that the India Meteorological Department record is missing storms, at least in those recent years. We furthermore use ocean surface wind speeds obtained from satellite scatterometers to estimate the number of extreme synoptic events occurring each summer over the Bay of Bengal, and find that this proxy for storm counts exhibits no statistically significant trend over the last three decades. Depression counts based on a reanalysis dataset also exhibit no statistically significant trend over the last three decades, but temporal changes in the observing network may render reanalysis-based storm-count trends unreliable.

In summary, our results demonstrate the existence of possible errors in the dataset on which previous claims of large trends in monsoon depression activity have been based. Although multiple issues exist with storm counts based on several reanalysis and satellite datasets, we were unable to find a statistically significant downward trend in depression counts using those datasets. This indicates a need for improved monitoring of monsoon depressions and more in-depth study of possible trends in their activity.