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

Citation: Naftali Y Cohen, and William R Boos, 2014: Has the number of Indian summer monsoon depressions decreased over the last thirty years? Geophysical Research Letters, 41, 7846–7853, doi: 10.1002/2014GL061895

Abstract

Monsoon depressions are cyclonic atmospheric vortices with outer radii near 1000 km that form within the larger-scale monsoon circulations of India and other regions. Recent studies have reported a downward trend in recent decades in the number of Indian summer monsoon depressions. In particular, the years 2002, 2010, and 2012 were noted for having the first summers, in over a century, in which no depressions formed. Here satellite and reanalysis data 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. Furthermore, the number of extreme synoptic events occurring each summer over the Bay of Bengal is estimated from satellite scatterometers and exhibits no statistically significant trend over the last three decades. These results raise questions about the validity of previously claimed large trends in monsoon depression activity in the Indian summer monsoon.

Keywords:

Classification, Gaussian filter, tracking algorithms, linear regression, nonlinear Poisson regressions, bootstrap, stochastic search, model averaging, Pearson correlation, identification algorithms, monsoon depression, atmospheric vortices, satellite and reanalysis data, satellite scatterometers, statistically significant trend, precipitation, floods, monsoon lows, India Meteorological Department, Bay of Bengal, reanalysis data, ocean surface wind speeds