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Publication Date
22 June 2020

Assessing Historical Variability of South Asian Monsoon Lows and Depressions with an Optimized Tracking Algorithm



Cyclonic low‐pressure systems (LPS) produce abundant rainfall in South Asia, where they are traditionally categorized as monsoon lows, monsoon depressions, and more intense cyclonic storms. The India Meteorological Department (IMD) has tracked monsoon depressions for over a century, finding a large decline in their number in recent decades, but their methods have changed over time and do not include monsoon lows. This study presents a fast, objective algorithm for identifying monsoon LPS and uses it to assess interannual variability and trends in reanalyses. Variables and thresholds used in the algorithm are selected to best match a subjectively analyzed LPS dataset while minimizing disagreement between four reanalyses in a training period. The streamfunction of 850 hPa horizontal wind is found to be optimal in this sense; it is less noisy than vorticity and represents the complete non‐divergent wind, even when flow is not geostrophic. Using this algorithm, LPS statistics are computed for five reanalyses, and none show a detectable trend in monsoon depression counts since 1979. Both the Japanese 55‐year Reanalysis (JRA‐55) and the IMD dataset show a step‐like reduction in depression counts when they began using geostationary satellite data, in 1979 and 1982 respectively; the 1958‐2018 linear trend in JRA‐55, however, is smaller than in the IMD dataset and its error bar includes zero. There are more LPS in seasons with above‐average monsoon rainfall and in La Niña years, but few other large‐scale modes of interannual variability are found to modulate LPS counts, lifetimes, or track length consistently across reanalyses.
“Assessing Historical Variability Of South Asian Monsoon Lows And Depressions With An Optimized Tracking Algorithm”. 2020. Journal Of Geophysical Research: Atmospheres. doi:10.1029/2020jd032977.
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