Assessing historical variability of South Asian monsoon lows and
depressions with an optimized tracking algorithm
Abstract
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
in high-resolution datasets. Variables and thresholds used in the
algorithm are selected to best match a subjectively analyzed LPS dataset
while minimizing disagreement between four atmospheric reanalyses in a
training period. The streamfunction of the 850 hPa horizontal wind is
found to be the best variable for tracking LPS; 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 also in
La Nin ̵̃a years, but few other large-scale modes of interannual climate
variability are found to modulate LPS counts, lifetimes, or track length
consistently across all reanalyses.