Automated identification of South Asian monsoon low pressure systems:
Historical variations across reanalysis products
Abstract
Synoptic-scale cyclonic vortices produce abundant rainfall in South
Asia, where these low pressure systems (LPS) are traditionally
categorized as monsoon lows, monsoon depressions, and more intense
cyclonic storms. The India Meteorological Department has tracked monsoon
depressions for over a century, finding a large decline in the number of
those storms in recent decades; their tracking methods, however, seem to
have changed over time and do not include monsoon lows, which can
produce intense rainfall despite their weak winds. This study presents a
fast and objective tracking algorithm that can identify monsoon LPS in
high-resolution datasets with a variety of grid structures. A
sensitivity analysis has been performed to select a set of atmospheric
variables and their corresponding thresholds for optimal tracking of
LPS. Approximately 250 combinations of variables and thresholds are used
to identify LPS over roughly a decade (the training period) in each of
four atmospheric reanalyses, and these combinations are ranked using a
skill score that compares the reanalyses with each other and with a
preexisting track dataset that was compiled by subjective identification
of LPS. This procedure finds the streamfunction of the 850 hPa
horizontal wind to be the best variable for tracking LPS. The
streamfunction is smoother than the vorticity field and represents the
complete non-divergent component of the wind even when the flow is not
geostrophic, unlike the geopotential height or sea level pressure. Using
this tracking algorithm, LPS statistics are then computed in five
reanalysis products that each span at least 40 years, with a primary
goal being to determine whether the large decrease in monsoon
depressions seen in the India Meteorological Department track dataset
since the 1970s can be found in any reanalysis. This trend assessment is
particularly relevant for the ERA5 reanalysis, which extends back to
1950 and which contains explicit climate forcings. In addition to
secular trends, this study assesses the decadal variation of LPS, as
well as interannual changes in LPS activity that are associated with the
El Niño-Southern Oscillation and the Indian Ocean Dipole.