Abstract
Abstract Monsoon depressions (MDs) bring substantial monsoon rainfall to northern and central India. These events usually form over the Bay of Bengal and travel across northern India toward Pakistan. Using European Centre for Medium-Range Weather Forecasts interim reanalysis, an MD-tracking algorithm, and an objective identification method, the authors find that about 40% of MDs interact with northerly intrusions of dry desert air masses as the MDs traverse the subcontinent. MD interactions with dry intrusions are often preceded by positive potential vorticity (PV) anomalies on the subtropical jet and low-level anticyclonic anomalies over the north Arabian Sea. Dry intrusions nearly halve the precipitation rate in the southwest quadrant of MDs, where MDs rain the most. However, dry intrusions increase the rainfall rate near the MD center. Similarly, ascent is reduced west of the MD center and enhanced at the MD center, especially in the upper troposphere. The reduced ascent west of MD centers is likely attributable to changes in vertical shear reducing differential cyclonic vorticity advection. Dry intrusions slightly reduce MDs’ propagation speed. For the mid- to upper-level vortex, this can be explained by anomalous westerlies reducing propagation by adiabatic advection. For the lower-tropospheric vortex, it is likely that reduced diabatic generation of PV plays a role in slowing propagation, along with reduced adiabatic advection.
Highlights
The variability of northern India’s summer monsoon includes frequent synoptic-scale disturbances
We explore the interaction between monsoon depressions and dry intrusions for the last 30 years of depressions
The potential vorticity (PV) increase near the center may be diabatically generated in the lower troposphere; we found evidence that the centers of Monsoon depressions (MDs) with dry intrusions are slightly more closed off to stirring from the outside; this may preserve the high PV near the center
Summary
The variability of northern India’s summer monsoon includes frequent synoptic-scale disturbances. The effect of the composite dry intrusion is most evident to the west and northwest of the depression center, with much lower wet-bulb potential temperature at both pressure levels. The anticyclonic PV anomaly in the lower troposphere west of the MD center induces a circulation that would advect the MD vortex southward We suggest that this combination of mechanisms slows MD propagation in the presence of dry intrusions. In most of the troposphere, the composite potential temperature is lower in depressions that encounter dry intrusions, especially west of the MD center, where convection and precipitation are suppressed While quantitative treatment of PV tendency terms is not possible with these data, Fig. 11 suggests that it is more the latter than the former, as the flow is mostly parallel with PV contours
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