The Interaction of Indian Monsoon Depressions with Northwesterly Midlevel Dry Intrusions

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...

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Bibliographic Details
Published in:Monthly weather review 2018-03, Vol.146 (3), p.679-693
Main Authors: Fletcher, Jennifer K., Parker, Douglas J., Hunt, Kieran M. R., Vishwanathan, Gokul, Govindankutty, Mrudula
Format: Article
Language:English
Subjects:
Adiabatic
Adiabatic flow
Advection
Aerodynamics
Air masses
Anomalies
Ascent
Deserts
Dry intrusions
Humidity
Interactions
Laboratories
Monsoon depressions
Monsoon rainfall
Monsoons
Potential vorticity
Precipitation
Precipitation rate
Propagation
Rain
Rainfall
Rainfall rate
Troposphere
Upper level vortexes
Upper troposphere
Vertical shear
Vorticity
Weather forecasting
Westerlies
Wind
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Summary: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.
ISSN:0027-0644
1520-0493
DOI:10.1175/MWR-D-17-0188.1