Quasi-biweekly oscillations of the South Asian monsoon and its co-evolution in the upper and lower troposphere

The Upper Tropospheric Quasi-Biweekly Oscillation (UQBW) of the South Asian monsoon is studied using the potential vorticity field on the 370 K isentrope. The UQBW is shown to be a common occurrence in the upper troposphere during the monsoon, and its typical evolution is described. We suggest that...

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Bibliographic Details
Published in:Climate dynamics 2017-11, Vol.49 (9-10), p.3159-3174
Main Authors: Ortega, Sebastián, Webster, Peter J., Toma, Violeta, Chang, Hai-Ru
Format: Article
Language:English
Subjects:
Anomalies
Anticyclones
Climatology
Cyclones
Dynamic height
Earth and Environmental Science
Earth Sciences
Environmental aspects
Evolution
Geophysics/Geodesy
Geopotential
Isentrope
Long wave radiation
Lower troposphere
Methods
Monsoons
Observations
Ocean currents
Oceanography
Oceans
Oscillations
Outgoing long-wave radiation
Planetary waves
Potential vorticity
Radiation
Rossby waves
South Asian monsoon
Tropical climate
Troposphere
Tropospheric circulation
Upper troposphere
Vorticity
Weather forecasting
Wind
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Summary:The Upper Tropospheric Quasi-Biweekly Oscillation (UQBW) of the South Asian monsoon is studied using the potential vorticity field on the 370 K isentrope. The UQBW is shown to be a common occurrence in the upper troposphere during the monsoon, and its typical evolution is described. We suggest that the UQBW is a phenomenon of both the middle and tropical latitudes, owing its existence to the presence of the planetary-scale upper-tropospheric monsoon anticyclone. The UQBW is first identified as Rossby waves originating in the northern flank of the monsoon anticyclone. These Rossby waves break when reaching the Pacific Ocean, and their associated cyclonic PV anomalies move southward to the east of Asia and then westward across the Indian Ocean and Africa advected by the monsoon anticyclone. A strong correlation, or co-evolution, between the UQBW and quasi-biweekly oscillations in the lower troposphere (QBW) is also found. In particular, analysis of vertically-integrated horizontal moisture transport, 850 hPa geopotential, and outgoing long-wave radiation show that the UQBW is usually observed at the same time as, and co-evolves with, the lower tropospheric QBW over South Asia. We discuss the nature of the UQBW, and its possible physical link with the QBW.
ISSN:0930-7575
1432-0894
DOI:10.1007/s00382-016-3503-y