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Unusual onset phase of the southwest monsoon 2009: Weak upper circulation

Naik, SS and Salvekar, PS and Deshpande, MS (2011) Unusual onset phase of the southwest monsoon 2009: Weak upper circulation. International Journal of Meteorology, 36 (365). pp. 301-312.

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The onset (23rd May) and the advancement of the southwest monsoon season for the year 2009 over the Indian region was unusual. The onset was one week earlier than normal, the progress of the monsoon was delayed for more than 3 weeks, but still the monsoon covered entire India by the 3rd July (12 days earlier than usual). An attempt is made to examine the mean circulation features and dynamics associated with the pre-onset to post-onset period of 2009 and are compared with the corresponding features obtained from a normal monsoon year (2000). Divergence, vorticity and vertical velocity are computed using NCEP reanalyzed daily horizontal wind, over the Asian region (25 °S-30 °N, 40 °E-110 °E) throughout the troposphere for the period 15th May-19th June for both years. For 2009, computed vorticity and vertical velocity indicated the large scale anti-cyclonic circulation in the lower levels over the central and western parts of Indian region, north of 10°N and downward motion is found west of 80 °E from the 5th-15th June. These features were responsible for the prolonged hiatus in the advancement of the monsoon over India during the 2nd and 3rd week of June. During the year 2000 (when monsoon season began at the usual time), computed vorticity and vertical velocity showed cyclonic circulation and upward motion throughout the troposphere during the period 1st June-19th June. To understand the phenomena carefully, the Kinetic Energy for different vertical levels are computed and the time series of Somali Jet (SJ) and Tropical Easterly Jet (TEJ) are obtained. It is found that both weak SJ and TEJ are not favourable to maintain meridional temperature gradient required for the development of monsoon in June 2009. © The International Journal of Meteorology.

Item Type: Article
Additional Information: Copyright of this article belongs to International Journal of Meteorology
Uncontrolled Keywords: atmospheric circulation; jet stream; monsoon; troposphere; vorticity, India
Subjects: Meteorology and Climatology
Depositing User: IITM Library
Date Deposited: 10 Dec 2015 05:44
Last Modified: 10 Dec 2015 05:44

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