Impact of Southern Oscillation on the Frequency of Monsoon Depressions in the Bay of Bengal

Authors: Singh, O.P.; Ali Khan, T.M.; Rahman, M.S.

Source: Natural Hazards, Volume 25, Number 2, February 2002 , pp. 101-115(15)

Publisher: Springer

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The impact of Southern Oscillation on the cyclogenesis over the Bay of Bengal during the summer monsoon has been investigated. The analysis of correlation coefficients (CCs) between the frequency of monsoon depressions and the Southern Oscillation Index (SOI) reveals that more depressions form during July and August of El Niño years. Due to this, the seasonal frequency of monsoon depressions remains little higher during El Niño epochs even though the correlations for June and September are not significant. The CCs for July and August are significant at the 99% level.

The El Niño-Southern Oscillation (ENSO) is known to affect Indian Monsoon Rainfall (IMR) adversely. The enhanced cyclogenesis over the Bay of Bengal during July and August is an impact of ENSO which needs to be examined closely. Increased cyclogenesis over the Bay of Bengal may be reducing the deficiency in IMR during El Niño years by producing more rainfall over the eastern parts of India during July and August. Thus there is a considerable spatial variation in the impact of ENSO on the monsoon rainfall over India and El Niño need not necessarily imply a monsoon failure everywhere in India.

The area of formation of monsoon depressions shifts eastward during El Niño years. Warmer sea surface temperature (SST) anomalies prevail over northwest and adjoining westcentral Bay of Bengal during premonsoon and monsoon seasons of El Niño years. May minus March SOI can provide useful predictions of monsoon depression frequency during July and August.

Keywords: Correlation Coefficient (CC); Cyclogenesis; El Niño/Southern Oscillation (ENSO); La Niña; Monsoon Depression; Multivariate ENSO Index (MEI); Standardized Anomaly; Summer Monsoon

Document Type: Regular Paper

Affiliations: SAARC Meteorological Research Centre (SMRC), Abhawa Bhaban, Agargaon Dhaka-1207, Bangladesh

Publication date: February 1, 2002

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