Ž . Global and Planetary Change 29 2001 135–147 www.elsevier.comrlocatergloplacha Probable reasons for enhanced cyclogenesis in the Bay of Bengal during July–August of ENSO years O.P. Singh ) , Tariq Masood Ali Khan, Md. Sazedur Rahman SAARC Meteorological Research Centre, 7th Floor, Abhawa Bhaban, Agargaon, Dhaka, Bangladesh Accepted 5 October 2000 Abstract This work presents the results of a study made to identify the probable causes of the higher frequency of monsoon depressions over the northern and adjoining central Bay of Bengal during July–August of ENSO years. For this purpose, Ž . Ž . 0.58 grid point satellite-derived NOAA–AVHRR sea surface temperature SST data and Multivariate ENSO Index MEI data for the period 1985–1998 have been used. SST distributions over various parts of Bay of Bengal have been obtained for Ž . Ž . MEI )1 Warm ENSO phase and MEI -y1 Cold phase . The analysis has been carried out for the entire summer monsoon season with special emphasis on July–August, during which the majority of monsoon depressions occur. q 2001 Published by Elsevier Science B.V. Ž . Ž . Ž . Keywords: El NinorSouthern Oscillation ENSO ; monsoon depression; sea surface temperature SST ; multivariate ENSO Index MEI ; ˜ Ž . Southern Oscillation Index SOI ; Summer monsoon; Bay of Bengal; La Nina; Cyclonic vorticity ˜ 1. Introduction Monsoon depressions are important synoptic sys- tems of the summer monsoon season lasting from June to September of each year. These systems develop at the eastern end of the monsoon trough located over extreme north of Bengal known as the Head Bay of Bengal, and generally move northwest- wards along the trough. The significant part of mon- soon rainfall over the eastern regions of India and Bangladesh is associated with these systems. On an average, four to six depressions form over the Bay of ) Corresponding author. Tel.: q 880-2-9124508; fax: q 880-2- 8111670. Ž . E-mail addresses: smrc@bol-online.com O.P. Singh , Ž . tariqmak@yahoo.com, tariqmak@pradeshta.net T.M. Ali Khan . Bengal every year during the summer monsoon sea- son, their frequency being highest during August. Monsoon systems exhibit large inter-annual varia- Ž . tions Shukla, 1987; Chang and Krishnamurti, 1987 . Ž . As El NinorSouthern Oscillation ENSO is known ˜ Ž to cause worldwide weather anomalies Chan, 1985; . Gray and Sheaffer, 1991; Rao, 1997 , it is expected that some components of inter-annual variations of Asian summer monsoon have their linkages with the Ž . ENSO Singh and Ali Khan, 1999 . The Bay of Bengal seems to respond to ENSO epochs actively. There seems to be an appreciable increase in the frequency of monsoon depressions during July– August of ENSO years. For instance, during the strongest ENSO event of 1982, six monsoon depres- sions formed over the Bay of Bengal in July–August against the normal frequency of about three. Simi- larly, during the recent ENSO of 1997, four depres- sions formed over the Bay whereas no depression 0921-8181r01r$ - see front matter q 2001 Published by Elsevier Science B.V. Ž . PII: S0921-8181 00 00090-4