ORIGINAL PAPER Impacts of ENSO and IOD on tropical cyclone activity in the Bay of Bengal Biranchi Kumar Mahala • Birendra Kumar Nayak • Pratap Kumar Mohanty Received: 7 May 2013 / Accepted: 3 August 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract The impacts of El Nin ˜o-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on tropical cyclone (TC) activity (intensity, frequency, genesis location, track and average lifetime) in the Bay of Bengal (BoB) are studied for the period 1891–2007 using cyclone e-Atlas of India Meteorological Department, Nin ˜o3.4 Index, Oceanic Nin ˜o Index and Dipole Mode Index (DMI). TCs in the present study include cyclones with maximum sustainable wind (MSW) C34 knots (referred as cyclonic storms) and severe cyclonic storms with MSW C48 knots. The study shows a total of 502 TCs over BoB during the 117-year study period at the rate of 4.29 TC per year. Seven-year running mean of TCs for the period 1891–2007 shows a decreasing trend. Correlation between Nin ˜o3.4 Index and DMI for the 117-year period is significant and positive and the sig- nificance level is higher (lower) for the period with higher (lower) TC frequencies. One- third monthly interval analysis for the 117-year period indicates first third (1–10) of November as the most favoured period of TC formation over BoB. 117-year study period is divided into years of ENSO (El Nin ˜ o, La Nin ˜ a and neutral ENSO) and IOD (?ve IOD, -ve IOD and no IOD) categories. Maximum frequency of TC is observed during La Nin ˜a years, -ve IOD years and also when La Nin ˜a co-occurred with -ve IOD. More severe cyclones are formed during La Nin ˜a and ?ve IOD years. Genesis location of TCs indicates that during La Nin ˜a (El Nin ˜o) years, the TCs are oriented in the south-east–north-west (south- west–north-east) direction. TCs in no IOD and -ve (?ve) IOD years are more (less) in northern BoB (north of 15° N), while in southern BoB (south of 15° N), TCs are more (less) during no IOD and ?ve (-ve) IOD condition. BoB is divided into four quadrants, and number of TCs in each quadrant is computed under different ENSO–IOD events. Peak B. K. Mahala (&) Department of Mathematics, KIIT Polytechnic, Bhubaneswar 751024, Odisha, India e-mail: biranchimahala@gmail.com B. K. Nayak Department of Mathematics, Utkal University, Vani Vihar, Bhubaneswar 751004, Odisha, India P. K. Mohanty Department of Marine Sciences, Berhampur University, Berhampur 760007, Odisha, India 123 Nat Hazards DOI 10.1007/s11069-014-1360-8