Intensification of Aila (May 2009) due to a warm core eddy in the north Bay of Bengal
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A very severe cyclonic storm “Aila” hit West Bengal on 26 May 2009. The storm intensified when it encountered with a warm core (SST = 31°C) anti-cyclonic eddy (ACE4) in the north Bay of Bengal. The storm intensity increased by 43% due to this eddy, which is comparable with that (34%) obtained from a best fit line (derived from several numerical experiments over north-west Pacific Ocean). The shallow mixed layer of the large-scale ocean and deep mixed layer inside the eddy appear to be crucial parameters besides translation speed of the storm (Uh), ambient relative humidity and thermal stratification below mixed layer, in the storm intensification. From the eddy size and Uh, the eddy feedback factor is found to be about 0.4 (i.e. 40%), which is close to the above. Since there exists an inverse relationship between Uh and UOHC (upper ocean heat content), slow (fast) moving storms require high (low) UOHC. The warm ACE4 with a high UOHC of 149 kj/cm2 (300% higher than the climatological value) and deep warm layer (D26 = 126 m) opposes the cooling induced by the storm and helps for the intensification of the storm through the supply of large enthalpy (latent + sensible) flux.
KeywordsAila Bay of Bengal Upper ocean heat content Warm eddy Mixed layer
The authors would like to thank Dr. S. R. Shetye, Director, N.I.O. and Dr. V. S. N. Murty, Scientist-in-Charge, NIO, Regional centre, Visakhapatnam (CSIR, New Delhi), for their support and encouragement. Mrs. Maneesha is thankful to CSIR (Council for Scientific and Industrial Research), New Delhi, for providing the Senior Research Fellowship (SRF). Thanks to all the project teams for making the data sets (SSHA,TMI SST, Argo data, RAMA buoys data, monthly mean vertical temperature profiles) available on the websites. The authors are thankful to the anonymous reviewers for their valuable suggestions, which helped to improve the original version of the manuscript. This is NIO contribution number 4969.
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