Sanjeev Kumar Patro

Bhubaneswar: For the two successive years (2019, 2020), severe cyclones took birth in the Bay of Bengal in the month of May. With the moderate La Nina conditions are on their way out, as per the latest El Nino Southern Oscillation forecast, what is in store in Bay of Bengal this May?

Meanwhile, after Asia Pacific Economic Cooperation (APEC) Climate centre, the multi-model probability forecast of International Research Institute for Climate and Society had made a forecast of 50 per cent probability of above-normal rainfall in Odisha during the months of May and June.

With regard to summer temperature in Odisha, the model forecast a no-normal summer this year, even as there has been no prediction about the State recording above normal temperature this summer if the model predictions are any indication.

As per the IMD’s January 2021 ENSO forecast, the sea surface temperature (SST) in the Bay of Bengal, and also the Arabian Sea, is predicted to remain normal to warmer than normal during the period of January-February-March to March-April-May. In simple terms, the SST in the Bay of Bengal is going to remain above normal this May.

SST Riddle

As per Indian National Centre for Ocean Information Services (INCOIS) report, possibly for the first time ever, before the formation of cyclone Amphan, the sea surface temperature in some pockets of the Bay of Bengal in the first two weeks of May was recorded as high as 34 degrees C vis-a-vis the normal temperature of 28 degrees Celsius.

“Our observations have recorded sea surface temperature going up to 32 degrees C earlier in Bay of Bengal but not 34 degrees C. This is what had favoured the formation of super cyclone Amphan greatly,” said a senior scientist at INCOIS.

In the month of May 2019, when the extremely severe cyclone Fani was formed, the Sea Surface Temperature in the Bay of Bengal had been measured at around 31-32 deg C.

SST Seeds Cyclones?

According to IMD Bhubaneswar Director HR Biswas, though SST is the prime factor behind cyclone genesis, it is not the only condition.

“Besides, high instability in the atmosphere, for a cyclonic system to develop there should be a prior disturbance. Also, as it needs moisture to grow, humidity has to remain very high. Along with the factors, low vertical wind shear (means difference in wind velocity from lower troposphere to higher level) and above all, a favourable rain-bearing phase called MJO (Madden Julian Oscillation) is required for a system to grow into a tropical cyclone,” he explained.

History Of Cyclones in May

As per IMD data, out of 711 tropical cyclones formed during the period of 1891 to 2019 May in the north Indian ocean (meaning the Bay of Bengal and the Arabian Sea), around 38 per cent were only depressions, 27 per cent were deep depressions, cyclonic storms (16 per cent), Severe Cyclonic storms (SCS) (9 per cent), Very SCS (6 per cent) and Extremely SCS were mere 3 per cent, including Fani.

Though most cyclones during May move either towards Bangladesh or Myanmar, On May 27, in 1823, a cyclone hit India for the first time in the month of May.

The other cyclone-hit years during the month of May are 1977, 1979, 1985, 1989(Balasore), 1990, 2000, 2010, 2019 (Puri).

As per an IMD research study, tropical cyclones in the month of May have a relationship with La Nina and El Nino conditions. Studies show normal La Nina years (like the year 2021) are not favourable for pre-monsoon cyclones.