The prediction of La Nina conditions from September this year may sound a stormy one for Odisha, especially when seen together with the prevalence of negative India Ocean Dipole (IOD) till the winter month of December.
The latest forecast by Asia Pacific Climate Centre (APCC) assigns an over 50-60 per cent probability of above-normal rainfall during October - the cyclone month - in Odisha. Significantly, when the same forecast predicts a below-normal rainfall in August and September.
Even, the GFS-model forecast by the US-based National Oceanic and Atmospheric Administration (NOAA) predicts a 15 to 50 per cent deficient rainfall across the districts in the State till August 17.
Both renowned weather forecasters predict the State witnessing above normal temperature between August to October. And the forecasts show the SST (sea surface temperature) will be remaining above normal till October.
La Nina - Negative IOD - Cyclone Link
A dozen of studies on the link between tropical cyclones in the Bay of Bengal and the climatic phenomenon like La Nina and negative IOD reveal big. Take a glance at the study conclusions below.
• As per a study conducted by scientists from the Indian National Centre for Ocean Information Services (INCOIS) and published in the Journal of Geophysical Research. a high of 20 cyclones had occurred during the La Nina years, while nine were in the El Nino period.
• Another study also shows that maximum frequency of tropical cyclone had been observed during the La Nina years, negative IOD years and also when La Nina co-occurred with negative IOD (As has been predicted to happen this year)
• IMD studies show that negative IOD results in stronger than usual cyclogenesis in the Bay of Bengal, but a suppressed cyclogenesis condition in the Arabian Sea.
• IMD study observes that La Nina conditions in post-monsoon season will ensure a decrease in rainfall, but if a cyclone develops then it may grow up to acquire a very severe status.
Instances On Ground
A look at the data available with IMD and NOAA shows that Odisha had witnessed some of the major cyclones during La Nina and negative IOD years. (see the image below)
• The years of 1999 -2000 had been La Nina years. And a Super Cyclone battered Odisha in the year 1999.
• Similarly, the El Nino years started in 2014 and lasted till 2016, Phailin hit Odisha in 2013 - the La Nina and negative IOD year.
• The year 2018 had weak La Nina conditions, Odisha had to brace up with very severe cyclones like Titli and severe cyclones like Daye.
Why Cyclone genesis Probability High?
As per the INCOIS study, during La Nina, the wind direction in the Pacific changes leading to an increase in the speed of winds blowing from Africa to Indonesia during the cyclone season. As a consequence to this increased wind speed, the warm water pile up in Indonesia, which then make Bay of Bengal warmer. The obvious rule then is a hotter ocean increases the probability of more cyclones in the bay.
Significantly, the study had also observed that during the La Nina years, the cyclone genesis location gets shifted to 87 degrees East in the Bay of Bengal, which means tropical cyclone formation takes place in the central Bay of Bengal (to the west of A&N Islands).