Marine heat waves could have provided a favourable condition for the cyclone ‘Biparjoy’ to form over the Arabian Sea and become a very severe cyclone, say scientists of the Indian National Centre for Ocean Information Systems (INCOIS) under the Ministry of Earth Sciences (MoES).
A warmer ocean water column helps build moisture content in the air and thus provides the necessary energy source for cyclonic storms to form over the ocean. When the ocean remains warmer beyond a threshold from the normal condition for a prolonged period, these events are referred to as marine heatwaves. A marine heat wave was prevailing in the eastern Arabian Sea since early May. The sea surface temperature was more than 31° C (1° C warmer than normal), during the entire May and early June this year.
Senior scientist Abhisek Chatterjee and his students Gouri Anil, and Lakshmi R. Shenoy investigated the trends and genesis of heat waves and found these events are becoming prolonged and sometimes persistent for the entire pre-monsoon season in the recent decade. Their study showed similar heatwave conditions to be prevailing during the other recent severe cyclones such as ‘Tauktae’ and ‘Nisagra’, which hit the west coast of India and caused extensive damage to property and life.
These events suggest that increased cyclogenesis over the Arabian Sea in the recent decade is linked to persistent extreme warm ocean conditions. The sea has been found to be warming at a much more rapid pace among the other tropical basins in recent decades.
The research done between 1982–2019 period showed that the frequency of heatwaves increased at a rate of about 1.5–2 events per decade in the northern Arabian Sea and the southeastern Arabian Sea close to the Indian west coast. The increasing trend of heatwave days is mainly evident in the post-2000 era and conspicuous after 2015.
As the number of heatwave days has increased significantly over the last two decades, the number of tropical cyclones has also increased over the Arabian Sea. While a multi-fold increase in heat waves was observed from the ‘80s and since the beginning of the satellite record, the years 2010 and 2016 have exhibited the maximum number of heatwave days when more than 75% of days of the pre-monsoon and summer monsoon season experience heatwaves.
While this accelerated trend of heatwaves is found to be driven by a rapid rise in the mean Sea Surface Temperature (SST) of the Arabian Sea in the recent decade, these events are also found to be associated with dominant climate modes such as ‘El Nino’.
Since El Nino usually also impacts the Indian Summer monsoon rainfall, the drought years over the subcontinent will likely co-occur with heat waves on the oceans. This rapid increase in heatwaves is likely to have an impact on the physio-biogeochemical processes of this basin as well, such as an increase in the harmful algal bloom and coral bleaching in the Arabian Sea, the scientists added.
“We have recently launched the marine heatwave advisory services for better understanding and preparedness considering the potential impact of such events, ”said INCOIS Director Srinivasa Kumar Tummala. Funding for the work published in ‘Ocean Science’ was provided from the MoES “Deep Ocean Mission” programme.
