Tropical storms, particularly cyclones, trigger blooms of phytoplanktons, microscopic marine algae over the Bay of Bengal and Arabian Sea, a study reveals1.

The study, by a team from the Indian Institute of Technology in Kharagpur, provides new insights into the cyclone-induced growth spurt of the algae, which are primary food producers, consume carbon dioxide and modify global sea-surface temperature.

Such phytoplankton blooms, the researchers say, could directly influence fisheries and modify the local but short-term carbon budget and sea surface temperature.

The scientists, led by Jayanarayanan Kuttippurath, analysed 51 tropical cyclones that occurred over the Bay of Bengal and Arabian Sea between 1997 and 2019. They assessed the amplitude of the phytoplankton blooms during pre-monsoon and post-monsoon seasons by studying the satellite-based data on chlorophyll-a concentrations of the algae.

They detected 30 cyclone-induced blooms in the Bay of Bengal and 18 in the Arabian Sea. The frequency of the cyclones and the cyclone-induced blooms is higher in the post-monsoon than in the pre-monsoon seasons.

The blooms in the post-monsoon periods lasted longer than those of the pre-monsoon period after the passage of the cyclones. The slow-moving cyclones that spend more time over the seas cause strong and sustained upwelling, a process that transports cold and nutrient-rich sub-surface water to the sea surface.

Such upwelling, when coupled with cold-core eddies near the cyclone tracks, triggers intense growth of phytoplanktons. The cyclones in La Niña years drove higher blooms than those in the normal and El Niño years, says Kuttippurath.


1. Kuttippurath, J. et al. Tropical storms trigger phytoplankton blooms in the deserts of north Indian Ocean. npg Clim. Atmos. Sci. 4, 11 (2021) doi: 10.1038/s41612-021-00166-x