A study on the physical and biogeochemical responses of the Bay of Bengal due to cyclone Madi

Madi was a category-2 cyclonic storm formed over the south-western Bay of Bengal (BoB) in December 2013. It formed on 6th December as a depression, and by 8^th December it became a very severe cyclonic storm. Some unique features of Madi were: (a) an unexpected reduction in the intensity during its north-westward movement, (b) sudden change in track by almost 180^o in a south-westerly direction, and (c) swift dissipation in the open ocean while moving over cold-core eddies during its south-westward movement. The rapid weakening in intensity before track reversal occurred mainly due to its passage over cold-core eddies, where the upper-ocean heat content was reduced due to eddy-pumping of cold water from the subsurface. An analysis of the eddy-feedback factor reiterated that the slower movement of the cyclone was due to the cold-core eddy. When the cyclone entered into the cold-core eddy region, it slowed-down and changed its track towards a south-westerly direction under the influence of prevailing north-easterly winds. The biogeochemical response of Madi estimated using the satellite remote sensing and Bio Argo data showed an increase in Chlorophyll-a (Chl-a) concentration from 0.2-0.4–2.7 mg/m³, while the net primary productivity (NPP) increased from 320 to 2500 mg C/m² /day, both of which were about 7 and 8 times respectively higher than the before-cyclone values. The CO₂ flux showed a 4 times increase from its pre-cyclone value of 3.5 mmol/m² /day, indicating that BoB becomes a strong source to the atmosphere during the cyclone Madi period.