Differential chlorophyll blooms induced by tropical cyclones and their relation to cyclone characteristics and ocean pre-conditions in the Indian Ocean

Variability of an ocean biological response induced by tropical cyclones and the factors responsible for the differential response is relatively unexplored in the Indian ocean. The aim of the current study is to analyse and identify the difference in the amplitude of chlorophyll blooms induced by tropical cyclones during the period of 1999-2016. The relationship of the amplitude of chlorophyll blooms to the cyclone characteristics (cyclone intensity and translation speed) and the oceanographic parameters (mixed layer depth and nutricline depth) is assessed. Analysis of chlorophyll blooms induced by 28 cyclones during the study period indicated that the amplitude of the chlorophyll concentration varies irrespective of the intensity of the cyclones. The results suggest that the translation speed exhibited by the cyclone is the key factor which controls the amplitude of blooms. The slower translation speed of the cyclone enhances the bloom intensity. Another factor which controls the bloom amplitude is the pre-existing shallow mixed layer depth. A shallow mixed layer modulates the light and nutrient availability which is essential in increasing the chlorophyll concentration. It is observed that shallower nutricline depth also favours an increase in the post-cyclone chlorophyll concentration. In-situ chlorophyll observations from Bio-Argo float during cyclones Hudhud and Vardah revealed that with its slower translation speed and shallower mixed layer depth, cyclone Hudhud could induce stronger bloom than cyclone Vardah, though the intensities of both the cyclones are the same at the location of float. This study implies that relatively weaker tropical cyclones can also induce strong chlorophyll blooms under favourable conditions and not all stronger cyclones induce blooms in the Indian Ocean.