[A-OS22] Physical, chemical and biological processes and variability in the Indian Ocean

[AOS22-04] Estimation of phytoplankton size classes response to the tropical cyclone Ockhi over the Arabian Sea

*Rebekah Shunmugapandi¹, Shirishkumar Gedam², Arun B Inamdar² (1.PhD Student,Centre of Studies in Resources Engineering, Indian Institute of Technology Bombay, India, 2.Professor,Centre of Studies in Resources Engineering, Indian Institute of Technology Bombay, India)

Keywords:Ockhi cyclone, Phytoplankton size classes, DINEOF, Arabian Sea

The influence of extreme events like cyclone profoundly interferes the phytoplankton community distribution over the ocean surface. With the intention of investigating the above-mentioned case, this study aims to perceive the response and dynamics of phytoplankton communities in the Arabian Sea (AS) to the tropical cyclone (TC), the cyclonic event Ockhi (29 November- 6 December 2017). To start with, MODIS-Aqua satellite-derived chlorophyll-a concentration (chl-a) data were utilized as the primary data to retrieve phytoplankton size classes (PSCs) (i.e micro, nano and pico phytoplankton) using the abundance-based approach. The inconsistencies in the data due to overcast were resolved by reconstructing the chl-a data using an empirical orthogonal function: DINEOF method. Besides, the corresponding sea surface temperature, sea surface salinity and wind speed data were used to estimate the response of phytoplankton communities during the TC. The results indicate that microphytoplankton were found to be dominant in the eastern AS during the post-cyclone, particularly on the passage of Ockhi cyclone track. The above scenario denotes an unusual incident because the eastern lower latitudes of AS are oligotrophic (less light and nutrient condition) in nature which is usually dominated by picophytoplankton. Therefore, on comparing with the wind, surface salinity and sea surface temperature, it is found that the freshwater discharges (high in nutrients) triggered the microphytoplankton production relative to the other size classes that led a short term community shift at the region of cyclone passage. From the results, it is witnessed that the Ockhi cyclone invoked a transient suitable condition for microphytoplankton to proliferate in the south-eastern part of the AS. The overall study sensitized relevant insight into the short-term community shift of the PSCs due to the effects of tropical cyclones.