Phytoplankton Blooms Triggered by Anticyclonic Eddy and Cyclonic Eddy during Tropical Cyclone Nada

Abstract

Tropical cyclone (TC) Nada occurred in the southwestern Bay of Bengal (BOB) in the winter of 2016. With high photosynthetically available radiation (PAR) and minimal precipitation, phytoplankton blooms occurred from offshore to nearshore respectively. In this study, there was an pre-existed ocean eddy pair on the left and right sides of Nada’s path before its passage, we studied the different mechanisms of phytoplankton blooms caused by TC Nada and the ocean eddy pair with satellite observation, multisource reanalysis products, and Argo float data. An anticyclonic eddy near Sri Lanka was weakened during Nada and horizontal transport of chlorophyll-a (Chl-a) balanced the inhibition effect of anticyclone on Chl-a. Near the anticyclonic eddy’s periphery, an upwelling above the thermocline was enhanced by Nada which promoted the uplift of nutrient-rich deep-water. Meanwhile, the large cyclonic eddy in the northern Bay of Bengal was strengthened by Nada, with the vorticity increasing from 0.19 s–1 to 0.28 s–1. Significant inertial oscillation (~2 days period) happened in the subsurface layer, leading to strong vertical mixing and upwelling and subsequently causing the offshore surface Chl-a bloom. This study provides new insights to evaluate typhoons and ocean eddies that induce biological responses in the future.