Influence of sequential tropical cyclones on phytoplankton blooms in the northwestern South China Sea

Abstract

Upper ocean responses to the passage of sequential tropical cyclones over the northwestern South China Sea (SCS) in 2011 were investigated using satellite remote sensing data, Argo reanalysis data, and an array of mooring data. We found that the sea surface low temperature region lasted for more than 38 days and two phytoplankton blooms occurred after the passage of sequential tropical cyclones. The upper ocean cooling reached 2–5°C with a right-side bias was observed along the typhoon track to about 200 km. The maintenance of low temperature region and the two phytoplankton blooms were mainly driven by upwelling and near-inertial turbulence mixing induced by the sequential tropical cyclones. The first phytoplankton bloom appeared on the 7th day after the passage of the three tropical cyclones, and the chlorophyll a (chl a) concentration increased by 226%, which may be mainly driven by typhoons induced upwelling. The second phytoplankton bloom occurred on the 30th day, the chl-a concentration increased by 290%. Further analysis suggested that only the typhoons with similar characteristics as Nesat and Nalgae can induce strong near-inertial oscillation (NIO). Strong turbulent mixing associated with the near-inertial baroclinic shear instability lasted for 26 days. The measured mean eddy diffusivity in the upper ocean was above 10−4 m2/s after typhoon Nesat. Enhancement of the turbulent mixing in the upper ocean helped to transport nutrient-rich cold waters from the deep layer to the euphotic layer, and is a major mechanism for the long-term maintenance of low temperature region as well as the second phytoplankton bloom.