Tropical storm-induced turbulent mixing and chlorophyll-a enhancement in the continental shelf southeast of Hainan Island

- Turbulent mixing and Chl-a southeast off Hainan are enhanced by storm Washi.
- Tropic storm Washi is a fast-moving and weak-strength storm.
- Near-inertial baroclinic energy rapidly dissipates as penetrating the thermocline.
- Baroclinic shear instability contributes to the strong mixing.
- The increase of Chl-a is attributed to the storm-induced turbulent mixing.

Based on moored observations and remote sensing data in July and August 2005, energy sources for enhancing turbulent mixing and possible mechanisms of phytoplankton bloom in the continental shelf southeast of Hainan Island under the influence of Washi, a fast-moving and weak tropical storm, are analyzed in this paper. Observations show that strong near-inertial internal waves were generated by the rapidly changing wind stress and the near-inertial energy was dissipated quickly across the thermocline. The strong turbulent mixing associated with the near-inertial baroclinic shear instability occurred with maximum eddy diffusivity above 3.2 × 10− 4 m2 s− 1, and the surface chlorophyll-a (Chl-a) concentration after the storm increased by 22.2%. The Chl-a concentration augment was inferred to be an upper ocean biophysical response to the enhanced near-inertial turbulent mixing which could increase the upward nutrient flux into the surface low eutrophic zone during the passage of Washi.