Strong enhancement of chlorophyll a concentration by a weak typhoon

TL;DR

This study shows that even a weak typhoon can significantly boost chlorophyll a levels in the ocean if it has a long forcing time, leading to strong phytoplankton blooms.

Contribution

It demonstrates that typhoon forcing time and location near the equator are critical factors in phytoplankton bloom enhancement, highlighting the importance of typhoon characteristics.

Findings

Typhoon Hagibis caused a 30% increase in annual chl a in the SCS.

Longer forcing time (>82 h) enhances upwelling and phytoplankton growth.

Weak typhoons can induce significant biological responses under certain conditions.

Abstract

Recent studies demonstrate that chlorophyll a (chl a) concentrations in the surface ocean can be significantly enhanced due to typhoons. The present study investigated chl a concentrations in the middle of the South China Sea (SCS) from 1997-2007. Only the Category1 (minimal) Typhoon Hagibis (2007) had a notable effect on the chl a concentrations. Typhoon Hagibis had a strong upwelling potential due to its location near the equator, and the forcing time of the typhoon (>82 h) was much longer than the geostrophic adjustment time (~63 h). The higher upwelling velocity and the longer forcing time increased the depth of the mixed-layer, which consequently induced a strong phytoplankton bloom that accounted for about 30% of the total annual chl a concentration in the middle of the SCS. The implication is that the forcing time of a typhoon should be long enough to establish a strong upwelling and consequently for the induction of significant upper ocean responses.