Abstract
A long-lasting phytoplankton bloom, characterized by high chlorophyll-a (Chl-a) concentrations in an eddy-like feature, was detected in MODIS satellite imagery of the northwestern Pacific following the passage of Typhoon Krosa in August 2019. Satellite datasets, Argo measurements, and regional ocean models were analyzed to determine the occurrence and abundance of high-Chl-a concentrations and the upper-ocean conditions associated with them before and after the passage of the typhoon. Remote sensing data revealed that the typhoon triggered sharp increases in surface Chl-a concentrations more than five times the pre-typhoon average, which lasted for two weeks. The elevated post-typhoon concentrations coincided with a pre-existing oceanic cyclone that was detected as an altimetry-based sea surface height anomaly. The typhoon looped around the oceanic cyclone and lingered for two days at slow speeds (less than 2 m/s), producing an unusual sea-surface cooling of up to approximately 9 °C in the cyclonic eddy region. Our model successfully captured the typhoon-induced cold-core cyclonic circulation, which corresponded to the region of high Chl-a concentration. Model–data comparisons revealed that the looping motion of the slow-moving typhoon enhanced the pre-existing cyclonic circulation, resulting in strong vertical mixing and upwelling, consequently initiating a phytoplankton bloom due to increased nutrient supply to the euphotic zone.
Highlights
Chlorophyll-a (Chl-a) concentration is an index of phytoplankton bloom on the ocean surface that plays a crucial role in the air–sea carbon cycle, a major climate-regulating factor [1,2]
Remote-sensed sea surface temperature (SST) and Chl-a data reveal the occurrence of a surface phytoplankton bloom with high Chl-a concentrations in a cold eddy in the northwestern Pacific following the passage of Typhoon Krosa in August 2019
We discussed the mechanisms producing high surface Chl-a concentrations during typhoon passage and explained the role of physical processes associated with ocean circulation
Summary
Chlorophyll-a (Chl-a) concentration is an index of phytoplankton bloom on the ocean surface that plays a crucial role in the air–sea carbon cycle, a major climate-regulating factor [1,2]. The upper-ocean layer is well stratified and the mixed layer depth is shallower than the euphotic layer throughout the year For these reasons, strong phytoplankton blooms are rarely detected in the subtropical regions despite the relatively more frequent cooling of sea surface temperature (SST) following typhoon passage [11]. An eddy-like phytoplankton bloom with high concentrations of Chl-a was observed in remote sensing images of the northwestern Pacific following the passage of Typhoon Krosa in August 2019 (Figure 1). Using satellite observations and ocean model simulation, Yin et al [13] determined that the cyclonic circulation associated with Typhoon Ketsana pumped cold, nutrient-rich subsurface waters up to the sea surface, producing SST cooling and high concentrations of Chl-a at the surface. Danataanalysis of satellite datasets, Argo data, and the output of a regional ocean circulation model
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