Upper Ocean Structure Determines the Contrasting Typhoon‐Induced Chlorophyll‐a Responses in the Northwest Pacific.

The frequent typhoons in the northwest Pacific drive oceanic responses, for example, changes in sea surface temperature (SST) and chlorophyll‐a (Chl‐a). A composite analysis shows that SST (Chl‐a) around the typhoon center decreases (increases). The SST begins to decrease 3 days before the typhoon's arrival and further decreases until 2 days following the typhoon's passage. The Chl‐a increases rapidly after the typhoon's arrival, and the maximum value is reached 3 days after the typhoon. Large oceanic responses are often associated with typhoons that have high wind speeds and slow translation speeds. From the perspective of the upper ocean structure, increasing changes in SST are identified with a shallower pre‐typhoon mixed layer depth (MLD). However, a significant dependence of the Chl‐a response on the pre‐typhoon MLD emerges only when the depth of typhoon‐induced mixing is greater than the pre‐typhoon MLD. This study helps to quantitatively describe typhoon‐induced changes with consideration of the determinant oceanic environment. Plain Language Summary: Typhoons in the northwestern Pacific induce strong oceanic responses. Using 17 years of satellite observations, the impacts of typhoons on sea surface temperature (SST) and chlorophyll‐a (Chl‐a) are investigated. The SST time series shows that the SST begins to decrease 2 days before the typhoon's arrival and continues to decrease until 2 days following the typhoon's passage. The Chl‐a has a weak peak 2 days prior to the typhoon's arrival, rapidly increases after the typhoon arrives, reaches the strongest response on the third day of the typhoon, and gradually decreases to a value slightly higher than the pre‐typhoon level. Prominent responses are associated with typhoons that have stronger intensity and slower translation speed. The pre‐typhoon upper ocean structure plays a dominant role in determining oceanic responses. Surface cooling is generally stronger where the pre‐typhoon mixed layer depth (MLD) is shallow. However, the change in Chl‐a shows a contrasting response in that the response prominently increases only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD. This study poses a quantitative approach to assess the influence of typhoons on the upper ocean from a statistical perspective with consideration of the upper ocean structure. Key Points: Sea surface temperature and chlorophyll‐a changes are composited to assess their responses to the passage of typhoons in the northwest PacificLarge responses are introduced by strong and slow‐moving typhoons, and surface cooling increases with shallow mixed layer depth (MLD)Responses in chlorophyll‐a increase only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD [ABSTRACT FROM AUTHOR]