Spatial Variation of Upper Ocean Chlorophyll‐ <i>a</i> Response Across the Track of a Typhoon in the Northwest Pacific

Abstract Typhoons typically enhance phytoplankton biomass in the upper ocean, as evidenced by elevated sea surface chlorophyll ‐a concentrations (Chl‐ a ). However, subsurface Chl ‐a responses remain debated, leading to uncertainty about whether vertically integrated Chl ‐a over upper depth increases or decreases. Using a coupled physical‐biogeochemical model, this study reveals that the depth‐averaged Chl ‐a response to a typhoon exhibits distinct spatial patterns relative to the storm's track. Near the typhoon's path, intense vertical mixing penetrates beyond the nutricline, transporting deep nutrients into the upper ocean. The supplied nutrients fuel phytoplankton growth in both surface and subsurface layers, thereby increasing depth‐averaged Chl‐ a over upper 150 m. However, the increase in subsurface Chl ‐a driven by biogeochemical processes is smaller than the decrease caused by physical processes, resulting in a net reduction of Chl ‐a in the subsurface layer. This ultimately leads to a significant increase in the surface layer, a minor decrease in the subsurface layer, and an overall rise in total Chl‐ a concentration. In contrast, areas farther from the track experience mixing that exceeds the mixed layer depth but remains shallower than the nutricline. Here, mixing redistributes subsurface Chl ‐a to the surface without uplifting new nutrients. Consequently, surface Chl ‐a rises, while subsurface levels decline, leaving the depth‐averaged Chl ‐a largely unchanged. These spatially divergent responses explain why observed vertical Chl ‐a profiles vary across different cross‐track locations. Clarifying the spatial variability of upper ocean Chl ‐a dynamics under typhoon forcing is critical for accurately assessing typhoons' ecological impacts, including their role in marine carbon cycling and productivity.