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[ACG41-P04] Typhoon response of phytoplankton observed in a marine ecosystem model
Strong winds associated with typhoons reduce sea surface temperatures through the exchange of thermal energy between the ocean and the atmosphere and through the effects of mixing and upwelling in the oceans. This typhoon-induced mixing and upwelling is considered to bring cold, nutrient-rich water from the deep ocean to surface layer, enhancing phytoplankton growth at the sea surface and stimulating ocean primary production.
Recently, the BGC-Argo profiling float observed the typhoon response in the ocean interior south of Japan. The results indicated that the surface phytoplankton bloom after the passage of Typhoon Trami was attributed to redistribution of subsurface chlorophyll throughout the mixed layer due to intense mixing, generating no net increase in phytoplankton biomass. However, BGC-Argo can only obtain data on the float position, so differences in response to typhoon Trami by sea area remain unclear. In this study, to investigate the typhoon response of the upper-ocean on the typhoon path from its genesis point to south of Japan, we used output from the physical-biological model of the North Pacific. The model consists of an eddy-resolving physical ocean model (OFES) coupled with a simple nitrogen-based NPZD model, and was forced by the Japanese 55-year Reanalysis (JRA55-do).
Typhoon Trami, which is targeted in this study, spawned in the western tropical Pacific (15°N, 143.7°E) on September 21, 2018, and moved northwestward while gradually slowing down its translation speed, reaching around 22°N, 128°E on September 27, and then moved northward at its slowest speed during its lifetime. It then moved northeastward at 25°N, 127°E on September 29, and reached off the coast of Kagoshima (30°N, 130°E), accelerating its speed. After crossing Japan, it turned into an extratropical cyclone on October 1.
To evaluate typhoon response at each latitude along the Typhoon Trami pathway, time vertical cross sections of temperature, phytoplankton concentration, and nutrient concentration were made. At 15-18°N, where the typhoon was weak, no changes in the vertical profiles of temperature were observed before and after the typhoon passed. At 19-24°N, after the typhoon passed, temperature decreased at all depths in the upper 200 m with an upward displacement of the isotherm. Nutrient isotherms also shifted upward at the same locations. These upward displacements of temperature and nutrient isotherms were attributed to typhoon-induced upwelling. In the subsurface chlorophyll maxima, phytoplankton decreased immediately after the typhoon passed and then recovered. In particular, the subsurface chlorophyll maximum was enhanced after the typhoon passed at 19-26°N than before the typhoon.
Next, to quantitatively compare the differences in the pre- and post-typhoon changes by location, we estimated the pre- and post-typhoon changes in integrated temperature, integrated phytoplankton, and integrated nutrients in the upper 200 m layer for each latitude (Figure 1). Temperature changes before and after the typhoon show a decrease in almost all latitudes. The largest decrease in temperature was observed at 21°N, and the decrease was gradually smaller north and south of that latitude. On the other hand, nutrient and phytoplankton increased throughout the entire 19 to 29 °N, with the greatest increase at 22-23 °N. The difference in temperature changes with latitude is similar to the difference in the typhoon translation speed, while the difference in nutrient and phytoplankton are opposite in phase to the typhoon translation speed. From this result, it can be inferred that slowdown of typhoon translation speed appears to be favorable for inducing an oceanic response.
In contrast with the previous study used BGC-Argo float, a net increase in phytoplankton was observed after the typhoon passed at locations where the typhoon was moving slowly, the temperature dropped significantly, and nutrients were transported to the upper layers.
Recently, the BGC-Argo profiling float observed the typhoon response in the ocean interior south of Japan. The results indicated that the surface phytoplankton bloom after the passage of Typhoon Trami was attributed to redistribution of subsurface chlorophyll throughout the mixed layer due to intense mixing, generating no net increase in phytoplankton biomass. However, BGC-Argo can only obtain data on the float position, so differences in response to typhoon Trami by sea area remain unclear. In this study, to investigate the typhoon response of the upper-ocean on the typhoon path from its genesis point to south of Japan, we used output from the physical-biological model of the North Pacific. The model consists of an eddy-resolving physical ocean model (OFES) coupled with a simple nitrogen-based NPZD model, and was forced by the Japanese 55-year Reanalysis (JRA55-do).
Typhoon Trami, which is targeted in this study, spawned in the western tropical Pacific (15°N, 143.7°E) on September 21, 2018, and moved northwestward while gradually slowing down its translation speed, reaching around 22°N, 128°E on September 27, and then moved northward at its slowest speed during its lifetime. It then moved northeastward at 25°N, 127°E on September 29, and reached off the coast of Kagoshima (30°N, 130°E), accelerating its speed. After crossing Japan, it turned into an extratropical cyclone on October 1.
To evaluate typhoon response at each latitude along the Typhoon Trami pathway, time vertical cross sections of temperature, phytoplankton concentration, and nutrient concentration were made. At 15-18°N, where the typhoon was weak, no changes in the vertical profiles of temperature were observed before and after the typhoon passed. At 19-24°N, after the typhoon passed, temperature decreased at all depths in the upper 200 m with an upward displacement of the isotherm. Nutrient isotherms also shifted upward at the same locations. These upward displacements of temperature and nutrient isotherms were attributed to typhoon-induced upwelling. In the subsurface chlorophyll maxima, phytoplankton decreased immediately after the typhoon passed and then recovered. In particular, the subsurface chlorophyll maximum was enhanced after the typhoon passed at 19-26°N than before the typhoon.
Next, to quantitatively compare the differences in the pre- and post-typhoon changes by location, we estimated the pre- and post-typhoon changes in integrated temperature, integrated phytoplankton, and integrated nutrients in the upper 200 m layer for each latitude (Figure 1). Temperature changes before and after the typhoon show a decrease in almost all latitudes. The largest decrease in temperature was observed at 21°N, and the decrease was gradually smaller north and south of that latitude. On the other hand, nutrient and phytoplankton increased throughout the entire 19 to 29 °N, with the greatest increase at 22-23 °N. The difference in temperature changes with latitude is similar to the difference in the typhoon translation speed, while the difference in nutrient and phytoplankton are opposite in phase to the typhoon translation speed. From this result, it can be inferred that slowdown of typhoon translation speed appears to be favorable for inducing an oceanic response.
In contrast with the previous study used BGC-Argo float, a net increase in phytoplankton was observed after the typhoon passed at locations where the typhoon was moving slowly, the temperature dropped significantly, and nutrients were transported to the upper layers.