Impacts of the unique landfall Typhoons Damrey on chlorophyll-a in the Yellow Sea off Jiangsu Province, China

Abstract

Typhoon Damrey was the rare typhoon to make a straight landfall in northern Jiangsu Province (35°N), China. The unique influence of typhoon Damrey and the following typhoons Haikui on the phytoplankton bloom and related dynamic mechanisms were fully investigated. Because of the complex regional features, e.g., nutrient distribution, stratification and sediment concentration, it induced different responses to the ocean along its track. In the near shore region, the Chlorophyll-a (Chl_a) concentration dropped prominently during typhoon, associating with strong wind events, because of the onshore advection of water with a low amount of Chl_a and coastal downwelling. For the shelf regions, no significant blooms were identified and the Chl_a values after typhoon were less than the preceding maximum values. Although a large amount of mixing happened in the water column, insufficient sunshine for photosynthesis caused by a high suspended sediment concentration (SSC) (10.8 g m−3) near the shore and a lack of nutrients in the bottom boundary of the open sea resulted in no Chl_a bloom in these two areas. For the open ocean area off the Yangtze River estuary, where subsurface nitrate concentration was high, the formation of a Chl_a bloom after strong winds was mainly determined by the intensity of the stratification in the upper ocean. The enhancement of nutrients from the subsurface layer became stronger (weaker) when the pycnocline was weakened (strengthened). As the pycnocline and stratification weakened, the Chl_a concentration was significantly higher than that on August 7 because of the intensified upward transport of nutrients from the deep layer. This study offers a comparison of the different mechanisms by which typhoons can impact Chl_a in mid-latitude regions and will be helpful for evaluating typhoon-induced biological responses in future.