Analysis Of The Impact And Mechanism Of Tropical Cyclone On The Thermocline Features Of Qiongdong Upwelling

The coast upwelling east of Hainan Island(Qiongdong Upwelling) is one the strong upwelling centers in the northern South China Sea(SCS). It has direction effects on directly effects the local ecosystem, fishery and climate. The tropical cyclones(TC)originated in the SCS and the west Pacific usually pass over the Qiongdong upwelling zone.The investigation of the thermocline response to tropical cyclone passage over the Qiongdong Upwelling zone would be helpful for further understanding of ocean-air interaction and production of ocean fishery.This study analyzes the statistical features of TCs passages over the upwelling zone east of Hainan Island and the induced variation of sea surface temperature(SST) from1982 to 2011. The results indicate that the SST variation in the studied area can be divided into three categories: increasing SST, little-changing SST, and decreasing SST. These are quite different from the case of open ocean, in which only decreasing SST occurs. Of the total 36 tropical cyclones passing through the study area, numbers of three categories are17, 16 and 3, respectively. The averaged SST increase is 2.1oC, large than the averaged decrease of-1.5oC. Comparing to the duration of tropical cyclone passing over the upwelling zone, the intensity and incidence angle of the TC have stronger effects on the variation of SST.In August 2013, tropical cyclone ‘Jebi’ passes over the Qiongdong Upwelling zone.Different from ‘right-hand’ decrease in the deep SCS, the SST increased in the upwelling zone with the maximum increase of 1oC on the right-hand side of the cyclone track. The repeated cruise observations along a cross-shelf section in the upwelling zone pre- and post- ‘Jebi’ are used for further analysis of the thermocline structures. The in-situ observations indicate that the shoreward tilt of isothermals, isohalines and isopycnals and the shoaling of thermoclines disappear after the passage of ‘Jebi’. The depths of mixed layer and thermocline deepen about 15 m in the near-shore region. The near-shore strong stratification with N2 of 10-3 s-2 due to the upwelling weakens to less than 10-4 s-2. In the off-shore region, the temperature in the upper layer decreases, and the depths of mixed layer and theroclines have little change. The thermocline are flatly located in layer of 20 –30 m with undulation signals of about 30 – 40 km.We suggest that high sea level nonlinear soliton induced by tropical cyclone propagate transports net heat as propagating from the open ocean to the coastal area, so that causes the SST to increase in the upwelling zone. The function of observed SST variation vs. the incidence angle of tropical cyclone track compares well with the theoretical predictions. In summary, the SST variation depends on the sum of local heat loss due to vertical advection and mixing, local heat transport by Ekman flow and remote heat transport by sea level soltion induced by tropical cyclone processes.