| As a strong local disturbance source in the ocean,tropical cyclones(TCs)produce cyclonic wind stress and strong mixing process,which have a great impact on the heat,energy and material exchange between the upper ocean and the deep ocean.To study the response of the upper ocean to TCs has an important scientific significance and practical application value for further understanding the coupling mechanism of biogeochemical and physical processes between the upper ocean and the lower atmosphere.At present,the research in this aspect is mainly focused on satellite remote sensing and numerical simulation,while there are few researches based on field observation,which is because of some limitations in the acquisition of field observation data.Therefore,it is necessary to carry out relevant research in representative sea areas.Based on a large number of mooring data and cross section observation data,combined with satellite remote sensing data,this paper carried out analysis of several TCs,focusing on the physical response mechanism of the upper ocean in the northen South China Sea(SCS),especially in the shallow water,and the effects of material transport and ecological chemistry.This paper is aimed to understand the response and mechanism of upper ocean to TCs in physical,chemical and ecological environment.The main research results are as follows:(1)Physical response:In July 2005,tropical storm Washi passed over the northwestern SCS.The sea surface temperature(SST)cooling caused by Washi mainly concentrated on both sides of its track,the maximum SST cooling was-3.2?that located around 30 km to the right of its track.After the passage of Washi,the mixed layer(ML)deepened 10?20 m,and strong near-inertial currents are generated by the storm with the max currents up to 0.2m·s-1in the upper ML and with a period of 35 h.The near-inertial kinetic energy(NKE)was transmitted downward,and only about 11.5%of the energy of the upper 30 m depth entered the water below 50 m.There was a strong upwelling in the low temperature zone during the period of the tropical storm,and the Ekman pumping velocity was up to 0.8×10-4m·s-1.Based on the field observation data,the turbulent eddy diffusivity was found to have increased by more than 3 times.These results show that the strong upwelling and vertical mixing should be the main reasons for the cooling process of TC.In September 2011,typhoon Nesat and typhoon Nalgae caused strong near-inertial oscillation in the northern SCS.The SST cooling induced by them showed a right-hand bias and recovered rather slowly.The maximum cooling occurred within 150 km on the right side of typhoon track,and the SST drop was 4.0?5.1?.The maximum near-inertial velocity caused by typhoon Nesat and Nalgae was 0.3 m·s-1and 0.26 m·s-1,respectively.And the influence depth of NKE was 300m and 200m,respectively.This is because the of the NKE caused by Nesat was not easily dissipated and can travel to the deep sea at a faster speed.In June 2017,the severe tropical storm Merbok passed over the northwestern SCS.The SST cooling caused by Merbok mainly concentrated along the track and showed the right-hand bias.The SST cooling area reached-2.0?was mainly concentrated within200km to the right side of the track and 100km on the left side of the track.The maximum SST drop was located within 100km to the right of the track,and the maximum temperature decrease was-3.0?.In general,the maximum SST cooling induced by typhoon Merbok is to the right of the typhoon's track.At the same time,there was a strong upwelling zone during the transit period of Merbok.(2)Biogeochemical response:There were two phytoplankton blooms in the process of typhoon Nesat and Nalgae.The first one occurred on the 7th day after the passage of Nalgae,and the concentration of chlorophyll-a(Chl-a)in the surface layer increased by2.26 times.The second phytoplankton bloom occurred 30 days after the passage of Nalgae,and the concentration of Chl-a in the surface ocean increased by 2.9 times.The near inertial current caused by typhoon increased the turbulent eddy diffusivity by 1 order of magnitude.The strong mixing caused phytoplankton blooms and was one of the major reasons for the long-term low temperature in the northwestern SCS in mid-October.Based on the cross section data before and after the passage of typhoon Merbok in June2017,by analyzing the changes of various hydrological elements and nutrients,it is determined that the strong mixing during the typhoon was the main process of material transport in the offshore area.The characteristics of high nutrient concentration,high temperature and low salt were found in the coastal waters before the passage of typhoon Merbok.During the passage of the typhoon,the significant decrease of SST and the increase of Chl-a indicated a strong upwelling process.After the passage of typhoon,the stratification of nutrient was weakened,and they were more well distributed in the whole water in the offshore area.Chl-a increased rapidly from 0.63?g·L-1to 0.91?g·L-1,and the peak value was in the mid-water,and gradually decreased towards the offshore area.The SST decreased about 2.5°C during the typhoon,and the region with the largest SST decrease was also the region with the largest increase in Chl-a concentration.Compared with the increase of fresh water flow,vertical mixing is the main process to control nutrient transport. |
PDF Full Text Request