The Delayed Ocean Response To Typhoon On The Northwestern Continental Shelf Of The South China Sea

Typhoon is a process with severe air-sea interaction.The typhoon passage induces significant variation in the hydrodynamic processes in the continental shelf seas,which directly affects the oceanic primary productivity,fishery and the coastal ocean economy.The continental shelf of the northwestern South China Sea(NWSCS)is an important ocean area for marine aquaculture and military base,where typhoons pass frequently.Understanding the characteristics and mechanisms of the ocean response to typhoon in the NWSCS is of great values in both scientific and practical areas.This paper analyzes the delayed ocean response to the super typhoon Mujigea(2015)in October 2015 on the continental shelf of the NWSCS using mutli-satellite remote sensing data and reanalysis data.The time series analysis indicates that there wasa unusual extended coastal ocean cooling event in the NWSCS after Mujigea(2015)passage.The cooling event had two different stages.The first stage occurredon October 3,and reached amaximum SST drop of –2? on October 7.After3-dwarming adjustment,a second cooling event occurred from October 10 to 15 with a maximum SST drop of –4?.The characteristics and mechanisms of the unusual second cooling event is different from that of the first cooling stage.Typhoon Mujigea(2015)passed through the NWSCS on October 3,2015,and landed on Zhanjiang on October 4.Thefirst cooling occurred immediately after the typhoon passage,lasted 4 days,and had the maximum SST dropon the right side of the typhoon track,similar to the generally observed cold wake after typhoon passage.During the first cooling stage,three cooling centers with maximum SST drop of –2?appearedsequentlyin regions near the Pearl River estuary,the 200 m isobaths in the edge of continental shelf off the Pearl River and the coastal areas from Yangjiang to Donghai Island.The SST variation in six regions distributed in offshore shore,the shelf and the deep water on two sides of the typhoon track are compared.The results show that themaximum SST drop in the offshore area was larger than that in the shelf and deep water,while the rate of warming recovery in deep waters was larger than that in the offshore area.The maximum cooling occurred on the same day on both sides of the typhoon track in the offshore area,while the maximum SST drop on the left side of the typhoon track was earlier than that on the right side in the shelf and deep waters.The unusual second cooling event suddenly occurred on October 10 after three-day warming recovery,when it had been 7 days after typhoon passage.This unique extended cooling event lasted for 5 days,and reached a maximum SST drop on October 15,when it has been 12 days after typhoon passage.The maximum SST drop,the decline rate and the cooling area of the second cooling were all larger than that of the first cooling.The unusual second cooling event is different from the first cooling and the typhoon cold wake reported in previous studies.Factors affecting the sea surface heat flux including the wind,precipitation,air temperature,surface current,and the sea surface height anomaly(SSHA)are analyzed.The results indicate that the air temperature dropping associated with cold air intrusion after typhoon led tothe second cooling event.The cold air spread southwestward to the coastal ocean of the NWSCS after typhoon landing,and the air temperature over the study ocean area sharply dropped before the second SST cooling event.The maximum air temperature drop reached –9? on October 11,2015,and the maximum SST drop occurred 4 days later.The lagged correlation coefficient of the air temperature drop and the SST drop reached0.8.The net air-sea heat flux at the second cooling stage was coherent with the air temperature dropping.The total heat loss was–8×106 J/m2 and could induce of a decline of –3.9? in the 0.5-m surface water,consistent with the maximum SST drop at the second stage.We further analyzed the mechanism of the second cooling and air temperature dropping.It was shown thatafter landing and moving northwestward to the Yunnan-Guizhou Plateau(YGP),typhoon Mujigea(2015)met the westerly wind front on October 5.The low-pressure andpositive-vorticity disturbances to the front triggered meridional air flow and low-pressure trough,thus induced a katabatic cold jet downward from the Qinghai-Tibet Plateau(QTP)passing through the YGP to the NWSCS,where the air temperature and SST sharply decreased.It is this katabatic cold jet intrusion induced by the typhoon led the unusual SST cooling event.Statistics of historical typhoons indicate that extended second cooling response occurred after typhoon in autumn and winter.The maximum SST drop exceeded –5?.The second SST cooling event and the katabatic cold jet induced by typhoon provides new insights to the ocean response to typhoon,and helps for weather prediction after typhoon landing.