Responses Of Cold-core Eddy To The Passage Of Typhoon

As one of the most violent processes of sea-air interaction,typhoon not only causes significant responses of physical dynamics and ecological environment,but also has an important impact on fishery and coastal ocean economy.The northwest Pacific Ocean is a sea area with frequent typhoons,rich in ocean mesoscale eddies,complex dynamic,biogeochemical processes and rich in marine resources,which play an important role in many processes such as air-sea flux exchanges,ocean energy cascades,and material transport and so on.To investigate the response of mesoscale eddies to typhoon is one of the hot issues in oceanography,which is helpful to further understand the interaction between the upper ocean and typhoon.A lot of important progress has been achieved on interaction between warm eddy and typhoon,but there is still a lack of understanding on the influence of typhoons on cold eddies and the associated non-geostrophic effects.In this study,the influence of typhoon Talim on the mesoscale cold eddy is analyzed using the multi-source satellite remote sensing data and re-analysis dataset.We focus on the dynamic and ecological responses of cold eddy to typhoons,and compared the spatial and temporal characteristics of Sea Level Anomaly（SLA）,Sea Surface Temperature（SST）,chlorophyll-a concentration,and the dynamic parameters,such as vorticity,strain and IROS parameter before and after typhoon Talim passage,providing deep understanding of the interaction between the upper ocean and typhoon.The results are as follows:（1）The cold eddy was significantly intensified during the passage of typhoon Talim.The maximum SLA occurred in the central of the cold eddy,which reached-32.7cm,and the area of the cold eddy was expanded twice.（2）The mixing and upwelling and sub-mesoscale processes induced by wind stress,entrained dissolved nutrients upward,raising surface nitrate concentrations.Combined with the relatively unstable thermo-dynamic structure near the cold eddy,the maximum drop in surface temperature reached more than 3°C.（3）The passage of typhoon Talim resulted in a more complete surface structure of the cold eddy,and the local horizontal shear and horizontal buoyancy gradients of the cold eddy increased significantly.The Rossby number,strain,and IROS parameter were increased by more than 2 times,maximum water strain and IROS parameter in cold eddy close to the typhoon path on the south side.（4）By means of dynamic diagnosis calculation,it can be found that the local non-geostrophic effect and turbulent mixing effect were significantly enhanced during the typhoon passage,which was conducive to the exchange of energy and momentum between the upper boundary layer and the ocean interior,and had an important impact on the supply of nutrient salts in the upper ocean.And the average chlorophyll concentration increased by 10 times,but only lasted for about a week.In addition,30 tropical cyclones’impacts on the northwest Pacific Ocean in 2017are further taken into account with the purpose of finding out the key factors for the dramatic changes of cold eddy caused by typhoon Talim.The statistical results showed that only 7 typhoons resulted in the enhancement of cold eddies,among which Talim was the only one that had caused the strong response of the ocean upper layer.In fact,the short bloom of phytoplankton on the surface during typhoon mainly depend on the transport of deeper nutrient-rich waters up to the euphotic zone.The upward transport of nutrient is mainly controlled by Ekman pumping and near-inertial mixing.Only when the wind stress shear frequency is close to the local inertial frequency,the near-inertial oscillation and turbulent mixing can be induced.During the passage of the cold eddy,typhoon Talim forced the sea surface for a relatively long time（48h）,injected more energy into the ocean interior（120k J）,formed a strong upwelling zone,and its pumping speed reached the maximum13.03?10^-5 m/s,which may be an important reason for the strong response of the cold eddy to the typhoon.This study is of great significance for the in-depth understanding of response process of cold eddy to typhoon as well as the associated ecological environment effect.