Research On Characteristics And Dynamic Mechanism Of Subsurface Eddies In The Northwestern Pacific Ocean

The Ocean is filled with mesoscale eddies with spatial scales ranging from tens to hundreds of kilometers and time scales ranging from weeks to months.Mesoscales eddies are of great significance for energy balance,water mass distribution,transport of water and nutrients.Mesoscale eddies can be classified as surface-intensified eddies and subsurface-intensified eddies.Based on Satellite data of the sea height,the study of surface-intensified eddies has been very clear.The core of subsurface-intensified eddy is in the subsurface layer,and its structure and generation mechanism are different from that of the surface-intensified eddies.In this study,subsurface eddies of the Northwestern Pacific Ocean were identified and tracked from 2003 to 2013,and their movement,occurrence,lifetime and nonlinear characteristics were statistically analyzed.The dynamics mechanism is also discussed.The main work and conclusions of this paper are as follows:(1)Subsurface eddies are widespread in the Northwestern Pacific Ocean.They are concentrated in four regions,namely,the sea to the east of the Philippines,the Kuroshio extension,the sea to the east of Taiwan Island and Ryukyu Island,and the north equatorial current.We tracked the subsurface eddies from 2003 to 2013 at a depth of 400 m,and detected a total of 2569 subsurface anticyclonic eddies and 2099 subsurface cyclonic eddies.The number of subsurface anticyclonic eddies is dominant especially in the Western Boundary Current.The sea east of the Philippines is the most important subsurface eddies producing area in the Northwest Pacific Ocean and it is also the most important place where subsurface eddies extinct.Most of the subsurface eddies in this area are generated locally.The western topography has a modulation effect on the extinction of these subsurface eddies.Subsurface eddies are highly nonlinear and can transport the source water,thus affecting the material and energy exchange of the middle water.Subsurface eddies propagate westward at a speed of 3-11cm/s,and the velocity fluctuates and decreases with the increase of latitude.(2)Based on the identification results of subsurface eddies,the mean temperature and salinity anomaly structure are obtained by using the synthetic method.The results show that the core of subsurface eddies is somewhere between 200 and 600 m and the maximum velocity is around 400 m.The temperature in the upper layer of subsurface anticyclonic eddies is slightly lower than the surrounding water,and the water in the core is higher.This dual-core structure leads to the formation of the double-convex lens structure of subsurface eddies.Water in subsurface cyclonic eddies is generally 0.2℃lower than that around it.The lower potential vorticity water is nearly homogeneous inside the subsurface anticyclonic eddies.In subsurface cyclonic eddies the opposite is the case.(3)The heat transport caused by subsurface eddies is also estimated in this study.The spatial distribution of heat transport in the northwestern Pacific Ocean due to subsurface eddies is given for the first time.The results show that the zonal heat transport caused by subsurface anticyclonic eddies is opposite to that caused by subsurface cyclonic eddies.However,the subsurface anticyclonic is dominant in both quantity and volume,the net heat transport is west.Similar to surface eddies,subsurface eddies in the tropical northwestern Pacific transport heat to equator.(4)Both barotropic and barocline instability are important factors for the generation of subsurface eddies in east of the Philippines.Combined with the spatial and temporal variations of the vertical flow field and distribution of barocline instability in the latitude bands of 9-17°N,we found that the interface of NEC and NEUC is complex for NEUC is characterized with multicore structure.The vertical shera between NEC and NEUC is the main factor of baroclinic instability.The vertical shear is also exacerbated by NEC moving.The energy transfer caused by barocline instability is the most important reason for the generation of subsurface eddies in the subsurface layer of the NEC.