Statistical Characteristics And Composite Three-dimensional Structures Of Mesoscale Eddies Near The Luzon Strait

The Luzon Strait (LS) is the main channel of the northwest Pacific (NWP) and the South China Sea (SCS), where mesoscale eddies are active in the sea areas near it. Although there have been accumulated a lot of eddy statistics results, the statistical analysis in terms of eddies in the adjacent waters of the LS is still not well documented. Meanwhile, there is a dispute about whether the NWP eddies can intrude into the SCS through the LS or not. Additionally, there are few investigations on eddy three-dimensional structures in these regions, and the volume, heat and salt transports related to eddies should be estimated.Based on the daily sea level anomaly (SLA) data from1993to2010, a modified eddy detection and tracking method was used to systematically analyze the statistical characteristics of eddies in the adjacent sea regions east of the LS (123°-130°E,16°-25°N) and west of the LS (112°-120°E,16°-25°N). A total of362cold eddy trajectories and329warm eddy trajectories, corresponding to6742cold eddies and6279warm eddies, were identified. The eddy occurrence frequencies in the areas southeast of the Taiwan Island, southwest of the Taiwan Island, west of the Luzon Island and southwest of the Dongsha Islands are high, while the one in the Kuroshio main-path area is close to zero. East of the LS, the eddy polarities northeast of the Taiwan Island, in the area of19°-21°N and in the area near the east coast of the Luzon Island are generally negative; West of the LS, except for the regions southwest of the Taiwan Island and in the LS, the eddy polarities are positive. Taking lifespan, propagation distance, radius, amplitude, eddy kinetic energy and vorticity into account, the eddies east of the LS are stronger than those west of the LS; cold eddies are stronger east of the LS, while warm eddies are stronger west of the LS. Under the influence of beta effect and background flows (or terrain), eddy propagation directions are changeable in these regions. East of the LS, most eddies move westward with velocities of5-20cm·s-1, which decrease with the increase of latitude; West of the LS, eddy propagation along the continental slope is southwestward with velocities of5-15cm·s-1, and west of Luzon Island is westward with velocities of 2-10cm·s-1. The result from analyzing40long-lifespan eddies shows that eddies grow quickly in the first60days and then decay gradually. The NWP strong eddies could not directly intrude into the SCS through the LS, but there are8eddies generated from the abruption or reduction of them and finally entered into the SCS. These eddies did not survive too long with an average lifespan of only47days; they all disappeared in the sea area east of119°E with a mean propagation distance of347km; their average propagation speed is about6.9cm·s-1, but the largest instantaneous velocity can be up to35.8cm·s-1; their sizes are small with a mean radius of71km and an average amplitude of6.8cm; their mean eddy kinetic energy is about284cm2·s-2, which is greater than those on both sides of the LS; their average vorticity is about7×10s-1, which is smaller than those on both sides of the LS.On this basis, the composite three-dimensional structures of eddies on both sides of the LS were also constructed by using the2002-2012weekly SLA records, Argo float profiles and CSIRO Atlas of Regional Seas (CARS)2009climatology with objective analysis. Furthermore, the volume, heat, and salt transports related to eddies were estimated. East of the LS, the largest potential temperature anomaly θ’ of the composite eddies can reach about±1.5℃. The composite cold eddy exists a double-core θ’structure with one core being located at150dbar and the other at400dbar. The θ’of the deeper core presents more significance which may be related to the accumulation of the upwelling cold water interrupted by the North Pacific Subtropical Mode Water. The composite warm eddy seems to have a single θ’core at200dbar. Its zonal distribution shows an obvious east-high-west-low pattern, which is possibly because the low latitude high temperature sea water carried by the Kuroshio rises the close-by climatological temperature. West of the LS, both types of the composite eddies have a single θ’core which is approximately at100dbar. The θ’of the cold eddy is larger than-1℃, while that of the warm one can exceed2℃. The salinity anomaly structures of the composite eddies east of the LS present an obvious sandwich-like pattern, but this pattern is barely detectable west of the LS due to a distinct Argo-CARS2009salinity bias. The polarity of the potential density anomaly is opposite to the θ’, but their spatial structures are similar indicating that the influence of temperature on density is more remarkable than salinity. The surface geostrophic current anomaly of the composite cold (warm) eddy east of the LS is about0.25(0.2) cm·s-1; and it is close to0.1(0.15) cm·s-1west of the LS. Being consistent with the statistics results, the composite eddies east of the LS are stronger than those west of the LS; the composite cold eddy is stronger east of the LS, while the composite warm eddy is stronger west of the LS. In the order of cold eddy east of the LS, warn eddy east of the LS, cold eddy west of the LS, and warm eddy west of the LS, trapping depths of the composite eddies are480,200,40, and150dbar. Based on the definition of trapping area, volume transports induced by the composite eddies are0.47,0.2,0.01, and0.17Sv; heat transports are-9.5,4.5,-0.05, and4.9×1011W; and salt transports are-11.2,-5.4,-0.4, and-15.3×103kg·s-1,respectively.