Mesoscale Eddies In The South China Sea:Identification And Statistical Characteristics Analysis

Eddies are common in the ocean, and range in diameter from centimeters to hundreds of kilometers. Mesoscale ocean eddies are characterized by currents which are weaker than the basic currents and flow in a roughly circular motion around the center of the eddy. The sense of rotation of these currents may either be cyclonic or anticyclonic. Mesoscale eddies carry enormous energies and play an important role in the ocean general circulations. They transport the energy and mass and impact the space distribution and structure of oceanographic elements such as temperature and salinity. As an important component of cross-scale ocean dynamics, mesoscale eddies directly direct affect the marine military activities and marine fishery, which brings interests to the oceanographers all over the globe. Using hydrologic observation data and satellite-observed data, previous studies indicate that mesoscale eddy activities are very active over the South China Sea (SCS), which is influenced by the wind mixing, bottom topography, Kuroshio intrusion and the instability of the basic currents.There are still large disagreements about the identification method of mesoscale eddies over the SCS. Some scholars identify the mesoscale eddies by change the eddy intensity and the minimum value of diameter artificially, which is difficult to conduct and will creates biases in eddy number and statistical characteristics because there is no consistent definition of mesoscale eddies. In this case, an algorithm which can identify and track the mesoscale eddies automatically is essential. While there are many achievements about the study of the mesoscale eddies over the SCS, the basic properties and dynamical mechanisms of the mesoscale eddies have not been understood totally due to the complex topography and dynamical conditions. Previous studies mostly concentrate on the motion and seasonality of the mesoscale eddies due to the limited length of date time. The interannual variations of mesoscale eddies over the SCS have not been investigated completely. So it is also an important issue to identify and examine the characteristics, seasonality, and interannual variations of the mesoscale eddied over the SCS with a practically automatic eddy detection method.Based upon the altimeter data for the years 1993-2012, the present study identifies the mesoscale eddies in the SCS with Okubo-Weiss method and examinesits time-space distribution andinterannual variation and investigates the underpinning dynamical mechanisms with wavelet analysis, EOF analysis, correlation analysis and some other methods. The main conclusions of this study are as follows:(1) Comparing with previous studies, the mesoscale eddy identification method used in this study is practical and reliable for both the high-resolution model data and satellite altimeter data.(2) Mesoscale eddies in the SCS mostly occur in waters below 2000m. Except for the northern SCS, the cold and warm eddies occur in the whole basin north of 10°N, especially for the regions southeast of Vietnam, west of Luzon, southwest of Taiwan and along the 2000m isobath over the northwestern basin. In addition, the generation and distribution of these eddies display pronounced seasonality. One of the interesting phenomenon is:The cyclonic (anticyclonic) eddies occupy the southern (northern) part of the regions southeast of Vietnam in winter and spring, while it’s completely opposite in summer.(3) The mesoscale eddy activities in the SCS display pronounced seasonality. Cyclonic (anticyclonic) eddies like occurring in summer (winter). Eddy kinetic energy is larger in summer and autumn, smaller in winter and spring. The eddy area is larger in autumn consistent with its larger eddy kinetic energy.(4) Our results show that there is prominent interannual variations of mesoscale eddies in the SCS. And it has a negative correlation with ENSO events suggested by the analysis of number of eddies, total eddy area and eddy kinetic energy:mesoscale eddy activities are weak (strong) during El Nino (La Nina) events. The wind anomalies can explain the above negative correlation. The wind stress curl decrease (increase) as climatological northeast wind fields weaken (strengthen) in El Nino (La Nina) years, leading to a weak (strong) eddy activities.