Decadal Variation of the Geostrophic Vorticity West of the Luzon Strait

Abstract

Merged satellite altimeter data (from 1993 to 2008) are used to study the decadal variation of geostrophic vorticity west of the Luzon Strait. The decadal variation of the geostrophic vorticity west of the Luzon Strait indicates that the Kuroshio intrusion and separated anticyclonic eddies from the Kuroshio are stronger in 1995-2000 than that in 2001- 2004. Between 1995 and 2000, along with the intensification of the anticyclone separated from the Kuroshio intrusion west of the Luzon Strait, the cyclone south of the Kuroshio intrusion is strengthened. This result proved the close relationship between eddy shedding and the growth of the cyclone south of the Kuroshio front. In 1995-2000, when the anticyclonic vorticity west of the Luzon Strait was increased, at the same time, the decrease thermocline thickness and the Kuroshio transport east of Luzon Island are found by analyzing the Simple Ocean Data Assimilation datasets. This suggests that the decadal change in the Kuroshio intrusion and eddy shedding in the Luzon strait is influenced by the decadal change in thermocline thickness and Kuroshio transport east of the Luzon Island. The South China Sea (SCS) is a semi-enclosed, marginal basin of the western Pacific. The Luzon Strait, located from 18.5°N to 22°N with widths around 400km and depths over 2000 m, is the only deep channel connecting the SCS with the Pacific. The Kuroshio, as one of the western boundary currents of the North Pacific, deforms while crossing the Luzon Strait (1). Anticyclonic eddies are found to be separated intermittently from the Kuroshio deformation in the Luzon Strait (2-4). The eddy shedding phenomenon is reported as an event of non-deterministic nature and with large variety in frequency and behavior (4-7), which indicates variation of time scale longer than interannual may exist in these phenomena. Furthermore, these studies are mainly limited in the description of the Sea Surface Height (SSH) and the geostrophic velocity, in which the strength or the Geostrophic Vorticity (GV) of the anticyclonic eddies left unknown. The GV anomaly is the anomaly vorticity of the geostrophic circulation calculated from the SSH anomaly data. It is very important in the study of potential vorticity interaction between the SCS and the Pacific. Moreover, it is suggested that the separation of the anticyclonic eddy from the Kuroshio intrusion is related with the cyclone generated by frontal instability in the south the Kuroshio intrusion (8), while the existence of the cyclone has not been proved by observation. By studying the GV anomaly west of the Luzon Strait, the relationship between the anticyclone from the Kuroshio intrusion and the cyclone from the south of the Kuroshio front will be discussed.