Abstract
This study reveals the features of the strong intraseasonal variability (ISV) of the upper-layer current in the northern South China Sea (NSCS) based on four long-time mooring observations and altimeter data. The ISV of the upper-layer current in the NSCS consists of two dominant periods of 10–65 days and 65–110 days. The ISV with period of 10–65 days is much strong in the Luzon Strait and decays rapidly westward along the slope. The ISV with the period of 65–110 days is relatively strong along the slope with two high cores at 115 and 119°E, whereas it is weak in the Luzon Strait. The 10–65-day ISV can propagate directly from the western Pacific into the NSCS for most of the time. However, due to its long wavelength, the 65–110-day ISV propagates into the NSCS indirectly, possibly similar to the wave diffraction phenomenon. The spatial differences between the two main frequency bands are primarily due to the baroclinic and barotropic instabilities. The spatial distribution of the upper-layer ISV is closely associated with the mesoscale eddy radius of the NSCS. The eddy radius is directly proportional to the strength of 65–110-day ISV, but it is inversely proportional to the strength of 10–65-day ISV.
Highlights
The South China Sea (SCS) is the largest marginal sea in the western Pacific Ocean
The energy of 10–65 days was stronger than that of 65–110 days, but this difference was relatively weak for eddy available potential energy (EPE), and EPE was stronger than eddy kinetic energy (EKE) for both periods. These results suggested that EPE was more closely related to the spatial differences between the two frequency bands
Based on four long-time mooring observations and satellite observations, a prominent intraseasonal variability (ISV) had been found in the northern SCS (NSCS), especially in the Luzon Strait and the northern slope with the ISV contributing more than 50% of the total variability
Summary
The South China Sea (SCS) is the largest marginal sea in the western Pacific Ocean. The Luzon Strait is the only deep passage for water exchange between the SCS and the western Pacific. Driven by the seasonally reversing monsoon, the upper circulation of the SCS exhibits cyclonic structure in winter, and it contains a cyclonic circulation in the northern part of the deep basin and an anticyclonic circulation in the southern part in summer (Wyrtki, 1961; Hu et al, 2000; Qu, 2000; Gan et al, 2006, 2016). The perpetual southwestward flow along the slope of the northern SCS (NSCS) enhances in winter and weakens in summer (Hu et al, 2000; Shu et al, 2018). The NSCS is one of the most active ISV areas, which is largely explained by the mesoscale eddies associated with the barotropic and baroclinic instabilities (Zhuang et al, 2010)
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