Interpretation of Subsurface Salinity EOFs on Southern ECS Shelf Using SODA Reanalysis: Large‐Scale Patterns and Seasonal to Interannual Variations

In the southern East China Sea (ECS), low‐salinity shelf water is distinguished from high‐salinity Kuroshio water. Neutral density solution of subsurface salinity in southern ECS with Empirical Orthogonal Function (EOF) analysis of Simple Ocean Data Assimilation (SODA) reanalysis data (1958‐2008) is presented. This study focuses on seasonal to interannual variations of large‐scale patterns. We choose isopycnal 24.5γ n${\gamma }^{n}$ as the representative of subsurface layers, which is the deepest of the Kuroshio subsurface that can outcrop to sea surface during winter and early spring. We conducted EOF both including and excluding the seasonal signal. The results show that annual cycles dominate the first two leading EOF modes, EOF‐1 representing inner‐shelf oscillation, and EOF‐2 outer‐shelf oscillation. However, EOF‐2 also shows significant interannual variability. To this end, we removed the seasonal signal from the dataset and recalculated EOF. We found that the interannual EOF‐1 is of cross‐shelf in spatial pattern and shows significant interannual variability. This suggests that seaward penetration of the shelf water and shelfward intrusion of the Kuroshio that mainly occur in winter and early spring are a pair of contradictions in the two‐way intrusions, which are modulated by significant interannual variability. We also found that the isopycnal extending abnormally northward and outcropping in early spring corresponds to significant non‐intrusions of the Kuroshio. In the underlying dynamics, oceanic stratification, besides topography uplifting, plays an important role in the subsurface intrusion of Kuroshio off northeast Taiwan.