Long-Term and Interannual Variation of the Steric Sea Level in the South China Sea and the Connection with ENSO

Multisource observation and model data sets, including satellite altimetry, satellite gravimetry (the Gravity Recovery and Climate Experiment mission [GRACE]), ocean model (Estimation of the Circulation and Climate of the Ocean [ECCO]), and oceanographic reanalysis data (Ishii), are used to explore the long-term and interannual sea-level variation (SLV) in the South China Sea (SCS) and the connection with El Nino-Southern Oscillation (ENSO). From 1993 to 2012, the sea level rose at a rate of 4.7 +/- 0.3 mm/y, and the steric component contributed approximately 40-55% of the increase shown in the Ishii data (1.9 +/- 0.3 mm/y) and the ECCO model (2.6 +/- 0.3 mm/yr). Using the GRACE observations from 2003 to 2012 for independent validation, the ECCO-derived steric trend was consistent with the mass-corrected altimetry result, whereas the Ishii data failed to capture the sea-level rise in the central basins. On the interannual scale, both the empirical orthogonal function (EOF) and the wavelet coherence (WTC) analysis indicate that the total SLV and the steric SLV have positive correlations with ENSO. The correlation is stronger between the Southern Oscillation index (SOI) and the ECCO-derived steric SLV than it is between the SOI and the Ishii-based steric SLV. In the time-frequency domain, the WTC shows a clear in-phase coherence in the 2-year cycle between the SOI and the ECCO-derived steric SLV over the entire time span; no significant coherence appears between the SOI and the Ishii-based steric SLV after 2003. The abnormal northerly winds and the increasing intrusion of low-temperature Kuroshio water into the SCS through the Luzon Strait during El Nino years may explain the connection between ENSO and the interannual steric SLV.