Low‐frequency western Pacific Ocean sea level and circulation changes due to the connectivity of the Philippine Archipelago

Abstract

Interannual‐to‐decadal sea level and circulation changes associated with the oceanic connectivity around the Philippine Archipelago are studied using satellite altimeter sea surface height (SSH) data and a reduced gravity ocean model. SSHs in the tropical North Pacific, the Sulu Sea and the eastern South China Sea (ESCS) display very similar low‐frequency oscillations that are highly correlated with El Niño and Southern Oscillation. Model experiments reveal that these variations are mainly forced by the low‐frequency winds over the North Pacific tropical gyre and affected little by the winds over the marginal seas and the North Pacific subtropical gyre. The wind‐driven baroclinic Rossby waves impinge on the eastern Philippine coast and excite coastal Kelvin waves, conveying the SSH signals through the Sibutu Passage‐Mindoro Strait pathway into the Sulu Sea and the ESCS. Closures of the Luzon Strait, Karimata Strait, and ITF passages have little impacts on the low‐frequency sea level changes in the Sulu Sea and the ESCS. The oceanic pathway west of the Philippine Archipelago modulates the western boundary current system in the tropical North Pacific. Opening of this pathway weakens the time‐varying amplitudes of the North Equatorial Current bifurcation latitude and Kuroshio transport. Changes of the amplitudes can be explained by the conceptual framework of island rule that allows for baroclinic adjustment. Although it fails to capture the interannual changes in the strongly nonlinear Mindanao Current, the time‐dependent island rule is nevertheless helpful in clarifying the role of the archipelago in regulating its multidecadal variations.