Interannual Variations of Subsurface Spiciness in the Philippine Sea: Observations and Mechanism

Abstract

Abstract The Philippine Sea (PS) is a key region connecting North Pacific subtropics to the equator via western boundary currents. Using available measurements from Argo profiling floats, satellite altimeters, and research surveys, the authors investigate the characteristics and mechanism of subsurface spiciness variability (represented by salinity changes between 23.5 and 24.5 σθ) in the PS. During the past decade, low-frequency salinity variability was dominated by interannual signals characterized by out-of-phase changes between the southern and northern PS with peak-to-peak amplitudes exceeding 0.1 psu. These salinity anomalies are mainly generated locally by anomalous cross-front geostrophic advections. In 2003, an anomalous cyclonic circulation developed in the PS, which transported greater (less) than normal high-salinity North Pacific Tropical Water to the northern (southern) PS and produced positive (negative) salinity anomalies there. In 2009, an anomalous anticyclone emerged, which produced negative (positive) salinity anomalies in the northern (southern) PS. These year-to-year variations are closely associated with ENSO cycle. During strong El Niño (La Niña) episodes, positive (negative) wind stress curl anomalies between 8° and 18°N evoke westward-propagating upwelling (downwelling) Rossby waves in the central Pacific and positive (negative) anomalous Ekman pumping in the western Pacific, resulting in the observed current and salinity changes in the PS. Further analysis suggests that these locally generated spiciness anomalies disperse quickly while propagating to the equatorial Pacific in the Mindanao Current (MC). In the meantime, anomalies advected from higher latitudes are nearly diminished upon reaching the PS. The western boundary of the North Pacific seems quite efficient in damping extratropical signals.