Observed Ocean Variability in the Mindanao Dome Region

Ocean variations at semiannual, annual, and interannual time scales in the Mindanao Dome (MD) region of the southern Philippine Sea were examined using data derived from underwater sensors on Triangle Trans-Ocean Buoy Network (TRITON) buoys at 8°N, 137°E; 5°N, 137°E; and 8°N, 130°E. Annual signal dominated above 300-m depth in the MD region. At 5°N, 137°E, saline water exceeding 35 psu was observed at 100–200-m depth from boreal winter to spring, seemingly associated with the meridional migration of the North Equatorial Countercurrent during these seasons. Thermocline ascent, probably related to the MD, was also observed from boreal winter to spring. An important mechanism of the annual variation of the MD at 5°N seems to be the annual variability of local wind, as mentioned in past studies. However, annual variability at 8°N seems to be due to Rossby waves originating west of 150°W rather than to local wind effects. Semiannual variation was also observed, with its amplitude reaching 40%–70% of the annual signal. With regard to interannual variability, ocean variation on the time scale of the El Niño–Southern Oscillation (ENSO) was seen; upper heat content (above 300-m depth) in the Mindanao Dome region decreased during the 2002–03 and 2006–07 El Niño periods and increased between those periods. Increasing upper heat content in this region after 2005 was probably associated with large negative anomalies of Ekman pumping (downwelling) that appeared from 2005 to 2006 east of 150°E and north of 5°N.