No abstract

Seasonality of the Mindanao Current (MC)/Undercurrent (MUC) system is investigated using moored acoustic Doppler current profiler (ADCP) measurements off Mindanao (8°N, 127.05°E) and ocean model simulations. The mooring observation during December 2010 to August 2014 revealed that the surface‐layer MC between 50–150 m is dominated by annual‐period variation and tends to be stronger in spring (boreal) and weaker in fall. Prominent semiannual variations were detected below 150 m. The lower MC between 150 and 400 m is stronger in spring and fall and weaker in summer and winter, while the northward MUC below 400 m emerges in summer and winter and disappears in spring and fall. In‐phase and out‐of‐phase current anomalies above and below 150 m were observed alternatively. These variations are faithfully reproduced by an eddy‐resolving ocean model simulation (OFES). Further analysis demonstrates that seasonal variation of the MC is a component of large‐scale upper‐ocean circulation gyre, while current variations in the MUC layer are confined near the western boundary and featured by shorter‐scale (200–400 km) structures. Most of the MC variations and approximately half of the MUC variations can be explained by the first and second baroclinic modes and caused by local wind forcing of the western Pacific. Semiannual surface wind variability and superimposition of the two baroclinic modes jointly give rise to the enhanced subsurface semiannual variations. The pronounced mesoscale eddy variability in the MUC layer may also contribute to the seasonality of the MUC through eddy‐current interaction.