Decadal Variations of the Mindanao Current During 1960–2010

Abstract

Results from an eddy-resolving ocean model simulation, an ocean reanalysis data, and reduced-gravity ocean (RGO) model simulations reach consensus in suggesting prominent decadal fluctuations of the Mindanao Current (MC) during 1960–2010. The MC was evidently enhanced during 1974–1982 and 2002–2008 and weakened during 1983–1990 and 1994–2001, with typical amplitude of ~2.0 Sv (1 Sv ≡ 106 m3/s) for decadal volume transport anomaly. These variations are closely associated with the strength of the westward flowing North Equatorial Current at 130°E and not evidently related with its bifurcation latitude. Experiments using a 1.5-layer RGO model are performed to understand the mechanisms. Decadal variations of the MC are primarily caused by off-equatorial wind changes in the tropical North Pacific, and winds in the latitude range of the MC (3–13°N) play the dominant role. Equatorial winds between 3°S and 3°N generally act to attenuate the MC variability, and subtropical winds have little contribution. For zonal distribution, local wind forcing between 120 and 160°E explains 67.8% of the MC decadal variability, while those in the central and eastern Pacific have weaker effects (25.0% and 7.2%, respectively). RGO model runs forced by nine wind data sets confirm the tight relationship between the MC and the western Pacific winds since the 1980s. The anomalous westerly winds under El Niño-like condition drive upwelling in the Philippine Sea through off-equatorial Ekman pumping that strengthens the MC. The relationship prior to 1980 is sensitive to the choice of wind data set, with four data sets yielding low or negative correlations.