Variability and Dynamics of the Kuroshio and Mindanao Current during the 2010/11 La Niña and in Late 2012

Abstract

Abstract Variability of two Pacific western boundary currents (WBCs)—the Kuroshio and the Mindanao Current—during the strong 2010/11 La Niña event is investigated using ship-based hydrographic observations and moored current-meter data collected off the east coasts of the Philippines. The geostrophic currents calculated using the hydrographic data show that, during the 2010/11 La Niña winter, the Kuroshio decreased by ∼10 Sv (1 Sv ≡ 106 m3 s−1), whereas the Mindanao Current increased by ∼5–10 Sv, relative to the normal winter in late 2012. The interannual variability based on the hydrographic data is confirmed by moored current-meter measurements and satellite altimeter geostrophic currents. A coastally trapped Kelvin wave model is used to explain the interannual variability of the two WBCs during the different ENSO phases. The good comparison of the simulated sea level anomalies around the Philippines with the altimeter data suggests that the interannual variability of the WBCs is associated with Kelvin wave propagation from the Sulawesi–Sulu Seas clockwise around the Philippine Archipelago. We identified that the Kelvin waves are excited by downwelling equatorial Rossby waves propagating into the Indonesian Seas during the La Niña. The transport anomalies of the WBCs are comparable to the total meridional transport anomalies integrated across the interior North Pacific Ocean, suggesting the importance of the WBCs in the heat charge–discharge processes of the western Pacific warm pool during ENSO events. Significance Statement The two western boundary currents (WBCs)—the Kuroshio and the Mindanao Current—play the role of closing the subtropical and tropical gyre circulation of the Pacific Ocean. Their variability during ENSO is unknown. Existing studies based on numerical modeling suggest that their variability is highly correlated with ENSO, with the Kuroshio stronger and Mindanao Current weaker during La Niña and vice versa during El Niño. Here, we use in situ hydrographic observations combined with mooring and satellite altimeter data to show that the Kuroshio transport decreases and the Mindanao Current transport increases during the 2010/11 La Niña, the dynamics of which are controlled by the Kelvin wave propagation from the Sulawesi–Sulu Seas clockwise around the Philippine Archipelago. The result is important for the warm pool dynamics during ENSO.