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ABSTRACT
The interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 7°59'N, 127°3'E during December 2010-August 2014. Depth-dependent interannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced-gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 1508-180°) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 120°-150°E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed.
1. Introduction
Low-latitude western boundary currents (LLWBCs) in the Pacific Ocean are of much importance in the tropical climate system for their role in the recharge/discharge of the El Niño-Southern Oscillation (ENSO) cycle and in regulating the western Pacific warm pool that is a key factor in the tropical Pacific Ocean (Hu and Cui 1991; Lukas et al. 1996; Jin 1997; Hu and Hu 2012; Hu et al. 2015). The Mindanao Current (MC) and the underlying Mindanao Undercurrent (MUC) are remarkable components of the LLWBCs in the Pacific Ocean (Hu and Cui 1989). They are suggested to be important in the global thermohaline circulation for their significant contribution to the Indonesian Throughflow (ITF) that connects different ocean basins (e.g., Gordon and Fine 1996; Sprintall et al. 2014). The MC is also a crucial pathway between the...