Glider observations of the Mindanao Current: Geostrophic velocity, transport and thermohaline structure

The Mindanao Current (MC), formed where the North Equatorial Current (NEC) bifurcates near the Philippines, is strong low-latitude western boundary current that transports North Pacific water to the equatorial Pacific and Indonesian Throughflow. Despite its importance to heat and fresh water transport in global ocean circulation, the MC has had few sustained observations, and the mean structure and variability of the current are not well resolved. Autonomous underwater Spray glider observations of the MC and the NEC took place from June 2009 to January 2014. Glider observations of the MC provide the mean geostrophic velocity structure, transport and transport variability, and thermohaline structure at length scales down to 10 km. Mean velocity of the MC exceeds 1 m s^-1, extending to a depth of 1000 m near the coast, with southward velocity extending 300 km offshore. The MC transports North Pacific Tropical Water (NPTW) and North Pacific Intermediate Water (NPIW). Offshore of the MC, the subthermocline Mindanao Undercurrent (MUC) transports South Pacific Water northward. Although spatially variable, the MUC is a persistent feature. Transport from the MUC may directly flow into the southern undercurrent below the NEC, where similar temperature and salinity characteristics are found. At length scales of 10 to 80 km, fine-scale thermohaline structure of the MC is analyzed along isopycnals. The isopycnal salinity variance of the MC is similar to that of the NEC, and shows enhanced variance where the MC is strongest, which may indicate increased stirring of water masses. Tracking the evolution of the salinity maximum of NPTW from the NEC into the MC may indicate how this water mass diffuses into the ocean interior.