Mindanao Current and Undercurrent: Thermohaline Structure and Transport from Repeat Glider Observations

Abstract

AbstractAutonomous underwater Spray gliders made repeat transects of the Mindanao Current (MC), a low-latitude western boundary current in the western tropical North Pacific Ocean, from September 2009 to October 2013. In the thermocline (<26 kg m?3), the MC has a maximum velocity core of ?0.95 m s?1, weakening with distance offshore until it intersects with the intermittent Mindanao Eddy (ME) at 129.25°E. In the subthermocline (>26 kg m?3), a persistent Mindanao Undercurrent (MUC), with a velocity core of 0.2 m s?1 and mean net transport, flows poleward. Mean transport and standard deviation integrated from the coast to 130°E is ?19 ± 3.1 Sv (1 Sv ≡ 106 m3 s?1) in the thermocline and ?3 ± 12 Sv in the subthermocline. Subthermocline transport has an inverse linear relationship with the Niño-3.4 index and is the primary influence of total transport variability. Interannual anomalies during El Niño are greater than the annual cycle for sea surface salinity and thermocline depth. Water masses transported by the MC/MUC are identified by subsurface salinity extrema and are on isopycnals that have increased finescale salinity variance (spice variance) from eddy stirring. The MC/MUC spice variance is smaller in the thermocline and greater in the subthermocline when compared to the North Equatorial Current and its undercurrents.