Mindanao Current

The Mindanao Current is a low-latitude western boundary current located in the North Pacific and formed at the Philippine Sea by the bifurcation of the NEC. The jet transports approximately ${\displaystyle 13-39}$ Sv (where ${\displaystyle 1}$ ${\displaystyle Sv=10^{6}m^{3}s^{-1}}$) to the equator.[3] It flows alongside the contours of the Philippine coast, approximately following the mean dynamic bathymetry based on underwater glider observations. The MC feeds the equatorial currents, including the North Equatorial Countercurrent and the Indonesian Throughflow which transports water from the Pacific to the Indian Ocean. Thus, the Mindanao Current serves a role in the global overturning circulation. It often interacts with the nearby Mindanao Eddy which is a semi-permanent cyclonic recirculation to the east. Beneath the Mindanao Current is an even less studied undercurrent, the Mindanao Undercurrent (MUC), which is deeper than ${\displaystyle 400}$ ${\displaystyle m}$ and lies offshore of the MC. Both the MC and its undercurrent are pathways for the exchange of water in the tropical Pacific.[2]

Identified by its salinity signature, the MC waters from the surface to ${\displaystyle 200}$ ${\displaystyle m}$ depth are North Pacific Tropical Water (NPTW) while the layer at ${\displaystyle 300-500}$ ${\displaystyle m}$ depth is North Pacific Intermediate water (NPIW). The NPTW is characterized by a salinity maximum of ${\displaystyle 35}$ ${\displaystyle psu}$ or Practical Salinity Units which is relatively high. As the water mass is advected southward by the MC, it evolves: the salinity maximum decays by ${\displaystyle 0.1}$ ${\displaystyle psu}$ along the Philippine coast. Once it reaches a latitude of 6°N, it becomes indiscernible. In contrast, the waters of the MUC transport Antarctic Intermediate Water which originates from the south Pacific. It is characterized by a salinity greater than ${\displaystyle 34.5}$ ${\displaystyle psu}$ and usually contains more oxygen than the NPIW.[2]

The MC originates from the southern branch of the NEC bifurcation. Its northern branch becomes the Kuroshio current which begins as a weak and variable current that strengthens at the Luzon Strait. The Kuroshio must travel a larger distance to gain speed than the Mindanao Current which is strong closer to the bifurcation location. It is fed by the southern portion of the NEC.[4]

The NEC is a wide current, spanning approximately 10-20°N. Towards 10°N, it is relatively shallow while towards 20°N, it deepens significantly (in the order of ${\displaystyle 100}$ ${\displaystyle m}$). As a consequence, the bifurcation location is northward with increasing depth, i.e., at the shallowest layer (${\displaystyle 0-100}$ ${\displaystyle m}$), the bifurcation occurs at ~12°N; at ${\displaystyle 100-300}$ ${\displaystyle m}$, it occurs at ~13°N; at depths of ${\displaystyle 300-600}$ ${\displaystyle m}$, it occurs at ~ 15°N. Thus, the origin of the Mindanao Current is more northern with depth.[2]

Observations from December 2010 to August 2014 show that the surface MC (specifically ${\displaystyle 50-150}$ ${\displaystyle m}$ depth) varies from season to season. It is stronger during boreal or Northern Hemisphere spring and weaker during fall. From ${\displaystyle 150-400}$ ${\displaystyle m}$, it is stronger in spring and fall and weaker in summer and winter. Seasonal variations in the MC are controlled by large-scale upper ocean circulation. The variations are often explained by local wind forcing of the western Pacific, both by local winds and winds in the Pacific’s interior which cause westward propagating Rossby waves.[5]

The Mindanao Current is difficult to observe and model because of the strength of the current, the extreme topography of the region, and complicating factors such as wind stress fluctuations and eddies.[2] The most extensive observations of the Mindanao Current were performed during the following research programs:

• The Western Equatorial Pacific Circulation Study (1985-1986)[9]
• Tropical Ocean Global Atmosphere (1985-1994)[10]
• Northwestern Pacific Ocean Circulation and Climate Experiment (2004-2015)
• South Pacific Ocean Circulation and Climate Experiment (Since 2005)[11]
• Origins of the Kuroshio and Mindanao Current initiative (2009-2014)[12]

• Ocean current
• Oceanic gyres
• Physical oceanography

• Map of the Mindanao Current