Fig. 1. Long-term mean horizontal flow field (left) and eddy-induced freshwater flux (right) at 605-m depth in OFES show the Mindanao Undercurrent is permanent but only up to ~12N. Eddies, though, can induce a northward freshwater flux offshore the Philippines.

Fig. 2. Long term-mean meridional velocity at 7N (left), whose geographic location is marked by the red line in Fig.1, and time variability of the two cores (right) show the MUC is highly variable and dominated by mesoscale (50-100 days) fluctuations.

OFES Helps Solve Mystery of Mindanao UndercurrentT. Qu and T.-L. Chiang

The anomalously fresh, oxygen-rich water in the subthermocline off the Philippine Coast is thought to come from the South Pacific via the Mindanao Undercurrent (MUC), but observations indicate the MUC reaches at most 13N. Eddy-resolving OFES simulations are helping to solve this mystery. OFES shows the MUC can reach no further than 12N as a permanent flow. Two branches of the MUC, inshore and offshore, are visible at 7N, the offshore carrying South Pacific fresh, oxygen-rich water. Both branches meander, and their northward transport fluctuates on periods mainly between 50 to 100 days. The fluctuations are indicative of strong eddy activity in the region’s sub-thermocline. The average transport is 3.7 Sv and comes mainly from the offshore branch. The eddies can carry South Pacific water further north than the MUC and may be the source of the anomalous fresh water off the Philippine Coast.