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