Effect of the Intensified Sub‐Thermocline Eddy on Strengthening the Mindanao Undercurrent in 2019

The northward‐flowing Mindanao Undercurrent (MUC) was directly measured by acoustic Doppler current profilers from a subsurface mooring at about 8°N, 127°E during 2 years (November 2017–December 2019). Its depth covers a range from 400 m to deeper than 1,000 m with its core appearing at around 900 m. The mean velocity of MUC's core was approximately 5.8 cm s−1 with a maximum speed of about 47.6 cm s−1. The MUC was observed as a quasi‐permanent current with strong intraseasonal variability (ISV) with a period of 70–80 days. Further analyses with an eddy‐resolving circulation model output suggest that the ISV is closely related to sub‐thermocline eddies (SEs). In this study, two types of SEs near the Philippine coast are disclosed: the westward propagating SE (SE‐1) and the quasi‐stational SE southeast of Mindanao Island (SE‐2). The SE‐1 has both cyclonic and anticyclonic polarities with the propagation speed of 7–8 cm s−1, while the SE‐2 is an anticyclonic eddy that moves erratically within 4–8°N, 127–130°E with the mean translation speed of about 11 cm s−1. Even though the SE‐1 plays an important role in modulating the MUC, our results show that the observed strong MUC event (May–July 2019) is evidently induced by the intensified SE‐2 that moves northwestward. This study emphasizes that the SE‐2 when intensified, receives more energy from the strengthened New Guinea Coastal Undercurrent and loses the energy northward along the Philippine coast by intensifying the MUC.