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 degrees N, 127 degrees 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 degrees N, 127-130 degrees 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.