Mindanao Undercurrent in relation to the intensified subthermocline eddies

Abstract

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 two 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 quasi-permanent current with strong intraseasonal variability with a period of 70-80 days. Further analyses with an eddy-resolving model output suggest that the intraseasonal variability is closely related to the subthermocline eddies (SE). In this study, two types of SE near the Philippine coast is disclosed: one is the westward propagating SE and the other is the southeastern SE. The westward propagating SE has both cyclonic and anticyclonic polarities with propagation speed of 7-8 cm s-1, while the southeastern SE is 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 westward propagating SE has more important role in modulating the MUC, our results show that the observed strong MUC event (May – July 2019) is evidently induced by the intensified southeastern SE that moves northwestward. Our finding emphasizes that the southeastern SE 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.