Structure and Variability of the Kuroshio and Luzon Undercurrent Observed by a Mooring Array

The spatial structure and variability of the western boundary currents east of Luzon Island at 18°N are investigated using measurements collected by acoustic Doppler current profilers (ADCPs) from three moorings between January 2018 and May 2020. During the observation period, the southward velocity of Luzon Undercurrent (LUC) has a maximum greater than 0.04 m s−1 ${\mathrm{s}}^{-1}$ at about 800 m on average and instantaneously reaches a maximum speed of about 0.47 m s−1 ${\mathrm{s}}^{-1}$. Observations indicate that the LUC and Kuroshio have significant intraseasonal variability and the LUC is reversed occasionally. We find that the strong upper‐layer anticyclonic eddy plays a key role in the weakening and reversal of LUC and that the LUC is a permanent mean undercurrent. A vertical mode decomposition analysis is employed and results show that the first two baroclinic modes explain the main vertical features of the LUC and Kuroshio at its origin. No reversal of the southward undercurrent occurs in the second baroclinic mode velocity. We conclude that the LUC variability is generally a result of the second baroclinic mode but the observed reversal of the LUC is mainly caused by the first mode. Plain Language Summary The western boundary currents east of Luzon, including the upper‐layer northward Kuroshio and the underlying southward Luzon Undercurrent (LUC), are of much importance in the climate system through transporting a large amount of mass and heat. However, the spatial structure of the currents and the existence of the LUC are not well documented. Here, we present observations of the Kuroshio and LUC from a mooring array at 18°N during January 2018–May 2020. The observed LUC shows significant variability with depths and time. We find that the LUC is a permanent undercurrent and that its variability is closely associated with the upper‐layer eddies. A cyclonic eddy tends to force a stronger LUC, while an extremely strong anticyclonic eddy may temporarily reverse the undercurrent. We conclude that the LUC variability is a result of the second baroclinic mode but the reversal of the LUC is mainly caused by the first mode. Key Points Kuroshio and Luzon Undercurrent (LUC) east of Luzon was directly measured by a mooring array Direct observations confirm the existence of the Luzon Undercurrent but it was reversed due to strong anti‐cyclonic eddies The LUC variability is found to be mainly a result of the first two baroclinic modes