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

Abstract

AbstractThe 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 at about 800 m on average and instantaneously reaches a maximum speed of about 0.47 m . 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.