Intraseasonal-to-interannual Variability of Subtropical Countercurrent, North Equatorial Current, and North Equatorial Undercurrent in the Western Pacific Ocean

Abstract

Currents obtained from moored ADCPs were analyzed from October 2007 to May 2012. Three subsurface moored buoy systems are located approximately across the Subtropical Countercurrent (STCC), near the northern boundary of, and at the center of the main axis of North Equatorial Current (NEC), respectively, in the western Pacific Ocean. Under the prevailing westward-flowing NEC, the eastward-flowing STCC and the North Equatorial Undercurrent (NEUC) sometimes appeared in the surface and in the subsurface layers, respectively, with an intra-seasonal time scale. While STCC appears as the result of vertical shear in the STCC-NEC system due to local wind stresses, NEUC seems to appear as the result of the combination of equatorial current system and local geostrophic balance. In order to figure out the meridional extent of the current variability of the STCC-NEC-NEUC system, five more pressure-equipped inverted echo sounder (PIES) have been added approximately along 135oE among the three moored ADCP stations since May 2012. The existence of the NEUC with three cores has been discussed by Qiu et al.(2013) with geostrophic currents from ARGO float data. However, it needs real measurements whether NEUC is totally fed from the convergence of Luzon and Mindanao Undercurrents or not, what drives NEUC in a variable intra-seasonal time scale, and how far it continues to flow eastwards, and under what condition it disf the main axis of North Equatorial Current (NEC), respectively, in the western Pacific Ocean. Under the prevailing westward-flowing NEC, the eastward-flowing STCC and the North Equatorial Undercurrent (NEUC) sometimes appeared in the surface and in the subsurface layers, respectively, with an intra-seasonal time scale. While STCC appears as the result of vertical shear in the STCC-NEC system due to local wind stresses, NEUC seems to appear as the result of the combination of equatorial current system and local geostrophic balance. In order to figure out the meridional extent of the current variability of the STCC-NEC-NEUC system, five more pressure-equipped inverted echo sounder (PIES) have been added approximately along 135oE among the three moored ADCP stations since May 2012. The existence of the NEUC with three cores has been discussed by Qiu et al.(2013) with geostrophic currents from ARGO float data. However, it needs real measurements whether NEUC is totally fed from the convergence of Luzon and Mindanao Undercurrents or not, what drives NEUC in a variable intra-seasonal time scale, and how far it continues to flow eastwards, and under what condition it dis