Subsurface structure and variability of the zonal currents in the Northwestern Tropical Pacific Ocean

Abstract

Seven subsurface moorings were deployed between 0°N and 7.5°N at 142°E in the western Pacific during August 2014–October 2015, obtaining unprecedented high-quality measurements of the North Equatorial Countercurrent (NECC) between 60 and 200 m and the underlying Northern Subsurface Countercurrent (NSCC) and North Equatorial Subsurface Current (NESC) between 300 and 600 m. During the observation period, the eastward-flowing NSCC showed a mean velocity of 5–15 cm s−1 between 2°N and 4°N, while the NESC manifested as intermittent westward currents of < 5 cm s−1 between 4°N and 6°N. Temporal variabilities in the NSCC and NESC are documented for the first time. Both the NSCC and NESC were strengthened when the NECC shifted to the south in boreal fall. The 2015–2016 El Niño induced significant strengthening of the NECC and weakening of the NESC. Further analysis combining ocean reanalysis data suggests that seasonal variations are likely coherent between the upper and lower (300–600 m) layers and associated with the meridional migration of the NECC. Seasonal variations of the three currents are likely first-mode baroclinic Rossby wave responses to basin-wide wind forcing. By contrast, interannual variations of the NSCC and NESC have more complicated vertical structure. El Niño events generally cause the strengthening and southward migration of the NECC and the weakening of the NESC. Ocean reanalysis products however show difficulties in realistically representing the observed subsurface structure and variability of the three currents, remarking the necessity of enhancing ocean in-situ observation systems.