The North Equatorial Current/Undercurrent volume transport and its 40-day variability from a mooring array along 130°E

Traditionally, the estimated volume transport of the North Equatorial Current/Undercurrent (NEC/NEUC) is based on geostrophic equations and/or model results; however, direct observational evidence has not been acquired. We focused on one-year mooring observation data collected along 130°E and calculated the NEC/NEUC volume transport and explore its variability. Results show that the mean NEC and NEUC volume transports calculated from the mean velocity structures in the upper 950 m are 39 Sv and 6 Sv, respectively. Analysis of daily mooring data indicated that the volume transport of the NEC is approximately 52 (±14) Sv and the volume transport of the NEUC is approximately 18 (±13) Sv. A significant 40-day variation existed for the volume transport of both the NEC and NEUC. Overall, the intraseasonal variability (ISV) of the NEC is vertically coherent with that of the NEUC. Observations indicated that the NEUC has three cores centered at approximately 8.5°N (∼500 m), 12.5°N (∼700 m), and 17.5°N (∼900 m), of which the middle core (12.5°N) is the strongest. The 40-day variability of the NEC and NEUC is related to the variability of local wind stress curl anomalies among various Madden-Julian Oscillation (MJO) phases. When local wind field generates a negative (positive) wind stress curl anomaly, a weaker NEC (NEUC) and stronger NEUC (NEC) would occur.