Structure and Variability of the North Equatorial Current/Undercurrent from Mooring Measurements at 130°E in the Western Pacific

Abstract

The mean structure and variability of the North Equatorial Current/Undercurrent (NEC/NEUC) are investigated with one-year Acoustic Doppler Current Profilers measurements from 4 subsurface moorings deployed at 10.5°N, 13°N, 15.5°N, and 18°N along 130°E in the western Pacific. The strong westward flowing NEC ranges from the sea surface down to 400 m, and the mean zonal velocity of the NEC at 10.5°N is around −30 cm/s at the depth of 70 m. Eastward flowing NEUC jets are detected below the NEC at 10.5°N and 13°N, and the depth of the NEUC could reach at least 900 m. The mean velocity of the NEUC is around 4.2 cm/s at 800 m. No eastward undercurrents are observed at 15°N and 18°N. The mooring measurements also reveal a strong intraseasonal variability of the currents at all 4 mooring sites, and the period is around 70–120 days. The vertical structure of this intraseasonal variability varies at different latitudes. The variability of the NEUC jets at 10.5°N and 13°N appears to be dominated by subthermocline signals, while the variability of the currents at 15.5°N and 18°N is dominated by surface-intensified signals.

On the Formation Dynamics of the North Equatorial Undercurrent

“…The LUC and the MUC were first discovered in the early 1990s using in situ conductivity-temperature-depth (CTD) measurement Cui 1989, 1991). This was further confirmed by additional data (Qu et al 1998) and direct measurements utilizing an acoustic Doppler current profiler (ADCP) deployed at the subsurface Zhang et al 2014). The southward maximum velocity of the LUC ranged from ;7 cm s 21 by geostrophic calculation to 27.5 cm s 21 by the ADCP, and the mean volume transport was 2-4 Sv (1 Sv [ 10 6 m 3 s 21 ).…”

“…The southward maximum velocity of the LUC ranged from ;7 cm s 21 by geostrophic calculation to 27.5 cm s 21 by the ADCP, and the mean volume transport was 2-4 Sv (1 Sv [ 10 6 m 3 s 21 ). The maximum velocity of the MUC was larger than that of the LUC, reaching 10-20 cm s 21 (Hu et al 1991;Zhang et al 2014), and the transport was 8-22 Sv (Hu et al 1991). Previous studies of the LUC and the MUC were mainly conducted independently, and their connections to the NEUC have not been adequately explored.…”

“…The NEC in the upper 500 m or so carries subtropical warm water westward and feeds into a higher latitude through the KC. Meanwhile, the subsurface current, namely, the North Equatorial Undercurrent (NEUC; Wang et al 1998), transports intermediate water eastward with a maximum velocity ranging from 0.02 to 0.05 m s 21 (Qiu et al 2013b;Qu et al 1998;Wang et al 2015;Yuan et al 2014;Zhang et al 2017). Although the NEUC is much weaker than the wind-driven NEC, it substantially affects the transport of the intermediate water across the WPO.…”

“…Qiu et al (2013a) pointed out that the jets were generated by the breaking-down eddies in the wind-induced westward propagating Rossby waves. However, utilizing mooring data, Zhang et al (2017) found that the zonal velocities in the upper and subthermocline layer were dominated by variabilities with different frequencies, and they suggested that the subthermocline jets could come from subthermocline eddies rather than wind-driven variability. Qiu et al (2013a) and Zhang et al (2017) focused on the mesoscale variability and intraseasonal variations in the NEUC.…”

“…However, utilizing mooring data, Zhang et al (2017) found that the zonal velocities in the upper and subthermocline layer were dominated by variabilities with different frequencies, and they suggested that the subthermocline jets could come from subthermocline eddies rather than wind-driven variability. Qiu et al (2013a) and Zhang et al (2017) focused on the mesoscale variability and intraseasonal variations in the NEUC. So far, the contribution from the largescale component to the NEUC remains unclear, which is critical to holistically characterizing the NEUC.…”