A Newly Observed Deep Countercurrent in the Subtropical Northwest Pacific

Abstract

The existence of a deep countercurrent in the subtropical northwest Pacific, named Subtropical Deep Countercurrent (STDCC), has been revealed by moored current-meter data obtained from a buoy system (∼21.6°N, ∼132.8°E) during 4 years (2010–2014). The climatological average of Argo-based absolute geostrophic velocities and ocean reanalyses have further confirmed the spatial picture of the eastward-flowing STDCC. STDCC is identified as a time-mean feature in the latitudinal range of 19°–25°N with a meridional scale of 200–300 km that persistently appears from 125° to 180°E and slightly shifts to the north as it goes east. Vertically, STDCC is mostly found below 1,000 m with its core located between the depths of 1,250 and 2,000 m, which corresponds to the undercurrent's meridional pattern with increasing depth northward and a meridional space of 2°–4°. Nevertheless, STDCC is distinctive where it lies along ∼27.6 sigma-theta, which is deeper than the North Equatorial Undercurrent's density layer (∼27.0 sigma-theta). The mean velocity of STDCC at 2,000 m observed by mooring measurements was approximately 0.2 ± 0.15 cm s−1 with a maximum speed of approximately 5.59 cm s−1. The ocean reanalyses also support the existence of STDCC with a mean velocity of approximately 0.2–0.6 cm s−1 west of 145°E and can be as large as 1.6 cm s−1 at the eastern region. The results show that STDCC is closely related to the westward-propagating eddies. Our finding emphasizes that the convergence of the cyclonic and anticyclonic eddies possibly generates time-mean zonally elongated eastward flows forming the STDCC feature.