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 (similar to 21.6 degrees N, similar to 132.8 degrees E) during 4 years (2010-2014). The climatological average of Argo-based absolute geostrophic velocities and ocean reanalyzes 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 degrees-25 degrees N with a meridional scale of 200-300 km that persistently appears from 125 degrees to 180 degrees 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 degrees-4 degrees. Nevertheless, STDCC is distinctive where it lies along similar to 27.6 - sigma theta, which is deeper than the North Equatorial Undercurrent's density layer (similar to 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 reanalyzes also support the existence of STDCC with mean velocity of approximately 0.2-0.6 cm s(-1) west of 145 degrees 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 STDCC feature.