Mesoscale variability of deep currents in the Northwest Pacific Basin

Abstract:

In our mooring observations during the past 15 years investigating the route and mean transport of the Lower Circumpolar Deep Water in the North Pacific Ocean, individual current meters have exhibited short-term current variability on timescales of 30―120 days at depths of 3000―5000 m. To clarify the spatial and temporal properties of this variability and its mechanism, we deployed nine moorings in May 2014 at 30°N, 147°E where the ocean depth is about 6200 m with relatively flat topography. The nine (3×3) moorings, of which five had current meters at 6000, 5000, 4000 and 3000 m and the remaining four had them at 6000 and 4000 m, were arrayed in a diamond shape, whose zonal and meridional widths are both 100 km. We also conducted full-depth CTD observations at 16 (4×4) stations alternating with the moorings. The isopycnal of σ₄ = 45.89 kg m^-3 was deepest (6033 m) at the northeastern corner and was shallowest (5595 m) at the southwestern corner. Isotherms and isohalines exhibited similar spatial variation in the deep layer.

 To understand current variability obtained from ongoing and previous mooring observations, we have also analyzed outputs from a global eddy-resolving ocean general circulation model. The model was COCO model, with 62 vertical levels with thickness increasing from 2 m at the surface to 660 m at the bottom (7200 m depth), and was driven by the CORE Ver.2 interannual forcing. We analyzed the modelled current in 1997―2006 at five locations corresponding to our mooring sites. The eastward and northward 10-year mean velocities at 4000 m depth at the five locations were -0.51 to 0.25 cm s^-1 and -1.03 to 1.27 cm s^-1, respectively. These averages were smaller than their standard deviations that exceeded 1.4 cm s^-1. The eddy kinetic energy (EKE) was 3.4 cm² s^-2 at the southernmost location (29°30’N, 147°E) and 11.1 cm² s^-2at the northernmost location (30°30’N, 147°E), increasing northward. These values were similar to those from previous mooring observations conducted near 30°N, 146°40’E in the late 1970’s. In the Northwest Pacific Basin, simulated EKE was highest at the Kuroshio Extension, and a high-EKE area extended about 500 km north and south of it.