[A-OS11] Ocean Mixing Frontiers

[AOS11-P06] Seasonal variations in salinity of the North Pacific Intermediate Water and vertical mixing intensity over the Okinawa Trough

*Hirohiko Nakamura¹, Ryuichiro Inoue², Ayako Nishina¹, Toshiya NAKANO³ (1.Kagoshima University, Faculy of Fisheries, 2.Japan Agency for Marine-Earth Science and Technology, 3.Nagasaki Meteorological Office)

Keywords:Seasonal variation, Vertical mixing, North Pacific Intermediate Water, Kuroshio, Near-inertial internal wave

Seasonal variations in vertical mixing intensity in the intermediate layer of the northern Okinawa Trough were deduced using the seasonal variations in salinity in the North Pacific Intermediate Water (NPIW) using two distinct datasets: the World Ocean Atlas 2013 and historical conductivity, temperature, and depth data in the Okinawa Trough. Water mass analysis of the NPIW with a simple advection model showed that the vertical mixing intensity in the intermediate layer is enhanced in the winter at a rate of 20%–50% of the annual mean amplitude value. To determine the driving force behind the seasonal variations in vertical mixing, we examined moored current-meter data in the northern Okinawa Trough during 2004–2011 and showed that the near-inertial internal wave energy in the intermediate layer was enhanced around January beneath the Kuroshio on the continental slope; this was due to strong southwestward flows associated with deep cyclonic eddies developing in the winter. We used density and velocity data from a realistic high-resolution ocean general circulation model simulation to hypothesize that the large-amplitude Kuroshio meander and associated deep cyclonic eddies in the winter generate near-inertial internal waves through centrifugal/inertial instability in the area of negative potential vorticity that appears not only beneath the Kuroshio near the western slope but also near the eastern slope of the trough. In addition, the symmetric instability along the offshore Kuroshio front and the wind energy input in the winter are suggested as possible sources of winter-enhanced near-inertial internal wave energy in the intermediate layer.