North pacific intermediate water transports in the mixed water region

Abstract

Initial mixing between the subtropical and subpolar waters of Kuroshio and Oyashio origin occurs in the mixed water region (interfrontal zone) between the Kuroshio and Oyashio. The relatively fresh water that enters the Kuroshio Extension from the Mixed Water Region is this already mixed subtropical transition water. Sub-tropical transition water in the density range 26.64-27.4 σθ can be considered to be the newest North Pacific Intermediate Water (NPIW) in the subtropical gyre; this density range is approximately that which is ventilated in the subpolar gyre with significant influence from the Okhotsk Sea. Freshening of the Kuroshio Extension core occurs between 140° and 165°E in the upper part of the NPIW (26.64-27.0 σθ), with the greatest freshening associated with the eastern side of the first and second Kuroshio meanders. Kuroshio Extension freshening in the lower part of the NPIW (27.0-27.4 σθ) occurs more gradually and farther to the east. There is nearly no distinction in water properties north and south of the Kuroshio Extension by 175°W. The upper part of the NPIW in the Mixed Water Region progresses from very intrusive and including much freshwater in the west, to much smoother and more saline water in the east. The lower part of the NPIW in the mixed water region progresses from very intrusive and fresh in the far west, to noisy and more saline at 152°E, to smooth and fresher in the east. These suggest a difference between the two layers in both advection direction and possibly transport across the Subarctic Front. Assuming that all waters in the region are an isopycnal mixture of subtropical and subpolar water, the zonal transport of subpolar water in the subtropical gyre at 152°E is estimated at about 3 Sv (Sv ≡ 106 m3 s-1). This could be approximately one-quarter of the Oyashio transport in this density range.