WOCE line P1W in the Sea of Okhotsk: 2. CFCs and the formation rate of intermediate water

Abstract

From August 30 to September 21, 1993, temperature, salinity, nutrient, and CFC data were collected along a section starting in the Pacific Ocean near Bussol' Strait in the Kuril Islands, crossing the basin and shelf region of the Sea of Okhotsk, and ending near the coastal town of Okhotsk at the northwest corner of the Sea of Okhotsk (World Ocean Circulation Experiment (WOCE) line P1W). Water on the continental shelf had potential density (σθ) as great as 27.0, with high CFC concentrations and low temperature and salinity. The Sea of Okhotsk Intermediate Water (SOIW) ranging from 26.8 to 27.4 σθ had uniform water properties along isopycnal surfaces that were distinctly different from either the North Pacific Intermediate Water (NPIW) or the shelf water found at the same density. Since the SOIW was colder and fresher than the NPIW, we postulate that SOIW is formed by sinking plumes and diapycnal mixing. Shelf‐derived water (SDW) as dense as 27.0 σθ is formed on the shelf during the winter by cooling and brine rejection during ice formation. Prevailing winds move the SDW off the shelf, where it sinks to an equilibrium depth and subsequently mixes isopycnally with North Pacific Water (NPW) flowing into the Sea of Okhotsk through the Kuril Islands. The observations on the shelf suggest that SDW directly modifies SOIW to 27.0 σθ. Using potential temperature (θ), salinity (S), and CFC data, we calibrated a simple box model to determine the input of SDW into the SOIW. Model results required a minimum of 0.9 Sv (1 Sv = 106 m3 s−1) of SDW in the formation of SOIW and produced a minimum export of 2.7 Sv of SOIW from the Sea of Okhotsk. Our minimum calculated export of SOIW into the North Pacific Ocean makes the Sea of Okhotsk a significant source of the salinity minimum present in the NPIW. Below 27.2 σθ the box model failed because the CFC concentrations of the SDW were too high. This suggests that SDW directly ventilates the SOIW to a maximum of 27.2 σθ. The unique properties of the SOIW below 27.2 σθ must be the result of diapycnal mixing in the Sea of Okhotsk and isopycnal mixing with inflowing NPW.