Modification of the intermediate waters in the northeastern subpolar Pacific

The modification of the intermediate layers in and around the Alaskan Gyre are investigated in relation to general circulation features of the region, using tracer data from the World Ocean Circulation Experiment P17N line. The Alaskan Gyre (AG) core lies between the North Pacific Current and the Alaskan Stream (AS) in the 52–54.5°N range and to the west of 144°W. The North Pacific Current feeds the Transition Zone (TZ) and the Western Subpolar Waters (WSW), which consequently constitute the inflow to the region from the northwestern Pacific. The Alaska Current (AC) is observed to the west of 144°W, between the North American coast and the AG.; The AC halocline (σ_&thetas; < 26.7) has distinct property distributions (low chlorofluorocarbon and oxygen saturations and high nutrient concentrations), which are correlated with eddies that are characterized by higher salt content and warmer temperatures (most significantly at 26.0 σ _&thetas;). The AG is found to act as a reservoir where water mass modification takes place and the AS transports a mixture of waters from the AG and the AC westward out of the region. The residence time estimate for the intermediate and upper layer water in the AG (σ_&thetas; ≤ 27.2) is about 2 years, which corresponds to a volume exchange between the gyre and the surrounding circulation of 4.7 Sv in the 0–1000 m range. The layers above 26.9 σ_&thetas; (∼0–200 m) are modified by vertical mixing mostly of WSW in the AG; the flux of modified waters out of the AG is approximately 0.9 Sv. Comparison of the present data with the 1984–1989 period suggests that mixing of water from different regimes has not changed dramatically.; Simulations with the PWP model provide estimates of isopycnic heat and salt fluxes that compare well to the observations, providing quantitative evidence for the lateral WSW input to the AG. The CFC-11 simulations confirm the prior calculation of the volumetric exchange rate between the WSW and the AG, and indicate that vertical penetration of CFC-11 within the AG cannot be a significant source of uncertainty. The effects of the anomalous climatological conditions during the 1977–1988 are most clearly observed on the halocline as the balance between upwelling and diffusion shifts in favor of upwelling during this period. The analyses of data from the North Pacific provide strong indications that the eastern half of the subpolar gyre is a watermass modification site down through intermediate layers, and the eastern boundary is a site for exchange between the subtropical and the subpolar gyres.