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 degrees N range and to the west of 144 degrees 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 degrees W, between the North American coast and the AG. The AC halocline (sigma(theta) < 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 sigma(theta)). 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 (sigma(theta) less than or equal to 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 sigma(theta) (similar to 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.