Interannual variability and sensitivity study of the ocean circulation and thermohaline structure in Prince William Sound, Alaska

Abstract

The interannual variability and sensitivity of ocean circulation and the thermocline structure of Prince William Sound, Alaska were examined using a 3D circulation model. A 4-year (1995–1998) simulation compared well with field observations of circulation and monthly mean sea surface temperature at NOAA Station 46060. Seasonal circulation regimes were characterized by an anticyclonic gyre in the central sound in January–April, and a strong cyclonic gyre in the central sound in September to December, while summer was the transition period of the two circulation regimes. The size, position and strength of the gyres and thermohaline depth in the central sound showed small interannual variability. Freshwater displayed a very strong seasonal cycle in the sound with minimum in April–May and maximum in October with a spatial distribution of more in the northwest sound and less in the other areas. The simulated freshwater thickness in the whole sound showed up to 20% interannual variability related to wind. The numerical oil spill drift experiments also showed a large interannual variability of possible oil spill trajectories. Sensitivity studies showed the relative importance of each model forcing: (1) wind has more impact on the surface circulation and mixed layer depth. Without wind, the surface current became weaker and the thermocline became shallower; (2) tidal current is a major current in the sound and important to surface and bottom mixing. Without tide, the thermocline depth became shallower; (3) the magnitude of the Alaska coastal current (ACC) inflow determined the outflow current through Montague Strait. Doubled ACC inflow could change the cyclonic circulation pattern in October in case of normal or less ACC inflow into a northward jet with a small accompanying anticyclonic gyre and strongly flush the west sound in addition to the central sound. Also, double ACC inflow would increase the mix-layer depth significantly; and (4) the surface T, S restoring is critical to maintain T, S seasonal cycle and surface circulation patterns. Salinity was the most important factor determining the central sound circulation patterns.