Atmospheric and tidal forcing of the exchange between Prince William Sound and the Gulf of Alaska

- Velocity profiles from moored ADCPs are used to quantify the nature of the exchange.
- The barotropic EOF mode constitutes more than 62% of the subtidal variance.
- Tides drive the largest fluctuations in transport, but have little net effect.
- Wind forcing is the dominant subtidal driver, peaking at 4 to 5-day time scales.
- Wind drives most of the subtidal transport by setting up sea level in and around PWS.

Current meter data from a series of oceanographic moorings spanning a total of five years was analyzed to quantify the tidal and subtidal exchange of water between Prince William Sound and the adjacent continental shelf in the northern Gulf of Alaska. Velocity profiles were used to quantify the exchange in terms of a transport through each of the two largest passages: Montague Strait and Hinchinbrook Entrance. Buoy wind and atmospheric pressure observations, as well as bottom pressure records, are then used to elucidate the role of atmospheric forcing on the exchange.An EOF analysis shows that the barotropic component accounts for 62% or more of the variance in the velocity profiles even after tides are removed by low-pass filtering, and thus the analysis is concerned primarily with depth-integrated transport. The estimated depth-integrated transport can reach ±0.6 Sv in Montague Strait, and ±1.5 Sv in Hinchinbrook Entrance. The largest fluctuations occur in response to the semidiurnal tides. Transport variations on subtidal time scales, which can reach −0.2 Sv in Montague Strait, and +0.6 Sv in Hinchinbrook Entrance, are shown by a frequency domain analysis to be dominated by easterly wind stress events which occur at periods of 2-5 days in both summer and winter. Atmospheric pressure has much less impact on transport, but there is some evidence that it might play a small role on time scales of a few weeks.Bottom pressure records suggest that easterly wind events set up a sea level height gradient in Hinchinbrook Entrance such that it tilts up to the east, which under geostrophy drives a barotropic flow into Prince William Sound. The same winds also raise the sea level in Hinchinbrook Entrance relative to Montague Strait, encouraging an outflow there in agreement with the ADCP observations. There is no evidence that the wind drives a vertically sheared bi-directional flow in either entrance, as has been observed in some estuaries. It is hypothesized that the lack of such a flow is possible because Prince William Sound has two major connections to the shelf, which alters the mass conservation requirement for each passage when compared to a system with just one entrance.