A Numerical Study of Long-Return Period Near-Bottom Ocean Currents in Lower Cook Inlet, Alaska

Abstract

Lower Cook Inlet (LCI) is an important waterway with very large tides and high marine productivity. Oceanographic forcing in LCI is complex due to a combination of tides, seasonal winds, and large freshwater discharges, as well as inflow from the Alaska Coastal Current. From an analysis of historical current meter data sets, deeper ocean currents of LCI were found to have large differences resulting from the dominance of large tides in the northeast portion of LCI while subtidal contributions to the deeper currents are more important relative to the reduced tidal currents in central and western parts of LCI. To compute the largest values of the near-bottom currents of LCI, a 3D hydrodynamical model was developed over a large model domain extending over the full 300 km length of Cook Inlet as well as a large portion of the adjoining Alaska continental shelf region. At the open model boundaries, nine major tidal height constituents were specified based on National Oceanic and Atmospheric Administration (NOAA) tidal gauge data. The model was forced by the spatially varying winds and freshwater discharges for the six gauged rivers in Cook Inlet. The model was verified using available current meter data in the study area. Model runs were carried out for 21 case studies to derive the near-bottom currents for return periods of 1, 10, and 100 years. Within LCI, the extremal values for near bottom currents arise from quite different forcing regimes. Tidal currents are completely dominant in the northeast portion of LCI while for central and western portions, remote wind forcing over the Alaskan continental shelf current, which generates the Alaska Coastal Current, becomes more important.