Observations of Shelf Exchange and High‐Frequency Variability in an Alaskan Fjord

Abstract

AbstractHigh‐frequency observations collected over 3 years are used to describe upper‐ocean variability in Behm Canal, a nonglacial fjord in Southeast Alaska. The fjord is sheltered by surrounding topography and connected to the outer continental shelf by a 400‐m‐deep strait. Summer conditions are characterized by strong near‐surface stratification, a sea breeze wind regime, and tidally dominated flows. In nonsummer months, baroclinic subinertial flows exceeding 0.5 m/s dominate the velocity record. The flow events represent a wind‐driven response to low‐pressure systems that impact the coast as they propagate across the Gulf of Alaska. The observations suggest that the storm systems generate downwelling events that propagate into the fjord with a mode‐one‐like vertical structure in the upper 60 m. Following the initial up‐fjord current pulse lasting approximately a day, a down‐fjord flow occurs lasting several days, the duration of the downwelling anomaly. These subinertial downwelling events likely are the dominant mechanism of shelf‐fjord exchange, with estimated fjord‐flushing times of 50 days. The downwelling events are accompanied by enhanced near‐inertial shear, which exhibits downward energy propagation. Evidence for enhanced energy near the maximum buoyancy frequency in the thermocline suggests high‐frequency internal wave trapping, with the primary energy source in the semidiurnal band associated with tidal and wind forcing. The observations highlight the importance of shelf‐fjord coupling through meteorological forcing and the existence of internal wave energy at a range of frequencies, which have implications for mixing and transport within the fjord.