Variability of subtidal current structure in a fjord estuary: Puget Sound, Washington

Abstract

Simultaneous observations of currents and water properties were made in a section across East Passage in Puget Sound over a 31‐day period between March and April 1983. These observations, supplemented with CTD profiles, shore‐based wind data, and far‐field current measurements, were used to describe the cross‐channel variability of subtidal currents. The mean circulation consisted of seaward flow in the near surface and landward flow below 7 m, with a core of maximum velocity at mid‐depth. Volume transport was 22,000 m3/s. Topographic effects force most of the seaward compensating flow through an adjacent channel, creating a clockwise circulation around an intervening island. Empirical orthogonal function analysis was used to describe the spatial structure of current fluctuations. At subtidal time scales, three circulation modes are evident: (1) a near‐surface (upper 20 m), wind‐driven shear; (2) a near‐bottom layer (150–200 m) dominated by density currents that propagate up estuary after generation during neap tidal mixing at the entrance sill to Puget Sound; and (3) an intermediate layer (25–75 m) influenced by partial refluxing of Colvos Passage water and modulated at fortnightly periods by nonlinear mixed tides. These current observations are the first to show recirculation of seaward flowing water from Colvos Passage to landward flow in East Passage.