A Role for Gravity Currents in Cross‐Sill Estuarine Exchange and Subsurface Inflow to the Southern Strait of Georgia

AbstractMultiyear current meter records from five sites are used to examine subsurface renewal processes in the southern Strait of Georgia. The lower layer inflow of dense oceanic‐derived water over the ~100‐m deep sill in Boundary Pass near the entrance to the strait reaches peak velocities during spring tides and rarely reverses direction. Corresponding changes in water density over the sill are inversely correlated with inflow velocity and, it is assumed, with temporal (t) variations in bottom layer thickness, h(t), arising from the fortnightly variations in tidal‐mixing. The dense bottom water crossing the sill transforms into a gravity current whose velocity is dependent on both h and sill depth, H. Gravity flow velocities, which peak at h(t)/H ~ 0.3 and weaken for h → 0 (neap tides) and h → H (spring tides), modulate the cross‐sill exchange with the strait. The density anomaly, Δρ, of the gravity flow determines its terminal depth and along‐strait extent. Lower‐density gravity flows generated during spring tides separate from the sloping bottom of the southern strait, but the higher‐density flows formed during neap tides can penetrate over 100 km northward to the deep central basin. The most extensive bottom intrusions occur monthly near neap tides in summer, when the underlying estuarine circulation delivers dense upwelled water from the coast to the sill. These deep inflows are accompanied by strong middepth outflow. There are typically three to five monthly replacement episodes each summer, but fewer during strong El Niño years due to reduced coastal upwelling.