Gravity Currents Facilitate Formation of High‐Frequency Internal Solitons and Bores at the Base of the Fraser Delta in the Southern Strait of Georgia

Abstract

AbstractWe use velocity and acoustic backscatter records from downward looking acoustic Doppler current profilers moored 47 m above the seafloor, 20 km apart in water depths of 100 and 240 m to examine internal wave dynamics near the seabed in the southern Strait of Georgia. The observations reveal that the lower layer of the strait becomes a seasonal (wintertime) waveguide for energetic internal “solibores”—internal bores trailed by large‐amplitude internal solitons—that propagate up the flank of the Fraser Delta. The rotor‐like bores have shoreward bottom velocities of 0.2 m s−1 and vertical velocities of 0.05 m s−1; the trailing wave trains attain amplitudes of tens of meters, periods as short as 10 min and durations of several hours. Establishment of the seabed waveguide required to support the high‐frequency internal motions begins in summer, when gravity currents originating from the shallow (~100 m) Skipjack‐Java sill in neighboring Boundary Pass, descend deep into the strait at monthly intervals. During the ensuing winter, lower‐density gravity currents form over the sill and subsequently override the denser bottom layer in the strait, increasing the buoyancy frequency of the guide. Based on the direction of solibore propagation, we speculate that the winter gravity currents also initiate generation of the internal waves during their descent over the rapidly deepening seafloor leading to the central strait. We further propose that the solibores are contributing to the formation and/or shoreward propagation of the O(10) m‐high sediment waves found on the delta foreslope.