Circulation and larval distribution in internal tidal bore warm fronts

Internal tidal bore warm fronts were observed during the summer of 1996 off the coast of Southern California. Warm bore fronts had concentrating currents resulting from high-frequency internal motions and from a larger two-way flow; the two-way flow featured surface currents onshore and bottom currents offshore. A sharp thermocline depression and high-frequency, large-amplitude internal motions followed the leading edge of the bore, with down-welling currents on the trailing side of the crest of the nonlinear internal waves and upwelling currents in front of the crest. Warm bores propagated onshore with a propagation speed, c, that ranged from 10.6 to 19.6 cm s21, while time-averaged frontal currents, ū, varied from 11.2 to 17.6 cm s21 in the shallowest bin. In one out of three cases u ̄. c, which implied that there were faster currents than the rate of advance of the front and which implied that the origin of surface frontal material is behind the front, not in front of it. Three invertebrate larval taxa were found at all sites across fronts, but only two intertidal barnacles, Pollicipes polymerus and Chthamalus spp., were con-centrated at the front’s surface, while the subtidal bryozoan Membranipora spp. was not. Frontal Pollicipes were more concentrated than were Chthamalus. The frontal downwelling currents observed suggested that concentrated larvae would have to swim upward in order to maintain depth. Pollicipes were abundant on the offshore warm side of the fronts but were absent or rare on the onshore colder side, suggesting that the origin of frontal Pollicipes wa