On the dynamics of current jets trapped to the flanks of mid‐ocean ridges

Time‐mean abyssal current observations over the flanks of the East Pacific Rise (EPR) at 9–10°N and at the Juan de Fuca Ridge at 45°N document the occurrence of paired along‐ridge current jets that are trapped to the ridge flanks and sheared across the ridge in an anticyclonic sense. A coincident feature, where local hydrothermal discharge effects are not in play, is the upward bowing of isopycnals over ridge crests and isopycnals plunging into ridge flanks. It would be tempting to explain the jets primarily as geostrophic responses to the doming/plunging isopycnal distribution, though that should lead to the question as to how the isopycnal perturbations originate. A numerical model of time‐dependent flow on a cross‐ridge (x–z) transect, forced in a way to be consistent with a yearlong, hourly sampled record of currents measured at the EPR ridge crest, is used to investigate some of the underlying physics. It will be shown that the jets can arise from oscillatory flows via eddy‐momentum, eddy‐heat, and eddy‐salt fluxes that ultimately cause the isopycnals to dome over the ridge. As the probable offspring of velocity‐velocity and velocity‐density correlations that depend upon oscillatory motion, the jets are likely examples of stratified topographic flow rectification. An ancillary feature is a slight yearlong‐averaged downward current O (0.1–0.5 mm/s) over the EPR ridge crest that crosses the time‐mean, upward bowing isopycnals in a counter‐intuitive vertical direction.