The generation of subsurface cyclones and jets through eddy–slope interaction

A mechanism for the generation of subsurface cyclones and jets when a warm ring smashes onto a continental slope and shelf is proposed based on the results of a primitive-equation three-dimensional numerical model. The warm ring initially ‘sits’ over a slope with an adjoining shelf in a periodic channel, and its subsequent evolution is examined. The ‘inviscid’ response is cyclonic ‘peeling-off’ of the on-slope portion of the warm ring. The cyclone propagates away (to the left looking on-slope) from the warm ring, and is bottom-intensified as well as slope-trapped (cross-slope scale ≈ Rossby radius). The near-surface flow ‘leaks’ further onto the shelf while subsurface currents are blocked by the slope. The ‘viscous’ response consists of the formation of a bottom boundary layer (BBL) with a temporally and spatially dependent displacement thickness. The BBL ‘lifts’ the strong along-slope (leftward) current or jet (speed >0.5 m s−1) away from the bottom. The jet, coupled with weak stratification within the BBL and convergence due to downwelling across the slope, becomes supercritical. Super-inertial disturbance in the form of a hydraulic jump or front, with strong upwelling and downwelling cell, and the jet, propagate along the slope as well as off-slope and upward into the water column. The upward propagation is halted at z≈ztrap when mixing smoothes out the ‘jump’ to an along-slope scale λtrap that allows the ambient jet to bend the propagation path horizontal. At this ‘matured’ stage, ztrap≈−250 m, λtrap≈50 km, and the jetʹs cross-slope and vertical scales are ≈30 km and 50 m, respectively. An example that illustrates the process under a more realistic setting in the Gulf of Mexico when the Loop Current impinges upon the west Florida slope is given. The phenomenon may be relevant to the recent oil industryʹs measurements in the Gulf, which at times indicate jets at z≈−150 m through −400 m over the slope.