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Linear and Nonlinear Stratified Spindown over Sloping Topography

Jessica A. Benthuysen, Ph.D., 2010
Leif Thomas, Stanford University and Steve Lentz, Advisors

In a stratified rotating fluid, frictionally driven circulations couple with the
buoyancy field over sloping topography. The impact of this coupling on the vertical circulation and the flow evolution is examined. Over a stratified slope, linear spindown of a geostrophic along‐isobath flow induces cross‐isobath Ekman flows. Ekman advection of buoyancy weakens the vertical circulation and slows spindown. Upslope (downslope) Ekman flows tend to inject (remove) potential vorticity into (from) the ocean. Nonlinear advection of momentum and buoyancy are examined in setting asymmetries in the vertical circulation and vertical relative vorticity field. During homogeneous spindown over a flat bottom, momentum advection weakens Ekman pumping and strengthens Ekman suction, while cyclonic vorticity decays faster than anticyclonic vorticity. During nonlinear stratified spindown over a slope, nonlinear advection of buoyancy enhances the asymmetry in Ekman pumping and suction, whereas anticyclonic vorticity can decay faster than cyclonic vorticity. During the adjustment of a spatially uniform geostrophic current over a shelfbreak, coupling between the Ekman flow and the buoyancy field generates Ekman pumping near the shelfbreak and forms a jet. The upwelling strength and length scale and the timescale for jet formation are considered. The results are applied to the Middle Atlantic Bight shelfbreak.