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Cross-frontal exchange associated with meanders

 

D. Randolph Watts

University of Rhode Island

 

Observations in the Gulf Stream and the Subantarctic Front (SAF) south of Australia have revealed strong dynamic linkage between near-surface and abyssal mesoscale eddy processes, and substantial cross-frontal motions associated with the growth of meanders. Vertical coupling is evident in the observed daily maps of temperature, horizontal current, and dynamic pressure.

Continuity of the vertical velocity field requires dynamical linkage via vortex stretching exerted by each layer upon the other. When steep upper layer troughs develop jointly with intense deep cyclones, the pressure-centers are observed to be offset vertically with a tilt opposite to the basic current shear --- a signature of baroclinic instability [Shay et al., 1995; Watts et al., 1995]. Where abyssal eddies provide a component of flow normal to the upper baroclinic front, this depth-independent reference turns the currents with depth. The deep eddies that are spun up produce cross-frontal flow along the sloping isopycnal surfaces, and hence drive a significant vertical component of flow at intermediate levels [Lindstrom et al., 1996].

Cross-frontal transports up to 5 Sv have been diagnosed in steep meanders of the Gulf Stream, and up to 1 Sv in a steep meander of the SAF. Much of this transport may oscillate and return in opposite phases of successive meanders; however, isopycnal float observations have demonstrated that some parcels cross out of the front at intermediate levels. Thus, summed along the whole Gulf Stream front or along the circumpolar SAF, this mesoscale-driven mechanism of cross-frontal exchange is estimated to produce net fluxes comparable in size to ocean-wide wind-driven and thermohaline-driven exchanges. It is suggested that substantial amounts of North Pacific Intermediate Water may cross the Kuroshio front by this process.

 

 

 

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