Eddy-Mean Flow Interaction in the Gulf Stream at 68°W. Part II: Eddy Forcing on the Time-Mean Flow

Abstract

Data front the SYNOP 68°W current meter array show a trough pattern in the 26-month mean Gulf Stream path at 68°W and a cyclone in the mean deep flow. In order to determine whether eddies are controlling the speed and direction of the time-mean Gulf Sream flow at 68°W a theory is developed that extends the Eliassen-Palm theory for zonal-mean flows to time-mean flows. The eddy force is evaluated as the three-dimensional divergence of an eddy stress tensor and is equivalent to the eddy advection of momentum Fmom and the rotated divergent eddy stretching potential vorticity flux Ftherm. The study region at 68°W is characterized by large amplitude trough formation events. It is shown that Ftherm is associated with the growth stage of the eddy trough formation events and tends to turn the upper-level time-mean flow southward into the trough pattern and accelerate and ?spin up? the deep-layer cyclone; Fmom has a vertically coherent pattern associated with the decay stage of the trough formation events and tends to offset Ftherm particularly in the deep layer.