DYNAMICS OF THE GULF-STREAM DEEP WESTERN BOUNDARY CURRENT CROSSOVER .2. LOW-FREQUENCY INTERNAL OSCILLATIONS

A low-frequency oscillation in the Gulf Stream/deep western boundary c urrent (DWBC) system is identified and its influences on several impor tant aspects of the basin-scale circulation are investigated. An eddy- resolving regional primitive equation model is used to demonstrate tha t feedbacks between the Gulf Stream, with its associated northern and southern recirculation gyres, and the upper core of the DWBC can lead to self-sustaining large amplitude internal oscillations of roughly de cadal frequency. The oscillator cycle is described as follows: The upp er core of the DWBC is entrained under the Gulf Stream through interac tion with the eddy-driven northern and southern recirculation gyres, a s described in Part I of this study. Once entrained, the low potential vorticity DWBC water stabilizes the Gulf Stream and suppresses the ed dy fluxes that maintained the interior recirculation gyres. This cause s the upper DWBC to switch to a southward path along the western bound ary, thus removing the source of the stabilizing low potential vortici ty water to the Gulf Stream. The Gulf Stream quickly returns to its un stable state and the resulting eddy fluxes spin up the northern and so uthern recirculation gyres. At this point, the upper DWBC is reentrain ed and the cycle begins again. The frequency and amplitude of the osci llations are controlled by the efficiency of the entrainment mechanism , as demonstrated by its sensitivity to variations in the model forcin g parameters. The oscillation strongly influences the penetration scal e of the Gulf Stream and distribution of eddy variability, the separat ion latitude of the Gulf Stream, the effective age of the DWBC south o f the crossover, and the pathways of the upper DWBC. The implications of such an oscillation on observing and modeling the thermohaline circ ulation an discussed.