Dynamics of the Gulf Stream/Deep Western Boundary Current Crossover. Part II: Low-Frequency Internal Oscillations

Abstract

A low-frequency oscillation in the Gulf Stream/deep western boundary current (DWBC) system is identified and its influences on several important aspects of the basin-scale circulation are investigated. An eddy-resolving regional primitive equation model is used to demonstrate that 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 decadal frequency. The oscillator cycle is described as follows: The upper core of the DWBC is entrained under the Gulf Stream through interaction with the eddy-driven northern and southern recirculation gyres, as described in Part I of this study. Once entrained, the low potential vorticity DWBC water stabilizes the Gulf Stream and suppresses the eddy fluxes that maintained the interior recirculation gyres. This causes the upper DWBC to switch to a southward path along the western boundary, thus removing the source of the stabilizing low potential vorticity water to the Gulf Stream. The Gulf Stream quickly returns to its unstable state and the resulting eddy fluxes spin up the northern and southern recirculation gyres. At this point, the upper DWBC is reentrained and the cycle begins again. The frequency and amplitude of the oscillations are controlled by the efficiency of the entrainment mechanism, as demonstrated by its sensitivity to variations in the model forcing parameters. The oscillation strongly influences the penetration scale of the Gulf Stream and distribution of eddy variability, the separation latitude of the Gulf Stream, the effective age of the DWBC south of the crossover, and the pathways of the upper DWBC. The implications of such an oscillation on observing and modeling the thermohaline circulation are discussed.