The entrainment and homogenization of tracers within the cyclonic gulf stream recirculation gyre

Abstract

The various distributions of tracer associated with the Northern Recirculation Gyre of the Gulf Stream (NRG) are studied to try to obtain information about the flow. An advective-diffusive numerical model is implemented to aid in the investigation. The model is composed of a gyre adjacent to a boundary current in which a source of tracer is specified at the upstream edge of the current. This set up attempts to simulate the lateral transfer of properties from the Deep Western Boundary Current (DWBC) to the NRG in the region where the two flows are in close contact west of the Grand Banks. The results of the model are analyzed in some detail. Tracer is entrained into the gyre as a plume which extends from the boundary current and spirals across streamlines toward the gyre center. The maintenance of the spiral during spin-up and its relationship to the occurrence of homogenization at steady state is examined. An asymmetry in the spiral exists due to the ellipticity of the gyre, which also effects homogenization. The anomalous properties that are fluxed into the NRG include salt, oxygen, and freon. These particular tracers are independent from each other, the former two because they are characterized by different vertical profiles in the deep layer. This results in a decay of oxygen but not salt, due to the presence of vertical mixing as discussed by Hogg et al. (1986, Deep-Sea Research, 33, 1139-1165). Their analysis is expanded upon here. The effect of vertical mixing on the gyre/boundary current system is examined within the context of the numerical model. Results are applied to recently collected water sample data from the region which leads to an estimate of the lateral and vertical eddy diffusion coefficients and an estimate of the amount of oxygen in the NRG that has diffused from the DWBC. The accumulation of freon within the NRG is considered in addition to salt and oxygen. Appreciable levels of freon have been present in the ocean only since 1950, and the atmospheric source functions have been increasing steadily since then. A simple overflow model is presented of the manner in which freon may be stirred in the Norwegian-Greenland basin prior to overflowing and entering the DWBC. Once in the boundary current the concentrations are diluted by way of mixing with surrounding water. Two different schemes are considered in which the immediate surrounding water accumulates a substantial amount of freon as time progresses. These models suggest that the freon-11:freon-12 ratio may not be a conserved quantity for the water in the core of the DWBC. It is found that the level of freon in the NRG is barely above the existing background level.