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.
