A tracer study of the deep Gulf Stream cyclonic recirculation

Abstract

An advective-diffusive numerical model is used to interpret water sample data from beneath the thermocline in the region of the Gulf Stream and deep western boundary current (DWBC), west of the Grand Banks. The nearly homogenized distributions of salinity and oxygen suggest that substantial cyclonic recirculation of the deep Gulf Stream comes in close contact with DWBC, which is the source of the tracers. The exchange of properties between the two flows is studied in order to relate tracer distributions to particular characteristics of the Gulf Stream gyre. Using a corresponding diffusion box model, it is shown that spin-up and the onset of homogenization in the gyre is controlled by diffusion from the boundary, and that the level of tracer in the gyre can be insensitive to the strength of lateral diffusivity (the salt case), but is strongly sensitive to vertical mixing (the oxygen case). The presence of vertical mixing, however, does not inhibit the occurrence of homogenization. The lateral (κ) and vertical (ν) diffusivities of the Gulf Stream gyre can be deduced by applying the model results to the observed oxygen distribution, and this leads to estimates of κ ∼ 10 6 cm 2 s −1. The tight lateral extent of the recirculation, as suggested by the model results, agrees well with existing current meter data from the region.