2.4.1. Regional studies of the DWBC.

The earlier studies showed an equatorward decrease in DWBC tracer concentrations, which is consistent with a Northern Hemisphere source. Jenkins and Rhines [1980] were the first to document the equatorward spreading of the tritium transient into the subtropics via the DWBC. They showed a high tritium plume of LNADW surrounding the Blake Bahama Outer Ridge, which is at most fifteen years old. By comparison, Olson et al. [1986] found the LNADW is not topographically confined east of the Bahamas. The spreading of the tracer core, when combined with maps of dynamic topography, suggested to Olson et al. [1986] that there is flow into the interior from the DWBC. The conclusion is consistent with a deep recirculation gyre offshore of the Bahamas, which is evident from float tracks [ Riser et al., 1978; Leaman et al., 1994], and recent direct velocity measurements [ Lee et al., 1990; Leaman and Harris, 1990].

Fine and Molinari [1988] identified two tracer maxima in a series of sections occupied between Abaco (26.5N) and Barbados (13N). These data were used to establish the basic two CFC core structure and continuity of the DWBC (see Figure 3). They identified the shallow tracer core as an additional water mass component of the DWBC at 5.0-3.9C (centered at 1200 m). The shallow core was found at progressively lower temperatures in the southward direction, because of the intrusion by overlying Antarctic Intermediate Water. The deep core at 2.5- 1.85C (centered at 3500 m) is LNADW. Below the SLSW at 3.9-3.2C is the LSW, which has intermediate CFC concentrations. Between the two high CFC cores, there is a high velocity core at Abaco [ Lee et al., 1990; Leaman and Harris, 1990]. The DWBC was located on the shoreward parts of the sections usually within 60 km of the boundary. Fine and Molinari [1988] used the tracer maxima to differentiate the DWBC from other high velocity areas offshore. They found differences in CFC structure and transports amongst the sections, which they attributed to exchange of water between the DWBC and the interior.