The lower limb or cold branch of the thermohaline circulation
in the North Atlantic Ocean is the DWBC. It transports newly
formed water masses from high northern latitudes first westward
[ McCartney, 1992], and then southward as a narrow intense
subsurface flow along the continental slope. For dynamic reasons
the southward flow of these high latitude waters is concentrated
along the western boundary. For a discussion of the dynamic
aspects of the DWBC see the review by Hogg and Johns in this
volume. The position of the DWBC up and down the continental slope
translates with time, possibly in response to changes in the source
waters [ Pickart, 1992b]. The existence of the DWBC in the
Atlantic was first shown by Wüst [1935], further
justified by Stommel [1958], and directly observed using
floats by Swallow and Worthington [1961]. The DWBC volume
transport was recently estimated to be about 20% that of the Gulf
Stream or 13 Sv (1 Sv = 10
m
/s) in the subpolar North
Atlantic [e.g., McCartney and Talley, 1984; Schmitz and
McCartney, 1993]. As the DWBC carries the products of the
newly convected water from the high northern latitudes southward,
there is mixing [e.g., Armi and Williams, 1991], and
advection in deep circulation gyres [e.g., Olson et al.,
1986; Hogg, 1983; Lee et al., 1990; Leaman and
Harris, 1990; Johns et al., 1993; Schmitz and
McCartney, 1993; Reid, 1994]. Some of these waters
eventually flow around Antarctica, and into the Indian and the
Pacific oceans, which takes about five hundred years [ Stuiver
et al., 1983]. The DWBC plays an important role in both the
global water mass distributions and oceanic heat flux.