He (1974-88) data from
near Bermuda in the central Saragasso Sea. Tritium increased
significantly in the intermediate layer (1200-1400 m) in 1976
(twelve years into the major bomb transient) and in the deep layer
(2200-2500 m) in 1986 (twenty-two years). The increases were
interpreted as marking the staggered arrival of the bomb transient.
There is remarkable agreement between the tritium increases and the
tritium/
He ages of twelve and twenty-two years, respectively for
intermediate and deep waters. Jenkins [1994] attributed the
agreement to the small affect that non-linear mixing has on DWBC
ages. Furthermore, the interior ages are dominated by the DWBC.
He concluded that the tracer ages give a mean southward flow rate,
which reflects considerable exchange between the DWBC and the
interior recirculation gyres.
Abell et al. [1994] examined CFC and hydrographic data
from the DWBC offshore of Abaco (26.5
N) from five cruises
between 1986 and 1992. Between February 1989 and June 1990, CFC
concentrations in the SLSW and LSW did not significantly increase.
However, over this same period, there was a large 40% annual
increase in CFC concentration in the LNADW. This was in contrast
to the 11% annual increase in the LNADW between June 1990 and
August 1992. There was also a large increase in decay corrected
tritium at Abaco between the February 1989 and June 1990. The
large increase in both transient tracers was interpreted as
indicative of enhanced deep water convection at the source regions,
because starting in the early 1970s conditions in the high northern
latitudes are considered favorable for convective overturning. A
link back to the high latitude source regions of the early to mid-
1970s is provided by the large increases observed in the tracers at
Abaco in 1990. This time scale is consistent with the tracer ages
at Abaco of eighteen years, and it implies an effective spreading
rate of at most 2 cm/s, in agreement with the float data average
from the region [ Leaman et al., 1994].