Propagation of Tracer Signals in Boundary Currents

The propagation of a range of tracer signals in a simple model of the deep western boundary current is examined. Analytical expressions are derived in certain limits for the transit-time distributions and the propagation times (tracer ages) of tracers with exponentially growing or periodic concentration histories at the boundary current’s origin. If mixing between the boundary current and the surrounding ocean is either very slow or very rapid, then all tracer signals propagate at the same rate. In contrast, for intermediate mixing rates tracer ages generally depend on the history of the tracer variations at the origin and range from the advective time along the current to the much larger mean age. Close agreement of the model with chlorofluorocarbon (CFC) and tritium observations in the North Atlantic deep western boundary current (DWBC) is obtained when the model is in the intermediate mixing regime, with current speed around 5 cm s 1 and mixing time scale around 1 yr. In this regime anomalies in temperature and salinity of decadal or shorter period will propagate downstream at roughly the current speed, which is much faster than the spreading rate inferred from CFC or tritium–helium ages (approximately 5 cm s 1 as compared with 2 cm s ). This rapid propagation of anomalies is consistent with observations in the subpolar DWBC, but is at odds with inferences from measurements in the tropical DWBC. This suggests that observed tropical temperature and salinity anomalies are not simply propagated signals from the north. The sensitivity of the tracer spreading rates to tracer and mixing time scales in the model suggests that tight constraints on the flow and transport in real DWBCs may be obtained from simultaneous measurements of several different tracers—in particular, hydrographic anomalies and steadily increasing transient tracers.