Temporal-geographical meltwater influences on the North Atlantic Conveyor: Implications for the Younger Dryas
The temporal and geographical roles of meltwater discharge (from the Laurentide ice sheet) on North Atlantic Deep Water (NADW) production are investigated utilizing a global, realistic geometry, coupled climate model which does not require the use of flux adjustments. Model results suggest that preconditioning by meltwater discharge (to the Mississippi) prior to the Younger Dryas (YD) is capable of pushing NADW beyond the limit of its sustainability. The diversion of meltwater to the St. Lawrence then merely serves to completely inhibit NADW production. The modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the YD and is intimately linked to changes in NADW formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Oceans, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provide a mechanism for interbasin teleconnection. Although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model, in the absence of additional feedbacks an equilibrium without the presence of NADW is possible. The inclusion of the wind stress/speed feedback is found to significantly contribute to the resumption of NADW production, as suggested by previous studies. Contrary to these same studies, however, the coupled model indicates an advective spin-up timescale is required for resumption of NADW production and hence the termination of the modeled YD-like climate event (as opposed to a decadal-century timescale). The reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies, or the duration, strength, and geographical location of the imposed meltwater applied.