À2 , only about one tenth of the estimated mean surface mixed layer heat flux to the sea ice. It is thus concluded that the vertical transport of heat from the Atlantic Water in the central basin is unlikely to have a significant impact to the Canada Basin ocean surface heat budget. Icebreaker conductivity-temperature-depth data from the Beaufort Gyre Freshwater Experiment show that the staircase is absent at the basin periphery. Turbulent mixing that presumably disrupts the staircase might drive greater flux from the Atlantic Water at the basin boundaries and possibly dominate the regionally averaged heat flux.
Small changes in the ways that the ocean transports heat to the overlying ice cover could have a substantial effect on future changes in Arctic ice cover.
The Younger Dryas-the last major cold episode on Earth-is generally considered to have been triggered by a meltwater flood into the North Atlantic. The prevailing hypothesis, proposed by Broecker et al. [1989 Nature 341:318-321] more than two decades ago, suggests that an abrupt rerouting of Lake Agassiz overflow through the Great Lakes and St. Lawrence Valley inhibited deep water formation in the subpolar North Atlantic and weakened the strength of the Atlantic Meridional Overturning Circulation (AMOC). More recently, Tarasov and Peltier [2005 Nature 435:662-665] showed that meltwater could have discharged into the Arctic Ocean via the Mackenzie Valley ∼4,000 km northwest of the St. Lawrence outlet. Here we use a sophisticated, high-resolution, ocean sea-ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the North Atlantic. Unlike the hypothesis of Broecker et al., freshwater from the St. Lawrence Valley advects into the subtropical gyre ∼3,000 km south of the North Atlantic deep water formation regions and weakens the AMOC by <15%. In contrast, narrow coastal boundary currents efficiently deliver meltwater from the Mackenzie Valley to the deep water formation regions of the subpolar North Atlantic and weaken the AMOC by >30%. We conclude that meltwater discharge from the Arctic, rather than the St. Lawrence Valley, was more likely to have triggered the Younger Dryas cooling.abrupt climate change | climate modeling | paleoclimate T he sudden release of meltwater from glacially dammed lakes located along the southern margin of the Laurentide Ice Sheet (LIS) is frequently cited as the main trigger for the Younger Dryas (YD) (1, 2)-a 1,200-y-long cold episode that began 12.9 kya (3). The basic premise suggests that at the onset of this sudden climatic transition, glacial runoff switched from the Gulf of Mexico to a more northerly outlet to allow thousands of cubic kilometers of meltwater to rapidly drain into the North Atlantic (1, 4) ( Fig. 1). It was originally hypothesized by Broecker et al. (1) that the subsequent freshening of the subpolar North Atlantic suppressed the sinking limb of the Atlantic Meridional Overturning Circulation (AMOC) and reduced the northward transport of heat to the poles. As a result of a weakened AMOC, the relatively warm climate of the Allerød episode abruptly ended and the YD began. This original meltwater diversion hypothesis focused on the likelihood that Lake Agassiz supplied freshwater to the ocean through an "eastern outlet," allowing meltwater to enter the North Atlantic via the St. Lawrence Valley (1, 4, 5).Since then, several studies have questioned the St. Lawrence Valley as a feasible drainage route to the ocean. Using dinoflagellates to reconstruct sea surface salinity at the mouth of the Gulf of St. Lawrence, de Vernal et al. (6) were forced to reject an eastern route based on a lack of evidence that the surface waters in this region freshened at the onset of the YD. A subsequent search for a freshwater sig...
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