The Absorption of Arctic Export Waters into the North Atlantic Current


Arctic Research Initiative
2009 Funded Project


The Labrador Current advects very cold, fresh arctic-origin water southward along the western subpolar continental shelf.  Because this is the main advective pathway for this water mass, the Labrador Current plays a critical role in determining where and how much freshwater enters the interior of the North Atlantic.  The freshwater transported by the Labrador Current is largely constrained to follow the boundary.  However, along its extended advection path portions of the flow are diverted offshore, resulting in the export of mass and freshwater from the boundary.   The most prominent of these export pathways is along the Grand Banks of Newfoundland in the southwest corner of the subpolar gyre.  Here, a significant portion of the Labrador Current turns offshore and back toward the north (retroflects), carrying most of the cold, fresh arctic water mass into the interior.  Immediately after it leaves the boundary the retroflected water encounters the North Atlantic Current, which is carrying a large volume of very salty water northward toward the interior of the subpolar gyre as a component of the Atlantic meridional overturning circulation.   The retroflection efficiently moves freshwater from the boundary into the interior, although the potential impact of this freshwater on the larger climate system depends on which processes are dominant in its interaction with the North Atlantic Current.  One possibility is that advection dominates and hence the freshwater is delivered intact into the central subpolar gyre where it may form a fresh buoyancy cap over convection regions.  Dire consequences have been described for such a scenario where the freshwater export increases with time, namely that shutdown of convection follows, with a collapse of the meridional overturning circulation.  However, it is more likely, based on our interpretation of the data, that mixing processes play a dominant role and that the retroflected water is absorbed by the North Atlantic Current.  This implies a more benign fate for the arctic water.  In a recent study, we examined the Labrador Current retroflection in a hydrographic climatology, describing its mean structure and examining its role in transporting freshwater offshore.  In this proposed study, we will build on these results making use of existing synoptic observations to investigate the processes by which the retroflected arctic waters, over a broader range of density classes, are mixed into the North Atlantic Current.  We will consider the mixing processes controlling the absorption of Labrador Current water into the North Atlantic Current and investigate the role of the Mann Eddy as a “blender” in this absorption process.  We anticipate that relative magnitudes of lateral mixing, winter convection, surface Ekman layer transport, Mann Eddy circulation strength, Gulf Stream Branch input, and variations in freshwater input will all contribute to varying degrees to the effectiveness of this absorption process.  The ultimate goal of this work is to determine the causes for the mean water mass distribution across the offshore limb of the Labrador Current retroflection, North Atlantic Current, and Mann Eddy and to diagnose their interannual co-evolution in terms of the variability of the water mass sources and absorption processes.  Understanding the fresh water absorption process will help in projecting the impacts of increasing arctic export on the north Atlantic overturning circulation.