Transformation and Fate of Overflows in the Northern North Atlantic

The largest full-depth changes in the modern instrumented oceanographic record have taken place in the Labrador Basin of the northwest Atlantic over the last 4 decades. The extreme amplitude of anomalous conditions there and the importance of their claimed effects for the thermohaline circulation and for climate (e.g. Bryden et al. 2005) justify attempts to identify the origin of change throughout the watercolumn of the subpolar Atlantic. At depths in the Labrador Basin greater than the limits of open-ocean deep convection (2,300 m or so), change is necessarily imported to the Basin by the two main dense water overflows that cross the Greenland–Scotland Ridge via the Denmark Strait and Faroe–Shetland Channel. Each of the constituent watermasses that form these overflows (see, for example, Rudels et al. 2002) will carry with them the imprint of time-varying climatic forcing in their source regions and of modifications en route, and their properties will also be subject to alteration by the processes of horizontal and vertical exchange from their spillways to the Labrador Basin. The purpose of this chapter is to identify from the hydrographic record those locations that are of primary importance for the transfer of ocean climate ‘signals’ into and between the two spreading overflow plumes, and if possible to trace the influence of these changes downstream to the Newfoundland Basin and beyond in the Deep Western Boundary Current