317 (-87%) 187 (-62%)

3840 (+159%) 3461 (+75%)

4157 (+3%) 3648 (+48%)

that control the interface between the freshwater outflows and the Atlantic circulation. East of Greenland, for example, predictive analysis based on the historical record has provided insight into the likelihood of future direct effects on the strength of overflow through Denmark Strait. Recognising that it is the density contrast across the Denmark Strait sill that drives the overflow and noting that both overflows have undergone a remarkably rapid and remarkably steady freshening over the past four decades (Dickson et al. 2002), Curry and Mauritzen (2005) use Whitehead's (1998) hydraulic equation to ask how much more fresh water would have to be added to the western parts of the Nordic seas to produce significant slowdown. They find that it's not going to happen anytime soon:-

At the observed rate, it would take about a Century to accumulate enough freshwater (e.g.

9000 km3) to substantially affect the ocean exchanges across the Greenland-Scotland

Ridge, and nearly two Centuries of continuous dilution to stop them. In this context, abrupt changes in ocean circulation do not appear imminent.

The fact that the freshening trend of both overflows at the sill has slowed to a stop over the last 10 years (see Yashayaev and Dickson 2007) has merely reinforced this conclusion.

West of Greenland, results remain much more equivocal regarding the local-to-regional impact of an increased flux of freshwater through the CAA. Though the relatively coarse global models of Goosse et al. (1997) and Wadley and Bigg (2002) find decreases of 10% and 35% (respectively) in the strength of the overturning circulation between closing and opening the CAA, Myers (2005) has subsequently used a high resolution regional model to suggest that very little (6-8%) of the freshwater exported from the Canadian Arctic gets taken up in the Labrador Sea Water of his model. In general terms then, it remains an open question as to whether a future increase in the freshwater outflow through Davis Strait would spread across the surface or skirt around the boundary of the Labrador Basin; a more complete observing system south of Davis Strait will be necessary to developing that understanding.

In summary then, the watercolumn of the Labrador Sea is of global climatic importance, acting as the receiving volume for time-varying inputs of fresh- and other watermasses from Northern Seas which are then stored, recirculated, transformed and discharged to modulate the abyssal limb of the Atlantic Meridional Overturning Circulation (AMOC). The extreme amplitude of anomalous conditions throughout the watercolumn of the Labrador Sea over the past four decades and the importance of their claimed effects for the thermohaline circulation and for climate justify a sustained ocean-observing effort to understand and test the behaviour of this system in climate models. Here we have placed emphasis on monitoring the changing balance of freshwater fluxes east and west of Greenland, and on investigating how each of these main freshwater outflows interfaces with the watercolumn of the NW Atlantic. In practice of course, each of the watermasses recruiting to the Labrador Basin will carry with them the imprint of time-varying climatic forcing in their source regions and of modifications en route, and their properties (volume, temperature, salinity, density, tracer-loading) will also be subject to alteration by the processes of horizontal and vertical exchange within the Labrador Basin itself. The key issue for climate may lie not so much in describing and attributing the diverse sources of change in this vertical stack of watermasses but in understanding whether and to what extent they interact and the effect of such interactions on deep ocean hydrography and circulation.

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