For the Observational Community

Despite major advances in observing and simulating the system, we remain undecided on many of the most basic issues that link change in our northern seas to climate. For example, while there is agreement that an increasing freshwater flux through Fram Strait to the North Atlantic is likely to be of climatic significance, we remain uncertain as to whether the impact on climate will result from local effects on overflow transport (e.g. from the changing density contrast across the Denmark Strait sill; Curry and Mauritzen 2005), from the regional effect of capping the water column of the NW Atlantic (leading to a reduction in vertical mixing, water mass transformation, and production of North Atlantic Deep Water), or from global-scale changes in the Ocean's thermohaline fields and circulation arising from an acceleration of the Global Water Cycle (Curry et al. 2003). Equally, we have yet to reconcile the subtleties of cause and effect revealed in our simulations of Arctic-Atlantic exchanges; for example, the finding by Oka and Hasumi (2006) that the deep-convective seesaw between the Labrador and Greenland Seas (Dickson et al. 1996) is controlled by changes in the freshwater transport through Denmark Strait, with the finding of Wu and Wood (2007, submitted) that the freshening recently observed in subpolar seas may ultimately be triggered by Labrador Sea deep convection. Despite this, there would probably be general acceptance of the conclusion of Jungclaus et al. (2005; from model experiments using ECHAM5 and the MPI-OM), that while the strength of the (Atlantic) overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, ... the variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport.

The significance of Fig. 12.5b is that it leads us into a complex of fairly specific questions relevant to the latitudinal exchange of freshwater with the Arctic through subarctic seas, and the way it might interface with the watercolumn of the NW Atlantic.

It suggests, fundamentally, that the impact on the AMOC will depend on the extent to which the freshwater efflux from the Arctic will be spread to depth on its arrival in the NW Atlantic. We already know from half a Century of repeat hydrography that the system of dense-water overflows from the Nordic seas has been the vehicle for the freshening of the deep and abyssal layers of the Labrador Basin, below the limits of convection (2,300 m or so) since the mid-1960s (Dickson et al. 2002). And this observation lends point to the more-specific questions posed by Fig. 12.5b: whether any future increase in the freshwater outflow from the Arctic is likely to be incorporated into the overflow system, or (effectively the same thing) whether any future increase of the freshwater efflux is likely to pass to the west or to the east of Greenland.

We know of only one model study that currently makes that prediction. Recent coupled experiments by Helmuth Haak and the MPI Group using ECHAM 5 and the MPI-OM (1.5 deg; l 40) suggest that although the freshwater flux is expected to increase both east and west of Greenland, the loss of the sea-ice component (which currently dominates the flux through Fram Strait) suggests we should expect a much greater total increase through the CAA by 2070-2099 (+48%) than through Fram Strait (+3% only; see Table 12.1). Such a stark shift in the balance of outflow should be evident even in intermittent observations, and the validation of this prediction should be one general task of a future observing system.

Both east and west of Greenland, the historical hydrographic record and some novel observing techniques are beginning to identify the more-localised processes

Table 12.1 Simulated Arctic Ocean freshwater flux (km3 year-1) through Fram Strait and the Canadian Arctic Archipelago in 2070-2099 compared with 1860-1999. Results of coupled experiments using ECHAM 5 and the MPI-OM (1.5°; l 40) (Adapted from Haak et al. 2005. See also Koenigk et al., Chapter 8, this volume)

Table 12.1 Simulated Arctic Ocean freshwater flux (km3 year-1) through Fram Strait and the Canadian Arctic Archipelago in 2070-2099 compared with 1860-1999. Results of coupled experiments using ECHAM 5 and the MPI-OM (1.5°; l 40) (Adapted from Haak et al. 2005. See also Koenigk et al., Chapter 8, this volume)

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