In the Atlantic, the meridional overturning circulation (MOC) is characterized by northward flow of light surface waters accompanied by a transport of heat and salt from the subtropics to the subpolar regions and farther into the Labrador Sea, Nordic Seas, and Arctic Ocean: the latter in common termed the Arctic Mediterranean. This flow of Atlantic Water keeps large parts of the region ice-free, and in combination with a gradual release of heat to the atmosphere it contributes to maintain the relatively mild winters of Europe [Seager et al., 2002]. In particular, during winter months, the cooling leaves the surface waters relatively salty and dense and allows sinking of surface waters to occur at a few locations: in the Greenland Sea, the Labrador Sea, and possibly in the Irminger Sea associated with strong wind events [Pickart et al., 2003]. The sinking water forms the southward flowing branch of the MOC at depth and spreads throughout the World Ocean. Whereas the Labrador Sea is directly connected to the North Atlantic, the Nordic Seas and Arctic Ocean are separated from the North Atlantic by the Greenland–Scotland Ridge. Here southward flow at depth is confined to a very limited number of passages, predominantly the Denmark Strait and the Faroe Bank Channel downstream of the Faroe Shetland Channel [Blindheim and Østerhus, 2005].
 In a future climate forced by increased concentrations of radiative active gasses emitted through human activities, increased atmospheric temperatures are expected to result in an enhanced hydrological cycle, i.e., more evaporation at low latitudes and more precipitation at high latitudes. This increased precipitation in the high-latitude sinking areas will imply a dilution and freshening of the surface waters, which may act to weaken the Atlantic MOC and the associated meridional heat transport [Hansen et al., 2004], as also shown by a number of studies with coupled climate models [e.g., Gregory et al., 2005].
 This scenario finds support in the increasing discharge from Siberian rivers [Peterson et al., 2002] in addition to an increasing number of studies reporting significant freshening of the major Atlantic water masses associated with the lower branch of the Atlantic MOC during the last four decades [Dickson et al., 2002, 2003; Curry et al., 2003; Curry and Mauritzen, 2005]. Also, long-term hydrographic measurements give evidence of a long-term decreasing trend in the interface level of dense water in the Nordic Seas with possible impacts on the southward flow of dense water with origin in the Nordic Seas [Hansen et al., 2001]. However, the sensitivity of the deep overflows to reservoir changes is possibly relatively weak [Curry and Mauritzen, 2005; Wilkenskjeld and Quadfasel, 2005]. In addition, observations of sea surface height indicate a weakening of the subpolar gyre over the past decade, which may not be attributed to local wind stress changes [Häkkinen and Rhines, 2004]. The decline in the subpolar gyre may also be connected to increasing salinities of the inflow branches of Atlantic Water to the Nordic Seas reported in the recent years [Mortensen and Valdimarsson, 1999; Hàtùn et al., 2005]. As seen, observed multidecadal freshening of the deep overturning water masses of the North Atlantic may be early imprints of global warming, and the observed patterns reflect the combined effect of changes in the intensity of the hydrological cycle and the strength of the Atlantic MOC. In fact, some indirect evidence of a possibly dramatic recession of the Atlantic MOC during the last decades has recently been deduced from historical hydrographic data [Bryden et al., 2005].
 The picture is complicated by the fact that the dominant mode of atmospheric variability of the North Atlantic region, the North Atlantic Oscillation (NAO), persisted in its negative phase during the 1960s but has since then systematically changed toward the positive phase with enhanced westerlies across the North Atlantic. As both model studies and theory suggest that the circulation of the North Atlantic and the ocean climate of the Nordic Seas are sensitive to the phase of the NAO (see reviews by Visbeck et al.  and Furevik and Nilsen ), changes in water mass characteristics and circulation observed during the last decades should be seen in light of this shift.
 It is presently not settled whether the shift in NAO is itself connected to the changed radiative forcing of the troposphere because of increased concentrations of greenhouse gasses or should be regarded as a natural climate fluctuation. Recently, Yin  identified a global warming signal with northward-shifted storm tracks that project onto the natural NAO pattern. Earlier, Shindell et al.  argued that the observed trend in NAO could be simulated using known external forcings. However, natural or anthropogenic in origin, it is essential to assess to what extend the known atmospheric evolution including the long-term shift in the NAO can account for the observed changes in ocean climate of the North Atlantic.
1.1. Ocean Hindcast
 Atmospheric imprints on recent changes of the ocean climate can be addressed using ocean hindcast simulations in which past oceanic dynamics and properties are simulated for a given time period using an ocean general circulation model (OGCM) constrained in part by historical atmospheric surface forcings. A number of such studies have been designed to reconstruct and assess the past natural variability and recent trends of the North Atlantic and the Arctic Mediterranean [Haak et al., 2003; Karcher et al., 2003; Nilsen et al., 2003; Zhang et al., 2004; Bentsen et al., 2004; Drange et al., 2005; Marsh et al., 2005; Gerdes et al., 2005]. The reconstruction or hindcast period is given by the availability of atmospheric forcing fields, usually obtained from data sets like the NCEP/NCAR reanalysis product available from 1948 onward [Kalnay et al., 1996; Kistler et al., 2001]. Intercomparison of model results [e.g., Drange et al., 2005] is complicated by major differences in the experimental design between studies, typically in the initial state for the hindcast obtained via a spin-up procedure of the model or in the use of restoring boundary conditions. Model spin-up period can vary from a few decades to a few centuries, largely dictated by the resolution of the OGCM or set by the scope of the study.
 Some restoring of model thermodynamic surface fields toward climatological values is most often needed to prevent model drift, in particular of salinity. Such relaxation is known to distort the free modes of internal ocean variability in the models by introducing an artificial feedback on salinity, and the actual configuration may be important for the result. State-of-the-art OGCM configurations can in some cases produce hindcast simulations without significant drift in upper ocean properties, avoiding the artificial feedback by applying seasonally resolved but annually repeated corrections of the surface salinity [Nilsen et al., 2003].
 Because of the potentially long memory of the ocean, hindcast studies must be regarded as a combined initial/boundary value problem. But while the boundary consists of the atmospheric forcing condition, the initial value, i.e., the complete oceanic state on, say, January 1948, is largely unknown. This is a principal problem with the hindcast methods, which must be properly dealt with. Usually, a probable initial ocean state is obtained from spin-up procedures that, as explained, vary significantly in complexity among studies. Most seek to precondition the ocean initial state for a smooth transition to the hindcast experiment where realistic forcing history is applied. Thus, the central idea behind most experimental designs is to reach a quasi-equilibrium state of the model ocean. Among the most straightforward designs are the use of climatological atmospheric forcing fields, possibly with the addition of some synoptic variability. Also used are designs where a single year or sequence of years is repeated a number of times. An example of a more advanced procedure is found by Bentsen et al.  where the spin-up simulation is initially forced by climatological atmospheric fields, followed by repetitions of daily reanalysis fields for a 5-year period central to the subsequent hindcast. It is difficult to evaluate the role of the exact spin-up procedure for the hindcast results, but regardless of the complexity, the central idea of an ocean in quasi-equilibrium can be questioned [e.g., Wunsch and Heimbach, 2006].
 Instead, several studies deal with the problem of the unknown ocean state and time history by applying an ensemble approach to explore the robustness of the model results to initial conditions. Yet, care must be exercised in order to obtain truly independent ocean initial states for the ensemble members considering the large decorrelation time of oceanic characteristics. For instance, if ensemble members are performed end-by-end, i.e., repeating the hindcast simulation using the same atmospheric forcing but using the end of the previous hindcast as initial condition for the next [e.g., Haak et al., 2003; Bentsen et al., 2004; Drange et al., 2005], the series of initial conditions hereby produced share the same forcing history and are therefore strictly speaking not independent. Also, one inevitably forces the ocean with an artificial periodicity (= the length of each hindcast simulation), which creates dependence between ocean states. At best, the computationally attractive end-by-end approach likely gives an underestimated indication of the spread of modeled variables because of internal ocean variability. At worst, it could by design invalidate significant sequences of the hindcast depending on the decorrelation timescale of the ocean and the tendency of exciting internal modes of variability in the particular OGCM configuration.
 The ensemble approach designed and applied in this study focuses on eliminating the role of initial ocean conditions on the reconstructed ocean climate history. In combination with a sufficiently, and unprecedented, large number of ensemble members, robust statistics for the evolution and characteristics of the major exchanges of the North Atlantic are produced. The selected approach builds upon an independent control simulation forced by atmospheric fields from randomly permuted years to ensure that the forcing exhibits a white spectrum without any preferred frequencies. From this, practically independent ocean states can be selected for the ensemble members, as will be described in detail later. The design includes a millennial scale spin-up period in order to reach an overall quasi-equilibrium of the model dynamics and basin scale water mass properties. This facilitates a discussion of interrelated changes in the large-scale Atlantic MOC, water mass properties, and key exchanges with the Nordic Seas and Labrador Sea rarely addressed in comparable hindcast studies.
 In the following section, given is a short description of the ocean model, the experimental design, and the characteristics of spin-up and control simulations. In Section 3, ensemble statistics are presented for the Atlantic MOC and Section 4 introduces the ensemble mean climatology of North Atlantic–Arctic Mediterranean exchanges. Long-term changes are discussed in Section 5 followed by the summary and conclusions in Section 6.