Abstract
A necessary step before assessing the performance of decadal predictions is the evaluation of the processes that bring memory to the climate system, both in climate models and observations. These mechanisms are particularly relevant in the North Atlantic, where the ocean circulation, related to both the Subpolar Gyre and the Meridional Overturning Circulation (AMOC), is thought to be important for driving significant heat content anomalies. Recently, a rapid decline in observed densities in the deep Labrador Sea has pointed to an ongoing slowdown of the AMOC strength taking place since the mid 90s, a decline also hinted by in-situ observations from the RAPID array. This study explores the use of Labrador Sea densities as a precursor of the ocean circulation changes, by analysing a 300-year long simulation with the state-of-the-art coupled model HadGEM3-GC2. The major drivers of Labrador Sea density variability are investigated, and are characterised by three major contributions. First, the integrated effect of local surface heat fluxes, mainly driven by year-to-year changes in the North Atlantic Oscillation, which accounts for 62% of the total variance. Additionally, two multidecadal-to-centennial contributions from the Greenland–Scotland Ridge outflows are quantified; the first associated with freshwater exports via the East Greenland Current, and the second with density changes in the Denmark Strait Overflow. Finally, evidence is shown that decadal trends in Labrador Sea densities are followed by important atmospheric impacts. In particular, a positive winter NAO response appears to follow the negative Labrador Sea density trends, and provides a phase reversal mechanism.
Highlights
The North Atlantic ocean is a major source of decadal variability (e.g. Kerr 2000; Frankcombe et al 2008; Vianna and Menezes 2013) with reported widespread climate impacts (Knight et al 2006; Zhang and Delworth 2006; Sutton and Dong 2012)
To separate the Labrador Sea contributions to the Atlantic Meridional Overturning Circulation (AMOC) from those of the boundary currents our analysis is focused on the Interior Labrador Sea (ILS; red box in Fig. 1f), a region with no direct influence of the boundary currents
Water mass properties in the Labrador Sea exhibit large decadal variability, which is believed to play an important role in the decadal variability of the wider North Atlantic Ocean
Summary
The North Atlantic ocean is a major source of decadal variability (e.g. Kerr 2000; Frankcombe et al 2008; Vianna and Menezes 2013) with reported widespread climate impacts (Knight et al 2006; Zhang and Delworth 2006; Sutton and Dong 2012). Delworth et al 1993; Eden and Willebrand 2001) suggest that Labrador Sea waters can influence both the Atlantic Meridional Overturning Circulation (AMOC) and the subpolar gyre (SPG) strength and in this way affect decadal variability in the wider North Atlantic. A decline is reported in float-derived estimates of the subsurface circulation (Palter et al 2016) and altimetryinferred estimates of the upper subpolar gyre strength (Häkkinen and Rhines 2004; Hakkinen and Rhines 2009) Other observational records, such as the deep densities in the Labrador Sea or the wider subpolar gyre, can offer invaluable indirect information on changes in ocean dynamics, as suggested by model outputs (Robson et al 2014a; Hermanson et al 2014). It could give rise to a negative phase of the Atlantic Multidecadal Variability (AMV), which is an ocean state associated with important climate impacts
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