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1. Introduction
Midlatitude atmospheric variability is known to strongly influence the underlying ocean, in particular by modulating surface heat fluxes and wind-induced Ekman circulations. The atmospheric influence on the North Atlantic Ocean is often viewed in terms of the North Atlantic Oscillation (NAO), which is strongly associated with a tripole pattern in sea surface temperatures (Bjerknes 1964; Visbeck et al. 2003; Marshall et al. 2001a; Eden and Willebrand 2001). The imprint of this atmospheric forcing associated with the NAO involves both local and far-field responses affecting the ocean heat storage over the North Atlantic. Anomalies in air-sea heat flux drive convection and interannual changes in local heat content over the subpolar gyre (Visbeck et al. 2003; Grist et al. 2010), while changes in wind stress drive variations in subtropical heat content (Lozier et al. 2008; Williams et al. 2014) and intergyre transfers between the subtropical and subpolar gyres (Marshall et al. 2001b). The combined effect of wind stress and air-sea buoyancy flux changes drive variations in the meridional overturning (Lozier et al. 2010; Robson et al. 2012), which in turn alters the gyre-scale convergence in heat transport (Williams et al. 2014; R. G. Williams et al. 2015) and controls multiyear and decadal changes in ocean heat content.
The NAO is a statistical measure for the state of the atmosphere and is typically defined from the mean sea level pressure using principal component analysis or a simple point difference (Hurrell and Deser 2010). As such, the NAO only empirically relates to the underlying atmospheric phenomena and may be affected by any circulation that projects onto its spatial pattern (Johnson et al. 2008). However, the majority of the variance of the NAO is known to represent variations of the North Atlantic eddy-driven jet stream (Thompson et al. 2003). Changes in both the strength and the latitude of the jet project onto the NAO, so that a positive NAO may indicate a strengthening, or a northward shift of the jet, or both (Woollings et al. 2010). The implications of the NAO and other weather regimes, including atmospheric blocking, on the surface ocean have been investigated using nonlinear, regime-based methods (Cassou et al. 2004, 2011; Barrier et al. 2014).
Although combined in the NAO, there is evidence that the...