A Damped Decadal Oscillation in the North Atlantic Climate System

Abstract

A simple stochastic atmosphere model is coupled to a realistic model of the North Atlantic Ocean. A northsouth SST dipole, with its zero line centered along the subpolar front, influences the atmosphere model, which in turn forces the ocean model by surface fluxes related to the North Atlantic Oscillation. The coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing. The oscillation consists of a fast wind-driven, positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies. The positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere. Using a novel diagnosing technique, it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation, and to show the important role played by anomalous geostrophic advection in sustaining the oscillation.

A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

“…10 shows that the negative LSW thickness anomaly in the western subpolar region up to 1976 is related to an anomalously weak MOC, while the positive LSW thickness anomaly after 1976 is related to an anomalously strong MOC. This is in agreement with LSW volume changing the zonal density gradient and, in turn, the MOC intensity (Eden and Greatbatch, 2003;Böning et al, 2006). We study these variations of the MOC intensity in the North Atlantic analysing circulation anomalies of the MOC stream function (Fig.…”

“…11): The MOC is divided by the SPF in two regimes, with the regime in the subpolar region showing larger variability. A similar MOC anomaly spanning comparable latitudes and depths inside the subpolar region is found by Greatbatch, 2003, andby Gulev et al, 2003, while multi-decadal gyre-specific changes of the MOC have been simulated by Lozier et al, 2010. The MOC in the subpolar region is at maximum in 1986 as also indicated by our MOC index at 47 • N, while the MOC intensity-changes in the subtropics are delayed approximately by five years, with maximum in 1991.…”

“…4d) is characterised by a cyclonic anomaly overlying the SPF, while a SPF north of its climatological position (between 1967 and 1995) is characterised by an anticyclonic anomaly. This circulation anomaly is similar to the inter-gyre gyre proposed by Marshall et al, 2001, and discussed by Eden and Greatbatch, 2003, and, therefore, we will also use the term inter-gyre gyre here. Additionally, we note that our circulation anomalies propagate from the north towards the south.…”

“…In response to a decreasing NAO, the isoline of zero WSC displaces southward, yielding a cyclonic circulation anomaly between the SPG and subtropical gyre (STG): an inter-gyre gyre (Marshall et al, 2001). Eden and Greatbatch, 2003, agree on the influence of the WSC on the formation of the inter-gyre gyre and state that it dominates the variability at interannual time scales. This circulation anomaly advects more cold subpolar water into the subtropical region in the Newfoundland Basin (NFB), displacing the SPF southwards, and more warm subtropical water into the subpolar region in the eastern North Atlantic, displacing the SPF north-westwards (Eden and Willebrand, 2001).…”

“…A few years after a change to positive NAO, SPG (Eden and Willebrand, 2001) and MOC (Eden and Greatbatch, 2003;Brauch and Gerdes, 2005;Böning et al, 2006) intensify, indicating that both are nearly in phase (Frankignoul et al, 2010). Figure 3 shows the low-pass filtered observed (panel a) and modelled (panel b) salinity anomalies at 500 m depth for selected years, which are representative for the upper layer of the SPG.…”