Using simulations of the last millennium to understand climate variability seen in palaeo-observations: similar variation of Iceland–Scotland overflow strength and Atlantic Multidecadal Oscillation

Abstract

Abstract.A recent palaeo-reconstruction of the strength of the Iceland-Scotland overflow during the last 600 years suggests that its low-frequency variability exhibits strong similarity with palaeo-reconstructions of the Atlantic Multidecadal Oscillation (AMO). The underlying mechanism of the similar variation remains unclear, however, based on palaeo-reconstructions alone. In this study we use simulations of the last millennium driven by external forcing reconstructions with three coupled climate models in order to investigate possible mechanisms underlying the similar variation of Iceland-Scotland overflow strength and AMO index. Similar variation of the two time series is also largely found in the model simulations. Our analysis indicates that the basin-wide AMO index in the externally forced simulations is dominated by the low-latitude sea surface temperature (SST) variability and is not predominantly driven by variations in the strength of the Atlantic meridional overturning circulation (MOC). This result suggests that a large-scale link through the strength of the MOC is not sufficient to explain the (simulated) similar variation of Iceland-Scotland overflow strength and AMO index. Rather, a more local link through the influence of the Nordic seas surface state and density structure, which are positively correlated with the AMO index, on the pressure gradient across the IcelandScotland ridge is responsible for the (simulated) similar variation. In the model simulation showing a weaker correlation between the Iceland-Scotland overflow strength and the AMO index, the wind stress in the Nordic seas also influences the overflow strength. Our study demonstrates that palaeo-climate simulations provide a useful tool to understand mechanisms and large-scale connections associated with the relatively sparse palaeo-observations.

On the reconstruction of ocean circulation and climate based on the “Gardar Drift”

“…However, the flow through the gap is difficult to simulate both due to the smoothing of the bathymetry in the coarse-resolution model and because the depth of the isopycnals in the model may be different from the real world. Other climate models (MPI-ESM and IPSLCM4) with similar resolution also exhibit similar behavior regarding the flow of ISOW [Lohmann et al, 2015]. Typically in these models, we also see that there is only a weak or no overflow across the shallow Iceland-Faroe Ridge [Lohmann et al, 2015], which is estimated by observations to be about 1 Sv [Beaird et al, 2013].…”

“…While previous studies have used models to investigate the processes behind overturning variations inferred from SS changes found in Gardar Drift sediment records [Lohmann et al, 2015;Thornalley et al, 2013], we use BCM to explore more explicitly the relationship between changes in the deep flow speed across the Gardar Drift and the overflow at the Iceland-Scotland Ridge. In addition, we use the model to evaluate the importance of overflow changes on Gardar Drift variability relative to other potential sources of Gardar Drift variability.…”

“…The corresponding control run of BCM has previously been used in several climate studies with focus on the North Atlantic and the Nordic Seas [Langehaug et al, 2012a[Langehaug et al, , 2012bMedhaug et al, 2012;Otterå et al, 2009;Smedsrud et al, 2013;Årthun and Eldevik, 2016], whereas the natural forced run [Lohmann et al, 2015;Otterå et al, 2010] has been less analyzed. The control simulation performs well in the North Atlantic region, where Paleoceanography 10.1002/2015PA002920 particularly the water mass exchange of Atlantic-derived water and Arctic-derived water across the Greenland-Scotland Ridge has been found to be comparable to observations [Langehaug et al, 2012a].…”

Variability in the Northern North Atlantic and Arctic Oceans Across the Last Two Millennia: A Review

“…This may be especially true since overflow waters propagating downstream can later influence the strength of the AMOC and SPG (Langehaug et al, ; Lohmann et al, ) and through their associated heat transports the SST field eventually. Although different from the DSO shown here, connections between SSTs across the Iceland‐Scotland Ridge and its deep overflow were explored in three climate models (Lohmann et al, ; including one of the MPI‐ESM simulations shown here), but as opposed than for the DSO, the authors found stronger overflows to be linked with warmer SSTs (and vice versa).…”

Evaluation of CMIP5 models over the northern North Atlantic in the context of forthcoming paleoclimatic reconstructions

“…Several modeling studies have questioned the response of NA temperature variations to the ocean's internal variability. Lohmann et al (2015) found the highest correlation coefficients between the AMO index and the NA SSTs in the tropical and subtropical regions, where the SSTs are mostly influenced by the external (volcanic, solar) radiative forcing (Otterå et al 2010). The opposite picture is evident in several General Circulation Models (GCMs), which produce the AMO in the absence of external forcing (Knight et al 2005).…”