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

摘要

A recent palaeo-reconstruction of the strength of the Iceland–Scotlandoverflow during the last 600 years suggests that its low-frequencyvariability exhibits strong similarity with palaeo-reconstructions of theAtlantic Multidecadal Oscillation (AMO). The underlying mechanism of thesimilar variation remains unclear, however, based on palaeo-reconstructionsalone. In this study we use simulations of the last millennium driven byexternal forcing reconstructions with three coupled climate models in orderto investigate possible mechanisms underlying the similar variation ofIceland–Scotland overflow strength and AMO index. Similar variation of thetwo time series is also largely found in the model simulations. Our analysisindicates that the basin-wide AMO index in the externally forced simulationsis dominated by the low-latitude sea surface temperature (SST) variability and is not predominantlydriven by variations in the strength of the Atlantic meridional overturningcirculation (MOC). This result suggests that a large-scale link through thestrength of the MOC is not sufficient to explain the (simulated) similarvariation of Iceland–Scotland overflow strength and AMO index. Rather, amore local link through the influence of the Nordic seas surface state anddensity structure, which are positively correlated with the AMO index, onthe pressure gradient across the Iceland–Scotland ridge is responsible forthe (simulated) similar variation. In the model simulation showing a weakercorrelation between the Iceland–Scotland overflow strength and the AMOindex, the wind stress in the Nordic seas also influences the overflowstrength. Our study demonstrates that palaeo-climate simulations provide auseful tool to understand mechanisms and large-scale connections associatedwith the relatively sparse palaeo-observations.