Abstract
We investigate the mechanism of a decadal-scale weakening shift in the strength of the subpolar gyre (SPG) that is found in one among three last millennium simulations with a state-of-the-art Earth system model. The SPG shift triggers multicentennial anomalies in the North Atlantic climate driven by long-lasting internal feedbacks relating anomalous oceanic and atmospheric circulation, sea ice extent, and upper-ocean salinity in the Labrador Sea. Yet changes throughout or after the shift are not associated with a persistent weakening of the Atlantic Meridional Overturning Circulation or shifts in the North Atlantic Oscillation. The anomalous climate state of the North Atlantic simulated after the shift agrees well with climate reconstructions from within the area, which describe a transition between a stronger and weaker SPG during the relatively warm medieval climate and the cold Little Ice Age respectively. However, model and data differ in the timing of the onset. The simulated SPG shift is caused by a rapid increase in the freshwater export from the Arctic and associated freshening in the upper Labrador Sea. Such freshwater anomaly relates to prominent thickening of the Arctic sea ice, following the cluster of relatively small-magnitude volcanic eruptions by 1600 CE. Sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events; a necessary causal link between the volcanic cluster and the SPG shift can therefore be excluded. Instead, preconditioning by internal variability explains discrepancies in the timing between the simulated SPG shift and the reconstructed estimates for the Little Ice Age onset.
Highlights
The subpolar gyre (SPG) influences the North Atlantic climate by modulating the transport of heat and salt between the North Atlantic and the Arctic oceans and, into the Nordic and Labrador seas, where deep water formation takes place (e.g., Grossmann and Klotzbach 2009)
A recent paleoceanographic reconstruction of the Atlantic mid-depth (600–1300 m) gyre circulation over the past 1500 years suggested that the SPG weakened between the Medieval Climate Anomaly (MCA; roughly the 950–1250 CE period; years hereafter always refer to the Common Era) and the Little Ice Age (LIA; roughly 1450–1850) (Copard et al 2012)
Schleussner and Feulner (2013) reported that the onset of the LIA can occur at different times in an ensemble of simulations with the Climber-3α model, which the authors attributed to the sensitivity of the subpolar variability to minor changes in the North Atlantic freshwater budget in their model. They were able to simulate an earlier LIA onset in the North Atlantic by forcing the Nordic Seas with a constant freshwater offset. It is unknown whether MPI-ESM-P features a similar sensitivity to the freshwater export from the Arctic, but we argue that an analogous temporal spread in the onset of the shift as in Schleussner and Feulner (2013) can be expected: only when a sufficient amount of freshwater is discharged toward the subpolar North Atlantic under the appropriate climate conditions, a SPG shift can occur
Summary
The subpolar gyre (SPG) influences the North Atlantic climate by modulating the transport of heat and salt between the North Atlantic and the Arctic oceans and, into the Nordic and Labrador seas, where deep water formation takes place (e.g., Grossmann and Klotzbach 2009). Variations in the strength and shape of the SPG can lead to major changes in, for example, the intensity of the Atlantic meridional overturning circulation (AMOC) (e.g., Hátún et al 2005; Moreno-Chamarro et al 2015) or the distribution of sea ice in the Arctic (e.g., Yoshimori et al 2010; Jungclaus et al 2014). Attribution and the physical mechanism responsible for such an abrupt SPG weakening remain unclear, as well as the connection between this event and the other climate changes reconstructed in the North Atlantic during the LIA. This study aims at better understanding the SPG variability on decadal and longer time scales, its related dynamics, and how it contributed to shaping the North Atlantic climate during the last millennium
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