Abstract
Despite the relative climate stability of the present interglacial, it has been punctuated by several centennial time scale climatic oscillations, the latest of which are often colloquially referred to as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA). The most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and/or volcanic activity and amplified by ocean-atmosphere-sea ice feedbacks. As such, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are widely believed to have been involved in the amplification of such climatic oscillations. The Labrador Sea is a key area of deep water formation. The waters produced here contribute approximately one-third of the volume transport of the deep limb of the AMOC and drive changes in the North Atlantic surface hydrography and subpolar gyre circulation. In this study, we present multi-proxy reconstructions from a high-resolution marine sediment core located south of Greenland that suggest an increase in the influence of polar waters reaching the Labrador Sea close to MCA-LIA transition. Changes in freshwater forcing may have reduced the formation of Labrador Sea Water and contributed towards the onset of the LIA cooling.
Highlights
The Labrador Sea is an active area of formation for the main intermediate water mass of the North Atlantic, known as Labrador Sea Water (LSW)
The production and spreading of LSW plays an important role in the Atlantic Meridional Overturning Circulation (AMOC) because (i) it influences the properties and volume transport of the Nordic overflows through vigorous entrainment and mixing [e.g., Price and Baringer, 1994; Bersch et al, 2007; Boessenkool et al, 2007], (ii) it regulates the surface circulation around the subpolar gyre (SPG) [e.g., Curry and McCartney, 2001; Lu et al, 2007], (iii) it fills the intermediate depth reservoir in the North Atlantic, exporting subpolar intermediate waters to the subtropics [Yashayaev et al, 2008; Bower et al, 2009], and (iv) it contributes to the upper component of the dense and deep water masses originating in the Arctic and North Atlantic, collectively termed North Atlantic Deep Water [Talley and McCartney, 1982]
The results record a cooling at ~1400 years A.D., which corresponds to the start of the Little Ice Age (LIA)
Summary
The Labrador Sea is an active area of formation for the main intermediate water mass of the North Atlantic, known as Labrador Sea Water (LSW). In 1970, the Labrador Sea received an unusually high discharge of Arctic sea ice and freshwater via the East Greenland Current (EGC), causing a widespread freshening of the upper water column in the North Atlantic [Dickson et al, 1988] This event was accompanied by a reduction in the depth of winter convection and production of LSW [Lazier, 1980; Curry et al, 1998]. The distribution, transport ( affected by changes in atmospheric circulation), and melting of anomalously large amounts of sea ice would have altered the freshwater budget in deep water formation sites, thereby weakening convection and reducing the northward ocean heat transport, which in turn would have reinforced the regional cooling and expansion of sea ice in the high latitudes [Sedláček and Mysak, 2009a, 2009b; Zhong et al, 2011; Miller et al, 2012; Lehner et al, 2013]. The records show substantial shifts in surface water conditions of the eastern Labrador Sea across the transition, which allow examination of the forcing and variability of the preconditioning of the Labrador Sea for winter convection and its potential role in the AMOC’s strength and climate over the last millennium
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