Surface changes in the North Atlantic meridional overturning circulation during the last millennium

Abstract

Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation dynamics likely amplified the relatively warm conditions during the Medieval Climate Anomaly and the relatively cool conditions during the Little Ice Age within the North Atlantic sector.

Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium

“…This reconstruction led to the inference that the AMOC strength was about 10 % weaker during the LIA than it is today, thus supporting previous suggestions that a reduced northward oceanic heat transport might have contributed to the anomalously cold conditions over Europe at that time (Broecker 2000). A recovery of the surface AMOC during the nineteenth and twentieth centuries following a minimum in the LIA was also reconstructed from 14 C shallowwater sea-shells off North Iceland (Wanamaker et al 2012).…”

“…In agreement with the FC proxy, a decrease of the surface AMOC over the last millennium with a recovery after 1945 was reconstructed from radiocarbon contents in sea-shells from the North Icelandic shelf (Wanamaker et al 2012). As pointed out by Rahmstorf et al (2015), however, this latter recovery contradicts recent instrumental results that describe an AMOC weakening since 1930 (Dima and Lohmann 2010).…”

“…Many CMIP5 historical simulations also describe a reduction of the AMOC strength during the last century (Drijfhout et al 2012;Cheng et al 2013), similarly to ECHO-G. Jungclaus et al (2014) explored this modern AMOC reduction in the context of the last millennium using an ensemble of transient Earth-System-model simulations, thus relating the weakening of the AMOC at subtropical and subpolar latitudes to reduced convection activity in the Labrador Sea. These authors also described a strengthening of the subpolar gyre during the twentieth century, which, we conjecture, might explain the recovery found in the surface AMOC reconstruction off North Iceland by Wanamaker et al (2012); however, the latter could also have been prominently influenced by changes in the Arctic circulation, wind patterns, and/or local currents. The majority of these studies thus would be consistent results shown in ECHO-G for the industrial period.…”

“…Some reconstructions simply rely on the use of geological proxy archives to some extent sensitive to changes in the AMOC or its components (e.g., Wanamaker et al 2012;Dylmer et al 2013). Others are based on model-based covariances between the AMOC and other oceanic variables that are easier to quantify, like Labrador Sea deep densities (Robson et al 2014) or mean surface temperatures in the warming hole region (Rahmstorf et al 2015).…”

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

“…In addition to the high temporal resolution of Arctica islandica, the reconstructions derived from its records can be crossvalidated, absolutely dated (Scourse et al 2006;Butler et al 2010) and offer significant advantages in evaluating long-term NA marine climate dynamics ). Records of Arctica islandica can be used to reconstruct sea water temperatures (Eagle et al 2013;Wanamaker et al 2016), salinity (Gillikin et al 2006), major NA climate modes like the Atlantic Multidecadal Oscillation (AMO) , ocean dynamics (see for NAO in Schöne et al 2003; see for AMOC in; Wanamaker et al 2012), hydrographic changes and ecosystem dynamics (Witbaard 1996;Witbaard et al 2003).…”