Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years

Abstract

The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle. The AMOC has been shown to be weakening in recent years ; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet . Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.

Last Century Warming Over the Canadian Atlantic Shelves Linked to Weak Atlantic Meridional Overturning Circulation

“…The AMOC reduction obtained with the UVic model (~17%) is consistent with the most recent estimate of weakening (Caesar et al, 2018;Rahmstorf et al, 2015;Thornalley et al, 2018). Both models used here produced a large-scale subsurface warming in the northwest Atlantic at 45°N (Figures S1 and S2) with a maximum of 1-3°C around 50°W.…”

“…org/10.1029/2018GL080083 1961 and 1995 (~0.02 Sv on average; Curry, 2005). This is especially true if we are to identify the forcing(s) responsible for the ongoing AMOC weakening (Bakker et al, 2016;Thornalley et al, 2018). It is therefore increasingly critical that we understand the impacts of climate change and freshwater release on convection in the Labrador Sea and its corresponding impact on AMOC intensity (Gregory et al, 2005).…”

“…Based on this relationship, it was estimated that AMOC intensity explains 89% of the variance in the temperature-based AMOC index with other factor having a minor influence on the observed temperature pattern (Caesar et al, 2018). Thus, the observed warming in the western North Atlantic and consequently in the Laurentian Channel could be linked to the weakened state of the AMOC (Caesar et al, 2018;Rahmstorf et al, 2015;Thornalley et al, 2018).…”

Regime shifts of Mediterranean forest carbon uptake and reduced resilience driven by multidecadal ocean surface temperatures

“…The AMO is linked to the Atlantic multidecadal overturning thermohaline circulation (AMOC), which in turn currently stands as a major modulator of the responses of the earth system to global warming (Caesar, Rahmstorf, Robinson, Feulner, & Saba, 2018;Chen & Tung, 2014;Hansen et al, 2016;Sgubin, Swingedouw, Drijfhout, Mary, & Bennabi, 2017;Thornalley et al, 2018). The AMO is linked to the Atlantic multidecadal overturning thermohaline circulation (AMOC), which in turn currently stands as a major modulator of the responses of the earth system to global warming (Caesar, Rahmstorf, Robinson, Feulner, & Saba, 2018;Chen & Tung, 2014;Hansen et al, 2016;Sgubin, Swingedouw, Drijfhout, Mary, & Bennabi, 2017;Thornalley et al, 2018).…”

Evaluating the link between the sulfur-rich Laacher See volcanic eruption and the Younger Dryas climate anomaly

“…Lehner et al (2013) identify a sea-ice-AMOC-atmospheric feedback that amplified an initial negative radiative forcing to produce the temperature pattern characterising the Little Ice Age. However, Rahmstorf et al (2015) argue that no identifiable change in AMOC occurred during the Little Ice Age, and Thornalley et al (2018) similarly suggest that AMOC only weakened near the end of the Little Ice Age. This implies that the drivers of Little Ice Age cooling were related to sea-ice, atmosphere, or oceanic (other than AMOC, e.g.…”