Atlantic Meridional Overturning Circulation (AMOC) in CMIP5 Models: RCP and Historical Simulations

Abstract

The Atlantic meridional overturning circulation (AMOC) simulated by 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the historical (1850–2005) and future climate is examined. The historical simulations of the AMOC mean state are more closely matched to observations than those of phase 3 of the Coupled Model Intercomparison Project (CMIP3). Similarly to CMIP3, all models predict a weakening of the AMOC in the twenty-first century, though the degree of weakening varies considerably among the models. Under the representative concentration pathway 4.5 (RCP4.5) scenario, the weakening by year 2100 is 5%–40% of the individual model's historical mean state; under RCP8.5, the weakening increases to 15%–60% over the same period. RCP4.5 leads to the stabilization of the AMOC in the second half of the twenty-first century and a slower (then weakening rate) but steady recovery thereafter, while RCP8.5 gives rise to a continuous weakening of the AMOC throughout the twenty-first century. In the CMIP5 historical simulations, all but one model exhibit a weak downward trend [ranging from −0.1 to −1.8 Sverdrup (Sv) century−1; 1 Sv ≡ 106 m3 s−1] over the twentieth century. Additionally, the multimodel ensemble–mean AMOC exhibits multidecadal variability with a ~60-yr periodicity and a peak-to-peak amplitude of ~1 Sv; all individual models project consistently onto this multidecadal mode. This multidecadal variability is significantly correlated with similar variations in the net surface shortwave radiative flux in the North Atlantic and with surface freshwater flux variations in the subpolar latitudes. Potential drivers for the twentieth-century multimodel AMOC variability, including external climate forcing and the North Atlantic Oscillation (NAO), and the implication of these results on the North Atlantic SST variability are discussed.

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

“…A similar weakening in the AMOC strength has also been simulated in historical CMIP5 runs (e.g. Drijfhout et al 2012;Cheng et al 2013). There is no clear imprint of the solar irradiance or volcanic eruptions (Ortega et al 2012), which are the dominant natural forcings at decadal and longer time scales (Fernández-Donado et al 2013).…”

“…Yet these reconstructions generally disagree with the one in Lund et al (2006). Rahmstorf et al (2015), for example, showed instead an AMOC weakening during the industrial period as compared to the LIA, in agreement with results from instrumental records (Dima and Lohmann 2010) and CMIP5 historical model simulations (Drijfhout et al 2012;Cheng et al 2013;Jungclaus et al 2014). These latter however generally disagree on the mechanisms underlying the AMOC variability, as well as on the AMOC sensitivity to external forcing (e.g., Servonnat et al 2010;Ortega et al 2012;Hofer et al 2011).…”

“…When applied in ECHO-G, this index reproduces most of the variability of the MOI, including the prominent fall during the last two centuries (not shown). 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.…”

Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans

“…The pattern and magnitude of the trends are very similar in the CESM-LENS ensemble mean including an absence of warming to the southeast of Greenland but with stronger warming in the Bering Sea and Greenland Sea (Figure 1b). The lack of warming south of Greenland results from a reduction in the poleward transport of warm water in the upper ocean by the Atlantic meridional overturning circulation (AMOC) and lengthened exposure to surface cooling as sinking at high latitudes decreases (Drijfhout et al, 2012;Cheng et al, 2013).…”

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

“…Additionally, errors could result from a lack of atmospheric processes in the models i.e. an underestimation of low cloud cover is a cause of significant errors in radiative fluxes (Kauker 2003), as the SST response to shortwave radiative forcing is thermally direct (Cheng et al 2013) and lowlevel stratiform liquid and mixed-phase clouds are found to be the most important contributors to the Arctic surface radiation balance (Shupe and Intrieri 2004). Another explanation could be the sea ice export and variation, so the changes of albedo and fresh water input of the area, during the 50 years of study (Halvorsen et al 2015).…”