Climate change

The North Atlantic basin stands out for the prominent multidecadal variability featured by the observed SST record, alternatively offsetting and enhancing the underlying warming trend (Deser and Blackmon 1993; Kushnir 1994; Ting et al. 2009). These fluctuations, due to their apparent oscillatory behavior, with a typical 60–70-yr time scale, have been associated with a low-frequency natural variability mode, termed the Atlantic multidecadal oscillation (AMO) or variability (AMV). This low-frequency fluctuation reverberates in a number of hydroclimatic, societally relevant features, including Mediterranean surface temperatures (Marullo et al. 2011; Mariotti and Dell’Aquila 2012), summertime climate over North America and western Europe (Sutton and Hodson 2005), rainfall variability over the Sahel (Mohino et al. 2011), and Atlantic hurricane activity (Trenberth and Shea 2006), among others.

Results from a study inspecting the origins of multidecadal variability in the North Atlantic sea surface temperature (NASST) are presented. The authors target in particular the 1940–75 “warm-to-cold” transition, an event that is generally framed in the context of the longer-term Atlantic multidecadal variability (AMV) cycle, in turn associated with the Atlantic meridional overturning circulation (AMOC) internal variability. Here the authors examine the ability of uninitialized, historical integrations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) archive to retrospectively reproduce this specific episode of twentieth-century climatic history, under a hierarchy of forcing conditions. For this purpose, both standard and so-called historical Misc CMIP5 simulations of the historical climate (combining selected natural and anthropogenic forcings) are exploited. Based on this multimodel analysis, evidence is found for a significant influence of anthropogenic agents on multidecadal sea surface temperature (SST) fluctuations across the Atlantic sector, suggesting that anthropogenic aerosols and greenhouse gases might have played a key role in the 1940–75 North Atlantic cooling. However, the diagnosed forced response in CMIP5 models appears to be affected by a large uncertainty, with only a limited subset of models displaying significant skill in reproducing the mid-twentieth-century NASST cooling.

Several mechanisms and processes have been invoked to explain the Atlantic cooling [see Hodson et al. (2014) for an extensive review]. These include the propagation from the Arctic to the subpolar basin of a large-scale, cold, and low-salinity anomaly (the so-called Great Salinity Anomaly; Dickson et al. 1988); a slowdown of the Atlantic branch of the meridional overturning circulation that could have determined the observed cooling pattern via a reduction in the poleward heat transport (Rahmstorf et al. 2015); the early 1960s occurrence of several volcanic eruptions of Mount Agung, which in turn might have determined a local cooling via different processes, ranging from the direct radiative effect associated with the injection of sulfate aerosols in the stratosphere, or indirectly through the dynamical adjustment of the atmospheric and oceanic circulation (Swingedouw et al. 2015); an approximately 10-yr decline in the solar radiation intensity that started in the late 1950s, possibly amplified over the Atlantic by regional processes (Hodson et al. 2014; see also Thiéblemont et al. 2015, for a discussion of the 11-yr solar cycle impact on the decadal variability in the Atlantic sector); and the increase in anthropogenic aerosols emissions over the United States and Europe, starting around the middle of the twentieth century and peaking during the 1970s–1980s (Smith et al. 2011), which was potentially responsible for the observed cooling of the North Atlantic ocean surface through direct radiative effects, and indirectly via the interplay with cloud albedo and life cycle (Booth et al. 2012; Bellucci et al. 2015).

The variety of physical mechanisms and processes identified as potential drivers of the mid-twentieth-century Atlantic cooling highlights that a clear causal attribution is still matter of debate.