The North Atlantic Oscillation as a driver of rapid climate change in the Northern Hemisphere

Abstract

Pronounced climate changes have occurred since the 1970s, including rapid loss of Arctic sea ice 1 , large-scale warming 2 and increased tropical storm activity 3 in the Atlantic. Anthropogenic radiative forcing is likely to have played a major role in these changes 4 , but the relative influence of anthropogenic forcing and natural variability is not well established. The above changes have also occurred during a period in which the North Atlantic Oscillation has shown marked multidecadal variations 5 . Here we investigate the role of the North Atlantic Oscillation in these rapid changes through its influence on the Atlantic meridional overturning circulation and ocean heat transport. We use climate models to show that observed multidecadal variations of the North Atlantic Oscillation can induce multidecadal variations in the Atlantic meridional overturning circulation and poleward ocean heat transport in the Atlantic, extending to the Arctic. Our results suggest that these variations have contributed to the rapid loss of Arctic sea ice, Northern Hemisphere warming, and changing Atlantic tropical storm activity, especially in the late 1990s and early 2000s. These multidecadal variations are superimposed on long-term anthropogenic forcing trends that are the dominant factor in long-term Arctic sea ice loss and hemispheric warming.There has been pronounced multidecadal variability of the North Atlantic Oscillation (NAO) over the past century 6 , with a negative phase of the NAO from the 1960s into the 1970s (Fig. 1a), associated with weakened westerly winds and cold ocean surface temperatures in the subpolar North Atlantic. This was followed by a rapid switch to a positive NAO phase from the 1980s into the early 2000s, along with strengthened westerly winds and rapid warming in the subpolar North Atlantic. These regional variations were coincident with long-term trends and multidecadal variability in Northern Hemisphere (NH) surface air temperature 2 and Arctic sea ice 1 . The relatively cool period of the 1970s was coincident with enhanced Arctic sea ice 7 , although observations before the satellite era are uncertain. This was followed by a rapid warming of the NH and reduction of Arctic sea ice from the late 1970s through the 2000s. There have been substantial multidecadal variations in Atlantic tropical storm activity 3 , with a relatively quiescent period from the late 1960s through the 1980s, followed by enhanced activity after the mid 1990s.Studies have suggested an important role for anthropogenic forcing in driving some of the above changes 4,8 , and yet the relative roles of natural variability and anthropogenic forcing are uncertain. In particular, the degree to which natural climate variability contributes to these past variations is a critical factor in our confidence in projections of future climate.We use simulations with three different climate models to explore the contribution of multidecadal variations of the NAO 9 to the above changes. The novel experimental design (see Methods) adds NAO-relate...

Linking the Tropical Northern Hemisphere Pattern to the Pacific Warm Blob and Atlantic Cold Blob

“…Figure 10b also reveals that the correlation coefficients between the TNH and the Pacific blob indices are higher than those between the TNH and the Atlantic blob indices, which implies that the TNH pattern accounts for more SST variability in the Pacific blob region than that in the Atlantic blob region. Other major forcing mechanisms also exist for the Atlantic cold blobs, such as those associated with the NAO, the AMOC, and the Greenland ice sheet melting (Delworth et al 2016;Yeager et al 2016;Rahmstorf et al 2015;Robson et al 2016;Duchez et al 2016;Schmittner et al 2016). This study indicates that TNH forcing is an additional, rather than an exclusive, explanation for the generation of the Atlantic blob.…”

“…The Atlantic cold blob was also suggested to affect downstream European climate (Duchez et al 2016). Several mechanisms have been proposed to explain the generation of the Atlantic cold blob, including atmospheric forcing associated with the North Atlantic Oscillation (NAO; Delworth et al 2016;Yeager et al 2016), an ocean circulation change linked to the Atlantic meridional overturning circulation (AMOC; Rahmstorf et al 2015;Robson et al 2016;Duchez et al 2016), and the melting of the Greenland ice sheet (Schmittner et al 2016). The general perspective of the scientific community at this point is to consider the Atlantic and Pacific blobs as two separate phenomena that are driven by different mechanisms.…”

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

“…McCormick et al (2012a) collected climatic evidence from the period of the Roman Empire (up to 800 CE) and reported eight large droughts in the western Empire (accessible through McCormick et al, 2012b). Domínguez-Castro et al (2014) summarized historical evidence from Muslim sources for southern Spain, a region exceptionally vulnerable to NAO-related droughts (Cook et al, 2016), from 711 to 1010 CE. They identified three major drought periods during this time.…”

“…Climate dynamics within this region is of crucial importance not only at regional scales (Scaife et al, 2008;Trigo et al, 2002), but also as a pacemaker for the whole Northern Hemisphere (Delworth et al, 2016). Inter-annual to multi-decadal climate variability in the North Atlantic sector is strongly influenced by large-scale variability patterns like the North Atlantic Oscillation (NAO; Hurrell et al, 2003).…”

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

“…Recent modelling suggests that reduced wind stress can immediately weaken AMOC, encouraging southward sea ice expansion and promoting cooling , illustrating a potential amplification mechanism following an initial aerosol-induced atmospheric circulation shift. Twentieth-century instrumental measurements further support this by demonstrating that westerly wind strength over the North Atlantic partially modulates AMOC (Delworth et al, 2016).…”