The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) (Box 1) is a system of ocean currents in the Atlantic that move warmer, upper waters northwards and cooler, deeper waters southwards. Accordingly, the AMOC is a major source of northward heat transport, accounting for 20-30% of total atmospheric and oceanic heat transport into the mid-latitudes 1 . The AMOC, therefore, has a key role in governing the climate of the North Atlantic region and beyond, influencing European air temperatures and precipitation, the frequency of Atlantic hurricanes and winter storms, spatial patterns of sea level and tropical monsoons 2,3 , and the global carbon budget 4 .The strength of the AMOC is typically 17 Sverdrups (Sv; 1 Sv = 10 6 m 3 s −1 ) 5 . However, both observations and models indicate that the AMOC exhibits substantial variability on daily to multi-decadal timescales. Coupled climate models suggest decadal variability can arise naturally due to internal interactions within the climate system [6][7][8] . The AMOC is also expected to respond to external forcing, including anthropogenic aerosols, volcanic eruptions and solar changes 9,10 , as well as anthropogenic greenhouse gas emissions 11 . Indeed, observations, reanalyses, models and proxies [12][13][14][15][16] indicate substantial contemporary decadal-scale changes in AMOC strength. The RAPID array at 26.5° N (refs 5,17 ), for example, revealed a statistically significant weakening from 2004 (refs 18,19 ), probably representing decadal variability rather than ongoing long-term weakening [20][21][22] . There are indications that the AMOC might be recovering in strength 12 .Despite evidence of decadal variability, many questions remain. For example, high-quality continuous observations, like the RAPID array, are short and sparse, making it difficult to assess longer-term AMOC variability and determine whether decadal changes are representative of those across the wider Atlantic. Moreover, there is uncertainty about the relative roles of internal variability and forced variability, owing to diverse AMOC variability 8,23 and externally forced AMOC trends 10,24 in models. Indeed, the AMOC might have already weakened over the twentieth century 25,26 , potentially implying that it is more sensitive to external forcing than previously thought. Understanding how and why the AMOC has changed on decadal timescales is thus crucial not only to understand the AMOC's role

Irminger Sea Is the Center of Action for Subpolar AMOC Variability

“…Since the subpolar AMOC time series from the eastern part of the OSNAP array only span the 2014–2018 period, we turn to an ocean reanalysis to confirm the robustness of the observed sea‐surface height and upper‐ocean density patterns. The GloSea5 ocean reanalysis is used for this purpose since its AMOC variability has been shown to be comparable to observations from both the RAPID and OSNAP arrays (Jackson et al., 2022; Moat et al., 2020; Zou et al., 2020).…”

“…A similar stability of the subtropical and subpolar AMOC since the 1990 has also been reported by Fu et al (2020) that combined repeated hydrographic sections with an inverse model constrained using satellite altimetry. Jackson et al (2022) concluded that there is no evidence for an AMOC slowdown both in the subtropics and the SPNA since 1980, although these authors recognize that a weakening on longer timescales such as that expected from anthropogenic warming may simply be obscured by the large interannual and decadal variability. Taken together, we conclude that there is yet no consensus regarding basin-wide North Atlantic slowdown of the overturning circulation.…”

“…Another complication is that, on decadal timescales, the overturning variability appears to differ in the North Atlantic between subtropical and subpolar latitudes (see Jackson et al., 2022, and references therein). Because of the lack of decadal‐scale coherency between these two regions, it is indeed uncertain which aspect of the AMOC available large‐scale proxies represent.…”

Performance based sub-selection of CMIP6 models for impact assessments in Europe

“…The AMOC also plays a significant role in the present and future European climate due to its role in the poleward transfer of heat and ocean circulation. It also impacts on the NA SST (Jackson et al, 2022;Zhang, 2008;Zhang et al, 2019;Yeager and Robson, 2017), thereby influencing the SST impact on European climate discussed above. The CMIP5 and CMIP6 ensemble both predict a reduction in the AMOC by the end of century for higher emissions pathways, (Menary et al, 2020;Bellomo et al, 2021).…”

Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean

“…S3d & h). According to ocean & sea-ice models forced with historical surface ux elds, the North Atlantic Oscillation (NAO) is the main driver of the AMOC variability at interannual to interdecadal time scales [49][50][51] . However, due to a baroclinic adjustment time of the subpolar North Atlantic Ocean circulation, it takes 3 ~ 15 years for the AMOC to respond to the NAO-induced surface buoyancy forcing and associated deep water formation 49,52 .…”