Likely accelerated weakening of Atlantic overturning circulation emerges in optimal salinity fingerprint

Abstract

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic forcing has been difficult to detect from the short direct measurements available due to strong interdecadal variability. Here, we present observational and modeling evidence for a likely accelerated weakening of the AMOC since the 1980s under the combined forcing of anthropogenic greenhouse gases and aerosols. This likely accelerated AMOC weakening signal can be detected in the AMOC fingerprint of salinity pileup remotely in the South Atlantic, but not in the classic warming hole fingerprint locally in the North Atlantic, because the latter is contaminated by the “noise” of interdecadal variability. Our optimal salinity fingerprint retains much of the signal of the long-term AMOC trend response to anthropogenic forcing, while dynamically filtering out shorter climate variability. Given the ongoing anthropogenic forcing, our study indicates a potential further acceleration of AMOC weakening with associated climate impacts in the coming decades.

Sensitivity of AMOC Fingerprints Under Future Anthropogenic Warming

“…AMOC intensity in the multi‐model ensemble mean (MMEM) averaged over the historical period 1980–2014 is ∼20 Sv, consistent with observations (McCarthy et al., 2015; Talley, 2013). Due to the combined forcing of increasing GHGs and decreasing anthropogenic aerosols, the AMOC has been significantly weakened since the 1980s in models (Figure 1a) and likely also in the real world (Zhu et al., 2023). The weakening trend continues in SSP245 and SSP585 runs.…”

“…Actually, a weaker warming hole over the SPG has been projected under a more strongly forced scenario run (Drijfhout et al., 2012), yet the exact mechanism at play remains to be clarified. In contrast, the salinity pile‐up fingerprint is dynamically controlled by AMOC perturbation as a whole on the mean climatological meridional salinity gradients (Zhu & Liu, 2020; Zhu et al., 2023) and thus would be independent of the driver and sources of AMOC change. Whether the relationship between AMOC and its fingerprints is stationary raises concerns, especially when these fingerprints are used to infer AMOC changes in a changing climate.…”

“…The classical warming hole fingerprint, which is characterized by a surface cooling over the North Atlantic (NA) subpolar gyre (SPG) region relative to background global warming, has been used as an "all-climate" AMOC fingerprint (Caesar et al, 2018;Rahmstorf et al, 2015). Different from most fingerprints that located locally in the subpolar NA (Caesar et al, 2021), a unique AMOC fingerprint from the South Atlantic with a higher signal-to-noise ratio was proposed recently, featuring a salinity pile-up over the subtropical South Atlantic (STSA) relative to the subtropical South Indo-Pacific (STSIP; Zhu & Liu, 2020;Zhu et al, 2023).…”

Separating Direct Heat Flux Forcing and Freshwater Feedback on AMOC Change Under Global Warming

“…The Atlantic meridional overturning circulation (AMOC) is a crucial component of Earth's climate system, responsible for transporting substantial heat from low latitudes northward into the mid‐high latitudes (Jackson et al., 2022) and exerting a profound influence on global climate patterns (Buckley & Marshall, 2016). Currently, the AMOC is experiencing a state of reduced intensity (Smeed et al., 2018; Zhu & Liu, 2020) and is projected to weaken further in the next century due to the impact of greenhouse gas (GHG)‐induced global warming (Cheng et al., 2013; Weijer et al., 2020; Zhu et al., 2023). However, significant uncertainty remains regarding the extent of AMOC weakening (Cheng et al., 2013; Jackson et al., 2022; Weijer et al., 2020).…”

Structural stability changes of the Atlantic Meridional Overturning Circulation

“…Zhu et al 2023) (Fig.5a). The belonging of the AMOC state to the "off" attractor is supported by reports of observed signi cant cooling of the upper ocean North Atlantic after 2005 56 (Robson et al 2016), of decreasing salinity trends in the subpolar North Atlantic after mid 200057 (Tesdal et al 2018) and by direct AMOC measurements indicating its weakening since 2004, in association with the SST-AMOC-TM pattern 58(Smeed et al 2018).…”