Observed fingerprints (EN4 analysis data for S S and S SA HadISST dataset for T NA) are shown as 5-year running means.
#South atlantic anomaly series#
Time series of observed (thick lines) and multi-model ensemble mean (MMEM, thin lines shading shows one standard deviation) ( a) anomaly of Atlantic meridional overturning circulation (AMOC) intensity, ( b) the salinity pileup fingerprint S S, ( c) the salinity pileup fingerprint S SA, ( d) monthly mean observation weight of salinity analysis over the subtropical South Atlantic (analysis value is more influenced by observations with weight closer to one), ( e) the classical warming hole fingerprint T NA and ( f) key external forcing of CO 2 (from NASA GISS, brown), anthropogenic (ASR_NH, blue, Methods) and volcanic (shown as global stratospheric aerosol optical depths at 550 nm from NASA GISS, grey) aerosols. 1a, e), consistent with previous model analysis for T NA as an AMOC fingerprint 13, 14.
In both CMIP5 and CMIP6 models, the forced responses of T NA do exhibit long-term trends largely consistent with multi-model ensemble mean (MMEM) AMOC (Fig. 1e, Methods), which is characterized by a surface cooling over the subpolar North Atlantic (SPNA) relative to background global warming in response to reduced northward AMOC heat transport, has been used as an “all-climate” fingerprint to infer AMOC trend and variability 13, 14. Notably, however, the classical ‘warming hole’ fingerprint (T NA, Fig. AMOC fingerprints used so far are located over the North Atlantic 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, most of which are not specifically designed for detecting long-term trend 9, 10, 11, 12. However, due to the presence of strong interdecadal variability and the shortness of direct measurements available 5, 6, 7, 8, whether this AMOC weakening has emerged in the real world has to be examined from a longer-term perspective, which can only be made indirectly with AMOC fingerprints. Modeling studies have long suggested an AMOC weakening response to global warming induced by anthropogenic greenhouse gasses (GHGs) 2, 3, 4.
The response of Atlantic meridional overturning circulation (AMOC) to global warming has profound impact on global climate 1. Given the ongoing anthropogenic forcing, our study indicates a potential further acceleration of AMOC weakening with associated climate impacts in the coming decades. 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. 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.
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. 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.
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