Fingerprints for Early Detection of Changes in the AMOC

Jackson, Laura; Wood, Richard

doi:10.1175/jcli-d-20-0034.1

Cited by 33 publications

(30 citation statements)

References 59 publications

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“…Although a negative trend can be inferred from these observations, their temporal coverage is not yet sufficient to infer a climatological AMOC weakening and a contribution by anthropogenic climate change, as opposed to natural decadal fluctuations. Therefore, several fingerprints of AMOC variability, based on different spatial averages of SST and salinity time series that are available for longer periods, have been proposed and investigated 11,[48][49][50][51] . Recent cooling anomalies of SSTs in the subpolar gyre region south of Greenland (Fig.…”

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confidence: 99%

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

2021

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The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic ocean basin. These results reveal spatially consistent empirical evidence that in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.The Atlantic Meridional Overturning Circulation (AMOC) is the key circulation system of the Atlantic ocean, transporting water masses northward at the surface, and southward at the bottom of the ocean 1 . The AMOC is the archetypical example of potentially multi-stable Earth system components 2 . Early studies based on box models 3 have indicated that the AMOC has two different stable states of operation, corresponding to a strong and a weak circulation mode. The AMOC's bistability and corresponding hysteresis has henceforth been confirmed in a hierarchy of models, from Earth system models of intermediate complexity (EMICs) 4,5 to comprehensively coupled atmosphere-ocean general circulation models (AOGCMs) 6 . It should be noted, however, that AMOC bistability has not been identified in a considerable number of state-of-the-art Earth system models 7 (ESMs). It remains debated whether this should be interpreted as evidence for a monostable AMOC under current climate conditions, or rather as evidence for excessive AMOC stability in these models 8,9 : most comprehensive climate models likely underestimate the freshwater export from the northern Atlantic ocean basin and hence suppress the associated destabilizing feedback 6,9-12 . Moreover, it has been argued that in comprehensive models, very high spatial resolution is needed to obtain a good representation of the AMOC response to freshwater forcing 13 . Indeed, AMOC bistability has been revealed in recent simulations with a comprehensive model with Eddy-permitting ocean module 14 .The strong AMOC state is currently attained, while the weak state has arguably been occupied recurringly during previous glacial intervals. Different lines of evidence from paleoclimate proxy records indicate that northern hemisphere temperatures have varied abruptly at millennial time scales during previous glacial episodes, with corresponding changes of the AMOC between its weak and strong modes 1,15,16 . Speleothem and ocean sediment records from arou...

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confidence: 99%

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

2021

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“…Indeed, a proxy for AMOC strength using the difference between 'warming hole' SSTs and global mean SSTs 25,123 suggests an AMOC weakening of 3 ± 1 Sv since the middle of the twentieth century. However, the interpretation of this proxy for changes in the AMOC 10,119,164,165 and its Stippling indicates statistically significant trends (P < 0.05). Analysis and regions adapted from ref.…”

Section: Linking To the Pastmentioning

confidence: 92%

“…Indirect evidence. In addition to observations, models and reanalyses, estimates of the AMOC can be determined by considering changes in the North Atlantic Ocean that are mechanistically and statistically associated with the AMOC 118,119 . Such proxies can be used to reconstruct AMOC timeseries, and are often developed using relationships derived from models owing to a lack of direct observations.…”

Section: Models and Reanalysesmentioning

confidence: 99%

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Jackson

Biastoch

Buckley

et al. 2022

Nat Rev Earth Environ

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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

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“…Previous studies have largely focused on time of emergence of an anthropogenically forced AMOC decline; that is, when a trend falls outside the range of its natural variability (Baehr et al, 2007, 2008; Jackson & Wood, 2020; Keller, Deutsch, et al, 2007; Keller, Kim, et al, 2007; Roberts & Palmer, 2012; Roberts et al, 2014; Santer et al, 1995; Vellinga & Wood, 2004; Williams et al, 2015). For example, Roberts et al (2014) estimate that the −0.53 Sv yr −1 AMOC trend, assessed from the first 8 years of RAPID, takes 18 years to be significantly different from the internal variability found in the CMIP5 preindustrial control simulations.…”

Section: Introductionmentioning

confidence: 99%

Detectability of an AMOC Decline in Current and Projected Climate Changes

Lobelle

Beaulieu

Livina

et al. 2020

Geophysical Research Letters

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Determining whether the Atlantic Meridional Overturning Circulation (AMOC)'s transport is in decline is challenging due to the short duration of continuous observations. To estimate how many years are needed to detect a decline, we conduct a simulation study using synthetic data that mimics an AMOC time series. The time series' characteristics are reproduced using the trend, variance, and autocorrelation coefficient of the AMOC strength at 26.5 • N from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) models under the RCP8.5 future scenario, and from RAPID observations (2004-2018). Our results suggest that the 14-year RAPID length has just entered the lower limits of the trend's "detection window" based on synthetic data generated using CMIP5 trends and variability (14-42 years; median = 24 years), but twice the length is required for detectability based on RAPID variability (29-67 years; median = 43 years). The annual RAPID trend is currently not statistically significant (−0.11 Sv yr −1 , p > 0.05). Plain Language Summary There are ongoing discussions in the scientific community about whether the Atlantic Meridional Overturning Circulation transport is slowing down. This is of interest due to the importance of this circulation in transporting heat from the tropics to the northern latitudes. A consensus about its decline is hard to reach due to the limited direct observational data available; with the longest continuous data being 14 years long from 2004 to 2018. We therefore conduct a simulation experiment to examine how many years of data are required to detect a decline in the circulation. We create simulations of the North Atlantic transport based on statistical properties from 20 general circulation models with future climate change projections (until 2100) and from the RAPID array observations (since 2004). Our results demonstrate that the length of data we currently have from observations has just entered the "detection window" of 14-42 years (based on model simulations). However, the RAPID observations do not currently exhibit a statistically significant trend.

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Fingerprints for Early Detection of Changes in the AMOC

Cited by 33 publications

References 59 publications

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Detectability of an AMOC Decline in Current and Projected Climate Changes

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