Spatial analysis of early-warning signals for a North Atlantic climate transition in a coupled GCM

Klus, Andrea; Prange, Matthias; Varma, Vidya; Schulz, Michael

doi:10.1007/s00382-018-4567-7

Cited by 8 publications

(4 citation statements)

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“…Following Zhang 48 and Chen and Tung 50 , a salinity-based AMOC index is constructed by averaging the salinity concentration over the North Atlantic. In addition, a smaller subset of this region south of Greenland is considered for a second salinity-based index S N N 2 , because recently EWS for an AMOC collapse in model simulations have been identified there 42 . Motivated by a recently revealed salinity pile up 46 a third and a fourth salinity-based index is constructed by averaging the salinity concentration in the North and South Atlantic basins, respectively (see Methods and Figs.…”

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confidence: 99%

“…Following Zhang48 and Chen and Tung 50 , the salinity-based AMOC index S N N 1 is constructed by averaging the salinity content of the Atlantic ocean basin from 45 • N to 65 • N using the EN4 dataset55 . Also a smaller subset of the northern North Atlantic region (54 • N to 62 • N; 62 • W to 26 • W) is taken for a second salinity-based index S N N 2 , because recently EWS for an AMOC collapse in model simulations have been identified in this region42 . Motivated by the results of Zhu and Liu46 , the salinity concentration in the North Atlantic basin from 10 • N to 40 • N is averaged to obtain the index S N .…”

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confidence: 99%

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

2021

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“…Transports of freshwater in the southern (de Vries and Weber 2005) and subtropical Atlantic (Mecking et al 2016;Jackson and Wood 2018a) have been suggested as potential indicators of an AMOC collapse. Some studies have looked for multivariate fingerprints based on water mass properties (Vellinga and Wood 2004;Roberts and Palmer 2012;Klus et al 2019), although Roberts and Palmer (2012) warned against simply using statistical techniques to look for patterns, without investigating the physical relationships. Various studies have highlighted the added value of considering a number of different metrics (Vellinga and Wood 2004;Roberts and Palmer 2012;Klus et al 2019), including different locations of the AMOC to assess spatial coherence (Bingham et al 2007;Feng et al 2014).…”

Section: Introductionmentioning

confidence: 99%

“…Although some of the experiments used here have been found to have a threshold beyond which the AMOC does not recover when freshwater is added (Jackson and Wood 2018a), we will not be covering the use of the time series statistics to indicate the approach of a threshold. Various papers (Lenton 2011;Lenton et al 2012;Boulton et al 2014;Nikolaou et al 2015;Klus et al 2019) have shown that this is potentially a useful indicator of approaching a threshold. However, although current techniques have been shown to indicate approaching a threshold (through increased variance or autocorrelation), they do not indicate how far away the threshold is.…”

Section: Introductionmentioning

confidence: 99%

Fingerprints for Early Detection of Changes in the AMOC

Jackson

Wood

2020

Journal of Climate

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Different strategies have been proposed in previous studies for monitoring the Atlantic meridional overturning circulation (AMOC). As well as arrays to directly monitor the AMOC strength, various fingerprints have been suggested to represent an aspect of the AMOC based on properties such as temperature and density. The additional value of fingerprints potentially includes the ability to detect a change earlier than a change in the AMOC itself, the ability to extend a time series back into the past, and the ability to detect crossing a threshold. In this study we select metrics that have been proposed as fingerprints in previous studies and evaluate their ability to detect AMOC changes in a number of scenarios (internal variability, weakening from increased greenhouse gases, weakening from hosing and hysteresis) in the eddy-permitting coupled climate model HadGEM3-GC2. We find that the metrics that perform best are the temperature metrics based on large-scale differences, the large-scale meridional density gradient, and the vertical density difference in the Labrador Sea. The best metric for monitoring the AMOC depends somewhat on the processes driving the change. Hence the best strategy would be to consider multiple fingerprints to provide early detection of all likely AMOC changes.

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Decreasing predictability as a precursor indicator for abrupt climate change

Xie

Mei

et al. 2021

Clim Dyn

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Spatial analysis of early-warning signals for a North Atlantic climate transition in a coupled GCM

Cited by 8 publications

References 78 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

Fingerprints for Early Detection of Changes in the AMOC

Decreasing predictability as a precursor indicator for abrupt climate change

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