Mechanisms for decadal scale variability in a simulated Atlantic meridional overturning circulation

Medhaug, Iselin; Langehaug, Helene R.; Eldevik, Tor; Furevik, Tore; Bentsen, Mats

doi:10.1007/s00382-011-1124-z

Cited by 73 publications

(76 citation statements)

References 75 publications

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“…6c). Because the overturning branch of the Arctic Mediterranean, including entrainment of Atlantic water into the overflows south of the GSR, is estimated to account for approximately 2/3 of the total AMOC (Dickson and Brown, 1994;Medhaug et al, 2012), stability of this northernmost branch of the Atlantic THC is important for sustaining a stable global overturning circulation (e.g. Jungclaus et al, 2006).…”

Section: Discussionmentioning

confidence: 99%

How northern freshwater input can stabilise thermohaline circulation

Lambert

Eldevik

Haugan

2016

Tellus A: Dynamic Meteorology and Oceanography

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A B S T R A C TThe North Atlantic thermohaline circulation (THC) carries heat and salt towards the Arctic. This circulation is partly sustained by buoyancy loss and is generally believed to be inhibited by northern freshwater input as indicated by the 'box-model ' of Stommel (1961). The inferred freshwater-sensitivity of the THC, however, varies considerably between studies, both quantitatively and qualitatively. The northernmost branch of the Atlantic THC, which forms a double estuarine circulation in the Arctic Mediterranean, is one example where both buoyancy loss and buoyancy gain facilitate circulation. We have built on Stommel's original concept to examine the freshwater-sensitivity of a double estuarine circulation. The net inflow into the double estuary is found to be more sensitive to a change in the distribution of freshwater than to a change in the total freshwater input. A double estuarine circulation is more stable than a single overturning, requiring a larger amount and more localised freshwater input into regions of buoyancy loss to induce a thermohaline 'collapse'. For the Arctic Mediterranean, these findings imply that the Atlantic inflow may be relatively insensitive to increased freshwater input. Complementing Stommel's thermal and haline flow regimes, the double estuarine circulation allows for a third: the throughflow regime. In this regime, a THC with warm poleward surface flow can be sustained without production of dense water; a decrease in high-latitude dense water formation does therefore not necessarily affect regional surface conditions as strongly as generally thought.

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Section: Discussionmentioning

confidence: 99%

How northern freshwater input can stabilise thermohaline circulation

Lambert

Eldevik

Haugan

2016

Tellus A: Dynamic Meteorology and Oceanography

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“…8b). The most plausible mechanism supported by most models is that a stronger overturning transports more oceanic heat to high latitudes where more of the sea ice melts (Delworth et al, 1993;Medhaug et al, 2011). Contradicting this, some models surprisingly show more sea ice for stronger AMOC.…”

Section: Discussionmentioning

confidence: 99%

“…Bjerknes (1964) suggested that the decadal to multidecadal climate variations were driven by slow changes in the ocean gyre circulation, while many later studies indicate that the main cause is variations in the AMOC, driven by density fluctuations in the convection regions. The mechanisms suggested for this relationship include variations in the meridional heat and salt transports causing basin scale fluctuations in surface temperatures known as the Atlantic Multidecadal Oscillation (AMO; Delworth and Mann, 2000;Latif et al, 2004;Knight et al, 2005), and the effect heat and salinity anomalies have on the density in the convective regions, and hence the strength of the deep water formation (e.g., Delworth et al, 1993;Marshall et al, 2001;Bentsen et al, 2004;Jungclaus et al, 2005;Medhaug et al, 2011).…”

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confidence: 99%

“…At present there is no consensus to what degree the AMOC variability is an ocean-only mode excited by (Frankcombe et al, 2009) or damped by (Hasselmann, 1976;Frankignoul et al, 1997) atmospheric forcing; an ocean-only mode with density fluctuations in the convection regions driven by advection of density anomalies from the south (e.g., Vellinga and Wu, 2004) or the northern high latitudes (e.g., Delworth et al, 1993); a fully coupled atmosphere-ocean or atmosphere-sea ice-ocean mode with the deep water formation rate dominated by variations in the local wind forcing (e.g., Dickson et al, 1996;Häkkinen, 1999;Eden and Willebrand, 2001; Deshayes and Frankignoul, 2008;Msadek and Frankignoul, 2009;Medhaug et al, 2011).…”

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confidence: 99%

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North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation

2011

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Abstract.Output from a total of 24 state-of-the-art Atmosphere-Ocean General Circulation Models is analyzed. The models were integrated with observed forcing for the period 1850-2000 as part of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. All models show enhanced variability at multi-decadal time scales in the North Atlantic sector similar to the observations, but with a large intermodel spread in amplitudes and frequencies for both the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The models, in general, are able to reproduce the observed geographical patterns of warm and cold episodes, but not the phasing such as the early warming (1930s-1950s) and the following colder period (1960s-1980s). This indicates that the observed 20th century extreme in temperatures are due to primarily a fortuitous phasing of intrinsic climate variability and not dominated by external forcing. Most models show a realistic structure in the overturning circulation, where more than half of the available models have a mean overturning transport within the observed estimated range of 13-24 Sverdrup. Associated with a stronger than normal AMOC, the surface temperature is increased and the sea ice extent slightly reduced in the North Atlantic. Individual models show potential for decadal prediction based on the relationship between the AMO and AMOC, but the models strongly disagree both in phasing and strength of the covariability. This makes it difficult to identify common mechanisms and to assess the applicability for predictions.

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“…Bentsen et al, 2004;Jungclaus et al, 2005;Msadek and Frankignoul, 2009;Medhaug et al, 2012;Escudier et al, 2013) suggest that AMOC variability is closely linked to convective mixing in the subpolar North Atlantic, but not strongly related to convective mixing in the Nordic Seas. Using multicenturylong control simulations with 10 coupled climate models, Ba et al (2014), however, find in some of the models significant correlation between AMOC variability and convective mixing in the Nordic Seas.…”

Section: Introductionmentioning

confidence: 99%

The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation

et al. 2014

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Abstract. We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is partly done by analysing long (order of 1000 years) control simulations with five coupled climate models. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45 • N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55 to 60 • N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland-Scotland Ridge. The latter might, however, be underestimated, as the models in general do not realistically simulate the flow path of the Iceland-Scotland overflow water south of the Iceland-Scotland Ridge. The influence of variations in subpolar deep water formation is, on multimodel average, larger than that of variations in the Denmark Strait overflow. This is true both at 45 • N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30 • N. At 30 • N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multimodel average, about half and one-third respectively of the decadal to multidecadal AMOC variance. Apart from analysing multimodel control simulations, we have performed sensitivity experiments with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. The sensitivity experiments indicate that variations in subpolar deep water formation and Nordic Seas overflows are not completely independent. We further conclude from these experiments that the decadal to multidecadal AMOC variability north of about 50 • N is mainly related to variations in Nordic Seas overflows. At 45 • N and south of this latitude, variations in both subpolar deep water formation and Nordic Seas overflows contribute to the AMOC variability, with neither of the processes being very dominant compared to the other.

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Mechanisms for decadal scale variability in a simulated Atlantic meridional overturning circulation

Cited by 73 publications

References 75 publications

How northern freshwater input can stabilise thermohaline circulation

How northern freshwater input can stabilise thermohaline circulation

North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation

The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation

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