Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

Mengel, Matthias; Levermann, Anders; Schleußner, Carl-Friedrich; Born, Andreas

doi:10.5194/esd-3-189-2012

Cited by 14 publications

(19 citation statements)

References 61 publications

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“…5). An important role for this advective-convective positive feedback was also concluded from the statistical analysis of unforced variability in 19 coupled climate models ) and from idealized simulations in a coarse-resolution model (Mengel et al 2012).…”

Section: Discussion and Summarymentioning

confidence: 99%

Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean

2015

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Decadal climate variability in the North Atlantic has received increased attention in recent years, because modeling results suggest predictability of heat content and circulation indices several years ahead. However, determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms. Here, the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region, the subpolar gyre (SPG), are not always consistent. A coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures. This confirms that surface heat flux indeed plays a leading role for this type of variability, as has been suggested in previous studies. The results further suggest that large variations in the SPG correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein. Performance of this conceptual model is tested against a statistical stochastic model. Hysteresis and the existence of two stable modes of the SPG circulation shape its response to forcing by atmospheric temperatures. The identification of the essential dynamics and the reduction to a minimal model of SPG variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability.

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Section: Discussion and Summarymentioning

confidence: 99%

Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean

2015

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“…From 1948 onwards, the anomalies according to the NCEP reconstruction are applied for all ensemble members. Subpolar oceanic resonance to atmospheric forcing in our model is enhanced to reproduce observed variability when a constant freshwater forcing offset is applied over the convective region in the Nordic seas (63.75-78.75 • N and 11.25 • W-10 • E) as shown in Mengel et al (2012). The offset applied in their model study is 15 mSv and thereby within the range of natural variability as estimated by Curry and Mauritzen (2005) for the second half of the 20th century.…”

Section: Experiments Designmentioning

confidence: 59%

“…To avoid discontinuities the 1948-2009 period is combined with reversed 2009-1948 time series (Mengel et al, 2012). The resulting 122 yr time series is cyclic and continuously applied during spin-up.…”

Section: Experiments Designmentioning

confidence: 99%

A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age

Schleußner

Feulner

2013

Clim. Past

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Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly succeeded by the Little Ice Age are of exceptional importance. The origin of these regional climate anomalies remains a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully coupled model of intermediate complexity Climber 3α forced with stochastically reconstructed wind-stress fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea ice–ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions in our model leads to a spin-up of the subpolar gyre and a weakened Atlantic meridional overturning circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for understanding the dynamics of past and future climate.

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“…Born et al (2012) identified this salinity advection feedback on subpolar convection as a potential driver of multi-stability of the horizon- tal subpolar gyre circulation (Levermann and Born, 2007) in a multi-model study. The existence of such a multi-stability was found to greatly enhance subpolar variability close to the system's internal threshold (Mengel et al, 2012). Atlantic variability is found to contribute significantly to Northern Hemisphere variations Knight et al, 2005Knight et al, , 2006 and eventually to global mean temperature (GMT).…”

Section: Introductionmentioning

confidence: 99%

“…Additionally, also a direct feedback between AMOC and NADO temperature and salinity via subpolar convection might play a role. Increased sea surface salinity as a consequence of a strengthened AMOC could lead to enhanced subpolar convection (Mengel et al, 2012) and as a consequence to a deep-ocean freshening and cooling (Yashayaev, 2007).…”

Section: Deep Ocean Heat Uptake and Densitymentioning

confidence: 99%

The role of the North Atlantic overturning and deep ocean for multi-decadal global-mean-temperature variability

et al. 2014

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Abstract. Earth's climate exhibits internal modes of variability on various timescales. Here we investigate multi-decadal variability of the Atlantic meridional overturning circulation (AMOC), Northern Hemisphere sea-ice extent and global mean temperature (GMT) in an ensemble of CMIP5 models under control conditions. We report an inter-annual GMT variability of about ±0.1 • C originating solely from natural variability in the model ensemble. By decomposing the GMT variance into contributions of the AMOC and Northern Hemisphere sea-ice extent using a graph-theoretical statistical approach, we find the AMOC to contribute 8 % to GMT variability in the ensemble mean. Our results highlight the importance of AMOC sea-ice feedbacks that explain 5 % of the GMT variance, while the contribution solely related to the AMOC is found to be about 3 %. As a consequence of multi-decadal AMOC variability, we report substantial variations in North Atlantic deep-ocean heat content with trends of up to 0.7 × 10 22 J decade −1 that are of the order of observed changes over the last decade and consistent with the reduced GMT warming trend over this period. Although these temperature anomalies are largely densitycompensated by salinity changes, we find a robust negative correlation between the AMOC and North Atlantic deepocean density with density lagging the AMOC by 5 to 11 yr in most models. While this would in principle allow for a self-sustained oscillatory behavior of the coupled AMOCdeep-ocean system, our results are inconclusive about the role of this feedback in the model ensemble.

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Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

Cited by 14 publications

References 61 publications

Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean

Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean

A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age

The role of the North Atlantic overturning and deep ocean for multi-decadal global-mean-temperature variability

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