Detecting potential changes in the meridional overturning circulation at 26˚N in the Atlantic

Baehr, Johanna; Keller, Klaus; Marotzke, Jochem

doi:10.1007/s10584-006-9153-z

Cited by 44 publications

(61 citation statements)

References 55 publications

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“…Within the limitations of the currently available observations, the correspondence between the level of variability in RAPID/MOCHA and ECHAM5/MPI-OM increases the confidence in the estimates of the period it takes to detect a change in the MOC based on such a model (e.g. Baehr et al, 2007aBaehr et al, , 2008. As the continuous instrumental record, both from the global observing system and from regional monitoring systems such as RAPID/MOCHA, extends in time, these estimated detection times will slowly become verifiable against observations.…”

Section: Discussionmentioning

confidence: 99%

“…Prior to deployment, Hirschi et al (2003) and Baehr et al (2004) tested the array in two numerical models, showing that such an array should be capable of capturing both the time-mean of the MOC as well as the daily to annual variability. The simulated array was also capable of detecting longterm trends within several decades as shown by Baehr et al (2008) with a (univariate) MOC timeseries from a climate simulation forced with the Intergovernmental Panel on Climate Change (IPCC) scenario A1B (Nakicenovic and Swart, 2000).…”

Section: Introductionmentioning

confidence: 99%

“…As pointed out by various authors (e.g. Wunsch and Heimbach, 2006;Baehr et al, 2008), a time series of such transects represents sparse sampling (once every few years or decades) with serious aliasing problems, complicating estimates of variability or trends. Dynamically consistent ocean state estimates from the ECCO project have recently become available covering either the timespan of the altimetric record after 1992 (Wunsch and Heimbach, 2006), or going back to the beginning of the NCEP/NCAR reanalysis period in 1952 (Köhl and Stammer, 2008).…”

Section: Introductionmentioning

confidence: 99%

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Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

et al. 2009

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Abstract.Daily timeseries of the meridional overturning circulation (MOC) estimated from the UK/US RAPID/MOCHA array at 26.5 • N in the Atlantic are used to evaluate the MOC as simulated in two global circulation models: (I) an 8-member ensemble of the coupled climate model ECHAM5/MPI-OM, and (II) the ECCO-GODAE state estimate. In ECHAM5/MPI-OM, we find that the observed and simulated MOC have a similar variability and time-mean within the 99% confidence interval. In ECCO-GODAE, we find that the observed and simulated MOC show a significant correlation within the 99% confidence interval. To investigate the contribution of the different transport components, the MOC is decomposed into Florida Current, Ekman and mid-ocean transports. In both models, the midocean transport is closely approximated by the residual of the MOC minus Florida Current and Ekman transports. As the models conserve volume by definition, future comparisons of the RAPID/MOCHA mid-ocean transport should be done against the residual transport in the models. The similarity in the variance and the correlation between the RAPID/MOCHA, and respectively ECHAM5/MPI-OM and ECCO-GODAE MOC estimates at 26.5 • N is encouraging in the context of estimating (natural) variability in climate simulations and its use in climate change signal-to-noise detection analyses. Enhanced confidence in simulated hydrographic and transport variability will require longer observational time series.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

et al. 2009

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“…For the time when hydrographic data is available, our method should be combined with direct calculations of the MOC strength or direct estimates of mass transports based on velocity measurements and hydrographic fields (e.g., Hirschi et al 2003;Bryden et al 2005;Baehr et al 2004Baehr et al , 2007a, as well as indirect methods (Latif et al 2004;Hu et al 2004;Levermann et al 2005). Global inverse calculations from hydrographic observations or other volume transport estimates typically yield a standard error of 2-6 Sv in the MOC strength (Roemmich and Wunsch 1985;Ganachaud and Wunsch 2000;Ganachaud 2003;Smethie and Fine 2001;Bryden et al 2005).…”

Section: Discussionmentioning

confidence: 99%

Estimating trends of Atlantic meridional overturning circulation from long-term hydrographic data and model simulations

2008

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The ocean meridional overturning circulation (MOC) plays a central role for the climate in the Atlantic realm. Since scenarios for future climate change indicate a significant reduction of the MOC under global warming, an assessment of variations and trends of the real MOC is important. Using observations at ocean weather ship (OWS) stations and along oceanic sections, we examine the hydrographic information that can be used to determine MOC trends via its signature in water mass properties obtained from model simulations with the climate model ECHAM5/MPI-OM. We show that temperature trends at mid-latitudes provide useful indirect measure of large-scale changes of deep circulation: A mid-depth warming is related to MOC weakening and a cooling to MOC strengthening. Based on our model experiments, we argue that a continuation of measurements at key OWS sites may contribute to a timely detection of a possible future MOC slowdown and to separate the signal from interannual-tomultidecadal MOC variability. The simulations suggest that the subsurface hydrographic information related to MOC has a lower variability than the MOC trend measured directly. Based on our model and the available long-term hydrographic data, we estimate non-significant MOC trends for the last 80 years. For the twenty-first century, however, the model simulations predict a significant MOC decline and accompanied mid-depth warming trend.

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“…To identify periods during the last millennium where the NH temperatures in the forced simulations leave the range of internal variability, we apply detection methods previously developed for the discrimination between anthropogenic climate change and natural variability (Santer et al, 1995;Baehr et al, 2008). In a first step, NH temperatures from the control run are sampled for a specific segment length (Santer et al, 1995) estimating linear trends for 2900 years.…”

Section: Nh Temperature Trendsmentioning

confidence: 99%

Climate and carbon-cycle variability over the last millennium

et al. 2010

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Abstract. A long-standing task in climate research has been to distinguish between anthropogenic climate change and natural climate variability. A prerequisite for fulfilling this task is the understanding of the relative roles of external drivers and internal variability of climate and the carbon cycle. Here, we present the first ensemble simulations over the last 1200 years with a comprehensive Earth system model including a fully interactive carbon cycle. Applying up-to-date reconstructions of external forcing including the recent lowamplitude estimates of solar variations, the ensemble simulations reproduce temperature evolutions consistent with the range of reconstructions. The 20th-century warming trend stands out against all pre-industrial trends within the ensemble. Volcanic eruptions are necessary to explain variations in pre-industrial climate such as the Little Ice Age; yet only the strongest, repeated eruptions lead to cooling trends that differ significantly from the internal variability across all ensemble members. The simulated atmospheric CO 2 concentrations exhibit a stable carbon cycle over the pre-industrial era with multi-centennial variations somewhat smaller than in the observational records. Early land-cover changes have modulated atmospheric CO 2 concentrations only slightly. We provide a model-based quantification of the sensitivity (termed γ ) of the global carbon cycle to temperature for a variety of climate and forcing conditions. We diagnose a disCorrespondence to: J. H. Jungclaus (johann.jungclaus@zmaw.de) tinct dependence of γ on the forcing strength and time-scales involved, thus providing a possible explanation for the systematic difference in the observational estimates for different segments of the last millennium.

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Detecting potential changes in the meridional overturning circulation at 26˚N in the Atlantic

Cited by 44 publications

References 55 publications

Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

Estimating trends of Atlantic meridional overturning circulation from long-term hydrographic data and model simulations

Climate and carbon-cycle variability over the last millennium

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