Spread of model climate sensitivity linked to double‐Intertropical Convergence Zone bias

Tian, Baijun

doi:10.1002/2015gl064119

Cited by 104 publications

(135 citation statements)

References 31 publications

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“…These studies generally indicate that models with strongly positive low-cloud feedback are more consistent with observations than models with weakly positive or negative feedback, suggesting ECS more likely lies in the upper range of model estimates. This is in line with other model-observation comparisons that also point toward higher ECS (Fasullo and Trenberth 2012;Sherwood et al 2014;Tian 2015). By contrast, studies focusing on Earth's energy budget generally point toward a lower ECS (Otto et al 2013), albeit with large uncertainties.…”

Section: Introductionsupporting

confidence: 64%

“…(Acronym expansions are available online at http://www.ametsoc.org/ PubsAcronymList.) (Lin 2007;Li and Xie 2014;Tian 2015). Identifying low-cloud regions with a fixed percentile of the relative humidity distribution has several advantages over using other common low-cloud proxies, such as fixed regions (Klein and Hartmann 1993;Qu et al 2014), or fixed threshold values of midtropospheric vertical velocities (Bony and Dufresne 2005;Vial et al 2013).…”

Section: Low-cloud Regionsmentioning

confidence: 99%

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Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

2016

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Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by 2(0.96 6 0.22)% K 21 (90% confidence level) for deseasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r 5 20.67) with the models' ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K; an ECS below 2.3 K becomes very unlikely (90% confidence).

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Section: Introductionsupporting

confidence: 64%

Section: Low-cloud Regionsmentioning

confidence: 99%

Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

2016

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“…Consequently, model biases in ITCZ width could have implications for estimated climate sensitivity, like the double-ITCZ biases that have already been shown to be related to climate sensitivity (Tian 2015). Despite its importance, the mechanisms responsible for setting the ITCZ width and for its sensitivity to the seasonal cycle, to interannual variability, and to changes in climate have not been elucidated.…”

Section: Introductionmentioning

confidence: 99%

Energetic Constraints on the Width of the Intertropical Convergence Zone

Byrne

Schneider

2016

Journal of Climate

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The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics.

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“…In comparing GCMderived values to those estimated from observations, they provide evidence favoring a reduction in low clouds under warming, supporting the existence of an associated positive feedback but with a relatively weak constraint on its precise value. More recently, Tian [41] relates the magnitude of the double-ITCZ bias to simulated ECS, finding low sensitivity models to be particularly flawed in representing its southern branch. Notwithstanding these findings and those of the broader EC literature, the exact linkages between the magnitude of subtropical cloud feedbacks and present-day climatological biases in CMIP models remain somewhat uncertain and a challenge to disentangle (e.g., [43]).…”

Section: Cloud-related Ecsmentioning

confidence: 99%

Recent Progress in Constraining Climate Sensitivity With Model Ensembles

Fasullo

Sanderson

Trenberth

2015

Curr Clim Change Rep

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Recently available model ensembles have created an unprecedented opportunity for exploring and narrowing uncertainty in one of climate's benchmark indices, equilibrium climate sensitivity. A range of novel approaches for constraining the raw sensitivity estimates from these ensembles with observations has also been proposed, applied, and explored in a diversity of contexts. Through subsequent analysis, an increased understanding of the relative merits and limitations of these methods has been gained and their refinement and optimal implementation continue to be actively studied and debated with the hopes of reducing uncertainty in one of climate science's most persistent and elusive measures.

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Spread of model climate sensitivity linked to double‐Intertropical Convergence Zone bias

Cited by 104 publications

References 31 publications

Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

Energetic Constraints on the Width of the Intertropical Convergence Zone

Recent Progress in Constraining Climate Sensitivity With Model Ensembles

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