Detectability of Changes in the Walker Circulation in Response to Global Warming*

Abstract Changes in the gradients in sea level pressure (SLP) and sea surface temperature (SST) along the equatorial Pacific are analyzed in observations and 101 numerical experiments performed with 37 climate models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The ensemble of numerical experiments simulates changes in the earth’s climate during the 1870–2004 period in response to changes in natural (solar variations and volcanoes) and anthropogenic (well-mixed greenhouse gases, ozone, direct aerosol forcing, and land use) radiative forcings. A reduction in the zonal SLP gradient is present in observational records and is the typical response of the ensemble, yet only 26 out of the 101 experiments exhibit a reduced SLP gradient within 95% statistical confidence of the observed value. The multimodel response indicates a reduction of the Walker circulation to historical forcings, albeit an order of magnitude smaller than the observed value. There are multiple nonexclusive interpretations of these results: (i) the observed trend may not be entirely forced and includes a substantial component from internal variability; (ii) there are problems with the observational record that lead to a spuriously large trend; and (iii) the strength of the Walker circulation, as measured by the zonal SLP gradient, may be less sensitive to external forcing in models than in the real climate system. Analysis of a subset of experiments suggests that greenhouse gases act to weaken the circulation, but aerosol forcing drives a strengthening of the circulation, which appears to be overestimated by the models, resulting in a muted response to the combined anthropogenic forcings.

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Inability of CMIP5 Models to Simulate Recent Strengthening of the Walker Circulation: Implications for Projections

Journal of Climate ◽

10.1175/jcli-d-13-00752.1 ◽

2014 ◽

Vol 28 (1) ◽

pp. 20-35 ◽

Cited By ~ 60

Author(s):

Greg Kociuba ◽

Scott B. Power

Keyword(s):

Climate Models ◽

Coupled Model ◽

Walker Circulation ◽

Internal Variability ◽

Cmip5 Models ◽

Eastern Equatorial Pacific ◽

Rcp 8.5 ◽

Intercomparison Project ◽

The One ◽

The Walker Circulation

Abstract This paper examines changes in the strength of the Walker circulation (WC) using the pressure difference between the western and eastern equatorial Pacific. Changes in observations and in 35 climate models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) are determined. On the one hand, 78% of the models show a weakening of the WC over the twentieth century, consistent with the observations and previous studies using CMIP phase 3 (CMIP3) models. However, the observations also exhibit a strengthening in the last three decades (i.e., from 1980 to 2012) that is statistically significant at the 95% level. The models, on the other hand, show no consensus on the sign of change, and none of the models shows a statistically significant strengthening over the same period. While the reasons for the inconsistency between models and observations is not fully understood, it is shown that the ability of the models to generate trends as large as the observed from internal variability is reduced because most models have weaker than observed levels of both multidecadal variability and persistence of interannual variability in WC strength. In the twenty-first-century future projections, the WC weakens in 25 out of 35 models, under representative concentration pathway (RCP) 8.5, 9 out of 11 models under RCP6.0, 16 out of 18 models under RCP4.5, and 12 out of 15 models under RCP2.6. The projected decrease is also consistent with results obtained previously using models from CMIP3. However, as the reasons for the inconsistency between modeled and observed trends in the last three decades are not fully understood, confidence in the model projections is reduced.

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Effective radiative forcing and adjustments in CMIP6 models

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9591-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9591-9618 ◽

Cited By ~ 9

Author(s):

Christopher J. Smith ◽

Ryan J. Kramer ◽

Gunnar Myhre ◽

Kari Alterskjær ◽

William Collins ◽

...

Keyword(s):

Climate Sensitivity ◽

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Internal Variability ◽

Cmip5 Models ◽

Model Intercomparison ◽

Aerosol Forcing ◽

Intercomparison Project ◽

Effective Radiative Forcing

Abstract. The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (±0.23) W m−2, comprised of 1.81 (±0.09) W m−2 from CO2, 1.08 (± 0.21) W m−2 from other well-mixed greenhouse gases, −1.01 (± 0.23) W m−2 from aerosols and −0.09 (±0.13) W m−2 from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m−2. The majority of the remaining 0.21 W m−2 is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from −0.63 to −1.37 W m−2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4×CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.

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Anthropogenic aerosol forcing – insights from multiple estimates from aerosol-climate models with reduced complexity

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-6821-2019 ◽

2019 ◽

Vol 19 (10) ◽

pp. 6821-6841 ◽

Cited By ~ 10

Author(s):

Stephanie Fiedler ◽

Stefan Kinne ◽

Wan Ting Katty Huang ◽

Petri Räisänen ◽

Declan O'Donnell ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Cloud Droplet ◽

Internal Variability ◽

Anthropogenic Aerosol ◽

Aerosol Forcing ◽

Effective Radiative Forcing ◽

Absolute Magnitudes ◽

Natural Aerosols

Abstract. This study assesses the change in anthropogenic aerosol forcing from the mid-1970s to the mid-2000s. Both decades had similar global-mean anthropogenic aerosol optical depths but substantially different global distributions. For both years, we quantify (i) the forcing spread due to model-internal variability and (ii) the forcing spread among models. Our assessment is based on new ensembles of atmosphere-only simulations with five state-of-the-art Earth system models. Four of these models will be used in the sixth Coupled Model Intercomparison Project (CMIP6; Eyring et al., 2016). Here, the complexity of the anthropogenic aerosol has been reduced in the participating models. In all our simulations, we prescribe the same patterns of the anthropogenic aerosol optical properties and associated effects on the cloud droplet number concentration. We calculate the instantaneous radiative forcing (RF) and the effective radiative forcing (ERF). Their difference defines the net contribution from rapid adjustments. Our simulations show a model spread in ERF from −0.4 to −0.9 W m−2. The standard deviation in annual ERF is 0.3 W m−2, based on 180 individual estimates from each participating model. This result implies that identifying the model spread in ERF due to systematic differences requires averaging over a sufficiently large number of years. Moreover, we find almost identical ERFs for the mid-1970s and mid-2000s for individual models, although there are major model differences in natural aerosols and clouds. The model-ensemble mean ERF is −0.54 W m−2 for the pre-industrial era to the mid-1970s and −0.59 W m−2 for the pre-industrial era to the mid-2000s. Our result suggests that comparing ERF changes between two observable periods rather than absolute magnitudes relative to a poorly constrained pre-industrial state might provide a better test for a model's ability to represent transient climate changes.

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Anthropogenic aerosol forcing – insights from multi-estimates from aerosol-climate models with reduced complexity

10.5194/acp-2018-639 ◽

2018 ◽

Author(s):

Stephanie Fiedler ◽

Stefan Kinne ◽

W. T. Katty Huang ◽

Petri Räisänen ◽

Declan O'Donnell ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Internal Variability ◽

Anthropogenic Aerosol ◽

Aerosol Forcing ◽

Aerosol Distribution ◽

Absolute Magnitudes ◽

Small Change ◽

Natural Aerosol

Abstract. The radiative forcing of anthropogenic aerosol remains a key uncertainty in the understanding of climate change. This study quantifies the model spread in aerosol forcing associated with (i) variability internal to the atmosphere and (ii) differences in the model representation of weather. We do so by performing ensembles of atmosphere-only simulations with four state-of-the-art Earth system models, three of which will be used in the sixth coupled model inter-comparison project (CMIP6, Eyring et al., 2016). In those models we reduce the complexity of the anthropogenic aerosol by prescribing the same annually-repeating patterns of the anthropogenic aerosol optical properties and associated effects on the cloud reflectivity. We quantify a comparably small model spread in the long-term averaged ERF compared to the overall possible range in annual ERF estimates associated with model-internal variability. This implies that identifying the true model spread in ERF associated with differences in the representation of meteorological processes and natural aerosol requires averaging over a sufficiently large number of annual estimates. We characterize the model diversity in clouds and use satellite products as benchmarks. Despite major inter-model differences in natural aerosol and clouds, all models show only a small change in the global-mean ERF due to the substantial change in the global anthropogenic aerosol distribution between the mid-1970s and mid-2000s, the ensemble mean ERF being −0.47 Wm−2 for the mid-1970s and −0.51 Wm−2 for the mid-2000s. This result suggests that inter-comparing ERF changes between two periods rather than absolute magnitudes relative to pre-industrial might provide a more stringent test for a model's ability for representing climate evolutions.

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Climate Drift in the CMIP5 Models*

Journal of Climate ◽

10.1175/jcli-d-12-00521.1 ◽

2013 ◽

Vol 26 (21) ◽

pp. 8597-8615 ◽

Cited By ~ 128

Author(s):

Alexander Sen Gupta ◽

Nicolas C. Jourdain ◽

Jaclyn N. Brown ◽

Didier Monselesan

Keyword(s):

Climate Models ◽

Coupled Model ◽

Deep Ocean ◽

Internal Variability ◽

Correction Method ◽

Physical Models ◽

Cmip5 Models ◽

Steric Sea Level ◽

Biogeochemical Models ◽

The Difference

Abstract Climate models often exhibit spurious long-term changes independent of either internal variability or changes to external forcing. Such changes, referred to as model “drift,” may distort the estimate of forced change in transient climate simulations. The importance of drift is examined in comparison to historical trends over recent decades in the Coupled Model Intercomparison Project (CMIP). Comparison based on a selection of metrics suggests a significant overall reduction in the magnitude of drift from phase 3 of CMIP (CMIP3) to phase 5 of CMIP (CMIP5). The direction of both ocean and atmospheric drift is systematically biased in some models introducing statistically significant drift in globally averaged metrics. Nevertheless, for most models globally averaged drift remains weak compared to the associated forced trends and is often smaller than the difference between trends derived from different ensemble members or the error introduced by the aliasing of natural variability. An exception to this is metrics that include the deep ocean (e.g., steric sea level) where drift can dominate in forced simulations. In such circumstances drift must be corrected for using information from concurrent control experiments. Many CMIP5 models now include ocean biogeochemistry. Like physical models, biogeochemical models generally undergo long spinup integrations to minimize drift. Nevertheless, based on a limited subset of models, it is found that drift is an important consideration and must be accounted for. For properties or regions where drift is important, the drift correction method must be carefully considered. The use of a drift estimate based on the full control time series is recommended to minimize the contamination of the drift estimate by internal variability.

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Multimodel Multisignal Climate Change Detection at Regional Scale

Journal of Climate ◽

10.1175/jcli3851.1 ◽

2006 ◽

Vol 19 (17) ◽

pp. 4294-4307 ◽

Cited By ~ 50

Author(s):

Xuebin Zhang ◽

Francis W. Zwiers ◽

P. A. Stott

Keyword(s):

Climate Change ◽

Twentieth Century ◽

Greenhouse Gases ◽

Regional Scale ◽

Internal Variability ◽

Anthropogenic Forcing ◽

Internal Climate Variability ◽

Aerosol Forcing ◽

Spatial Domains ◽

Northern Hemispheric

Abstract Using an optimal detection technique and climate change simulations produced with two versions of two GCMs, we have assessed the causes of twentieth-century temperature changes from global to regional scales. Our analysis is conducted in nine spatial domains: 1) the globe; 2) the Northern Hemisphere; four large regions in the Northern Hemispheric midlatitudes covering 30°–70°N including 3) Eurasia, 4) North America, 5) Northern Hemispheric land only, 6) the entire 30°–70°N belt; and three smaller regions over 7) southern Canada, 8) southern Europe, and 9) China. We find that the effect of anthropogenic forcing on climate is clearly detectable at global through regional scales. The effect of combined greenhouse gases and sulfate aerosol forcing is detectable in all nine domains in annual and seasonal mean temperatures observed during the second half of the twentieth century. The effect of greenhouse gases can also be separated from that of sulfate aerosols over this period at continental and regional scales. Uncertainty in these results is larger in the smaller spatial domains. Detection is improved when an ensemble of models is used to estimate the response to anthropogenic forcing and the underlying internal variability of the climate system. Our detection results hold after removal of North Atlantic Oscillation (NAO)-related variability in temperature observations—variability that may or may not be associated with anthropogenic forcing. They also continue to hold when our estimates of natural internal climate variability are doubled.

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The Key Role of Ozone-Depleting Substances in Weakening the Walker Circulation in the Second Half of the Twentieth Century

Journal of Climate ◽

10.1175/jcli-d-17-0906.1 ◽

2019 ◽

Vol 32 (5) ◽

pp. 1411-1418 ◽

Cited By ~ 1

Author(s):

Lorenzo M. Polvani ◽

Katinka Bellomo

Keyword(s):

Twentieth Century ◽

Pacific Ocean ◽

Greenhouse Gases ◽

Radiative Forcing ◽

Climate Model ◽

Walker Circulation ◽

Surface Warming ◽

Ozone Depleting Substances ◽

The Antarctic ◽

The Walker Circulation

It is widely appreciated that ozone-depleting substances (ODS), which have led to the formation of the Antarctic ozone hole, are also powerful greenhouse gases. In this study, we explore the consequence of the surface warming caused by ODS in the second half of the twentieth century over the Indo-Pacific Ocean, using the Whole Atmosphere Chemistry Climate Model (version 4). By contrasting two ensembles of chemistry–climate model integrations (with and without ODS forcing) over the period 1955–2005, we show that the additional greenhouse effect of ODS is crucial to producing a statistically significant weakening of the Walker circulation in our model over that period. When ODS concentrations are held fixed at 1955 levels, the forcing of the other well-mixed greenhouse gases alone leads to a strengthening—rather than weakening—of the Walker circulation because their warming effect is not sufficiently strong. Without increasing ODS, a surface warming delay in the eastern tropical Pacific Ocean leads to an increase in the sea surface temperature gradient between the eastern and western Pacific, with an associated strengthening of the Walker circulation. When increasing ODS are added, the considerably larger total radiative forcing produces a much faster warming in the eastern Pacific, causing the sign of the trend to reverse and the Walker circulation to weaken. Our modeling result suggests that ODS may have been key players in the observed weakening of the Walker circulation over the second half of the twentieth century.

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Did tropical precipitation improve in CMIP6 simulations?

10.5194/egusphere-egu2020-3579 ◽

2020 ◽

Author(s):

Stephanie Fiedler ◽

Traute Crueger ◽

Roberta D’Agostino ◽

Karsten Peters ◽

Tobias Becker ◽

...

Keyword(s):

Climate Models ◽

Climate Model ◽

Southern Oscillation ◽

Spatial Scales ◽

Precipitation Amount ◽

Coupled Model ◽

Internal Variability ◽

Atmospheric Composition ◽

Tropical Precipitation ◽

General Aspects

<p>Climate models are known to have biases in tropical precipitation. We assessed to what extent simulations of tropical precipitation have improved in the new Coupled Model Intercomparison Project (CMIP) phase six, using state-of-the-art observational products and model results from the earlier CMIP phases three and five. We characterize tropical precipitation with different well-established metrics. Our assessment includes (1) general aspects of the mean climatology like precipitation associated with the Intertropical Convergence Zone and shallow cloud regimes in the tropics, (2) solar radiative effects including the summer monsoons and the time of occurrence of tropical precipitation in the course of the day, (3) modes of internal variability such as the Madden-Julian Oscillation and the El Ni&#241;o Southern Oscillation, and (4) changes in the course of the 20th century. The results point to improvements of CMIP6 models for some metrics, e.g., the occurrence of drizzle events and consecutive dry days. However, no improvements of CMIP6 models are identified for other aspects of tropical precipitation. These include the area and intensity of the global summer monsoon as well as the diurnal cycle of the tropical precipitation amount, frequency and intensity.</p><p>All our metrics taken together, CMIP6 models show no systematic improvement of tropical precipitation across different temporal and spatial scales. The model biases in the spatial distribution of tropical precipitation are typically larger than the changes associated with anthropogenic warming. Given the pace of climate change as compared to the pace of climate model improvements, we suggest to use novel modeling approaches to understand the responseof tropical precipitation to changes in atmospheric composition.</p>

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What Caused the Observed Twentieth-Century Weakening of the Walker Circulation?

Journal of Climate ◽

10.1175/2011jcli4101.1 ◽

2011 ◽

Vol 24 (24) ◽

pp. 6501-6514 ◽

Cited By ~ 44

Author(s):

Scott B. Power ◽

Greg Kociuba

Keyword(s):

Twentieth Century ◽

Climate Models ◽

Southern Oscillation ◽

Walker Circulation ◽

Natural Variability ◽

First Century ◽

External Forcing ◽

Wide Range ◽

Twenty First Century ◽

The Walker Circulation

Abstract The Walker circulation (WC) is one of the world’s most prominent and important atmospheric systems. The WC weakened during the twentieth century, reaching record low levels in recent decades. This weakening is thought to be partly due to global warming and partly due to internally generated natural variability. There is, however, no consensus in the literature on the relative contribution of external forcing and natural variability to the observed weakening of the WC. This paper examines changes in the strength of the WC using an index called BoxΔP, which is equal to the difference in mean sea level pressure across the equatorial Pacific. Change in both the observations and in World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. The annual average BoxΔP declines in the observations and in 15 out of 23 models during the twentieth century (results that are significant at or above the 95% level), consistent with earlier work. However, the magnitude of the multimodel ensemble mean (MMEM) 1901–99 trend (−0.10 Pa yr−1) is much smaller than the magnitude of the observed trend (−0.52 Pa yr−1). While a wide range of trends is evident in the models with approximately 90% of the model trends in the range (−0.25 to +0.1 Pa yr−1), even this range is too narrow to encompass the magnitude of the observed trend. Twenty-first-century changes in BoxΔP under the Special Report on Emissions Scenarios (SRES) A1B and A2 are also examined. Negative trends (i.e., weaker WCs) are evident in all seasons. However, the MMEM trends for the A1B and A2 scenarios are smaller in magnitude than the magnitude of the observed trend. Given that external forcing linked to greenhouse gases is much larger in the twenty-first-century scenarios than twentieth-century forcing, this, together with the twentieth-century results mentioned above, would seem to suggest that external forcing has not been the primary driver of the observed weakening of the WC. However, 9 of the 23 models are unable to account for the observed change unless the internally generated component of the trend is very large. But indicators of observed variability linked to El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation have modest trends, suggesting that internally variability has been modest. Furthermore, many of the nine “inconsistent” models tend to have poorer simulations of climatic features linked to ENSO. In addition, the externally forced component of the trend tends to be larger in magnitude and more closely matches the observed trend in the models that are better able to reproduce ENSO-related variability. The “best” four models, for example, have a MMEM of −0.2 Pa yr−1 (i.e., approximately 40% of the observed change), suggesting a greater role for external forcing in driving the observed trend. These and other considerations outlined below lead the authors to conclude that (i) both external forcing and internally generated variability contributed to the observed weakening of the WC over the twentieth century and (ii) external forcing accounts for approximately 30%–70% of the observed weakening with internally generated climate variability making up the rest.

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Reduced complexity model intercomparison project phase 1: Protocol, results and initial observations

10.5194/gmd-2019-375 ◽

2020 ◽

Cited By ~ 1

Author(s):

Zebedee R. J. Nicholls ◽

Malte Meinshausen ◽

Jared Lewis ◽

Robert Gieseke ◽

Dietmar Dommenget ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Phase 1 ◽

Coupled Model ◽

Surface Air Temperature ◽

Internal Variability ◽

Second Phase ◽

Model Intercomparison ◽

Intercomparison Project ◽

Reduced Complexity

Abstract. Here we present results from the first phase of the Reduced Complexity Model Intercomparison Project (RCMIP). RCMIP is a systematic examination of reduced complexity climate models (RCMs), which are used to complement and extend the insights from more complex Earth System Models (ESMs), in particular those participating in the Sixth Coupled Model Intercomparison Project (CMIP6). In Phase 1 of RCMIP, with 14 participating models namely ACC2, AR5IR (2 and 3 box versions), CICERO-SCM, ESCIMO, FaIR, GIR, GREB, Hector, Held et al. two layer model, MAGICC, MCE, OSCAR and WASP, we highlight the structural differences across various RCMs and show that RCMs are capable of reproducing global-mean surface air temperature (GSAT) changes of ESMs and historical observations. We find that some RCMs are capable of emulating the GSAT response of CMIP6 models to within a root-mean square error of 0.2 °C (of the same order of magnitude as ESM internal variability) over a range of scenarios. Running the same model configurations for both RCP and SSP scenarios, we see that the SSPs exhibit higher effective radiative forcing throughout the second half of the 21st Century. Comparing our results to the difference between CMIP5 and CMIP6 output, we find that the change in scenario explains approximately 46 % of the increase in higher end projected warming between CMIP5 and CMIP6. This suggests that changes in ESMs from CMIP5 to CMIP6 explain the rest of the increase, hence the higher climate sensitivities of available CMIP6 models may not be having as large an impact on GSAT projections as first anticipated. A second phase of RCMIP will complement RCMIP Phase 1 by exploring probabilistic results and emulation in more depth to provide results available for the IPCC's Sixth Assessment Report author teams.

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