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Buwen Dong
,
Jonathan M. Gregory
, and
Rowan T. Sutton

Abstract

Climate model simulations consistently show that surface temperature over land increases more rapidly than over sea in response to greenhouse gas forcing. The enhanced warming over land is not simply a transient effect caused by the land–sea contrast in heat capacities, since it is also present in equilibrium conditions. This paper elucidates the transient adjustment processes over time scales of days to weeks of the surface and tropospheric climate in response to a doubling of CO2 and to changes in sea surface temperature (SST), imposed separately and together, using ensembles of experiments with an atmospheric general circulation model. These adjustment processes can be grouped into three stages: immediate response of the troposphere and surface processes (day 1), fast adjustment of surface processes (days 2–5), and adjustment of the whole troposphere (days 6–20).

Some land surface warming in response to doubled CO2 (with unchanged SSTs) occurs immediately because of increased downward longwave radiation. Increased CO2 also leads to reduced plant stomatal resistance and hence restricted evaporation, which increases land surface warming in the first day. Rapid reductions in cloud amount lead in the next few days to increased downward shortwave radiation and further warming, which spreads upward from the surface, and by day 5 the surface and tropospheric response is statistically consistent with the equilibrium value. Land surface warming in response to imposed SST change (with unchanged CO2) is slower. Tropospheric warming is advected inland from the sea, and over land it occurs at all levels together rather than spreading upward from the surface. The atmospheric response to prescribed SST change in about 20 days is statistically consistent with the equilibrium value, and the warming is largest in the upper troposphere over both land and sea. The land surface warming involves reduction of cloud cover and increased downward shortwave radiation, as in the experiment with CO2 change, but in this case it is due to the restriction of moisture supply to the land (indicated by reduced soil moisture), whereas in the CO2 forcing experiment it is due to restricted evaporation despite increased moisture supply (indicated by increased soil moisture). The warming over land in response to SST change is greater than over the sea and is the dominant contribution to the land–sea warming contrast under enhanced CO2 forcing.

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Ruijie Zhang
,
Buwen Dong
,
Zhiping Wen
,
Yuanyuan Guo
, and
Xiaodan Chen

Abstract

Air–sea coupling system in the southwestern Indian Ocean (SWIO; 35°–55°S, 40°–75°E) exhibits predominant multidecadal variability that is the strongest during austral summer. It is characterized by an equivalent barotropic atmospheric high (low) pressure over warm (cold) sea surface temperature (SST) anomalies and a poleward (equatorward) shift of the westerlies during the positive (negative) phase. In this study, physical processes of this multidecadal variability are investigated by using observations/reanalysis and CMIP6 model simulations. Results suggest that the multidecadal fluctuation can be explained by the modulation of the Atlantic meridional overturning circulation (AMOC) and the local air–sea positive feedback in the SWIO. In both observations/reanalysis and CMIP6 model simulations, the AMOC fluctuation presents a significantly negative correlation with the multidecadal SST variation in the SWIO when the AMOC is leading by about a decade. The mechanisms are that the preceding AMOC variation can cause an interhemispheric dipolar pattern of SST anomalies in the Atlantic Ocean. Subsequently, the SST anomalies in the midlatitudes of the South Atlantic can propagate to the SWIO by the oceanic Rossby wave under the influence of the Antarctic Circumpolar Current (ACC). Once the SST anomalies reach the SWIO, these SST anomalies in the oceanic front can affect the baroclinicity in the lower troposphere to influence the synoptic transient eddy and then cause the atmospheric circulation anomaly via the eddy–mean flow interaction. Subsequently, the anomalous atmospheric circulation over the SWIO can significantly strengthen the SST anomalies through modifying the oceanic meridional temperature advection and latent and sensible heat flux.

Open access
Shuheng Lin
,
Buwen Dong
,
Song Yang
,
Shan He
, and
Yamin Hu

Abstract

This study examines the fidelity of 47 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the influence of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) during the ENSO decaying summer. The response of the SEASM to ENSO shows a large model spread among the models, some of which even simulate opposite signs of SEASM anomalies compared to the observed values. The bad-performance models (BPMs) are therefore selected to be compared with both the good-performance models (GPMs) and observations to explore the possible causes of the deficiency. Results show that in the BPMs, the ENSO-related warm sea surface temperature (SST) anomalies extend too far westward in the western equatorial Pacific (WEP) and they do not dissipate in the El Niño decaying summer in comparison with those in the GPMs and observations, interfering with the effect of ENSO on the SEASM. The slow decay of WEP SST anomalies from the El Niño mature winter to the decaying summer in the BPMs is mainly caused by a weak negative shortwave radiation feedback due to a low sensitivity of convection to local SST anomalies, which is related to the cold bias in climatological SST over this region. On the other hand, from the mature winter to the decaying summer of El Niño, the El Niño–related anomalous eastward current does not reverse to a westward current in the BPMs, which also contributes to the slow decay of WEP SST anomalies via inducing excessively persistent warm zonal advection.

Significance Statement

We investigate the possible causes of the diverse impacts of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) among 47 CMIP6 models. We find that a plausible reason for the deficiency of some models in simulating the influence of ENSO on the monsoon is that the sea surface temperature (SST) anomalies associated with ENSO are unrealistic in the western equatorial Pacific (WEP) in these models. Further diagnoses indicate that the unrealistic WEP SST anomalies are related to the cold bias of the climatological SST, which could lead to a weak negative shortwave radiation feedback and excessively persistent warm zonal advection. The information provided in this study is useful for improving the skill of the climate models in representing the ENSO–SEASM relationship.

Open access
Nicholas M. J. Hall
,
Paul J. Valdes
, and
Buwen Dong

Abstract

A 5-yr simulation of the last glacial maximum using the UGAMP GCM is presented. It has a full seasonal cycle, T42 resolution, and interactive land surface and sea ice. Boundary conditions of SST, sea ice extent and land ice elevation are taken from the CLIMAP dataset and orbital parameters and carbon dioxide concentration are adjusted. It is compared with a 10-yr simulation of present-day climate using the same model.

The results are analyzed in terms of processes leading to the maintenance of the atmospheric circulation and temperature structure, midlatitude transient behavior, precipitation, and eventually accumulation of ice over the glaciers. The model responds in a similar manner to previous studies in global mean statistics but differs in its treatment of regional climates. Changes in sea ice and orography are equally important in determining the positions of the upper-level jets. The Atlantic jet and storm track in particular are much stronger than in the present-day simulation, and the associated distribution of precipitation and snowfall changes accordingly. Both major ice sheets are maintained by snowfall at the center and ablation at the edges at a reasonable rate through the annual cycle.

The results with a full seasonal cycle are compared to perpetual integrations by the authors and found to be very similar in most measures.

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Buwen Dong
,
Rowan T. Sutton
,
Len Shaffrey
, and
Nicholas P. Klingaman

Abstract

There is still no consensus about the best methodology for attributing observed changes in climate or climate events. One widely used approach relies on experiments in which the time periods of interest are simulated using an atmospheric general circulation model (AGCM) forced by prescribed sea surface temperatures (SSTs), with and without estimated anthropogenic influences. A potential limitation of such experiments is the lack of explicit atmosphere–ocean coupling; therefore a key question is whether the attribution statements derived from such studies are in fact robust. In this research the authors have carried out climate model experiments to test attribution conclusions in a situation where the answer is known—a so-called perfect model approach. The study involves comparing attribution conclusions for decadal changes derived from experiments with a coupled climate model (specifically an AGCM coupled to an ocean mixed-layer model) with conclusions derived from parallel experiments with the same AGCM forced by SSTs derived from the coupled model simulations. Results indicate that attribution conclusions for surface air temperature changes derived from AGCM experiments are generally robust and not sensitive to air–sea coupling. However, changes in seasonal mean and extreme precipitations, and circulation in some regions, show large sensitivity to air–sea coupling, notably in the summer monsoons over East Asia and Australia. Comparison with observed changes indicates that the coupled simulations generally agree better with observations. These results demonstrate that the AGCM-based attribution method has limitations and may lead to erroneous attribution conclusions in some regions for local circulation and mean and extreme precipitation. The coupled mixed-layer model used in this study offers an alternative and, in some respects, superior tool for attribution studies.

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Ed Hawkins
,
Buwen Dong
,
Jon Robson
,
Rowan Sutton
, and
Doug Smith

Abstract

Decadal climate predictions exhibit large biases, which are often subtracted and forgotten. However, understanding the causes of bias is essential to guide efforts to improve prediction systems, and may offer additional benefits. Here the origins of biases in decadal predictions are investigated, including whether analysis of these biases might provide useful information. The focus is especially on the lead-time-dependent bias tendency. A “toy” model of a prediction system is initially developed and used to show that there are several distinct contributions to bias tendency. Contributions from sampling of internal variability and a start-time-dependent forcing bias can be estimated and removed to obtain a much improved estimate of the true bias tendency, which can provide information about errors in the underlying model and/or errors in the specification of forcings. It is argued that the true bias tendency, not the total bias tendency, should be used to adjust decadal forecasts.

The methods developed are applied to decadal hindcasts of global mean temperature made using the Hadley Centre Coupled Model, version 3 (HadCM3), climate model, and it is found that this model exhibits a small positive bias tendency in the ensemble mean. When considering different model versions, it is shown that the true bias tendency is very highly correlated with both the transient climate response (TCR) and non–greenhouse gas forcing trends, and can therefore be used to obtain observationally constrained estimates of these relevant physical quantities.

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Buwen Dong
,
Rowan T. Sutton
,
Ellie Highwood
, and
Laura Wilcox

Abstract

In this study, the atmospheric component of a state-of-the-art climate model [the Hadley Centre Global Environment Model, version 2–Earth System (HadGEM2-ES)] has been used to investigate the impacts of regional anthropogenic sulfur dioxide emissions on boreal summer Sahel rainfall. The study focuses on the transient response of the West African monsoon (WAM) to a sudden change in regional anthropogenic sulfur dioxide emissions, including land surface feedbacks but without sea surface temperature (SST) feedbacks. The response occurs in two distinct phases: 1) fast adjustment of the atmosphere on a time scale of days to weeks (up to 3 weeks) through aerosol–radiation and aerosol–cloud interactions with weak hydrological cycle changes and surface feedbacks and 2) adjustment of the atmosphere and land surface with significant local hydrological cycle changes and changes in atmospheric circulation (beyond 3 weeks).

European emissions lead to an increase in shortwave (SW) scattering by increased sulfate burden, leading to a decrease in surface downward SW radiation that causes surface cooling over North Africa, a weakening of the Saharan heat low and WAM, and a decrease in Sahel precipitation. In contrast, Asian emissions lead to very little change in sulfate burden over North Africa, but they induce an adjustment of the Walker circulation, which leads again to a weakening of the WAM and a decrease in Sahel precipitation. The responses to European and Asian emissions during the second phase exhibit similar large-scale patterns of anomalous atmospheric circulation and hydrological variables, suggesting a preferred response. The results support the idea that sulfate aerosol emissions contributed to the observed decline in Sahel precipitation in the second half of the twentieth century.

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Wei Chen
,
Buwen Dong
,
Laura Wilcox
,
Feifei Luo
,
Nick Dunstone
, and
Eleanor J. Highwood

ABSTRACT

Observations indicate large changes in temperature extremes over China during the last four decades, exhibiting as significant increases in the amplitude and frequency of hot extremes and decreases in the amplitude and frequency of cold extremes. An ensemble of transient experiments with the fully coupled atmosphere–ocean model HadGEM3-GC2, including both anthropogenic forcing and natural forcing, successfully reproduces the spatial pattern and magnitude of observed historical trends in both hot and cold extremes. The model-simulated trends in temperature extremes primarily come from the positive trends in clear-sky longwave radiation, which is mainly due to the increases in greenhouse gases (GHGs). An ensemble of sensitivity experiments with Asian anthropogenic aerosol (AA) emissions fixed at their 1970s levels tends to overestimate the trends in temperature extremes, indicating that local AA emission changes have moderated the trends in these temperature extremes over China. The recent increases in Asian AA drive cooling trends over China by inducing negative clear-sky shortwave radiation directly through the aerosol–radiation interaction, which partly offsets the strong warming effect by GHG changes. The cooling trends induced by Asian AA changes are weaker over northern China during summer, which is due to the warming effect by the positive shortwave cloud radiative effect through the AA-induced atmosphere–cloud feedback. This accounts for the observed north–south gradients of the historical trends in some temperature extremes over China, highlighting the importance of local Asian AA emission changes on spatial heterogeneity of trends in temperature extremes.

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Xiaoxuan Zhao
,
Riyu Lu
,
Buwen Dong
,
Xiaowei Hong
,
Junqi Liu
, and
Jianqi Sun

Abstract

In this study, we investigate circulation, convection, and sea surface temperature (SST) anomalies associated with interannual variability of the cross-equatorial flow (CEF) intensity over the Maritime Continent (MC) in boreal summer. Observational diagnostics show that strengthened CEF is associated with large-scale circulation anomalies featuring weakened Walker circulation, upper-level northeasterly anomalies across the MC, and lower-level cyclonic anomalies over the tropical western North Pacific (WNP). Further analyses indicate that strengthened CEF is associated with both La Niña–like SST anomalies in the preceding winter and El Niño–like SST anomalies in the simultaneous summer. These relationships between CEF and ENSO are established by two key convection regions: enhanced convection over WNP and depressed convection over the MC. A linear baroclinic model is applied here to further discuss the causality between circulation and convection. Results suggest that both the WNP heating and MC cooling can induce the strengthened CEF. Moreover, the stability of the relationship between CEF and El Niño–Southern Oscillation (ENSO) is also discussed. Results show that the relationship between CEF and SST anomalies in the simultaneous summer is stable and remains significant, whereas that between CEF and SST anomalies in the preceding winter experienced a decadal strengthening around 1997/98 from insignificant to significant. After 1998, the preceding winter ENSO is followed by strong summer SST anomalies in the MC that significantly affect CEF via modulating local convection. However, this ENSO–summer MC SST relationship is weak before 1997, failing to establish the relationship between the preceding ENSO and CEF.

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