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This study analyzes the multidecadal changes in the severity of extreme meteorological droughts at the regional scale in China during 1951–2017. Dominance analysis is applied to multiple linear regression models to quantify the relative influence of global warming and internal variability on the meteorological drought severity in nine climate regions of China to understand which drivers are the most significant for each region and how they are likely to influence the severity of droughts in the near future (10–20 years). The influence of internal variability is represented by the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). The PDO and global warming are found to have a stronger influence on the multidecadal variability of drought severity in China than the AMO. Global warming is found to be the more dominant driver of multidecadal variability of drought severity in the western parts of China, whereas the PDO is found to have a more dominant influence in the eastern parts of China. In the near future, global warming and the PDO are both likely to contribute to reduction of drought severity in Xinjiang, Northwest, and Tibet regions. The positive phase of the PDO is also expected to reduce the severity of droughts in Inner Mongolia and South China. On the other hand, both the PDO and global warming are expected to contribute to increase in drought severity in North and Southwest China in the near future. Key Points: Effect of global warming and PDO has been much stronger than that of AMO on the multidecadal variability of drought severity in ChinaGlobal warming is found to be dominant driver in Xinjiang, Tibet and Northwest regionsPDO is found to be the dominant driver in Inner Mongolia, North, East, and South China [ABSTRACT FROM AUTHOR]

Atlantic multidecadal variability (AMV) of sea surface temperature exhibits an important influence on the climate of surrounding continents. It remains unclear, however, the extent to which AMV is due to internal climate variability (e.g., ocean circulation variability) or changes in external forcing (e.g., volcanic/anthropogenic aerosols or greenhouse gases). Here, the sources of AMV are examined over a 340-yr period using proxy indices, instrumental data, and output from the Last Millennium Ensemble (LME) simulation. The proxy AMV closely follows the accumulated atmospheric forcing from the instrumental North Atlantic Oscillation (NAO) reconstruction (r = 0.65)—an "internal" source of AMV. This result provides strong observational evidence that much of the AMV is generated through the oceanic response to atmospheric circulation forcing, as previously demonstrated in targeted modeling studies. In the LME there is a substantial externally forced AMV component, which exhibits a modest but significant correlation with the proxy AMV (i.e., r = 0.37), implying that at least 13% of the AMV is externally forced. In the LME simulations, however, the AMV response to accumulated NAO forcing is weaker than in the proxy/observational datasets. This weak response is possibly related to the decadal NAO variability, which is substantially weaker in the LME than in observations. The externally forced component in the proxy AMV is also related to the accumulated NAO forcing, unlike in the LME. This indicates that the external forcing is likely influencing the AMV through different mechanistic pathways: via changes in radiative forcing in the LME and via changes in atmospheric circulation in the observational/proxy record. [ABSTRACT FROM AUTHOR]

Features of decadal–multidecadal variations of the Asian summer rainfall are revealed by analysis of the reconstructed Asian summer precipitation (RAP) dataset from 1470 to 2013. Significant low-frequency periodicities of the all-Asian rainfall (AAR) index (AARI) are found on decadal (8–10 yr), quasi-bidecadal (22 yr), and multidecadal (50–54 yr) time scales, as well as centennial time scales. The decadal and multidecadal peaks are mainly from the "monsoon Asia" area and the Maritime Continent, while the 22-yr peak is from the "arid Asia" area. A remarkable change of leading frequency from multidecadal to decadal after AD 1700 is detected across the entire Asian landmass. The leading empirical orthogonal function (EOF) modes on the decadal and multidecadal time scales exhibit a uniform structure similar to that on the interannual time scale, suggesting a cross-time-scale, in-phase variation of the rainfall across continental Asia and the Maritime Continent. Enhanced AAR on a decadal time scale is found associated with the mega-La Niña sea surface temperature (SST) pattern over the Pacific. The AARI–mega-ENSO (El Niño–Southern Oscillation) relationship is persistently significant except from 1820 to around 1900. Enhanced decadal AAR is also found to be associated with extratropical North Atlantic warming. The AARI–AMO (Atlantic multidecadal oscillation) relationship, however, is nonstationary. On the multidecadal time scale, the AAR is significantly related to the AMO. Mechanisms associated with the decadal–multidecadal variability of AAR are also discussed. [ABSTRACT FROM AUTHOR]

The relations of the low-level extratropical cyclones in the southeastern Pacific and South Atlantic with the sea surface temperature (SST) anomalies associated with the Atlantic multidecadal oscillation (AMO) during the summer and winter of the 1979–93 cold AMO (CAMO) and 2003–17 warm AMO (WAMO) are analyzed. During both seasons and in both AMO phases, the cyclone trajectories defined by cyclone local counts exceeding 10 events per grid box occur approximately in the areas with the AMO-related positive SST anomalies. The cyclone densities in most latitudes during both seasons are higher in the CAMO than in the WAMO. Thus, the cyclone density in the study domain presents a reduction trend during the 1979–2017 period. The large-scale northward SST anomalous gradients between the bands north and south of 40°S increase the long-wave baroclinicity in the midlatitudes in the WAMO, and the southward SST anomalous gradients decrease it in the CAMO. Consequently, the short-wave baroclinicity is higher in the WAMO than in the CAMO in the southeastern Pacific midlatitudes. Thus, the cyclones are more energetic in the WAMO than in the CAMO. In the South Atlantic region off the Argentinean coast, both the barotropic and baroclinic conversion terms are positive, indicating an increase of the kinetic energy of the short waves. The low-level cyclones in the southeastern Pacific and South Atlantic are modulated by the AMO. As far as we know, the relation of the SH low-level extratropical cyclones to the AMO documented here was not studied before. [ABSTRACT FROM AUTHOR]

Several climate field reconstruction methods assume stationarity between the leading patterns of variability identified during the instrumental calibration period and the reconstruction period. We examine how and to what extent this restrictive assumption may generate uncertainties in reconstructing past tropical Pacific climate variability. Based on the Last Millennium (850–2005 CE) ensemble simulations conducted with the Community Earth System Model and by developing a series of pseudoproxy reconstructions for different calibration periods, we find that the overall reconstruction skill for global and more regional-scale climate indices depends significantly on the magnitude of externally forced global mean temperature variability during the chosen calibration period. This effect strongly reduces the fidelity of reconstructions of decadal to centennial-scale tropical climate variability, associated with the interdecadal Pacific oscillation (IPO) and centennial-scale temperature shifts between the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). In contrast, our pseudoproxy-based analysis demonstrates that reconstructions of interannual El Niño–Southern Oscillation (ENSO) variability are more robust and less affected by changes in calibration period. [ABSTRACT FROM AUTHOR]

Over the last half century, the Arctic sea ice cover has declined dramatically. Current estimates suggest that, for the Arctic as a whole, nearly one-half of the observed loss of summer sea ice cover is not due to anthropogenic forcing but rather is due to internal variability. Using the 40 members of the Community Earth System Model Large Ensemble (CESM-LE), our analysis provides the first regional assessment of the role of internal variability on the observed sea ice loss. The CESM-LE is one of the best available models for such an analysis, because it performs better than other CMIP5 models for many metrics of importance. Our study reveals that the local contribution of internal variability has a large range and strongly depends on the month and region in question. We find that the pattern of internal variability is highly nonuniform over the Arctic, with internal variability accounting for less than 10% of late summer (August–September) East Siberian Sea sea ice loss but more than 60% of the Kara Sea sea ice loss. In contrast, spring (April–May) sea ice loss, notably in the Barents Sea, has so far been dominated by internal variability. [ABSTRACT FROM AUTHOR]

North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV. [ABSTRACT FROM AUTHOR]

Rapid warming has led to an aggregated environmental degradation over the Tibetan Plateau (TP) in the last few decades, including accelerated glacier retreat, early snowmelt, permafrost degradation, and forest fire occurrence. Attribution of this warming in recent decades has mainly been focused on anthropogenic forcing. Yet, linkages to the Atlantic multidecadal variability (AMV), an essential part of the climate system causing decadal to centennial fluctuations of temperature, remains poorly understood for the TP, especially at long time scales. Using well-replicated tree-ring width records, we reconstructed 358 years of summer minimum temperature (MinT) of the whole TP. This reconstruction matches the recent warming signal recorded since the 1980s, and captures 63% of the variance in 1950–2005 instrumental records. A teleconnection from the North Atlantic to the TP is further identified based in observations and simulations with an atmospheric general circulation model (AGCM). We propose that half of the multidecadal variability of TP summer MinT can be explained by the AMV over the past three and a half centuries. Both observations and AGCM simulations indicate that the AMV warm phase induces a zonal dipole response in sea level pressure across the Atlantic–Eurasia region, with anomalously high surface pressure and corresponding downward atmospheric motion over the TP. We propose that the descending motion during warm AMV phases causes negative rainfall and positive temperature anomalies over the TP. Our findings highlight that the AMV plays a role in the multidecadal temperature variability over the TP. [ABSTRACT FROM AUTHOR]

This study applies the maximum temperatures at more than 2000 Chinese stations to investigate the dominant modes of China summer heat waves (HWs). The first empirical orthogonal function (EOF) mode of the HW days reflects an increased frequency of HWs in northern China (NC), while the second and third modes represent two distinct interannual modes, with key regions over the Yangtze River valley (YRV) and southern China (SC), respectively. The NC HWs are possibly associated with the Atlantic–Eurasian teleconnection, showing zonally propagating wave trains over the North Atlantic and Eurasian continent. The YRV HWs are proposed to be linked to the North Atlantic Oscillation, which may trigger a southeastward-propagating wave train over northern Russia and East Asia that results in a high pressure anomaly over the YRV. The SC HWs are obviously dominated by the Indian Ocean and northwest Pacific warm SSTs owing to the transition from the preceding El Niño to La Niña, which excites above-normal highs over SC. The anomalously high pressures over NC, the YRV, and SC are usually accompanied by descending air motions, clear skies, decreased precipitation, and increased solar radiation, which jointly cause a drier and hotter soil condition that favors the emergence of HWs. The GFDL HiRAM experiments are able to reproduce the historical evolution of NC and SC HWs, but fail to capture the YRV HWs. The correlation coefficient between model PC1 (PC2) and observed PC1 (PC3) for the period of 1979–2008 is 0.65 (0.38), which significantly exceeds the 95% (90%) confidence level, indicating that this model has a more faithful representation for the SST-forced HWs. [ABSTRACT FROM AUTHOR]

In this study, we analyze the nonstationarity in meteorological droughts at the multidecadal scale in different parts of the contiguous United States during 1901–2017. We develop metrics to compare the drought risk calculated under the assumptions of stationarity and nonstationarity and identify their spatial and temporal patterns. By analyzing the variability of drought risk in the past and exploring its ongoing patterns, we evaluate in which regions of the contiguous United States the assumption of stationarity can be safely used for drought risk planning and management. We find statistically significant interdecadal changes in the probability distribution functions of drought severity in parts of the Northwest, upper Midwest, the Northeast, eastern parts of Great Plains and in parts of Arizona, New Mexico, Utah, and Nevada in the Southwest. In these regions, the nonstationary risk has been significantly higher than the stationary estimate of risk in the past, which shows that the assumption of stationarity can lead to the underestimation of drought risk in these regions. The multidecadal drought risk shows low variability in California, parts of northern and western Great Plains, Ohio Valley, and in the Southeast, since the statistical properties of droughts have not changed significantly in these regions during 1901–2017. However, the meteorological drought risk has increased in California and the Southeast in the recent decades due to the influence of global warming and hence the assumption of stationarity for risk estimation may lead to underestimation of drought risk in future in these regions if this effect of global warming persists. Plain Language Summary: Traditionally, statistical approaches adopted by water resource managers for planning and design of water resource systems and infrastructure are based on the assumption of stationarity; that is, it is assumed that the probabilistic characteristics of the hydrological and meteorological processes do not change with time, and hence, the planning and designs for future can be based on the past observations. In this paper, we have evaluated the validity of the stationarity assumption for meteorological drought risk estimation at the multidecadal scale by comparing drought risk calculated under the assumptions of stationarity and nonstationarity, respectively, in different parts of the continental United States. We find statistically significant nonstationarity in meteorological droughts in the Northwest, upper Midwest, the Northeast, eastern Great Plains and in parts of Nevada, Utah, Arizona, and New Mexico in the Southwest United States, which results in high interdecadal variability of drought risk in these regions. This result demonstrates that the assumption of stationarity can lead to underestimation of drought risk in these regions, thereby exposing water resource systems to failure under severe droughts. Key Points: The assumption of stationarity for meteorological drought risk estimation is evaluated in different parts of the CONUS at the multidecadal scaleNonstationarity is identified with statistical significance in the Northwest, upper Midwest, the Northeast, eastern Great Plains and in parts of Nevada, Utah, Arizona, and New Mexico in the Southwest United StatesInterdecadal changes in drought severity have not been statistically significant in California, Ohio valley, northern and western Great Plains, and the Southeast United States [ABSTRACT FROM AUTHOR]

Observed September Arctic sea ice has declined sharply over the satellite era. While most climate models forced by observed external forcing simulate a decline, few show trends matching the observations, suggesting either model deficiencies or significant contributions from internal variability. Using a set of perturbed climate model experiments, we provide evidence that atmospheric teleconnections associated with the Atlantic multidecadal variability (AMV) can drive low-frequency Arctic sea ice fluctuations. Even without AMV-related changes in ocean heat transport, AMV-like surface temperature anomalies lead to adjustments in atmospheric circulation patterns that produce similar Arctic sea ice changes in three different climate models. Positive AMV anomalies induce a decrease in the frequency of winter polar anticyclones, which is reflected both in the sea level pressure as a weakening of the Beaufort Sea high and in the surface temperature as warm anomalies in response to increased low-cloud cover. Positive AMV anomalies are also shown to favor an increased prevalence of an Arctic dipole–like sea level pressure pattern in late winter/early spring. The resulting anomalous winds drive anomalous ice motions (dynamic effect). Combined with the reduced winter sea ice formation (thermodynamic effect), the Arctic sea ice becomes thinner, younger, and more prone to melt in summer. Following a phase shift to positive AMV, the resulting atmospheric teleconnections can lead to a decadal ice thinning trend in the Arctic Ocean on the order of 8%–16% of the reconstructed long-term trend, and a decadal trend (decline) in September Arctic sea ice area of up to 21% of the observed long-term trend. [ABSTRACT FROM AUTHOR]

Previous studies have documented a poleward shift in the subsiding branches of Earth's Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models' historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability. [ABSTRACT FROM AUTHOR]

Meteorological droughts in the continental United States (CONUS) are known to oscillate at the multidecadal time scale in response to the sea surface temperatures (SST) variability over the Pacific Ocean and the North Atlantic Ocean. While previous studies have focused on understanding the influence of SST oscillations on drought frequency over the CONUS, this information has not been integrated with global warming for future drought risk assessment at the decadal scale. In this study, we use the support vector machines (SVMs) to handle correlation between input variables for quantifying the influence of internal variability [Atlantic multidecadal oscillation (AMO) and Pacific decadal oscillation (PDO)] and global warming on the decadal changes in the severity of seasonal droughts over the CONUS during 1901–2015. The regional drivers of drought severity identified using SVMs are used for the assessment of decadal drought risk in the near future. We find internal variability as the dominant driver of decadal changes in drought severity in the southern and central Great Plains and global warming as the dominant driver for the southeastern and southwestern United States. In the southern Plains, the existing pattern of increasing drought severity is likely to persist in the near future if AMO and PDO remain in their positive and negative phases, respectively, while global warming is likely to contribute to increasing drought severity in the Southeast and Southwest. This study suggests an emerging role of global warming in drought risk over the southern states, where near-term climate change adaptation is necessary. [ABSTRACT FROM AUTHOR]

The Indus River basin is highly vulnerable to water scarcity due to increasing population, unsustainable management practices, and climate change. Yet the regional hydroclimate and precipitation dynamics remain poorly understood. Using running trend and spectral analysis with multiple gauge-based, remote sensing, and reanalysis precipitation datasets, this study analyzes precipitation temporal variability, its subregional variations, and the main seasonal drivers, particularly the South Asian monsoon. The results uncover remarkable alternation of long-term positive and negative interdecadal precipitation trends in the basin over the past half century. These trends have led to substantial changes in water input over the region at the time scales comparable to climate assessment periods (30 years), and therefore this high intrinsic variability must be accounted for in climate change adaptation studies. This study also reconstructs onset and withdrawal dates of the South Asian monsoon that exhibit interdecadal variability, but their dominant modes differ from that of annual precipitation. The findings hypothesize that higher-frequency variability in El Niño–Southern Oscillation is likely to have a pronounced impact on monsoon onset and duration in the studied region. [ABSTRACT FROM AUTHOR]

The North Atlantic has shown large multidecadal temperature shifts during the twentieth century. There is ongoing debate about whether this variability arises primarily through the influence of atmospheric internal variability, through changes in ocean circulation, or as a response to anthropogenic forcing. This study isolates the mechanisms driving Atlantic sea surface temperature variability on multidecadal time scales by using low-frequency component analysis (LFCA) to separate the influences of high-frequency variability, multidecadal variability, and long-term global warming. This analysis objectively identifies the North Atlantic subpolar gyre as the dominant region of Atlantic multidecadal variability. In unforced control runs of coupled climate models, warm subpolar temperatures are associated with a strengthened Atlantic meridional overturning circulation (AMOC) and anomalous local heat fluxes from the ocean into the atmosphere. Atmospheric variability plays a role in the intensification and subsequent weakening of ocean overturning and helps to communicate warming into the tropical Atlantic. These findings suggest that dynamical coupling between atmospheric and oceanic circulations is fundamental to the Atlantic multidecadal oscillation (AMO) and motivate approaching decadal prediction with a focus on ocean circulation. [ABSTRACT FROM AUTHOR]

The European precipitation response to El Niño (EN) has been found to present interdecadal changes, with alternated periods of important or negligible EN impact in late winter. These periods are associated with opposite phases of multi-decadal sea surface temperature (SST) variability, which modifies the tropospheric background and EN teleconnections. In addition, other studies have shown how SST anomalies in the equatorial Pacific, and in particular, the location of the largest anomalous SST, modulate the stratospheric response to EN. Nevertheless, the role of the stratosphere on the stationarity of EN response has not been investigated in detail so far. Using reanalysis data, we present a comprehensive study of EN teleconnections to Europe including the role of the ocean background and the stratosphere in the stationarity of the signal. The results reveal multidecadal variability in the location of EN-related SST anomalies that determines different teleconnections. In periods with relevant precipitation signal over Europe, the EN SST pattern resembles Eastern Pacific EN and the stratospheric pathway plays a key role in transmitting the signal to Europe in February, together with two tropospheric wavetrains that transmit the signal in February and April. Conversely, the stratospheric pathway is not detected in periods with a weak EN impact on European precipitation, corresponding to EN-related SST anomalies primarily located over the central Pacific. SST mean state and its associated atmospheric background control the location of EN-related SST anomalies in different periods and modulate the establishment of the aforementioned stratospheric pathway of EN teleconnection to Europe too. [ABSTRACT FROM AUTHOR]

Assessing the ability of atmospheric models to capture observed climate variations when driven by observed sea surface temperature (SST), sea ice concentration (SIC) and radiative forcings is a prerequisite for the feasibility of near term climate predictions. Here we achieve ensembles of global atmospheric simulations to assess and attribute the reproducibility of the boreal winter atmospheric circulation against the European Centre for Medium Range Forecasts (ECMWF) twentieth century reanalysis (ERA20C). Our control experiment is driven by the observed SST/SIC from the Atmospheric Model Intercomparison Project. It is compared to a similar ensemble performed with the ECMWF model as a first step toward ERA20C. Moreover, a two-tier methodology is used to disentangle externally-forced versus internal variations in the observed SST/SIC boundary conditions and run additional ensembles allowing us to attribute the observed atmospheric variability. The focus is mainly on the North Atlantic Oscillation (NAO) variability which is more reproducible in our model than in the ECMWF model. This result is partly due to the simulation of a positive NAO trend across the full 1920-2014 integration period. In line with former studies, this trend might be mediated by a circumglobal teleconnection mechanism triggered by increasing precipitation over the tropical Indian Ocean (TIO). Surprisingly, this response is mainly related to the internal SST variability and is not found in the ECMWF model driven by an alternative SST dataset showing a weaker TIO warming in the first half of the twentieth century. Our results may reconcile the twentieth century observations with the twenty-first century projections of the NAO. They should be however considered with caution given the limited size of our ensembles, the possible influence of other sources of NAO variability, and the uncertainties in the tropical SST trend and breakdown between internal versus externally-forced variability. [ABSTRACT FROM AUTHOR]

Several studies have shown a statistically significant correlation between Atlantic multidecadal variability (AMV) and Indian summer monsoon rainfall (ISMR) since 1871 when instrumental data are available. In the instrumental records, both ISMR and North Atlantic sea surface temperatures (SSTs) have multidecadal variability with a period close to 60 yr, where periods of warm (cold) North Atlantic SSTs are accompanied by periods of wetter (dryer) ISMR and lower (higher) frequencies of dry years. We have studied both AMV and ISMR for the period from 1481 to present using several proxy reconstructions from both regions, as well as an extended instrumental data set for ISMR, to investigate multidecadal variability in the ISMR and the teleconnection to AMV. Previous studies investigating the relationship between AMV and ISMR in instrumental data have only used the period from 1871 onwards, whereas rain gauge data from the year 1844 are studied here, extending the instrumental record by 26 yr. We find that the observed link between AMV and ISMR is present in the extended instrumental data. We also find that multidecadal variability is present in the ISMR in all proxy records; however, all the proxy records for both ISMR and AMV diverge before the 1800s. In addition, the observed correlation between AMV and ISMR has weakened in the last decade. These results emphasise that it is not appropriate to use single proxy reconstructions to study past climates.

Atlantic multidecadal variability (AMV) has long been thought to be an expression of low-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). However, alternative hypotheses have been forwarded, including that AMV is primarily externally forced. Here, we review the current state of play by assessing historical simulations made for the sixth coupled model intercomparison project (CMIP6). Overall, the importance of external forcing is sensitive to the type of AMV index used, due to the importance of globally coherent externally forced signals in the models. There are also significant contrasts between the processes that drive internally and externally forced AMV, but these processes can be isolated by exploring the multivariate expression of AMV. Specifically, internal variability in CMIP6 models is consistent with an important role of ocean circulation and AMOC and the externally forced AMV is largely a surface-flux forced mechanism with little role for the ocean. Overall, the internal multivariate fingerprint of AMV is similar to the observed, but the externally forced fingerprint appears inconsistent with observations. Therefore, climate models still suggest a key role for ocean dynamics, and specifically AMOC, in observed AMV. Nevertheless, models remain deficient in a number of areas, and a stronger role for externally forced dynamical changes cannot be ruled out. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Recently, multidecadal variability in the Southern Ocean has been found in a strongly eddying global ocean circulation model. In this paper, we study the Lorenz energy cycle of this so‐called Southern Ocean Mode (SOM). The Lorenz energy cycle analysis provides details on the energy pathways associated with the SOM. It shows that ocean eddies and the baroclinic energy pathway together with variations in the kinetic energy input by the wind are crucial aspects of the variability. It is also shown how convective mixing, which is induced by the SOM in particular in the Weddell Gyre, is responsible for the large‐scale multidecadal variability in Antarctic Bottom Water and Atlantic Meridional Overturning Circulation. Plain Language Summary: Recently, large fluctuations of the oceanic heat content were discovered in the Atlantic sector of the Southern Ocean in an ocean simulation. These changes are periodic and repeat every 50 years, which is why we cannot confirm them from out limited observational record. We investigate this phenomenon through a mechanical energy perspective and find that it is likely caused by an interplay between eddies (the oceanic equivalent of atmospheric storms with 10‐ to 100‐km diameter) and the average flow around Antarctica. In addition, we find connected sinking events that influence the ocean's overturning strength. If this phenomenon is found in the real ocean, it may influence the global mean surface temperature significantly over decades. Key Points: The multidecadal varying Lorenz energy cycle of a global ocean model is analyzedA mechanical energy description of the Southern Ocean Mode is providedA detailed physical mechanism for the Southern Ocean Mode is proposed [ABSTRACT FROM AUTHOR]

This study examines the atmospheric and oceanic heat transports in preindustrial control and historical runs of 15 fully coupled global climate models from the CMIP5 project. The presence of Bjerknes compensation is confirmed in all models by the strong anticorrelation and approximately equal magnitude of the anomalies of these heat transports. Previous studies of Bjerknes compensation in the absence of external forcing have all shown the strongest compensation at high latitudes, where the warm ocean meets the cold Arctic atmosphere. In this study, however, it is found that many of the 15 models have a second and often dominant peak of compensation in the northern midlatitudes, where strong air–sea interaction is often associated with the midlatitude storm tracks. It has also been suggested that variations in heat transport in the ocean lead those in the atmosphere, but this work has found no clear and robust support for this, as only half the models show such a relationship. In the historical simulations where external forcings are applied, Bjerknes compensation continues to be present, but many models show pronounced trends in the heat transports. All of the models show multidecadal variability in heat transports in both preindustrial control and historical simulations. Any anthropogenic climate change signal could potentially be masked or amplified by the natural variability governed by Bjerknes compensation. Given its presence in the CMIP5 models, which are the basis of so much policy and adaptation planning, an improved understanding of Bjerknes compensation may have socioeconomic relevance for the future. [ABSTRACT FROM AUTHOR]

The impact of North American and European (NAEU) anthropogenic aerosol emissions on Eurasian summer climate during the twentieth century is studied using historical single- and all-forcing (including anthropogenic aerosols, greenhouse gases, and natural forcings) simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Intermodel agreement on significant linear trends during a period of increasing NAEU sulfate emissions (1900–74) reveals robust features of NAEU aerosol impact, supported by opposite changes during the subsequent period of decreasing emissions. Regionally, these include a large-scale cooling and associated anticyclonic circulation, as well as a narrowing of the diurnal temperature range (DTR) over Eurasian midlatitudes. Remotely, NAEU aerosols induce a drying over the western African and northern Indian monsoon regions and a strengthening and southward shift of the subtropical jet consistent with the pattern of temperature change. Over Europe, the temporal variations of observed temperature, pressure, and DTR tend to agree better with simulations that include aerosols. Throughout the twentieth century, aerosols are estimated to explain more than a third of the simulated interdecadal forced variability of European near-surface temperature and more than half between 1940 and 1970. These results highlight the substantial aerosol impact on Eurasian climate, already identifiable in the first half of the twentieth century. This may be relevant for understanding future patterns of change related to further emission reductions. [ABSTRACT FROM AUTHOR]

Precipitation regime shifts in the Sahel region have dramatic humanitarian and economic consequences such as the severe droughts during the 1970s and 1980s. Though Sahel precipitation changes during the late twentieth century have been extensively studied, little is known about the decadal variability prior to the twentieth century. Some evidence suggests that during the second half of the nineteenth century, the Sahel was as rainy as or even more rainy than during the 1950s and 1960s. Here, we reproduce such an anomalous Sahel humid period in the late nineteenth century by means of climate simulations. We show that this increase of rainfall was associated with an anomalous supply of humidity and higher-than-normal deep convection in the mid- and high troposphere. We present evidence suggesting that sea surface temperatures (SSTs) in the Atlantic basin played the dominant role in driving decadal Sahel rainfall variability during this early period. [ABSTRACT FROM AUTHOR]

The atmospheric response to global sea surface temperatures is the leading cause of rainfall variability in the West African Sahel. On interannual periodicities, El Niño–Southern Oscillation, the Atlantic equatorial mode, and Mediterranean warm/cold events primarily drive variations of summer rainfall over the Sahel. Nevertheless, the rainfall response to these modes of interannual SST variability has been suggested to be unstable throughout the observational record. This study explores changes in the leading patterns of covariability between Sahel rainfall and SSTs, analyzing the dynamical mechanisms at work to explain the nonstationary relationship between anomalies in these two fields. A new network of rain gauge stations across West Africa is used for the first time to investigate these instabilities during the period 1921–2010. A hypothesis is raised that the underlying SST background seems to favor some interannual teleconnections and inhibit others in terms of the cross-equatorial SST gradients and associated impacts on the location of the intertropical convergence zone. Results of this study are relevant for improving the seasonal predictability of summer rainfall in the Sahel. [ABSTRACT FROM AUTHOR]

Extracted nonstationary oscillations and nonlinear trends of precipitation and sea surface temperature (SST) data reveal that rainfall variability in Mexico City is mainly composed by a long‐term positive trend, a multidecadal oscillation highly correlated with the Atlantic Multidecadal Oscillation (AMO), and year‐to‐year variability. The precipitation trend, lasting for more than a century, cannot be attributed to global warming or urbanization alone; rather, it can be thought of as part of a natural oscillation composed of alternating wet‐dry anomalies with a period of a couple of centuries, as past evidence indicates. To further test the dependence of the AMO‐related component, yearly SST time series were derived from a simplified model of the atmosphere‐ocean system forced by white noise. The simulated SST time series exhibits AMO‐like variability entirely consistent with the observed one, implying that North Atlantic SST multidecadal variability can be seen as the integrated response of surface ocean layers to external stochastic atmospheric forcing. Plain Language Summary: In this work, nonlinear trends and nonstationary oscillations are extracted from both annual accumulated precipitation and sea surface temperature (SST) data. Our results show that the precipitation variability in Mexico City is mainly composed by a very long‐term positive trend of more than a century, a multidecadal oscillation highly correlated with the Atlantic Multidecadal Oscillation (AMO), and year‐to‐year variability. The long‐term positive trend cannot be caused by global warming or urbanization alone; rather, it can be seen as the positive part of a natural oscillation composed of alternating wet‐dry anomalies with a period of a couple of centuries, as there are past evidence supporting this interpretation. In order to get a depth understanding of the origin of the AMO‐related component, yearly SST time series were obtained by using a simple box model of the ocean‐atmosphere system forced by white noise (short timescale atmospheric forcing). This simple model is able to simulate SST anomalies that exhibit AMO‐like variability that is entirely consistent with the observed one, implying that North Atlantic SST multidecadal variability can be seen simply as the response of near‐surface layers of the ocean to stochastic atmospheric forcing. Key Points: Long‐term secular positive precipitation trends in Mexico City cannot be accounted for by man‐induced climate change aloneThe Atlantic Multidecadal Oscillation can be explained as an integrated response of the upper ocean to surface stochastic forcingLong‐term positive trends in precipitation in Mexico City can be thought of as the increasing portion of a longer period oscillation [ABSTRACT FROM AUTHOR]

Numerical simulations of a realistic ocean general circulation model forced by prescribed surface fluxes are used to study the origin and structure of intrinsic interdecadal variability of the ocean circulation. When eddy-induced turbulent diffusivities are low enough, spontaneous oscillations of the Atlantic meridional overturning circulation (AMOC) with periods O(20) yr and amplitude O(1) Sv (1 Sv ≡ 106 m3 s−1) emerge. The transition from the steady to the oscillatory regime is shown to be consistent with a supercritical Hopf bifurcation of the horizontal Peclet number. Adding atmospheric thermal damping is shown to have a very limited influence on the domain of existence of intrinsic variability. The spatial structure of the mode consists of a dipole of sea surface temperature (SST)/sea surface height (SSH) anomalies centered at about 50°N with stronger variance in the western part of the subpolar gyre, in agreement with the observed Atlantic multidecadal oscillation (AMO) signature in this region. Specific features include a westward propagation of temperature anomalies from the source region located on the western flank of the North Atlantic Current (NAC) and a one-quarter phase lag between surface and subsurface (800 m) temperature anomalies. Local linear stability calculations including viscous and diffusive effects confirm that the North Atlantic Current is baroclinically unstable on scales of O(1000) km with growth rates of O(1) yr−1. Both the spatial structure of the mode and the period agree in magnitude with in situ measurements in the North Atlantic, suggesting that this intrinsic ocean mode participates in the observed Atlantic bidecadal climate variability. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal oscillation (AMO) has been shown to play a major role in the multidecadal variability of the Northern Hemisphere, impacting temperature and precipitation, including intertropical convergence zone (ITCZ)-driven precipitation across Africa and South America. Studies into the location of the intertropical convergence zone have suggested that it resides in the warmer hemisphere, with the poleward branch of the Hadley cell acting to transport energy from the warmer hemisphere to the cooler one. Given the impact of the Atlantic multidecadal oscillation on Northern Hemisphere temperatures, we expect the Atlantic multidecadal oscillation to have an impact on the location of the intertropical convergence zone. We find that the positive phase of the Atlantic multidecadal oscillation warms the Northern Hemisphere, resulting in a northward shift of the intertropical convergence zone, which is evident in the Pacific climate proxy record. Using a coupled climate model, we further find that the shift in the intertropical convergence zone is consistent with the surface energy imbalance generated by the Atlantic multidecadal oscillation. In this model, the Pacific changes are driven in large part by the warming of the tropical Atlantic and not the extratropical Atlantic. [ABSTRACT FROM AUTHOR]

In this paper we examine various options for the calculation of the forced signal in climate model simulations, and the impact these choices have on the estimates of internal variability. We find that an ensemble mean of runs from a single climate model [a single model ensemble mean (SMEM)] provides a good estimate of the true forced signal even for models with very few ensemble members. In cases where only a single member is available for a given model, however, the SMEM from other models is in general out-performed by the scaled ensemble mean from all available climate model simulations [the multimodel ensemble mean (MMEM)]. The scaled MMEM may therefore be used as an estimate of the forced signal for observations. The MMEM method, however, leads to increasing errors further into the future, as the different rates of warming in the models causes their trajectories to diverge. We therefore apply the SMEM method to those models with a sufficient number of ensemble members to estimate the change in the amplitude of internal variability under a future forcing scenario. In line with previous results, we find that on average the surface air temperature variability decreases at higher latitudes, particularly over the ocean along the sea ice margins, while variability in precipitation increases on average, particularly at high latitudes. Variability in sea level pressure decreases on average in the Southern Hemisphere, while in the Northern Hemisphere there are regional differences. [ABSTRACT FROM AUTHOR]

The multidecadal fluctuations in the patterns and teleconnections of the winter mean Arctic Oscillation (AO) are investigated based on observational and reanalysis datasets. Results show that the Atlantic center of the AO pattern remains unchanged throughout the period 1920–2010, whereas the Pacific center of the AO is strong during 1920–59 and 1986–2010 and weak during 1960–85. Consequently, the link between the AO and the surface air temperature over western North America is strong during 1920–59 and 1986–2010 and weak during 1960–85. The time-varying Pacific center of the AO motivates a revisit to the nature of the AO from the perspective of decadal change. It reveals that the North Pacific mode (NPM) and North Atlantic Oscillation (NAO) are the inherent regional atmospheric modes over the North Pacific and North Atlantic, respectively. Their patterns over the North Pacific and North Atlantic remain stable and change little with time during 1920–2010. The Atlantic center of the AO always resembles the NAO over the North Atlantic, but the Pacific center of the AO only resembles the NPM over the North Pacific when the NPM–NAO coupling is strong. These results suggest that the AO seems to be fundamentally rooted in the variability over the North Atlantic and that the annular structure of the AO very likely arises from the coupling of the atmospheric modes between the North Pacific and North Atlantic. [ABSTRACT FROM AUTHOR]

Two independent atmospheric general circulation models reveal that the positive (negative) phase of Atlantic multidecadal variability (AMV) can reduce (amplify) the variance of the shorter time-scale (e.g., ENSO related) precipitation fluctuations in the United States, especially in the Southwest, as well as decrease (increase) the long-term seasonal mean precipitation for the cold season. The variance is modulated because of changes in 1) dry day frequency and 2) maximum daily rainfall intensity. With positive AMV forcing, the upper-level warming originating from the increased precipitation over the tropical Atlantic Ocean changes the mean vertical thermal structure over the United States continent to a profile less favorable for rain-inducing upward motions. In addition, a northerly low-level dry advection associated with the local overturning leaves less available column moisture for condensation and precipitation. The opposite conditions occur during cold AMV periods. [ABSTRACT FROM AUTHOR]

The relationship between climate models' internal variability and their response to external forcings is investigated. Frequency-dependent regressions are performed between the outgoing top-of-atmosphere (TOA) energy fluxes and the global-mean surface temperature in the preindustrial control simulations of the CMIP5 archive. Two distinct regimes are found. At subdecadal frequencies the surface temperature and the outgoing shortwave flux are in quadrature, while the outgoing longwave flux is linearly related to temperature and acts as a negative feedback on temperature perturbations. On longer time scales the outgoing shortwave and longwave fluxes are both linearly related to temperature, with the longwave continuing to act as a negative feedback and the shortwave acting as a positive feedback on temperature variability. In addition to the different phase relationships, the two regimes can also be seen in estimates of the coherence and of the frequency-dependent regression coefficients. The frequency-dependent regression coefficients for the total cloudy-sky flux on time scales of 2.5 to 3 years are found to be strongly (r2 > 0.6) related to the models' equilibrium climate sensitivities (ECSs), suggesting a potential "emergent constraint" for Earth's ECS. However, O(100) years of data are required for this relationship to become robust. A simple model for Earth's surface temperature variability and its relationship to the TOA fluxes is used to provide a physical interpretation of these results. [ABSTRACT FROM AUTHOR]

Multidecadal hydroclimate variability has been expressed as "megadroughts" (dry periods more severe and prolonged than observed over the twentieth century) and corresponding "megapluvial" wet periods in many regions around the world. The risk of such events is strongly affected by modes of coupled atmosphere-ocean variability and by external impacts on climate. Accurately assessing the mechanisms for these interactions is difficult, since it requires large ensembles of millennial simulations as well as long proxy time series. Here, the Community Earth System Model (CESM) Last Millennium Ensemble is used to examine statistical associations among megaevents, coupled climate modes, and forcing from major volcanic eruptions. El Niño-Southern Oscillation (ENSO) strongly affects hydroclimate extremes: larger ENSO amplitude reduces megadrought risk and persistence in the southwestern United States, the Sahel, monsoon Asia, and Australia, with corresponding increases in Mexico and the Amazon. The Atlantic multidecadal oscillation (AMO) also alters megadrought risk, primarily in the Caribbean and the Amazon. Volcanic influences are felt primarily through enhancing AMO amplitude, as well as alterations in the structure of both ENSO and AMO teleconnections, which lead to differing manifestations of megadrought. These results indicate that characterizing hydroclimate variability requires an improved understanding of both volcanic climate impacts and variations in ENSO/AMO teleconnections. [ABSTRACT FROM AUTHOR]

Near-term climate projections are needed by policymakers; however, these projections are difficult because internally generated climate variations need to be considered. In this study, temperature change scenarios in the near-term period 2017-35 are projected at global and regional scales based on a refined multi-model ensemble approach that considers both the secular trend (ST) and multidecadal variability (MDV) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. The ST and MDV components are adaptively extracted from each model simulation by using the ensemble empirical mode decomposition (EEMD) filter, reconstructed via the Bayesian model averaging (BMA) method for the historical period 1901-2005, and validated for 2006-16. In the simulations of the “medium” representative concentration pathways scenario during 2017-35, the MDV-modulated temperature change projected via the refined approach displays an increase of 0.44°C (90% uncertainty range from 0.30 to 0.58°C) for global land, 0.48°C (90% uncertainty range from 0.29 to 0.67°C) for the Northern Hemispheric land (NL), and 0.29°C (90% uncertainty range from 0.23 to 0.35°C) for the Southern Hemispheric land (SL). These increases are smaller than those projected by the conventional arithmetic mean approach. The MDV enhances the ST in 13 of 21 regions across the world. The largest MDV-modulated warming effect (46%) exists in central America. In contrast, the MDV counteracts the ST in NL, SL, and eight other regions, with the largest cooling effect (220%) in Alaska. [ABSTRACT FROM AUTHOR]

Twentieth-century trends in seasonal temperature and precipitation over the African continent are analyzed from observational datasets and historical climate simulations. Given the agricultural economy of the continent, a seasonal perspective is adopted as it is more pertinent than an annual-average one, which can mask offsetting but agriculturally sensitive seasonal hydroclimate variations. Examination of linear trends in seasonal surface air temperature (SAT) shows that heat stress has increased in several regions, including Sudan and northern Africa where the largest SAT trends occur in the warm season. Broadly speaking, the northern continent has warmed more than the southern one in all seasons. Precipitation trends are varied but notable declining trends are found in the countries along the Gulf of Guinea, especially in the source region of the Niger River in West Africa, and in the Congo River basin. Rainfall over the African Great Lakes-one of the largest freshwater repositories-has, however, increased. It is shown that the Sahara Desert has expanded significantly over the twentieth century, by 11%-18% depending on the season, and by 10% when defined using annual rainfall. The expansion rate is sensitively dependent on the analysis period in view of the multidecadal periods of desert expansion (including from the drying of the Sahel in the 1950s-80s) and contraction in the 1902-2013 record, and the stability of the rain gauge network. The desert expanded southward in summer, reflecting retreat of the northern edge of the Sahel rainfall belt, and to the north in winter, indicating potential impact of the widening of the tropics. Specific mechanisms for the expansion are investigated. Finally, this observational analysis is used to evaluate the state-of-the-art climate simulations from a comparison of the twentieth-century hydroclimate trends. The evaluation shows that modeling regional hydroclimate change over the African continent remains challenging, warranting caution in the development of adaptation and mitigation strategies. [ABSTRACT FROM AUTHOR]

Multidecadal internal climate variability centered in the North Atlantic is evident in sea surface temperatures and is assumed to be related to variations in the strength of the Atlantic meridional overturning circulation (AMOC). In this study, the extent to which variations in the AMOC may also alter hemispheric and global surface air temperature trends and ocean heat content during the past century is examined. Forty-seven realizations of the twentieth-century climate change from two large ensembles using the Community Earth System Model (CESM) are analyzed. One of the ensembles shows a much wider spread in global mean surface air temperature between its members. This ensemble simulates diverging trends of the AMOC strength during the twentieth century. The presence and strength of deep convection in the Labrador Sea controls these trends. The AMOC strength influences the air–sea heat flux into the high-latitude ocean, where a strengthening of the AMOC leads to decreased storage of heat in the Atlantic, and a larger fraction of the heat taken up by the global ocean accumulates in the top 300 m, compared to the case of a weakening AMOC. The spread in the amount of heat stored in the global ocean below 300 m is similar across the CESM members as in a set of CMIP5 models, confirming the AMOC as a “control knob” on deep-ocean heat storage. By influencing the ocean heat uptake efficiency and by shifting the pattern of heat uptake, global surface air temperatures are significantly altered on a multidecadal time scale by AMOC variability. [ABSTRACT FROM AUTHOR]

Six recurrent thermal regimes are identified over continental North America from June to September through a k-means clustering applied to daily maximum temperature simulated by ECHAM5 forced by historical SSTs for 1930–2013 and validated using NCEP–DOE AMIP-II reanalysis over the 1980–2009 period. Four regimes are related to a synoptic wave pattern propagating eastward in the midlatitudes with embedded ridging anomalies that translate into maximum warming transiting along. Two other regimes, associated with broad continental warming and above average temperatures in the northeastern United States, respectively, are characterized by ridging anomalies over North America, Europe, and Asia that suggest correlated heat wave occurrences in these regions. Their frequencies are mainly related to both La Niña and warm conditions in the North Atlantic. Removing all variability beyond the seasonal cycle in the North Atlantic in ECHAM5 leads to a significant drop in the occurrences of the regime associated with warming in the northeastern United States. Superimposing positive (negative) anomalies mimicking the Atlantic multidecadal variability (AMV) in the North Atlantic translates into more (less) warming over the United States across all regimes, and does alter regime frequencies but less significantly. Regime frequency changes are thus primarily controlled by Atlantic SST variability on all time scales beyond the seasonal cycle, rather than mean SST changes, whereas the intensity of temperature anomalies is impacted by AMV SST forcing, because of upper-tropospheric warming and enhanced stability suppressing rising motion during the positive phase of the AMV. [ABSTRACT FROM AUTHOR]

This study evaluates different hypotheses of the origin of the Little Ice Age, focusing on the long-term response of Arctic sea ice and oceanic circulation to solar and volcanic perturbations. The authors analyze the Last Millennium Ensemble of climate model simulations carried out with the Community Earth System Model at the National Center for Atmospheric Research. The authors examine the duration and strength of volcanic perturbations, and the effects of initial and boundary conditions, such as the phase of the Atlantic multidecadal oscillation. They evaluate the impacts of these factors on decadal-to-multicentennial perturbations of the cryospheric, oceanic, and atmospheric components of the climate system. The authors show that, at least in the Last Millennium Ensemble, volcanic eruptions are followed by a decadal-scale positive response of the Atlantic multidecadal overturning circulation, followed by a centennial-scale enhancement of the Northern Hemispheric sea ice extent. It is hypothesized that a few mechanisms, not just one, may have to play a role in consistently explaining such a simulated climate response at both decadal and centennial time scales. The authors argue that large volcanic forcing is necessary to explain the origin and duration of Little Ice Age–like perturbations in the Last Millennium Ensemble. Other forcings might play a role as well. In particular, prolonged fluctuations in solar irradiance associated with solar minima potentially amplify the enhancement of the magnitude of volcanically triggered anomalies of Arctic sea ice extent. [ABSTRACT FROM AUTHOR]

In this study the mechanisms for low-frequency variability of summer Arctic sea ice are analyzed using long control simulations from three coupled models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three key predictors (Atlantic and Pacific oceanic heat transport into the Arctic and the Arctic dipole) across all three models. In all three models, an enhanced Atlantic (Pacific) heat transport into the Arctic induces summer Arctic sea ice decline and surface warming, especially over the Atlantic (Pacific) sector of the Arctic. A positive phase of the Arctic dipole induces summer Arctic sea ice decline and surface warming on the Pacific side, and opposite changes on the Atlantic side. There is robust Bjerknes compensation at low frequency, so the northward atmospheric heat transport provides a negative feedback to summer Arctic sea ice variations. The influence of the Arctic dipole on summer Arctic sea ice extent is more (less) effective in simulations with less (excessive) climatological summer sea ice in the Atlantic sector. The response of Arctic sea ice thickness to the three key predictors is stronger in models that have thicker climatological Arctic sea ice. [ABSTRACT FROM AUTHOR]

There is observational and modeling evidence that low-frequency variability in the North Atlantic has significant implications for the global climate, particularly for the climate of the Northern Hemisphere. This study explores the representation of low-frequency variability in the Atlantic region in historical large ensemble and preindustrial control simulations performed with the Community Earth System Model (CESM). Compared to available observational estimates, it is found that the simulated variability in Atlantic meridional overturning circulation (AMOC), North Atlantic sea surface temperature (NASST), and Sahel rainfall is underestimated on multidecadal time scales but comparable on interannual to decadal time scales. The weak multidecadal North Atlantic variability appears to be closely related to weaker-than-observed multidecadal variations in the simulated North Atlantic Oscillation (NAO), as the AMOC and consequent NASST variability is impacted, to a great degree, by the NAO. Possible reasons for this weak multidecadal NAO variability are explored with reference to solutions from two atmosphere-only simulations with different lower boundary conditions and vertical resolution. Both simulations consistently reveal weaker-than-observed multidecadal NAO variability despite more realistic boundary conditions and better resolved dynamics than coupled simulations. The authors thus conjecture that the weak multidecadal NAO variability in CESM is likely due to deficiencies in air-sea coupling, resulting from shortcomings in the atmospheric model or coupling details. [ABSTRACT FROM AUTHOR]

Abstract: We find that summertime air quality in the eastern U.S. displays strong dependence on North Atlantic sea surface temperatures, resulting from large‐scale ocean‐atmosphere interactions. Using observations, reanalysis data sets, and climate model simulations, we further identify a multidecadal variability in surface air quality driven by the Atlantic Multidecadal Oscillation (AMO). In one‐half cycle (~35 years) of the AMO from cold to warm phase, summertime maximum daily 8 h ozone concentrations increase by 1–4 ppbv and PM2.5 concentrations increase by 0.3–1.0 μg m−3 over much of the east. These air quality changes are related to warmer, drier, and more stagnant weather in the AMO warm phase, together with anomalous circulation patterns at the surface and aloft. If the AMO shifts to the cold phase in future years, it could partly offset the climate penalty on U.S. air quality brought by global warming, an effect which should be considered in long‐term air quality planning. [ABSTRACT FROM AUTHOR]

Two large ensembles (LEs) of historical climate simulations are used to compare how various statistical methods estimate the sea surface temperature (SST) changes due to anthropogenic and other external forcing, and how their removal affects the internally generated Atlantic multidecadal oscillation (AMO), Pacific decadal oscillation (PDO), and the SST footprint of the Atlantic meridional overturning circulation (AMOC). Removing the forced SST signal by subtracting the global mean SST (GM) or a linear regression on it (REGR) leads to large errors in the Pacific. Multidimensional ensemble empirical mode decomposition (MEEMD) and quadratic detrending only efficiently remove the forced SST signal in one LE, and cannot separate the short-term response to volcanic eruptions from natural SST variations. Removing a linear trend works poorly. Two methods based on linear inverse modeling (LIM), one where the leading LIM mode represents the forced signal and another using an optimal perturbation filter (LIMopt), perform consistently well. However, the first two LIM modes are sometimes needed to represent the forced signal, so the more robust LIMopt is recommended. In both LEs, the natural AMO variability seems largely driven by the AMOC in the subpolar North Atlantic, but not in the subtropics and tropics, and the scatter in the AMOC-AMO correlation is large between individual ensemble members. In three observational SST reconstructions for 1900-2015, linear and quadratic detrending, MEEMD, and GM yield somewhat different AMO behavior, and REGR yields smaller PDO amplitudes. Based on LIMopt, only about 30% of the AMO variability is internally generated, as opposed to more than 90% for the PDO. The natural SST variability contribution to global warming hiatus is discussed. [ABSTRACT FROM AUTHOR]

The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific-North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo-west Pacific, an arrangement of tropical oceans and Pacific-North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case. [ABSTRACT FROM AUTHOR]

In a recent article, Cheung et al. applied a semiempirical methodology to isolate internal climate variability (ICV) in CMIP5 models and observations. The essence of their methodology is to subtract the scaled CMIP5 multimodel ensemble mean (MMEM) from individual model simulations and from the observed time series of several surface temperature indices. Cheung et al. detected large differences in both the magnitude and spatial patterns of the observed and simulated ICV, as well as large differences between the historical (simulated) ICV and preindustrial (PI) control CMIP5 simulations. Here it is shown that subtraction of the scaled MMEM from CMIP5 historical simulations produces a poor estimate of the modeled ICV due to the difference between the scaled MMEM and a given model's true forced signal masquerading as ICV. The resulting phase and amplitude errors of the ICV so estimated are large, which compromises most of Cheung et al.'s conclusions pertaining to characterization of ICV in the historical CMIP5 simulations. By contrast, an alternative methodology based on forced signals computed from individual model ensembles produces a much more accurate estimate of the ICV in CMIP5 models, whose magnitude is consistent with the PI control simulations and is much smaller than any of the semiempirical estimates of the observed ICV on decadal and longer time scales. [ABSTRACT FROM AUTHOR]

In a comment on a 2017 paper by Cheung et al., Kravtsov states that the results of Cheung et al. are invalidated by errors in the method used to estimate internal variability in historical surface temperatures, which involves using the ensemble mean of simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to estimate the forced signal. Kravtsov claims that differences between the forced signals in the individual models and as defined by the multimodel ensemble mean lead to errors in the assessment of internal variability in both model simulations and the instrumental record. Kravtsov proposes a different method, which instead uses CMIP5 models with at least four realizations to define the forced component. Here, it is shown that the conclusions of Cheung et al. are valid regardless of whether the method of Cheung et al. or that of Kravtsov is applied. Furthermore, many of the points raised by Kravtsov are discussed in Cheung et al., and the disagreements of Kravtsov appear to be mainly due to a misunderstanding of the aims of Cheung et al. [ABSTRACT FROM AUTHOR]