Atmospheric General Circulation Model Research Articles

Evaluation of a Coupled Modeling Approach for the Investigation of the Effects of SST Mesoscale Variability on the Atmosphere

AbstractThis study further evaluates the modeling approach of Jia et al. (2019), https://doi.org/10.1029/2019gl081960 (JEA19) to investigate the potential effects of mesoscale sea surface temperature (SST) variability on the atmospheric circulation. The approach employs a global atmospheric circulation model coupled to a slab ocean model to produce two ensembles of simulations: one in which the SST exhibits realistic mesoscale variability, and another in which the mesoscale SST variability is suppressed. The results of the present study suggest that the modeling approach yields the desired SST differences between the two ensembles at the mesoscales. They also show, however, that the approach can lead to undesirable SST differences at the large scales, if the prescribed pair of oceanic heat flux fields have large scale differences. In the experiments of JEA19, such forced, large scale SST differences dominate over the large scale differences that may develop in response to changes in the large scale atmospheric circulation by nonlinear interactions with the SST. This result suggests that finding a proper pair of estimates of the oceanic heat flux fields is necessary for the investigation of the potential upscale impact of mesoscale SST variability on the atmosphere by the approach of JEA19. The paper concludes with proposing a potential improved strategy to obtain such a pair of estimates.

Enhanced Turbulent Heat Fluxes Improve Meiyu‐Baiu Simulation in High‐Resolution Atmospheric Models

AbstractBased on 10 pairs of atmospheric general circulation models from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project, we find that high‐resolution models (HRMs, ∼50 km) perform better than low‐resolution models (LRMs, ∼100 km) in simulating the East Asian summer monsoon (EASM) rain belt. We attribute the increased rainfall to enhanced vertical advection of water vapor in the HRMs which is mainly contributed by enhanced vertical motion. Based on moist static energy analyses, the stronger vertical motion in the HRMSs is mainly balanced by enhanced net energy flux into the atmosphere column in the “Meiyu‐Baiu” (MB) region due to the increase of air‐sea turbulent latent heat fluxes along the Kuroshio and sensible heat fluxes over land. The increased net energy flux in the HRMs drives stronger ascending motion, resulting in more realistic rainfall in the MB region. Higher saturated specific humidity determined by finer sea surface temperature, less near‐surface specific humidity along the Kuroshio, and stronger near‐surface winds on western side of the western North Pacific Subtropical High strengthened by the enhanced pressure gradient together contribute to higher ventilation efficiencies and enhanced ocean forcing along the Kuroshio, leading to more turbulent latent heat fluxes in the HRMs. Additionally, drier and less cloudy atmosphere in the HRMs result in more downward shortwave radiation heating over the land, which partly contributes to the increased sensible heat flux. This understanding of the sensitivity of simulated MB to model resolution has implications for precipitation simulation and prediction over East Asia.

Zonal wave 3 pattern in the Southern Hemisphere generated by tropical convection

A distinctive feature of the Southern Hemisphere extratropical atmospheric circulation is the quasi-stationary zonal wave 3 pattern. This pattern is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the zonal wave 3 pattern has substantial impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the presence of three major landmasses in the Southern Hemisphere extratropics. Here we use an atmospheric general circulation model to show that the stationary zonal wave 3 pattern is instead driven by zonally asymmetric deep convection in the tropics, with little influence from extratropical orography or landmasses. Localized regions of deep convection in the tropics form a local Hadley cell, which in turn creates a wave source in the subtropics that excites a poleward- and eastward-propagating wave train, forming quasi-stationary waves in the Southern Hemisphere high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, fundamentally control the zonal wave 3 pattern, with implications for southern high-latitude climate, ocean circulation and sea ice. The zonal wave 3 circulation pattern in the Southern Hemisphere is driven by tropical convection, according to results from an atmospheric general circulation model.

Summer monsoon over northeastern India during the last millennium

AbstractAnalysing three high resolution coupled model simulations for the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) regimes, available as the last millennium simulations of the PMIP3, we find that Northeastern Indian summer monsoon rainfall (NEISMR) did not change appreciably from the MCA to the LIA. This is in contrast to the signals in the rest of the Indian region. Our results from all the models suggest that, during the MCA, the simulated 100‐hPa tropical easterly jet (TEJ) becomes relatively more intense than that during LIA, due to a stronger Tibetan High. This strengthening of the TEJ results in an associated increase in relative vorticity at the 500 hPa over the Head Bay of Bengal and neighbouring east coast of the Bay of Bengal. This results in higher moisture availability and increased summer monsoon rainfall in the neighbourhood, including the central Indian region, during the MCA. However, the simulations do not show any such changes over northeastern India, indicating a relatively stable simulated NEISMR from MCA through LIA. Furthermore, just as the current day observations, the simulated correlations between the NEISMR with various concurrent ENSO indices are weak and statistically insignificant during the MCA and LIA regimes. Interestingly, an analysis of time‐slice simulations for the MCA and LIA from an atmospheric general circulation model broadly agree with the above conclusions, indicating that the tropical ocean and atmospheric coupling may not have played a major role in the northeast climate.

Changes in river water resources of the Russian Federation's economic regions forecasted based on the CMIP5 runoff data

The article presents predictive estimates of changes in river water resources in eleven economic regions of the Russian Federation for the period of 2030 and 2050 based on the use of river runoff data of the atmospheric and ocean general circulation models (AOGCM) of the CMIP5 project. An overview of the design schemes of the AOGCM river runoff used in the research is made. The methodology uses an ensemble approach. The predictive estimates were carried out for two forecast periods 2021-2040 and 2041-2060 in relation to the period 1981-2000, which was chosen as the base one. The calculations were made on the basis of an ensemble of 24 selected models for both the moderately aggressive RCP4.5 scenario and the “hard” RCP8.5 scenario. The results obtained were averaged over 20-year periods and compared with the corresponding values for the period 1981-2000. According to the results obtained, in the coming decades there is no reason to expect any significant changes in river flow as a result of climate change. For most of the country's territory, a slight increase in annual runoff is most likely, which is within the limits of its natural variability. The most abundant river water resources will be the Asian part of the Russian Federation and the adjacent eastern part of the European territory of Russia. The most problematic from the point of view of water resources provision will be the Central Black Earth, North Caucasus and Volga economic regions. There will be more water in those regions where it is sufficient, and less where it is most needed. It should be noted that the technique presented in this article is considered by the authors primarily as one of the alternative forecasting methods that must be used in conjunction with other accepted methods. However, given the continuous process of improving the AOGCMs, as well as the quality of the information they provide, this approach should be considered as one of the most promising.

RETRACTED ARTICLE: Tropical coastal climate change based on image defogging algorithm and swimming rehabilitation training

This paper first designs and implements an image defogging algorithm based on FPGA. The hardware design mainly includes SDRAM control module, SD card control module, data transmission control module, and convolution neural network calculation module. The SDRAM control module controls the working state of SDRAM; the SD card control module controls the working state of SD card; the data transmission control module controls the data transmission of reading and writing SDRAM; and the convolution neural network module is responsible for the image defogging algorithm. Based on this point, this paper uses the medium complex ocean atmosphere combination model to estimate the SST diversity and climate characteristics of the tropical coastal Pacific and Atlantic Ocean in recent years, and studies and analyzes the influencing factors of the prediction. The atmospheric part of the coupled model is the global atmospheric circulation model CCM3, and the oceanic part is a simplified gravity model of Zebiak Cane type. In addition, an air filter is added to the coupled model to test the impact of natural noise during broadcasting. Finally, the article investigated the positive effect of swimming rehabilitation training on chronic neck pain symptoms of office workers; Objectives were to study the effect of various swimming rehabilitation trainings on neck pain of office workers and investigate the effect of different swimming rehabilitation training cycles on chronic neck pain symptoms of office workers. Objective was to provide a theoretical basis for the development of exercise rehabilitation plan to improve the chronic pain symptoms of office workers. The results showed that breaststroke training and freestyle training were effective in treating neck pain of office workers. In this paper, the application of adaptive image defogging algorithm and the correction of coastal climate change in the rehabilitation training research has promoted the healthy development and progress of swimming rehabilitation training.

Impacts of Arctic Sea Ice on Cold Season Atmospheric Variability and Trends Estimated from Observations and a Multi-model Large Ensemble

AbstractTo examine the atmospheric responses to Arctic sea-ice variability in the Northern Hemisphere cold season (October to following March), this study uses a coordinated set of large-ensemble experiments of nine atmospheric general circulation models (AGCMs) forced with observed daily-varying sea-ice, sea-surface temperature, and radiative forcings prescribed during the 1979-2014 period, together with a parallel set of experiments where Arctic sea ice is substituted by its climatology. The simulations of the former set reproduce the near-surface temperature trends in reanalysis data, with similar amplitude, and their multi-model ensemble mean (MMEM) shows decreasing sea-level pressure over much of the polar cap and Eurasia in boreal autumn. The MMEM difference between the two experiments allows isolating the effects of Arctic sea-ice loss, which explain a large portion of the Arctic warming trends in the lower troposphere and drives a small but statistically significant weakening of the wintertime Arctic Oscillation. The observed interannual co-variability between sea-ice extent in the Barents-Kara Seas and lagged atmospheric circulation is distinguished from the effects of confounding factors based on multiple regression, and quantitatively compared to the co-variability in MMEMs. The interannual sea-ice decline followed by a negative North Atlantic Oscillation-like anomaly found in observations is also seen in the MMEM differences, with consistent spatial structure but much smaller amplitude. This result suggests that the sea-ice impacts on trends and interannual atmospheric variability simulated by AGCMs could be underestimated, but caution is needed because internal atmospheric variability may have affected the observed relationship.

The long-term transport and radiative impacts of the 2017 British Columbia pyrocumulonimbus smoke aerosols in the stratosphere

Abstract. Interactions of meteorology with wildfires in British Columbia, Canada, during August 2017 led to three major pyrocumulonimbus (pyroCb) events that resulted in the injection of large amounts of smoke aerosols and other combustion products at the local upper troposphere and lower stratosphere (UTLS). These plumes of UTLS smoke with elevated values of aerosol extinction and backscatter compared to the background state were readily tracked by multiple satellite-based instruments as they spread across the Northern Hemisphere (NH). The plumes were observed in the lower stratosphere for about 8–10 months following the fire injections, with a stratospheric aerosol e-folding time of about 5 months. To investigate the radiative impacts of these events on the Earth system, we performed a number of simulations with the Goddard Earth Observing System (GEOS) atmospheric general circulation model (AGCM). Observations from multiple remote-sensing instruments were used to calibrate the injection parameters (location, amount, composition and heights) and optical properties of the smoke aerosols in the model. The resulting simulations of three-dimensional smoke transport were evaluated for a year from the day of injections using daily observations from OMPS-LP (Ozone Mapping Profiler Suite Limb Profiler). The model-simulated rate of ascent, hemispheric spread and residence time (or e-folding time) of the smoke aerosols in the stratosphere are in close agreement with OMPS-LP observations. We found that both aerosol self-lofting and the large-scale atmospheric motion play important roles in lifting the smoke plumes from near the tropopause altitudes (∼ 12 km) to about 22–23 km into the atmosphere. Further, our estimations of the radiative impacts of the pyroCb-emitted smoke aerosols showed that the smoke caused an additional warming of the atmosphere by about 0.6–1 W/m2 (zonal mean) that persisted for about 2–3 months after the injections in regions north of 40∘ N. The surface experienced a comparable magnitude of cooling. The atmospheric warming is mainly located in the stratosphere, coincident with the location of the smoke plumes, leading to an increase in zonal mean shortwave (SW) heating rates of 0.02–0.04 K/d during September 2017.

The influence of regional climatic changes on the flood regime in European Russia in the 21st Century

Possible changes in the average annual maximum snow reserves and flood runoff in European Russia (ER), on the basis of global climate modelling data was estimated. The data on precipitation and temperature from 5 AOGCMs (atmospheric and ocean general circulation models) of the CMIP5 project, based on the best reproduction of the current climate were used. The multidirectional changes in the maximum snow reserves and flood runoff are expected in ER, although there is a tendency in the southern territories towards a decrease in these characteristics; this intensifies near the end of the 21st Century and when using data from the ‘hard’ scenario of greenhouse gas emissions.

Interdecadal variation of summer rainfall in the Greater Mekong Subregion and its possible causes

This paper investigates the interdecadal variation in summer rainfall over the Greater Mekong Subregion (GMS) during 1981–2020 and its possible causes, using Climate Hazards Group Infrared Precipitation with Station rainfall datasets, European Centre for Medium-range Weather Forecasts Reanalysis fifth generation data and the atmospheric general circulation model ECHAM6. The dominant mode of summer rainfall in the GMS features a seesaw pattern with an increase in rainfall over the central–southern GMS and a decrease in rainfall in northwestern Myanmar–Yunnan. The dominant mode of the GMS summer rainfall shows a change in regime around 2001/2002. Interdecadal variability in rainfall is largely related to sea surface temperature anomalies (SSTAs) over the Western Pacific Warm Pool (WPWP). Warmer SSTAs in the WPWP lead to interdecadal variation in summer rainfall in the GMS by exciting an anomalous cyclone in the lower troposphere over the southern GMS–South China Sea. This is accompanied by anomalous ascending motions in the central–southern GMS and anomalous descending motions in northern Myanmar–Yunnan. The Matsuno–Gill mechanism, which links SSTAs with interdecadal variations in the GMS summer rainfall, is further confirmed by numerical experiments.

EFSO at Different Geographical Locations Verified with Observing System Experiments

AbstractAn ensemble-based forecast sensitivity to observations (EFSO) diagnosis has been implemented in an atmospheric general circulation model–ensemble Kalman filter data assimilation system to estimate the impacts of specific observations from the quasi-operational global observing system on weekly short-range forecasts. It was examined whether EFSO reasonably approximates the impacts of a subset of observations from specific geographical locations for 6-h forecasts, and how long the 6-h observation impacts can be retained during the 7-day forecast period. The reference for these forecasts was obtained from 12 data-denial experiments in each of which a subset of three radiosonde observations launched from a geographical location was excluded. The 12 locations were selected from three latitudinal bands comprising (i) four Arctic regions, (ii) four midlatitude regions in the Northern Hemisphere, and (iii) four tropical regions during the Northern Hemisphere winter of 2015/16. The estimated winter-averaged EFSO-derived observation impacts well corresponded to the 6-h observation impacts obtained by the data denials and EFSO could reasonably estimate the observation impacts by the data denials on short-range (from 6 h to 2 day) forecasts. Furthermore, during the medium-range (4–7 day) forecasts, it was found that the Arctic observations tend to seed the broadest impacts and their short-range observation impacts could be projected to beneficial impacts in Arctic and midlatitude North American areas. The midlatitude area was located just downstream of dynamical propagation from the Arctic toward the midlatitudes. Results obtained by repeated Arctic data-denial experiments were found to be generally common to those from the non-repeated experiments.

Contrasting the Role of Regional and Remote Circulation in Driving Asian Monsoon Biases in MetUM GA7.1

AbstractMonsoon precipitation affects nearly half of the world's population, but monsoon biases are a long‐standing problem in climate simulations. We apply dynamical nudging either globally or regionally to demonstrate the role of regional and remote circulation in generating Asian monsoon biases in an atmospheric general circulation model. Monsoon precipitation biases are substantially reduced in response to global nudging but may also be exacerbated over the warm oceanic equatorial areas because of unconstrained sub‐grid convection. Regional nudging over Asia appears to be more efficient than nudging outside Asia in reducing seasonal precipitation biases over eastern China and India. This suggests a predominant role of local circulation anomalies in generating monsoon precipitation errors in these regions. An exception is the summer precipitation bias over eastern China, which is more strongly controlled by remote circulation. Besides seasonal mean rainfall, nudging can also improve the simulated interannual and intraseasonal precipitation variability over the subtropics. This results in a better skill in reproducing the observed El Niño teleconnections to India and China and the monsoon onset date. Improved understanding of the origin of Asian monsoon biases and the contribution from regional and remote circulation advances our knowledge of the interplay between the Asian monsoon and large‐scale circulation, which can be beneficial to the simulation and interpretation of monsoon projections.

Robust atmospheric river response to global warming in idealized and comprehensive climate models

AbstractAtmospheric rivers (ARs), narrow intense moisture transport, account for much of the poleward moisture transport in midlatitudes. While studies have characterized AR features and the associated hydrological impacts in a warming climate in observations and comprehensive climate models, the fundamental dynamics for changes in AR statistics (e.g., frequency, length, width) are not well understood. Here we investigate AR response to global warming with a combination of idealized and comprehensive climate models. To that end, we developed an idealized atmospheric GCM with Earth-like global circulation and hydrological cycle, in which water vapor and clouds are modeled as passive tracers with simple cloud microphysics and precipitation processes. Despite the simplicity of model physics, it reasonably reproduces observed dynamical structures for individual ARs, statistical characteristics of ARs, and spatial distributions of AR climatology. Under climate warming, the idealized model produces robust AR changes similar to CESM large ensemble simulations under RCP8.5, including AR size expansion, intensified landfall moisture transport, and an increased AR frequency, corroborating previously reported AR changes under global warming by climate models. In addition, the latitude of AR frequency maximum shifts poleward with climate warming. Further analysis suggests the thermodynamic effect (i.e., an increase in water vapor) dominates the AR statistics and frequency changes while both the dynamic and thermodynamic effects contribute to the AR poleward shift. These results demonstrate that AR changes in a warming climate can be understood as passive water vapor and cloud tracers regulated by large-scale atmospheric circulation, whereas convection and latent heat feedback are of secondary importance.

Interdecadal Shift of the Relationship between ENSO and Winter Synoptic Temperature Variability over the Asian–Pacific–American Region in the 1980s

AbstractIn this study, the interdecadal variability of the relationship between ENSO and winter synoptic temperature variability (STV) over the Asian–Pacific–American region is investigated based on observational data from 1951 to 2018. An interdecadal shift in the ENSO–STV relationship occurred in the 1980s over Eastern China, changing from significant in period 1 (P1; 1951–87) to insignificant in period 2 (P2; 1988–2018). But the ENSO–STV relationship is significantly stable over North America for the whole period. In addition, a possible reason for this interdecadal shift in the ENSO–STV relationship over Eastern China is also investigated. During P1, the ENSO pattern is significantly correlated to the temperature gradient over Northeast Asia, which is the key region influencing the intensification of extratropical eddies. The intensification of extratropical eddies over Northeast Asia is directly associated with the magnitude of STV over Eastern China. But in P2, the ENSO pattern is not related to the temperature over Northeast Asia. Therefore, the change in the ENSO pattern from P1 to P2 contributes to the interdecadal shift in the ENSO–STV relationship in the 1980s over Eastern China by influencing the temperature gradient over Northeast Asia, while ENSO can influence the temperature gradient over North America for the whole period. Furthermore, the possible role of the ENSO patterns in P1 and P2 is also examined by using an atmospheric general circulation model, highlighting that the pattern of SST variation is a determining factor in regulating STV in different regions.

A zonally-oriented teleconnection pattern induced by heating of the western Tibetan Plateau in boreal summer

The thermal effect of the Tibetan Plateau (TP) on the northern hemisphere climate has long been a hot topic of scientific research. However, the global effects of the TP heat source are still unclear. We investigate the teleconnection patterns coincident with the TP heat source in boreal summer using both observational data and numerical models including a linearized baroclinic model and an atmospheric general circulation model. The western TP shows the most intense variability in atmospheric heating and the most active connection to atmospheric circulations. The surface sensible heating component of the western TP heat source is associated with a high-latitude wave train propagating from North Japan to central North America through the Bering Sea and Canada. The radiative heating component is accompanied by a wavenumber-4 wave train over Eurasia. We focus on the global zonally-oriented pattern that is connected with the latent heat release from the western TP, referred to here as the TP–circumglobal teleconnection (TP-CGT). The TP-CGT pattern is triggered by the western TP latent heating in two parts starting from the TP: an eastward-propagating wave train trapped in the westerly jet stream and a westward Rossby wave response. The TP-CGT accounts for above 18% of the total variance of the circumglobal teleconnection pattern and modulates mid-latitude precipitation by superimposition. The western TP is the key region in which diabatic heating can initiate the two atmospheric responses concurrently, and the heating over northeastern Asia or the Indian Peninsula is unable to induce the circumglobal pattern directly. The unique geographical location and strong tropospheric heating also make the western TP as a “transit area” of transferring the indirect impact of the Indian summer monsoon (ISM) to the TP-CGT. These results enhance our understanding of the relationship between the circumglobal teleconnection and the ISM and is helpful for improving the prediction of the circumglobal teleconnection variability.

A local sigma-point unscented Kalman filter for geophysical data assimilation

The prior covariance calculated in the Reduced-rank Sigma-Point Kalman filter (RSPKF) data assimilation method can be suboptimal as a result of finite number of sigma points, effects of sampling error, process error, and other factors. In this study, we analyse the performance of RSPKF by applying a localization scheme that combines local analysis and global generation of sigma points. The analysis at each model grid for the entire domain are updated using only the observations local to the analysis grid. The localization enables a high rank approximation of the background error covariance in a local subspace of greatly lower dimension than the global domain using a small number of sigma points. The global-analysis vector is constructed combining the local analyses at all model grid points. The global analysis-covariance matrix is generated in the ensemble subspace and the global sigma points are constructed following the RSPKF algorithm. Numerical experiments of our method utilized the Lorenz-96 model. The performance of the localization scheme is assessed in the presence of varying parameters such as the number of sigma points (k), inflation factor (ϕ), localization radius (d) and the number of model variables (Nx). When the localization is implemented, the number of sigma points required to achieve the minimum RMSE is significantly reduced compared to a case where no localization is used, for three different cases of model variables (Nx). We also show that the approximate number of sigma points used to obtain optimal estimate is independent of the state dimension Nx of the model. This further highlights the importance of localization in RSPKF, making it a potential candidate for data assimilation in oceanic or atmospheric General Circulation Models (GCMs).

Teleconnections of the Quasi‐Biennial Oscillation in a multi‐model ensemble of QBO‐resolving models

AbstractThe Quasi‐biennial Oscillation (QBO) dominates the interannual variability of the tropical stratosphere and influences other regions of the atmosphere. The high predictability of the QBO implies that its teleconnections could lead to increased skill of seasonal and decadal forecasts provided the relevant mechanisms are accurately represented in models. Here modelling and sampling uncertainties of QBO teleconnections are examined using a multi‐model ensemble of QBO‐resolving atmospheric general circulation models that have carried out a set of coordinated experiments as part of the Stratosphere‐troposphere Processes And their Role in Climate (SPARC) QBO initiative (QBOi). During Northern Hemisphere winter, the stratospheric polar vortex in most of these models strengthens when the QBO near 50 hPa is westerly and weakens when it is easterly, consistent with, but weaker than, the observed response. These weak responses are likely due to model errors, such as systematically weak QBO amplitudes near 50 hPa, affecting the teleconnection. The teleconnection to the North Atlantic Oscillation is less well captured overall, but of similar strength to the observed signal in the few models that do show it. The models do not show clear evidence of a QBO teleconnection to the Northern Hemisphere Pacific‐sector subtropical jet.

Improved Simulation of Midlatitude Climate in a New Channel Model Compared to Contemporary Global Climate Models

AbstractA midlatitude channel model (MCM), zonally global but meridionally bounded, is constructed for the Northern Hemisphere (0°–360°, 26°N–60°N), based on the Weather Research and Forecasting model. The MCM simulates the midlatitude at a higher resolution than typically possible in a global climate model (GCM). On the other hand, compared to four lateral boundaries in a standard regional or limited‐area model, MCM is bounded on two meridional sides only allowing the simulated atmosphere to develop more freely. Based on a 4‐year simulation at ∼0.33° horizontal grid‐spacing, the MCM realistically captures the annual mean and seasonal cycle of the midlatitude atmosphere and the meridional heat transport by the stationary and transient eddies that dominate the winter weather. Moreover, a comparison of MCM with the ensemble mean of 20 contemporary atmospheric GCMs reveals that the MCM performs better than the GCM ensemble mean. Possible applications of this new modeling configuration are discussed.

Contributors to linkage between Arctic warming and East Asian winter climate

Previous modelling and observational studies have shown discrepancies in the interannual relationship of winter surface air temperature (SAT) between Arctic and East Asia, stimulating the debate about whether Arctic change can influence midlatitude climate. This study uses two sets of coordinated experiments (EXP1 and EXP2) from six different atmospheric general circulation models. Both EXP1 and EXP2 consist of 130 ensemble members, each of which in EXP1 (EXP2) was forced by the same observed daily varying sea ice and daily varying (daily climatological) sea surface temperature (SST) for 1982–2014 but with different atmospheric initial conditions. Large spread exists among ensemble members in simulating the Arctic–East Asian SAT relationship. Only a fraction of ensemble members can reproduce the observed deep Arctic warming–cold continent pattern which extends from surface to upper troposphere, implying the important role of atmospheric internal variability. The mechanisms of deep Arctic warming and shallow Arctic warming are further distinguished. Arctic warming aloft is caused primarily by poleward moisture transport, which in conjunction with the surface warming coupled with sea ice melting constitutes the surface-amplified deep Arctic warming throughout the troposphere. These processes associated with the deep Arctic warming may be related to the forcing of remote SST when there is favorable atmospheric circulation such as Rossby wave train propagating from the North Atlantic into the Arctic.

Recent trends in summer atmospheric circulation in the North Atlantic/European region: is there a role for anthropogenic aerosols?

AbstractThe variability of the westerly jet stream and storm track is crucial for summer weather and climate in the North Atlantic/European region. Observations for recent decades show notable trends in the summer jet from 1970s to 2010s, characterized by an equatorward migration over the North Atlantic accompanied by a poleward migration and weakening of the Mediterranean jet over Europe. These changes in atmospheric circulation were associated with more cyclonic storms traveling across the UK into northern Europe, and fewer over the Mediterranean, leading to wet summers in northern Europe and dry summers in southern Europe.In this study we investigate the potential drivers and processes that may have been responsible for the observed changes in summer atmospheric circulation, with a particular focus on the role of anthropogenic aerosols (AA). We conduct attribution experiments with an atmospheric general circulation model (AGCM) forced with observed changes in sea surface temperatures/sea ice extent (SST/SIE), greenhouse gas concentrations and AA precursor emissions. Comparison between the model results and observations strongly suggests that fast responses to AA changes were likely the primary driver of the observed poleward migration and weakening of the Mediterranean jet, with changes in SST/SIE playing a secondary role. The simulated response shows good agreement with the observed changes in both magnitude and vertical structure, which suggests that common mechanisms - involving aerosol-radiation and aerosol-cloud interactions - are responsible. By contrast, changes in the North Atlantic jet are influenced in the model experiments by changes in both Atlantic SST/SIE (which may themselves have been influenced by changes in AA) and fast responses to AA. In this case, however, there are significant differences between the model response and the observed changes; we argue these differences may be explained by biases in the model climatology.