Atmospheric Model Intercomparison Project Research Articles

Brazilian Annual Precipitation Analysis Simulated by the Brazilian Atmospheric Global Model

The strategy for assessing simulations produced by climate models established as part of the Atmospheric Model Intercomparison Project (AMIP) delivers an outline for model analysis, verification/validation, and intercomparison. Numerical models are continuously being developed to find the best representation for the amount and distribution of precipitation in Brazil to improve the country’s precipitation forecast. This article describes the key features of the Brazilian Global Atmospheric Model (BAM) (developed by the Center for Weather Forecasting and Climate Studies of the National Institute for Space Research (CPTEC/INPE)) and analyses of its performance for annual rainfall climate simulations. This study considered the representation of the annual precipitation in Brazil mainly during the rainy season in the central part of Brazil by the BAM. The model was run over the 1990 to 2015 period using spectral Eulerian model dynamics with a 70-horizontal resolution of approximately 1.0∘× 1.0∘ and 42 vertical sigma levels. The analysis was divided into two stages: the annual precipitation and the rainy season precipitation. Model precipitation analyses were performed using statistical methods, such as the mean and standard deviation, comparing modeled data with observed data from two datasets, data from the XAV (observed data from INMET, ANA, and DAEE), and the Climate Prediction Center (CPC). In general, the BAM model simulations reasonably replicated the configuration of the spatial distribution of precipitation in the Brazilian territory almost entirely, especially compared with the XAV. The accumulated precipitation in the southern region presented great variation, accumulating from 750 mm year−1 in the extreme south to 1750 mm year−1 in the north of this region. Average values of the BAM accumulated precipitation ranged from 1000 to 2000 mm year−1, within the expected average, compared to observed values of 750–1500 mm year−1 (CPC and XAV, correspondingly). Although there was an underestimation of the accumulated precipitation by the model, the model reasonably reproduced the precipitation during the rainy season. The performed assessment identified model aspects that need to be improved.

Intermodel Spread in Walker Circulation Responses Linked to Spread in Moist Stability and Radiation Responses

AbstractThe response of the Pacific Walker circulation (WC) to long‐term warming remains uncertain. Here, we diagnose contributions to the WC response in comprehensive and idealized general circulation model (GCM) simulations. We find that the spread in WC response is substantial across both the Coupled Model Intercomparison Project (CMIP6) and the Atmospheric Model Intercomparison Project (AMIP) models, implicating differences in atmospheric models in the spread in projected WC strength. Using a moist static energy (MSE) budget, we evaluate the contributions to changes in the WC strength related to changes in gross moist stability (GMS), horizontal MSE advection, radiation, and surface fluxes. We find that the multimodel mean WC weakening is mostly related to changes in GMS and radiation. Furthermore, the spread in WC response is related to the spread in GMS and radiation responses. The GMS response is potentially sensitive to parameterized convective entrainment which can affect lapse rates and the depth of convection. We thus investigate the role of entrainment in setting the GMS response by varying the entrainment rate in an idealized GCM. The idealized GCM is run with a simplified Betts‐Miller convection scheme, modified to represent entrainment. The weakening of the WC with warming in the idealized GCM is dampened when higher entrainment rates are used. However, the spread in GMS responses due to differing entrainment rates is much smaller than the spread in GMS responses across CMIP6 models. Therefore, further work is needed to understand the large spread in GMS responses across CMIP6 and AMIP models.

An assessment of land energy balance over East Asia from multiple lines of evidence and the roles of the Tibet Plateau, aerosols, and clouds

Abstract. With high emissions of aerosols and the known world's “Third Pole” of the Tibet Plateau (TP) in East Asia, knowledge on the energy budget over this region has been widely concerned. This study first attempts to estimate the present-day land energy balance over East Asia by combining surface and satellite observations as well as the atmospheric reanalysis and Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. Compared to the global land budget, a substantially larger fraction of atmospheric shortwave radiation of 5.2 % is reflected, highly associated with the higher aerosol loadings and more clouds over East Asian land. While a slightly smaller fraction of atmospheric shortwave absorption of 0.6 % is unexpectedly estimated, possibly related to the lower water vapor content effects due to the thinner air over the TP to overcompensate for the aerosol and cloud effects over East Asian land. The weaker greenhouse effect and fewer low clouds due to the TP are very likely the causes of the smaller fraction of East Asian land surface downward longwave radiation. Hence, high aerosol loadings, clouds, and the TP over East Asia play vital roles in the shortwave budgets, while the TP is responsible for the longwave budgets during this regional energy budget assessment. The further obtained cloud radiative effects suggest that the presence of clouds results in a larger cooling effect on the climate system over East Asian land than that over the globe. This study provides a perspective to understand fully the roles of potential factors in influencing the different energy budget assessments over regions.

An Interdecadal Enhancement of the Impact of ENSO on the Summer Northeast Asian Circulation around 1999/2000 through the Silk Road Pattern

Abstract In this study, we find that the negative relationship between El Niño–Southern Oscillation (ENSO) and summer Northeast Asia (NEA; 30°–50°N, 110°–140°E) circulation, especially the geopotential height anomalies in the upper troposphere (H200), is weakened from the early 1970s, remains stable since the middle 1980s, and is strengthened dramatically after 1999/2000. The recent transitions of the ENSO–NEA H200 relationship are closely connected with the variation of circumglobal teleconnection (CGT)/Silk Road pattern (SRP), which is further modified by the interdecadal shift of the ENSO–Indian summer monsoon rainfall (ISMR) relationship and ENSO evolution. During 1980–99 when the continuing ENSOs dominate, the weakened ENSO–ISMR relationship leads to an inactive ENSO-related CGT/SRP wave train in the upper-level troposphere and then a weakened connection between ENSO and H200 over the NEA. On the other hand, when the emerging ENSOs dominate after 1999/2000, the restored ENSO–ISMR relationship reinforces the ENSO-related CGT/SRP wave train and then enhances ENSO–NEA H200 relationship. This mechanism is well simulated in the Atmospheric Model Intercomparison Project models (AMIP) and Pacific Ocean–Global Atmosphere (POGA) experiments.

Links between winter dust over the Tibetan Plateau and preceding autumn sea ice variability in the Barents and Kara Seas

The Tibetan Plateau (TP) is characterized by heavily local dust activities, however, the mechanism of interannual variations of winter dust frequency over the TP remain poorly understood. Previous studies showed the autumn Arctic sea ice could significantly influence the winter climate over Eurasia. Whether autumn sea ice affects winter dust activity over the TP or not? Here, we used an integrated surface database to investigate possible mechanisms for interannual variability in the frequency of winter dust events above the TP. This variability, which is thought to be mainly caused by local dust emissions, shows significant correlations with sea ice concentration (SIC) in the Barents and Kara Seas during the preceding autumn. Low Barents–Kara SIC is accompanied by reduced snow depth over northern Eurasia between autumn and winter, which can enhance the Eurasian mid-latitude westerly jet stream. This strengthening increases the cyclogenesis and occurrence of strong surface wind speeds in winter, especially over the TP. In addition, a lower SIC is closely associated with reduced precipitation and snow cover in late autumn and winter over the TP, which in turn enhances warming of the land surface and reduces the area of frozen ground. These anomalies in atmospheric circulation patterns and local surface conditions promote dust events above the TP during winter. The ensemble means of Atmospheric Model Intercomparison Project experiments from Phase 6 of the Coupled Model Inter-comparison Project and the Community Atmosphere Model version 4 can generally reproduce the atmospheric circulation anomalies associated with decreased Barents–Kara SIC. This study reveals the crucial effect that SIC anomalies in the Barents and Kara Seas have on winter dust activities over the TP.

On the Effect of Historical SST Patterns on Radiative Feedback

AbstractWe investigate the dependence of radiative feedback on the pattern of sea‐surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea‐ice over the historical record from 1871 to near‐present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long‐term climate sensitivity feedbacks (diagnosed from corresponding atmosphere‐ocean GCM abrupt‐4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long‐term CO2 increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a “pattern effect” (defined as the difference between historical and long‐term CO2 feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1 for the time‐period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1 when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014.

Evaluation of the interannual variability in the East Asian summer monsoon in AMIP and historical experiments of CAS FGOALS-f3-L

The evaluation of East Asian summer monsoon (EASM) simulations could improve our understanding of Asian monsoon dynamics and climate simulations. In this study, by using Phase 6 of the Coupled Model Intercomparison Project (CMIP6) experiments of the Atmospheric Model Intercomparison Project (AMIP) and historical runs of the Chinese Academy of Sciences (CAS) Flexible Global Ocean–Atmosphere–Land System (FGOALS-f3-L) model, the model simulation skill for the interannual variability in the EASM was determined. According to multivariate empirical orthogonal function (MV-EOF) analysis, the major mode of the EASM mainly emerged as a Pacific-Japan pattern in the western Pacific accompanied by a local anticyclonic anomaly with a total variance of 24.6%. The historical experiment could suitably reproduce this spatial pattern and attained a closer total variance than that attained by the AMIP experiment. The historical experiment could also better simulate the time frequency of the EASM variability than the AMIP experiment. However, the phase of principal component 1 (PC1) was not suitably reproduced in the historical experiment since no initialization procedure was applied at the beginning of the integration in the historical simulation process, whereas the sea surface temperature (SST) was preset in the AMIP experiment. Further analysis revealed that air–sea interactions in the Indian Ocean and tropical western Pacific were important for the model to provide satisfactory EASM simulations, while El Niño–Southern Oscillation (ENSO) simulation was possibly related to the climate variability in the EASM simulations, which should be further analyzed.摘要对东亚夏季季风(EASM)模拟的评估可以提高我们对亚洲季风动力和气候模拟的理解. 在这项研究中, 通过使用中国科学院(CAS)全球海洋-大气-陆地系统(FGOALS-f3-L)模式参加的第六次耦合模式相互比较计划(CMIP6)中的大气模式相互比较计划(AMIP)和历史(historical)试验, 明确了EASM的年际变率的模拟能力. 通过多变量经验正交函数(MV-EOF)分析发现, 观测的EASM的主导模态为西太平洋上的太平洋-日本模态, 并伴有局部反气旋异常. 主导模态的方差贡献率为24.6%. 历史(historical)试验可以基本再现这种空间模态, 其方差贡献率较AMIP试验更接近于观测. 与AMIP试验相比, 历史(historical)试验还能更好地模拟EASM变率的时间频率. 然而, 由于历史(historical)模拟没有在积分开始时应用初始化过程, 而AMIP试验受到海表面温度(SST)的约束, 因此主成分(PC1)的位相在历史(historical)试验中没有得到较好地再现. 进一步分析发现, 印度洋和西太平洋热带地区的海气相互作用对EASM的模拟非常重要, 而EASM气候变率的模拟可能与厄尔尼诺-南方涛动(ENSO)的模拟能力有关, 这值得进一步分析.

Hotspots of Monthly Land Precipitation Variations Affected by SST Anomalies

Abstract Climate predictability at seasonal to interannual time scales is mainly associated with sea surface temperature anomalies (SSTAs). How to quantitatively assess the impact of SSTAs on climate variability and predictability is an unresolved topic. Using a novel metric [bulk connectivity (BC)], the integrated influences of global SSTAs on precipitation anomalies over land are examined in observations and compared with Atmospheric Model Intercomparison Project (AMIP) simulations in 1957–2018. The hotspots of the land precipitation variation affected by global SSTA are identified, and the seasonality is evaluated. Such hotspots indicate the regions of land precipitation predictability caused by SSTAs. The hotspots are observed in the Sahel region in September–March, in the Indochina Peninsula in April and May, and in southwestern United States in December–March, which are mostly linked to the influence of El Niño–Southern Oscillation (ENSO). The overall impact of SSTAs on land precipitation is larger in the Southern Hemisphere than in the Northern Hemisphere. The spatial variations of BC and hotspots in the observations are partially reproduced in the AMIP simulations. However, an individual run in the AMIP simulations underestimates the integrated influence of global SSTA on land precipitation anomalies, while the ensemble mean amplifies the integrated influence, and both show a challenge in capturing the seasonality of the SST influence, particularly the time of the strongest impact. The results of the BC metric can serve as a benchmark to evaluate climate models and to identify the predictability sources.

Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models

Abstract Two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations, in the scope of the current Coupled Model Intercomparison Project (CMIP6), are compared with their fully coupled counterparts. The atmospheric models simulate less barotropic atmospheric circulation variability over the North Atlantic and more barotropic atmospheric circulation variability over the North Pacific when compared with reanalysis variability, at intraseasonal and interannual scales. The coupled climate simulations have smaller global barotropic variability than the corresponding AMIP simulations. The smaller variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the North Atlantic. The results suggest that the reduction of the biases, in the North Pacific barotropic atmospheric variability, of coupled climate simulations is achieved through compensating biases in the mean sea surface temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic.

Indian Ocean warming as key driver of long-term positive trend of Arctic Oscillation

Arctic oscillation (AO), which is the most dominant atmospheric variability in the Northern Hemisphere (NH) during the boreal winter, significantly affects the weather and climate at mid-to-high latitudes in the NH. Although a climate community has focused on a negative trend of AO in recent decades, the significant positive trend of AO over the last 60 years has not yet been thoroughly discussed. By analyzing reanalysis and Atmospheric Model Inter-comparison Project (AMIP) datasets with pacemaker experiments, we found that sea surface temperature warming in the Indian Ocean is conducive to the positive trend of AO from the late 1950s. The momentum flux convergence by stationary waves due to the Indian Ocean warming plays an important role in the positive trend of AO, which is characterized by a poleward shift of zonal-mean zonal winds. In addition, the reduced upward propagating wave activity flux over the North Pacific due to Indian Ocean warming also plays a role to strengthen the polar vortex, subsequently, it contributes to the positive trend of AO. Our results imply that the respective warming trend of tropical ocean basins including Indian Ocean, which is either anthropogenic forcing or natural variability or their combined effect, should be considered to correctly project the future AO’s trend.

Deceleration of Madden–Julian Oscillation Speed in NICAM AMIP‐Type Simulation Associated With Biases in the Walker Circulation Strength

AbstractThe eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30‐year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model, following the atmospheric model intercomparison project protocol with a slab ocean was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when it is embedded within the stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian Ocean and eastern Pacific. However, the simulated MJO events occurred disproportionately during El Niño events and displayed a slow bias. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of the MJO in long term simulations.

Systematic improvement in simulated latent and sensible heat fluxes over tropical oceans in AMIP6 models compared to AMIP5 models with the same resolutions

Recent studies have shown that most Atmospheric Model Intercomparison Project (AMIP5) models that participated in the fifth phase of the coupled model intercomparison project (CMIP5) overestimate surface latent (QLH) and sensible (QSH) heat fluxes over tropical oceans. This study aims to quantify and understand any improvement in simulated QLH and QSH in AMIP6 models compared to AMIP5 models with the same horizontal resolutions. To accomplish this, we compare the Ocean Atmosphere air-sea Flux (OAFlux) dataset to nine AMIP5 and nine AMIP6 models integrated over 30 years (1979–2008). We found that all the AMIP5 and AMIP6 models overestimate both components of fluxes, but all of the AMIP6 models performed better than their AMIP5 counterparts over the tropical oceans (30°S-30°N). Yet, systematic spatial biases remain, which leads to only a small improvement in the simulated fluxes. For example, the root mean squared error (RMSE) and mean bias in QLH in AMIP5 are 30 and 21 W m−2 compared to 28 and 19 W m−2 in AMIP6. This is an improvement of 2 W m−2 for both RMSE and bias in QLH, yielding a reduction in QLH bias by 10% and RMSE by 7%. For QSH, an improvement of 1 W m−2 is seen in both bias and RMSE, yielding a reduction in QSH bias by 25% and RMSE by 13%. The primary reason for the improvement in QLH in AMIP6 is the better representation of the 10m wind speed and air-sea humidity difference than those in AMIP5. The improvement in QSH is due to an improvement in air-sea temperature difference. These results offer guidance to the modeling community to improve model simulation of near-surface meteorological variables in the tropics.

Evaluation of East Asian Meiyu from CMIP6/AMIP simulations

East Asian Meiyu simulated by 35 global atmospheric models from the 6th Coupled Model Intercomparison Project (CMIP6)/Atmospheric Model Intercomparison Project (AMIP) were systematically evaluated for 1998–2014. The results show that most of the CMIP6/AMIP models can hardly reproduce the observed spatial pattern and interannual variability of East Asian Meiyu. The spatial pattern is relatively better simulated over Southern Korea and Japan where 14 out of 35 models have realistically simulated precipitation, as compared with the lower reaches of the Yangtze River where only 7 out of 35 models can well reproduce the Meiyu precipitation. For the Meiyu interannual variations, GFDL-CM4 and GFDL-ESM4 have the closest variance among the models versus the TRMM observations, while 12 out of 35 models show smaller variances. We explored the relationships of Meiyu precipitation with large-scale circulation and tropical sea surface temperature (SST), and showed that these relationships from CESM2-WACCM-FV2, EC-Earth3-CC, and MPI-ESM1-2-HAM agree well with those based on TRMM precipitation, MERRA2 reanalysis-derived large-scale atmospheric fields, and observed SST. It is found that the models with a better simulation of Meiyu precipitation can capture the relationship between Meiyu precipitation and the SST in the eastern equatorial Pacific and Indian Ocean more realistically. A performance ranking of the 35 individual CMIP6/AMIP models is further provided. It is shown that the top 20% of models based on interannual variability score tend to simulate a more realistic western Pacific subtropical high than the bottom 20% of models. And the top 20% of models based on comprehensive ranking measure simulate a more realistic EAP pattern than the bottom 20% of models.

The enhanced role of atmospheric reduced nitrogen deposition in future over East Asia–Northwest Pacific

The atmospheric nitrogen deposition plays a crucial role in natural ecosystem, and the changes in emissions substantially affect the amount of nitrogen deposition. Along with the decrease in NOx emissions and increase in NH3 emissions, the reduced nitrogen deposition may play a more important role in future. However, to what extent these changes may modify the reduced nitrogen deposition across East Asia, which is fulfilled with a large amount of nitrogen deposition, to the northwestern Pacific has not yet to be clear. Based on the results of multi-model ensemble of Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), the future changes of reduced nitrogen (NHx) deposition is firstly examined. Here we show the changes of NHx deposition flux is substantially modulated by both climate change and emissions, exhibiting an increasing trend over East Asia-Northwest Pacific in future under representative concentration pathways (RCP) 8.5 scenario, largely controlled by increase of NH3 emissions, contrasting to the oxidized nitrogen deposition which is projected to decrease. Specifically, the ratio of NHx to total nitrogen deposition in eastern China increases from 38% at present to 56% by the end of the century under RCP 8.5, indicative of a transition in the form of dominant nitrogen deposition from oxidized to reduced one. The increase is clearly discernable over the marginal seas and northwestern Pacific. Moreover, we identify a meridional shift of high wet NHx deposition from northern China in summer to southern China in the other seasons. Based on simulations from regional models Weather Research and Forecasting (WRF) and Community Multi-scale Air Quality (CMAQ), we find that the synergistically nonlinear modulation of NHx concentration and precipitation triggers the north-south shift of wet NHx deposition. The findings in this study indicate a potentially more important role of reduced nitrogen deposition on the natural ecosystem in future.

It’s the Heat and the Humidity: The Complementary Roles of Temperature and Specific Humidity to Recent Changes in the Energy Content of the Near‐Surface Atmosphere

AbstractGlobal climate change is a change in the planetary energy (E) balance. It is usually expressed as a change in near‐surface air temperature (Ta), but changes in energy content due to Ta (ET) represent only part of the near‐surface energy balance, which also includes E changes due to specific humidity (ESH). We analyzed MERRA‐2 and ERA5 reanalysis data and 15 CMIP6 Atmospheric Model Intercomparison Project (AMIP) models from 1980 to 2014. Some 44%, 39%, and 50% of MERRA‐2 pixels showed significant increases in ET, ESH, and E, respectively. The average increase in ET (ESH) was 16.9 (13.6) J kg−1 year−1, but AMIP models estimated larger average ET (21.6 J kg−1 year−1) and ESH (16.7 J kg−1 year−1). Global Ta would have increased at nearly double the observed rate if energy was not partitioned into latent heat. Results demonstrate the critical role that specific humidity plays in recent climate changes.

Variations in Summer Extreme High-Temperature Events over Northern Asia and the Possible Mechanisms

AbstractIn this study, interannual and interdecadal variations in the extreme high-temperature event (EHE) frequency over northern Asia (NA) and the associated possible mechanisms are explored. On an interannual time scale, the first two empirical orthogonal function modes of the NA EHE frequency exhibit a meridional dipole pattern (EOF1) and diagonal tripolar pattern (EOF2), respectively. The higher NA EHE frequency is related to anomalous local highs, reduced mid- to low clouds, and more solar radiation. The warmer ground further heats the overlying atmosphere through longwave radiation and sensible heat. The warm temperature advection in the lower troposphere and the drier soil conditions also favor higher EHE frequency. Further analysis reveals that the EOF1 mode is related to the Polar–Eurasian teleconnection pattern (POL), while the EOF2 mode is associated with North Atlantic Oscillation (NAO) and Pacific–Japan/East Asia–Pacific pattern (PJ/EAP). The fitted EHE frequency based on the atmospheric factors (POL, NAO, and PJ/EAP) can explain the interannual variation in the regionally averaged EHE frequency by 33.8%. Furthermore, three anomalous sea surface temperature (SST) patterns over the North Atlantic–Mediterranean Sea region and around the Maritime Continent are associated with the two EHE modes by intensifying the pronounced atmospheric teleconnections. Analysis on the simulation of five models in the Atmospheric Model Intercomparison Project experiment further confirms the impact of the pronounced SST patterns on the POL, NAO and PJ/EAP. In addition, NA EHE frequency experienced a significant interdecadal increase around the mid-1990s, which could be associated with the phase shift of the Atlantic multidecadal oscillation and long-term global warming trend.

Assessing the representation of South American monsoon features in Brazil and U.K. climate model simulations

AbstractThis paper assesses how well the CPTEC/INPE Brazilian Global Atmospheric Model (BAM‐1.2) and the atmospheric component of the UK Met Office Hadley Centre Global Environment Model (HadGEM3‐GC3.1) represent the main South American monsoon features. Climatological (1981–2010) ensemble means of Atmospheric Model Intercomparison Project (AMIP)‐type climate simulations are evaluated. The assessment evaluated the models’ ability to represent the South America austral summer and winter precipitation contrast and associated circulation, key South American monsoon system elements, the association between south‐east Brazil and South America precipitation, and climatological (1997/1998 to 2013/2014) distributions of rainy season onset and demise dates over south‐east Brazil (15°S–25°S, 40°W–50°W) and the core monsoon region (10°S–20°S, 45°W–55°W). Despite some identified deficiencies, both models depict the monsoon region and represent the main features, including (1) the north‐west–south‐east precipitation band and associated ascending motion over central South America; (2) the upper‐level Bolivian High and the north‐east South America trough during the summer; (3) the lower‐level South Atlantic and Pacific subtropical anti‐cyclones and (4) the low‐level jet east of the Andes. Both models represent upper‐level divergence and lower‐level convergence over the core monsoon region, and upper‐level convergence and lower‐level divergence over the Pacific and Atlantic anti‐cyclones associated with the regional Walker circulation during the pre‐monsoon (spring) and peak monsoon (summer) seasons. Convection over South America is weaker in BAM‐1.2 than observed, consistent with continental precipitation deficit. The models reproduce the dipole‐like precipitation pattern between south‐east Brazil and south‐eastern South America during the austral summer but overestimate these patterns spatial extent over the South Atlantic. Both models simulate the main observed climatological features of rainy season onset and demise dates for the two above defined investigated regions. HadGEM3 overestimates onset dates interannual variability. These results can contribute towards understanding climate and land‐use change implications for environmental sustainability and for recommending climate adaptation strategies.

Evaluation of the seasonality and spatial aspects of the Southern Annular Mode in CMIP6 models

AbstractThe simulation skills of the Southern Annular Mode (SAM) pattern for the CMIP6 models are evaluated in this study. Both the Atmospheric Model Intercomparison Project (AMIP) and historical experiments are utilized to understand the effect of SST forcing and air–sea interaction. The spatial correlation coefficient, standard deviation, and root‐mean‐square error (RMSE) are used as the objective metrics. The results suggest that the multimodel mean (MME) for both AMIP and historical data could capture the basic spatial patterns of the SAM for all seasons, featuring the opposite sign between the middle and high latitudes with a zonal three‐wave structure in the middle latitudes (positive phase). However, the overall skill of the AMIP simulation is higher than that of the historical simulation, which could be attributed to realistic SST forcing being applied. The simulation skill varies for different seasons: the skill for boreal spring (March–May [MAM]) is the lowest, while it is highest in boreal winter (December–February [DJF]). Further analysis shows that the simulations of the SAM asymmetric part are the key component for the simulation of the spatial pattern of the SAM, especially sensitive in MAM. The wave–current interaction simulation at mid–high latitudes is suggested to be improved for the advancement of SAM simulations in future model development.

Decadal change of extreme consecutive dry days in spring over the middle and lower reaches of the Yangtze River around the early 2000s: The synergistic effect of mega-El Niño/Southern Oscillation, Atlantic Multidecadal Oscillation, and Arctic sea ice

Base on daily station precipitation data from 1960 to 2019, the variability of extreme consecutive dry days (extreme-CDDs) over the middle and lower reaches of the Yangtze River (MLRYR) during spring sowing time (March–April) is investigated. It is found that the extreme-CDDs over MLRYR experienced a significant decadal increase around the early 2000s. Associated with this decadal change, the Mongolian high and western North Pacific anticyclone (WNPA) are significantly intensified and weakened, respectively. The intensified Mongolian high and weakened WNPA lead to anomalous northerlies and water vapor divergence over MLRYR, providing favorable atmospheric conditions for more extreme-CDDs over the region. Further mechanism analyses suggest that the transition of mega-El Niño/Southern Oscillation (mega-ENSO) from negative-phase to positive-phase contributes to the decadal weakening of WNPA. And the phase transition of Atlantic Multidecadal Oscillation (AMO) and decadal decrease of sea ice over the Barents Sea lead to intensified Mongolian high through exciting atmospheric wave train. Multiple linear regression shows that there could be a synergistic role of mega-ENSO, AMO, and sea ice over the Barents Sea in the decadal change in extreme-CDDs over MLRYR around the early 2000s. Analysis on the simulation of 14 models in the Atmospheric Model Intercomparison Project (AMIP) experiment from phase 6 of the Coupled Model Intercomparison Project (CMIP6) shows that the models can reproduce the observed decadal intensification of the Mongolian high and weakening of WNPA around the early 2000s, which indicates the contribution of mega-ENSO, AMO, and sea ice over the Barents Sea to the decadal changes in Mongolian high, WNPA and extreme-CDDs over MLRYR.

Revisiting the Tropical Atlantic Influence on El Niño–Southern Oscillation

AbstractThe influence of the tropical Atlantic on El Niño–Southern Oscillation (ENSO) is examined using sensitivity experiments with the SINTEX-F general circulation model with prescribed sea surface temperature (SST) distributions based on observations for the period 1982–2018. In the control experiment (CTRL) observed SSTs are prescribed over the global oceans; in the sensitivity experiment observed SSTs are prescribed in the tropical Atlantic only, while in other regions the climatological annual cycle is prescribed. A composite analysis of the model output suggests that cold SST events in the northern tropical Atlantic during boreal spring are associated with near-surface wind changes over the equatorial and subtropical Pacific that are conducive to the development of El Niño, consistent with previous studies. The amplitude of these changes, however, is at most 20% of those observed during typical El Niño events. Likewise, warm events in the equatorial Atlantic produce only about 10% of the wind changes seen in the western equatorial Pacific during the developing phase of typical La Niña events. Similar results are obtained from a partial regression analysis performed on an ensemble of atmosphere-only simulations from phase 6 of the Atmospheric Model Intercomparison Project (AMIP) although the equatorial Atlantic influence is stronger in AMIP. Further analysis of the AMIP models indicates that model biases do not have a major impact on the Atlantic-to-Pacific influence. Overall, the results suggest that the tropical Atlantic has a rather weak influence on ENSO development and mostly acts to modulate ongoing events rather than initiate them.