How well do integrated assessment models simulate climate change?

Integrated assessment models (IAMs) are regularly used to evaluate different policies of future emissions reductions. Since the global costs associated with these policies are immense, it is vital that the uncertainties in IAMs are quantified and understood. We first demonstrate the significant spread in the climate system and carbon cycle components of several contemporary IAMs. We then examine these components in more detail to understand the causes of differences, comparing the results with more complex climate models and earth system models (ESMs), where available. Our results show that in most cases the outcomes of IAMs are within the range of the outcomes of complex models, but differences are large enough to matter for policy advice. There are areas where IAMs would benefit from improvements (e.g. climate sensitivity, inertia in climate response, carbon cycle feedbacks). In some cases, additional climate model experiments are needed to be able to tune some of these improvements. This will require better communication between the IAM and ESM development communities.     

How well are daily intense rainfall events captured by current climate models over Africa?

The ability of state-of-the-art climate models to capture the mean spatial and temporal characteristics of daily intense rainfall events over Africa is evaluated by analyzing regional climate model (RCM) simulations at 90- and 30-km along with output from four atmospheric general circulation models (AGCMs) and coupled atmosphere–ocean general circulation models (AOGCMs) of the Climate Model Intercomparison Project 5. Daily intense rainfall events are extracted at grid point scale using a 95th percentile threshold approach applied to all rainy days (i.e., daily rainfall ≥1 mm day^?1) over the 1998–2008 period for which two satellite-derived precipitation products are available. Both RCM simulations provide similar results. They accurately capture the spatial and temporal characteristics of intense events, while they tend to overestimate their number and underestimate their intensity. The skill of AGCMs and AOGCMs is generally similar over the African continent and similar to previous global climate model generations. The majority of the AGCMs and AOGCMs greatly overestimate the frequency of intense events, particularly in the tropics, generally fail at simulating the observed intensity, and systematically overestimate their spatial coverage. The RCM performs at least as well as the most accurate global climate model, demonstrating a clear added value to general circulation model simulations and the usefulness of regional modeling for investigating the physics leading to intense events and their change under global warming.     

How well do CMIP5 Earth System Models simulate present climate conditions in Europe and Africa?

The present study assesses the ability of seven Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 5 to reproduce present climate conditions in Europe and Africa. This is done from a downscaling perspective, taking into account the requirements of both statistical and dynamical approaches. ECMWF’s ERA-Interim reanalysis is used as reference for an evaluation of circulation, temperature and humidity variables on daily timescale, which is based on distributional similarity scores. To additionally obtain an estimate of reanalysis uncertainty, ERA-Interim’s deviation from the Japanese Meteorological Agency JRA-25 reanalysis is calculated. Areas with considerable differences between both reanalyses do not allow for a proper assessment, since ESM performance is sensitive to the choice of reanalysis. For use in statistical downscaling studies, ESM performance is computed on the grid-box scale and mapped over a large spatial domain covering Europe and Africa, additionally highlighting those regions where significant distributional differences remain even for the centered/zero-mean time series. For use in dynamical downscaling studies, performance is specifically assessed along the lateral boundaries of the three CORDEX domains defined for Europe, the Mediterranean Basin and Africa.     

How well do the current state-of-the-art CMIP5 models characterise the climatology of the East Asian winter monsoon?

Previous studies have revealed some common biases in coupled general circulation model’s simulations of the East Asian (EA) winter monsoon (EAWM), including colder surface air temperature and more winter precipitation over the EA region. In this study, we examined 41 fully coupled atmosphere–ocean models from fifth phase of the Coupled Model Intercomparison Project (CMIP5), which will be widely used in the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), and address whether the current state-of-the-art CMIP5 models can characterise the climatology of the East Asian winter monsoon. We also compared the results with the models from third phase of CMIP, which was extensively used in the fourth assessment report of the IPCC. The results show that the cold surface air temperature (SAT) bias is lessened and the precipitation amount decreased with the current CMIP5 models. Moreover, the CMIP5 models performbetter at predicting surface winds and high-level jet streams than the CMIP3 models. Moreover, CMIP5 models show more model consistency in most EAWM parameters, and the interannual variability of the SAT is closer to the observations. We also examined the change in the radiation energy budget in the CMIP5 models and compared with CMIP3 models. Although the improvements are significant, deficiencies still exist in the simulation of the EAWM, e.g., the stronger EA major trough and the stronger zonal sea level pressure gradient.     

Traditional El Nino and El Nino Modoki Revisited: Is El Nino Modoki Linearly Independent of Traditional El Nino?

The present study revisited the first two leading modes of tropical Pacific sea surface temperature anomalies (SSTA) during the period of 1979-2008. It is suggested that the so-called El Nino Modoki, which is captured by the second mode, exists objectively and exhibits obvious differences from traditional El Nino, which is captured by the first mode, in terms of its spatial characteristics. Furthermore, the authors found that El Nino Modoki is linearly independent of traditional El Nino; hence, it cannot be described as part of the traditional El Nino evolution, and vice versa.     

How well do coupled models replicate ocean energetics relevant to ENSO?

Accurate replication of the processes associated with the energetics of the tropical ocean is necessary if coupled GCMs are to simulate the physics of ENSO correctly, including the transfer of energy from the winds to the ocean thermocline and energy dissipation during the ENSO cycle. Here, we analyze ocean energetics in coupled GCMs in terms of two integral parameters describing net energy loss in the system using the approach recently proposed by Brown and Fedorov (J Clim 23:1563?C1580, 2010a) and Fedorov (J Clim 20:1108?C1117, 2007). These parameters are (1) the efficiency ?? of the conversion of wind power into the buoyancy power that controls the rate of change of the available potential energy (APE) in the ocean and (2) the e-folding rate ?? that characterizes the damping of APE by turbulent diffusion and other processes. Estimating these two parameters for coupled models reveals potential deficiencies (and large differences) in how state-of-the-art coupled GCMs reproduce the ocean energetics as compared to ocean-only models and data assimilating models. The majority of the coupled models we analyzed show a lower efficiency (values of ?? in the range of 10?C50% versus 50?C60% for ocean-only simulations or reanalysis) and a relatively strong energy damping (values of ??^?1 in the range 0.4?C1?years versus 0.9?C1.2?years). These differences in the model energetics appear to reflect differences in the simulated thermal structure of the tropical ocean, the structure of ocean equatorial currents, and deficiencies in the way coupled models simulate ENSO.     

Indices of El Nino and El Nino Modoki： An Improved El Nino Modoki Index

In recent years, El Nino Modoki (pseudo-El Nino) has been distinguished as a unique large-scale ocean warming phenomenon happening in the central tropical Pacific that is quite different from the traditional El Nino. Empirical Orthogonal Function (EOF) analysis is used to successfully separate El Nino and El Nino Modoki. The abilities of the NINO3 index, NINO3.4 index, NINO1+2 index and NINO4 index in characterizing the El Nino are explored in detail. It is suggested that the NINO3 index is comparatively optimal to monitor the El Nino among the four NINO indices, since other NINO indices either cannot well distinguish El Nino and El Nino Modoki signals or are easily disturbed by El Nino Modoki signals. Further, an improved El Nino Modoki index (IEMI) is introduced to better represent the El Nino Modoki that is captured by the second leading EOF mode of monthly tropical Pacific sea surface temperature anomalies (SSTA). The IEMI is an improvement of the El Nino Modoki index (EMI) through adjusting the inappropriate weight coefficients of the three boxes of EMI, and it effectively overcomes the lack of EMI in monitoring the two historical El Nino Modoki events and also avoids the possible risk of EMI in excluding the interference of El Nino signal, indicating the realistic and potential advantages.     

How accurately do coupled climate models predict the leading modes of Asian-Australian monsoon interannual variability?

Accurate prediction of the Asian-Australian monsoon (A-AM) seasonal variation is one of the most important and challenging tasks in climate prediction. In order to understand the causes of the low accuracy in the current prediction of the A-AM precipitation, this study strives to determine to what extent the ten state-of-the-art coupled atmosphere-ocean-land climate models and their multi-model ensemble (MME) can capture the two observed major modes of A-AM rainfall variability–which account for 43% of the total interannual variances during the retrospective prediction period of 1981–2001. The first mode is associated with the turnabout of warming to cooling in the El Niño-Southern Oscillation (ENSO), whereas the second mode leads the warming/cooling by about 1 year, signaling precursory conditions for ENSO. The first mode has a strong biennial tendency and reflects the Tropical Biennial Oscillation (Meehl in J Clim 6:31–41, 1993). We show that the MME 1-month lead prediction of the seasonal precipitation anomalies captures the first two leading modes of variability with high fidelity in terms of seasonally evolving spatial patterns and year-to-year temporal variations, as well as their relationships with ENSO. The MME shows a potential to capture the precursors of ENSO in the second mode about five seasons prior to the maturation of a strong El Niño. However, the MME underestimates the total variances of the two modes and the biennial tendency of the first mode. The models have difficulties in capturing precipitation over the maritime continent and the Walker-type teleconnection in the decaying phase of ENSO, which may contribute in part to a monsoon “spring prediction barrier” (SPB). The NCEP/CFS model hindcast results show that, as the lead time increases, the fractional variance of the first mode increases, suggesting that the long-lead predictability of A-AM rainfall comes primarily from ENSO predictability. In the CFS model, the correlation skill for the first principal component remains about 0.9 up to 6 months before it drops rapidly, but for the spatial pattern it exhibits a drop across the boreal spring. This study uncovered two surprising findings. First, the coupled models’ MME predictions capture the first two leading modes of precipitation variability better than those captured by the ERA-40 and NCEP-2 reanalysis datasets, suggesting that treating the atmosphere as a slave may be inherently unable to simulate summer monsoon rainfall variations in the heavily precipitating regions (Wang et al. in J Clim 17:803–818, 2004). It is recommended that future reanalysis should be carried out with coupled atmosphere and ocean models. Second, While the MME in general better than any individual models, the CFS ensemble hindcast outperforms the MME in terms of the biennial tendency and the amplitude of the anomalies, suggesting that the improved skill of MME prediction is at the expense of overestimating the fractional variance of the leading mode. Other outstanding issues are also discussed.     

Statistical relationship between two types of El Niño events and climate variation over the Korean Peninsula

Recently, a new type of El Niño (Warm-Pool El Niño) is more often observed than the conventional El Niño (Cold-Tongue El Niño); each has a distinctive spatial pattern. The two types of El Niño have different teleconnections; therefore their impacts on a specific region can be considerably different. In this study, we focus on statistical relationship between climate variation in Korea and the two types of El Niño. When the two types of El Niño are not separately considered, the statistical relation between climate variables in Korea and the El Niño events is weak in general. When the two types of El Niño are separately considered, however, each type exhibits significant relationship with climate variation in Korea. Therefore, consideration of two types of El Niño separately can potentially improve climate prediction over the Korean Peninsula.     

Simulation of two types of El Niño from different convective parameters

Many recent studies have reported the presence of two types of El Niño events in observation: Cold Tongue (CT) El Niño and Warm Pool (WP) El Niño. We investigate the sensitivity of a model simulating two types of El Niño by changing a convective triggering parameter (Tokioka parameter). When deep convections are highly suppressed with a large Tokioka parameter, the model is capable of simulating distinct two-types of El Niño. However, the model has a problem in simulating two-types of El Niño distinctively when the Tokioka parameter is small, because the location of the maximum precipitation anomaly related to the CT El Niño is significantly shifted westward, leading to an atmospheric response pattern similar to that of the WP El Niño. Our results suggest that the mean precipitation over the eastern equatorial Pacific and the resultant zonal distribution in atmospheric feedback associated with ENSO can be one of the crucial factors for simulating two-types of El Niño.     

Influence of two types of El Niños on the East Asian climate during boreal summer: a numerical study

The sea surface temperature anomaly pattern differs between the central Pacific (CP) and eastern Pacific (EP) El Niños during boreal summer. It is expected that the respective atmospheric response will be different. In order to identify differences in the responses to these two phenomena, we examine the Community Atmosphere Model Version 4 simulations forced with observed monthly sea surface temperature during 1979–2010 and compare with the corresponding observations. For CP El Niño, a triple precipitation anomaly pattern appears over East Asia. During EP El Niño, the triple pattern is not as significant as and shifts eastward and southward compared to CP El Niño. We also examine the influence of CP La Niña and EP La Niña on East Asia. In general, the impact of CP (EP) La Niña on tropics and East Asia seems to be opposite to that of CP (EP) El Niño. However, the impacts between the two types of La Niña are less independent compared to the two types of warm events. Both types of El Niño (La Niña) correspond to a stronger (weaker) western North Pacific summer monsoon. The sensitivity experiments support this result. But the CP El Niño (La Niña) may have more significant influence on East Asia summer climate than EP El Niño (La Niña), as the associated low-level anomalous wind pattern is more distinct and closer to the Asian continent compared to EP El Niño (La Niña).     

How do weather characteristics change in a warming climate?

The possible change in the characteristics of weather in the future should be considered as important as the mean climate change because the increasing risk of extremes is related to the variability on daily time scales. The weather characteristics can be represented by the climatological mean interdiurnal (day-to-day) variability (MIDV). This paper first assessed the phase five of the Coupled Model Intercomparison Project coupled climate models’ capability to represent MIDV for the surface maximum and minimum temperature, surface wind speed and precipitation under the present climate condition. Based on the assessment, we selected three best models for projecting future change. We found that the future changes in MIDV are characterized by: (a) a marked reduction in surface maximum and minimum temperature over high latitudes during the cold season; (b) a stronger reduction in the surface minimum temperature than in the maximum temperature; (c) a reduction in surface wind speed over large parts of lands in Northern Hemisphere (NH) during NH spring; (d) a noticeable increase in precipitation in NH mid-high latitudes in NH spring and winter, and in particular over East Asia throughout most of the year.     

On the long-term memory characteristic in land surface air temperatures: How well do CMIP6 models perform?

Stemming from the multi-scale interactions of various processes, long-term memory (LTM) has become a well-recognized property in the climate system. Whether a dynamic model can reproduce the observed LTM is a widely used criterion for model evaluation, especially regarding its ability in simulating natural variabilities. While many works have shown poor model skill in simulating the LTM of land surface air temperature (LSAT), it is not yet known whether CMIP6 models offer any improvement. In this study, the performances of 60 CMIP6 models in simulating the LTM characteristics in LSAT were evaluated. Results showed that most models reproduced the LTM in the global-mean LSAT, among which AWI-ESM-1-1-LR and E3SM-1-0 performed best. All 60 models reproduced the variation in LTM with latitude. CNRM-CM6-1 and HadGEM3-GC31-LL performed best in simulating the LTM of LSAT at the global scale. The multi-model mean (MMM) performed better than any single model. The biases of the MMM and CRUTEM5, and among the 60 models, were significant in the equatorial and coastal regions, which may be attributable to the simulation differences of the models in terms of their ocean–atmosphere coupling processes.摘要利用去趋势涨落分析 (DFA) 方法计算序列的长程记忆性 (LTM) , 以CRUTEM5数据集的结果作为观测参照, 评估了60个参与第六次国际耦合模式比较计划 (CMIP6) 的气候模式对地表气温LTM的再现能力. 结果表明: 大部分模式可以再现全球平均地表气温序列的LTM特征, 其中AWI-ESM-1-1-LR和E3SM-1-0的模拟效果最好; 60个模式均能模拟LTM随纬度带的变化; 综合来说, 全球水平上CNRM-CM6-1和HadGEM3-GC31-LL对地表气温LTM的模拟性能最好; 多模式平均相比单一模式模拟性能更好; 多模式平均与观测结果的偏差以及模式之间的模拟差异显著体现在赤道和沿海区域, 这种偏差可能源于模式对海气耦合过程的模拟差异.     

Simulations of two types of El Niño events by an optimal forcing vector approach

In this paper, an optimal forcing vector (OFV) approach is proposed. The OFV offsets tendency errors and optimizes the agreement of the model simulation with observation. We apply the OFV approach to the well-known Zebiak–Cane model and simulate several observed eastern Pacific (EP) El Niño and central Pacific (CP) El Niño events during 1980–2004. It is found that the Zebiak–Cane model with a proper initial condition often reproduces the EP-El Niño events; however, the Zebiak–Cane model fails to reproduce the CP-El Niño events. The model may be much more influenced by model errors when simulating the CP-El Nino events. As expected, when we use the OFV to correct the Zebiak–Cane model, the model reproduces the three CP-El Niño events well. Furthermore, the simulations of the corresponding winds and thermocline depths are also acceptable. In particular, the thermocline depth simulations for the three CP-El Niño events lead us to believe that the discharge process of the equatorial heat content associated with the CP-El Niño is not efficient and emphasizes the role of the zonal advection in the development of the CP-El Nino events. The OFVs associated with the three CP-El Niño events often exhibit a sea surface temperature anomaly (SSTA) tendency with positive anomalies in the equatorial eastern Pacific; therefore, the SST tendency errors occurring in the equatorial eastern Pacific may dominate the uncertainties of the Zebiak–Cane model while simulating CP-El Nino events. A further investigation demonstrates that one of the model errors offset by the OFVs is of a pattern similar to the SST cold-tongue cooling mode, which may then provide one of the climatological conditions for the frequent occurrence of CP-El Nino events. The OFV may therefore be a useful tool for correcting forecast models and then for helping improve the forecast skill of the models.     

Different influences of two types of El Niños on the Indian Ocean SST variations

By comparing correlation of sea surface temperature (SST) and vertical circulation with canonical El Niño and El Niño Modoki, we find that El Niño Modoki has an effect on the Indian Ocean different from traditional El Niño. There exists obvious Indian Ocean basin mode (IOBM) after canonical El Niño, while insignificant SST anomalies exist in the Indian Ocean after El Niño Modoki. Anomalous downdraft and updraft appear over the eastern and western Indian Ocean, respectively, during canonical El Niño, while anomalous updraft is weak over the Indian Ocean during El Niño Modoki. Besides, the strength of El Niño Modoki is slightly weaker than that of canonical El Niño. According to previous studies, two mechanisms can explain IOBM after canonical El Niño: tropospheric temperature (TT) mechanism and ocean dynamics. However, both of them do not exist during El Niño Modoki. Comparing with the complicated oceanic processes, it is convenient to verify the observed TT anomalies and test the possible mechanism using the simple model. Therefore, we pay more attention on the question why TT mechanism does not work during El Niño Modoki. Using a linear barocinic model (LBM), we demonstrate that the strength of SST anomalies and cold SST anomalies in the eastern Pacific have an influence on TT anomalies. Especially, cold SST anomalies in the eastern Pacific cancel the effects of warm SST anomalies in the central Pacific on TT anomalies. It suggests that the SST anomalies in the eastern Pacific are important for the TT mechanism in two types of El Niño.     

Impacts of two types of El Niño on atmospheric circulation in the Southern Hemisphere

Based on NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmo- spheric Research） reanalysis data from 1979 to 2010, the impacts of two types of E1 Nino on atmospheric circulation in the Southern Hemisphere （SH） are analyzed. It is shown thaL when a warming event occurs in the equatorial eastern Pacific （EP E1 Nino）, there is a negative sea level pressure （SLP） anomaly in the east- ern Pacific and a positive one in the western Pacific. Besides, there exists a negative anomaly between 40°S and 60°S and a positive anomaly to the south of 60°S. When a warming event in the central Pacific （CP E1 Nino） occurs, there appears a negative SLP anomaly in the central Pacific and a positive SLP anomaly in the eastern and western Pacific, but the SLP anomalies are not so evident in the SH extratropics. In particular, the Pacific-South America （PSA） pattern induced by the CP E1 Nino is located more northwestward, with a weaker anomaly compared with the EP E1 Nino. This difference is directly related with the different position of heating centers associated with the two types of E1 Nino events. Because the SST anomaly associated with CP E1 Nino is located more westward than that associated with EP El Nino, the related heating center tends to move westward and the response of SH atmospheric circulation to the tropical heating changes accordingly, thus exciting a different position of the PSA pattern. It is also noted that the local meridional cell plays a role in the SH high latitudes during EP E1 Nino. The anomalous ascending motion due to the enhancement of convection over the eastern Pacific leads to an enhancement of the local Hadley cell and the meridional cell in the middle and high latitudes, which in turn induces an anomalous descending motion and the related positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea.