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.     

Is climate change affecting wolf populations in the high arctic?

Gobal climate change may affect wolves in Canadas High Arctic (80° N) acting through three trophic levels (vegetation, herbivores, and wolves). A wolf pack dependent on muskoxen and arctic hares in the Eureka area of Ellesmere Island denned and produced pups most years from at least 1986 through 1997. However, when summer snow covered vegetation in 1997 and 2000 for the first time since records were kept, halving the herbivore nutrition-replenishment period, muskox and hare numbers dropped drastically, and the area stopped supporting denning wolves through 2003. The unusual weather triggering these events was consistent with global-climate-change phenomena.     

Are atmospheric biases responsible for the tropical Atlantic SST biases in the CNRM-CM5 coupled model?

In this study, the CNRM-CM5 model is shown to simulate too warm SSTs in the tropical Atlantic as most state-of-the-art CMIP5 models. The warm bias develops within 1 or 2 months in decadal experiments initialised in January using an observationally derived state. To better quantify the role of the atmospheric biases in initiating this warm SST bias, several sensitivity experiments have been performed. In a first set of experiments, the surface solar net heat flux sent to the ocean model is academically corrected over the southeastern tropical Atlantic Ocean. This correction locally reduces the warm SST bias by more than 50 % with some remote impacts over equatorial regions. In contrast, the solar heat flux correction has locally little impact on the spring cooling. A second set of experiments quantifies the role of surface winds, using a nudging technique. When applied in a narrow equatorial region, the wind correction mainly improves the SST annual cycle amplitude along the Equator. It promotes not only the spring cooling along the Equator in preconditioning the mixed-layer depth but also in the southeastern Atlantic along the African coast. These local and remote effects are attributed to the more realistic representation of the oceanic equatorial circulation, driven by corrected winds. These results are consistent with those reported by Wahl et al. (Clim Dyn 36:891–906, 2011) in a very similar study with the Kiel Climate Model. The solar and wind biases have comparable effects in their study, although the importance of off-equatorial winds is less clear in our study. Diagnosing the wind energy flux provides a physical understanding of the equatorial region. When combining the corrections of both the equatorial wind and the southeastern solar heat flux, no obvious feedback between them is evidenced. The present study also emphasizes the need to consider two time-scales, the annual mean and the seasonal cycle, as well as two regions, the equatorial and the southeastern Atlantic regions, to comprehensively address the Atlantic SST bias. As pointed out in Richter (Clim Dyn, doi:10.1007/s00382-012-1624-5, 2013), the need to improve the atmospheric component of the CNRM-CM model is emphasized, even though strong positive coupling feedbacks are highlighted.     

Indian summer monsoon influence on the climate in the North Atlantic–European region

Previous studies have shown that climate anomalies over the North Atlantic–Europe (NAE) can influence the Indian summer monsoon (ISM) variability. It is, however, still an outstanding question whether the latter has a significant impact on the former. In this study, observational evidences indicate that the interannual variability of ISM is closely linked to the climate anomalies over NAE. A strong ISM is often associated with significant above normal precipitation over most of western Europe. Meanwhile, positive surface air temperature (SAT) anomalies are usually observed over the Mediterranean, accompanied by below normal SAT in Western Europe during a strong ISM summer. The situation is just opposite during a weak ISM summer. A global primitive equation model is utilized to assess the mechanism of the above observed connection.     

On the relationships between features of the depth–latitude meridional overturning streamfunctions across global coupled climate models

A comparative analysis of the state and response of the latitude–depth meridional overturning streamfunctions in the Climate Model Inter-comparison Project 3 (CMIP3) model set is presented. Simulated overturning strengths of the North Atlantic cell tend to converge towards observational estimates. The models whose simulations of the North Atlantic cell are closest to observational estimates indicate a 29.5 ± 13 % decrease in the maximum intensity of that cell by 2,100. In contrast, agreement with regard to the state and the response to anthropogenic radiative forcing of the global Southern Ocean abyssal cell is poor among the models. A weak relationship between the mean state and the response of the abyssal cell can be used to constrain the reduction of the Southern abyssal cell by 2,100 to 29.3 ± 20.7 %, in rough agreement with the decrease predicted in the Northern cell. The biases across the CMIP3 models in the Northern deep cell and Southern abyssal cell cannot be related dynamically by a buoyancy-based seesaw-like argument. The absence or presence of characteristic relationships between the state and evolution of different features of the overturning streamfunction indicate that the main reasons for across-model spread are how each model deals with subgrid-scale processes and viscosity. This highlights the fact that subgrid-scale parameterizations and resolution improvements should be a priority of model development. These factors are able to explain qualitatively the inter-model differences between the Northern overturning cells of the different models. Across-model differences in the winds over the Southern Ocean are responsible for much of the disparity in the overturning circulation cells of the Southern Ocean.     

Is the interdecadal circumglobal teleconnection pattern excited by the Atlantic multidecadal Oscillation?

年代际环球遥相关型(ID-CGT)是夏季北半球大气环流年代际变化的主导模态,其位相的时间演变与大西洋多年代际振荡(AMO)基本同步。本研究利用5个大气环流模式的敏感性试验,研究给定AMO型的海表面温度异常能否强迫出ID-CGT型响应。结果显示,5个模式中的2个模拟出了沿西风急流分布的波列状响应,表明ID-CGT至少部分是由AMO型的海温异常所激发。此外,模式模拟的结果显示,在年代际尺度上,AMO可能通过ID-CGT影响夏季北半球陆表气温。     

Future projections of the surface heat and water budgets of the Mediterranean Sea in an ensemble of coupled atmosphere–ocean regional climate models

Within the CIRCE project “Climate change and Impact Research: the Mediterranean Environment”, an ensemble of high resolution coupled atmosphere–ocean regional climate models (AORCMs) are used to simulate the Mediterranean climate for the period 1950–2050. For the first time, realistic net surface air-sea fluxes are obtained. The sea surface temperature (SST) variability is consistent with the atmospheric forcing above it and oceanic constraints. The surface fluxes respond to external forcing under a warming climate and show an equivalent trend in all models. This study focuses on the present day and on the evolution of the heat and water budget over the Mediterranean Sea under the SRES-A1B scenario. On the contrary to previous studies, the net total heat budget is negative over the present period in all AORCMs and satisfies the heat closure budget controlled by a net positive heat gain at the strait of Gibraltar in the present climate. Under climate change scenario, some models predict a warming of the Mediterranean Sea from the ocean surface (positive net heat flux) in addition to the positive flux at the strait of Gibraltar for the 2021–2050 period. The shortwave and latent flux are increasing and the longwave and sensible fluxes are decreasing compared to the 1961–1990 period due to a reduction of the cloud cover and an increase in greenhouse gases (GHGs) and SSTs over the 2021–2050 period. The AORCMs provide a good estimates of the water budget with a drying of the region during the twenty-first century. For the ensemble mean, he decrease in precipitation and runoff is about 10 and 15% respectively and the increase in evaporation is much weaker, about 2% compared to the 1961–1990 period which confirm results obtained in recent studies. Despite a clear consistency in the trends and results between the models, this study also underlines important differences in the model set-ups, methodology and choices of some physical parameters inducing some difference in the various air-sea fluxes. An evaluation of the uncertainty sources and possible improvement for future generation of AORCMs highlights the importance of the parameterisation of the ocean albedo, rivers and cloud cover.     

Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean�Catmosphere simulations

In this paper we use a coupled ocean?Catmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the tropical Atlantic, a region where strong coupled ocean?Catmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures an MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the ??warm water path?? in favor of the ??cold water path??. As part of the atmospheric response, there is a northward shift of the intertropical convergence zone (ITCZ). The changes in trade winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the equatorial undercurrent and spread away from the equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome. Surprisingly, the main effect at the surface is small on the equator and relatively large at the Guinea Dome. In the atmosphere, the northward shift of the ITCZ is associated with a band of negative precipitation anomalies and higher salinities over the Tropical South Atlantic. An important implication of these results is that the modified water characteristics due to a shut down of the Agulhas leakage remain largely unaffected when crossing the equatorial Atlantic and therefore can affect the deepwater formation in the North Atlantic. This supports the hypothesis that the Agulhas leakage is an important source region for climate change and decadal variability of the Atlantic.     

Assessing the link between Atlantic Niño 1 and drought over West Africa using CORDEX regional climate models

The skill of Coordinated Regional Climate Downscaling Experiment (CORDEX) models (ARPEGE, CCLM, HIRHAM, RACMO, REMO, PRECIS, RegCM3, RCA, WRF and CRCM) in simulating the climate (precipitation, temperature and drought) of West Africa is determined using a process-based metric. This is done by comparing the CORDEX models’ simulated and observed correlation coefficients between Atlantic Niño Index 1 (ATLN1) and the climate over West Africa. Strong positive correlation is observed between ATLN1 and the climate parameters at the Guinea Coast (GC). The Atlantic Ocean has Niño behaviours through the ATLN indices which influence the climate of the tropics. Drought has distinct dipole structure of correlation with ATLN1 (negative at the Sahel); precipitation does not have distinct dipole structure of correlation, while temperature has almost a monopole correlation structure with ATLN1 over West Africa. The magnitude of the correlation increases with closeness to the equatorial eastern Atlantic. Correlations between ATLN1 and temperature are mostly stronger than those between ATLN1 and precipitation over the region. Most models have good performance over the GC, but ARPEGE has the highest skill at GC. The PRECIS is the most skilful over Savannah and RCA over Sahel. These models can be used to downscale the projected climate at the region of their highest skill.     

Madden–Julian Oscillation simulated in BCC climate models

This study evaluates the ability of four versions BCC (Beijing Climate Center or National Climate Center) models (BCC_AGCM2.1, BCC_AGCM2.2, BCC_CSM1.1 and BCC_CSM1.1m) in simulating the MJO phenomenon using the outputs of the AMIP (Atmospheric Model Intercomparison Project) and historical runs. In general, the models can simulate some major characteristics of the MJO, such as the intensity, the periodicity, the propagation, and the temporal/spatial evolution of the MJO signals in the tropics. There are still some biases between the models and the observation/reanalysis data, such as the overestimated total intraseasonal variability, but underestimated MJO intensity, shorter significant periodicity, and excessive westward propagation. The differences in the ability of simulating the MJO between AMIP and historical experiments are also significant. Compared to the AMIP runs, the total intraseasonal variability is reduced and more realistic, however the ratio between the MJO and its westward counterpart decreases in the historical runs. This unrealistic simulation of the zonal propagation might have been associated with the greater mean precipitation over the Pacific and corresponded to the exaggeration of the South Pacific Convergence Zone structure in precipitation mean state. In contrast to the T42 versions, the improvement of model resolution demonstrate more elaborate topography, but the enhanced westward propagation signals over the Arabia Sea followed. The underestimated (overestimated) MJO variability over eastern Indian Ocean (Pacific) was assumed to be associated with the mean state. Three sets of sensitive experiments using BCC_CSM1.1m turn out to support this argument.     

Why does the Antarctic Peninsula Warm in climate simulations?

The Antarctic Peninsula has warmed significantly since the 1950s. This pronounced and isolated warming trend is collectively captured by 29 twentieth-century climate hindcasts participating in the version 3 Coupled Model Intercomparison Project. To understand the factors driving warming trends in the hindcasts, we examine trends in Peninsula region’s atmospheric heat budget in every simulation. We find that atmospheric latent heat release increases in nearly all hindcasts. These increases are generally anthropogenic in origin, and account for about 60% of the ensemble-mean warming trend in the Peninsula. They are driven primarily by well-understood features of the anthropogenic intensification of global hydrological cycle. As sea surface temperature increases, moisture contained in atmospheric flows increases. When such flows are forced to ascend the Peninsula’s topography, enhanced local latent heat release results. The mechanism driving the warming of the Antarctic Peninsula is therefore clear in the models. Evidence for a similar mechanism operating in the real world is seen in the increasing snow accumulation rates inferred from ice cores drilled in the Peninsula. However, the relative importance of this mechanism and other processes previously identified as potentially causing the observed warming, such as the recent sea ice retreat in the Bellingshausen Sea, is difficult to assess. Thus the relevance of the simulated warming mechanism to the observed warming is unclear, in spite of its robustness in the models.     

Evaluating climate change at the Croatian Adriatic from observations and regional climate models’ simulations

The 2m temperature (T2m) and precipitation from five regional climate models (RCMs), which participated in the ENSEMBLES project and were integrated at a 25-km horizontal resolution, are compared with observed climatological data from 13 stations located in the Croatian coastal zone. The twentieth century climate was simulated by forcing RCMs with identical boundary conditions from the ERA-40 reanalysis and the ECHAM5/MPI-OM global climate model (GCM); climate change in the twenty-first century is based on the A1B scenario and assessed from the GCM-forced RCMs’ integrations. When forced by ERA-40, most RCMs exhibit cold bias in winter which contributes to an overestimation of the T2m annual cycle amplitude and the errors in interannual variability are in all RCMs smaller than those in the climatological mean. All models underestimate observed warming trends in the period 1951–2010. The largest precipitation biases coincide with locations/seasons with small observed amounts but large precipitation amounts near high orography are relatively well reproduced. When forced by the same GCM all RCMs exhibit a warming in the cold half-year and a cooling (or weak warming) in the warm period, implying a strong impact of GCM boundary forcing. The future eastern Adriatic climate is characterised by a warming, up to +5 °C towards the end of the twenty-first century; for precipitation, no clear signal is evident in the first half of the twenty-first century, but a reduction in precipitation during summer prevails in the second half. It is argued that land-sea contrast and complex coastal configuration of the Croatian coast, i.e. multitude of island and well indented coastline, have a major impact on small-scale variability. Orography plays important role only at small number of coastal locations. We hypothesise that the parameterisations related to land surface processes and soil hydrology have relatively stronger impact on variability than orography at those locations that include a relatively large fraction of land (most coastal stations), but affecting less strongly locations at the Adriatic islands.     

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.     

May we engineer the climate?

Not only is the science of climate engineering uncertain; the legal issues are also highly uncertain. Although existing international law does not specifically limit the freedom of states to undertake climate engineering, the international community would likely demand a say should climate engineering move from the realm of speculation to concrete proposals. The experience of other environmental regimes, however, suggests that developing an international decision-making mechanism would be difficult, and that the international community might opt for a simple prohibition on climate engineering on grounds of precaution.