2016 .

(41 publications)

J. Vial, S. Bony, J.-L. Dufresne, and R. Roehrig. Coupling between lower-tropospheric convective mixing and low-level clouds: Physical mechanisms and dependence on convection scheme. Journal of Advances in Modeling Earth Systems, 8:1892-1911, December 2016. [ bib | DOI | ADS link ]

Several studies have pointed out the dependence of low-cloud feedbacks on the strength of the lower-tropospheric convective mixing. By analyzing a series of single-column model experiments run by a climate model using two different convective parametrizations, this study elucidates the physical mechanisms through which marine boundary-layer clouds depend on this mixing in the present-day climate and under surface warming. An increased lower-tropospheric convective mixing leads to a reduction of low-cloud fraction. However, the rate of decrease strongly depends on how the surface latent heat flux couples to the convective mixing and to boundary-layer cloud radiative effects: (i) on the one hand, the latent heat flux is enhanced by the lower-tropospheric drying induced by the convective mixing, which damps the reduction of the low-cloud fraction, (ii) on the other hand, the latent heat flux is reduced as the lower troposphere stabilizes under the effect of reduced low-cloud radiative cooling, which enhances the reduction of the low-cloud fraction. The relative importance of these two different processes depends on the closure of the convective parameterization. The convective scheme that favors the coupling between latent heat flux and low-cloud radiative cooling exhibits a stronger sensitivity of low-clouds to convective mixing in the present-day climate, and a stronger low-cloud feedback in response to surface warming. In this model, the low-cloud feedback is stronger when the present-day convective mixing is weaker and when present-day clouds are shallower and more radiatively active. The implications of these insights for constraining the strength of low-cloud feedbacks observationally is discussed.

S. Leroux, G. Bellon, R. Roehrig, M. Caian, N. P. Klingaman, J.-P. Lafore, I. Musat, C. Rio, and S. Tyteca. Inter-model comparison of subseasonal tropical variability in aquaplanet experiments: Effect of a warm pool. Journal of Advances in Modeling Earth Systems, 8:1526-1551, December 2016. [ bib | DOI | ADS link ]

This study compares the simulation of subseasonal tropical variability by a set of six state-of-the-art AGCMs in two experiments in aquaplanet configuration: a zonally symmetric experiment, and an experiment with a warm pool centered on the equator. In all six models, the presence of the warm pool generates zonal asymmetries in the simulated mean states in the form of a “Gill-type” response, made more complex by feedbacks between moisture, convective heating and circulation. Noticeable differences appear from one model to another. Only half the models simulate mean low-level equatorial westerlies over the warm pool area. The presence of the warm pool can also favor the development of large-scale variability consistent with observed Madden-Julian Oscillation (MJO) characteristics, but this happens only in half the models. Our results do not support the idea that the presence of the warm pool and/or of mean low-level equatorial westerlies are sufficient conditions for MJO-like variability to arise in the models. Comparing spectral characteristics of the simulated Convectively Coupled Equatorial Waves (CCEWs) in the aquaplanet experiments and the corresponding coupled atmosphere-ocean (i.e., CMIP) and atmosphere-only (i.e., AMIP) simulations, we also show that there is more consistency for a given model across its configurations, than for a given configuration across the six models. Overall, our results confirm that the simulation of subseasonal variability by given model is significantly influenced by the parameterization of subgrid physical processes (most-likely cloud processes), both directly and through modulation of the mean state.

J. Galewsky, H. C. Steen-Larsen, R. D. Field, J. Worden, C. Risi, and M. Schneider. Stable isotopes in atmospheric water vapor and applications to the hydrologic cycle. Reviews of Geophysics, 54:809-865, December 2016. [ bib | DOI | ADS link ]

The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long-term data sets and comprehensive modeling capabilities now available. Theories for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large-scale mixing, deep convection, and kinetic fractionation. The technologies for in situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite- and ground-based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope-enabled microphysics schemes using higher-order moments for water and ice size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.

W. Li, Z. Jiang, J. Xu, and L. Li. Extreme Precipitation Indices over China in CMIP5 Models. Part II: Probabilistic Projection. Journal of Climate, 29:8989-9004, December 2016. [ bib | DOI | ADS link ]

A. Voigt, M. Biasutti, J. Scheff, J. Bader, S. Bordoni, F. Codron, R. D. Dixon, J. Jonas, S. M. Kang, N. P. Klingaman, R. Leung, J. Lu, B. Mapes, E. A. Maroon, S. McDermid, J.-y. Park, R. Roehrig, B. E. J. Rose, G. L. Russell, J. Seo, T. Toniazzo, H.-H. Wei, M. Yoshimori, and L. R. Vargas Zeppetello. The tropical rain belts with an annual cycle and a continent model intercomparison project: TRACMIP. Journal of Advances in Modeling Earth Systems, 8:1868-1891, December 2016. [ bib | DOI | ADS link ]

This paper introduces the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP). TRACMIP studies the dynamics of tropical rain belts and their response to past and future radiative forcings through simulations with 13 comprehensive and one simplified atmosphere models coupled to a slab ocean and driven by seasonally varying insolation. Five idealized experiments, two with an aquaplanet setup and three with a setup with an idealized tropical continent, fill the space between prescribed-SST aquaplanet simulations and realistic simulations provided by CMIP5/6. The simulations reproduce key features of present-day climate and expected future climate change, including an annual-mean intertropical convergence zone (ITCZ) that is located north of the equator and Hadley cells and eddy-driven jets that are similar to present-day climate. Quadrupling CO2 leads to a northward ITCZ shift and preferential warming in Northern high latitudes. The simulations show interesting CO2-induced changes in the seasonal excursion of the ITCZ and indicate a possible state dependence of climate sensitivity. The inclusion of an idealized continent modulates both the control climate and the response to increased CO2; for example, it reduces the northward ITCZ shift associated with warming and, in some models, climate sensitivity. In response to eccentricity-driven seasonal insolation changes, seasonal changes in oceanic rainfall are best characterized as a meridional dipole, while seasonal continental rainfall changes tend to be symmetric about the equator. This survey illustrates TRACMIP's potential to engender a deeper understanding of global and regional climate and to address questions on past and future climate change.

J. Quaas, M. F. Quaas, O. Boucher, and W. Rickels. Regional climate engineering by radiation management: Prerequisites and prospects. Earth's Future, 4:618-625, December 2016. [ bib | DOI | ADS link ]

Radiation management (RM), as an option to engineer the climate, is highly controversial and suffers from a number of ethical and regulatory concerns, usually studied in the context of the objective to mitigate the global mean temperature. In this article, we discuss the idea that RM can be differentiated and scaled in several dimensions with potential objectives being to influence a certain climate parameter in a specific region. Some short-lived climate forcers (e.g., tropospheric aerosols) exhibit strong geographical and temporal variability, potentially leading to limited-area climate responses. Marine cloud brightening and thinning or dissolution of cirrus clouds could be operated at a rather local scale. It is therefore conceivable that such schemes could be applied with the objective to influence the climate at a regional scale. From a governance perspective, it is desirable to avoid any substantial climate effects of regional RM outside the target region. This, however, could prove impossible for a sustained, long-term RM. In turn, regional RM during limited time periods could prove more feasible without effects beyond the target area. It may be attractive as it potentially provides the opportunity to target the suppression of some extreme events such as heat waves. Research is needed on the traceability of regional RM, for example, using detection and attribution methods. Incentives and implications of regional RM need to be examined, and new governance options have to be conceived.

W. Yu, L. Tian, C. Risi, T. Yao, Y. Ma, H. Zhao, H. Zhu, Y. He, B. Xu, H. Zhang, and D. Qu. δ18O records in water vapor and an ice core from the eastern Pamir Plateau: Implications for paleoclimate reconstructions. Earth and Planetary Science Letters, 456:146-156, December 2016. [ bib | DOI | ADS link ]

This study is the first to examine δ18O in daily water vapor at Taxkorgan on the eastern Pamir Plateau. The results show that changes in observed and simulated δ18O values in water vapor/precipitation at the event scale (using a LMDZ-iso model) were mainly affected by temperature. The influences of humidity, precipitation amount, and different moisture sources, such as the westerlies, local evaporated moisture, and polar air masses, on δ18O values are comparatively weak. The combination of the δ18O record from the Muztagata ice core, 58 km away from the study area, and the LMDZ-iso simulated annual δ18O pattern in precipitation at Taxkorgan also demonstrated that temperature, and particularly the temperature of the regions over which the southern branch of the westerlies flows, is the most important factor controlling δ18O variations. The results from this study area, which is dominated by the westerlies throughout the year, are markedly different from those derived from parts of the Tibetan Plateau that are dominated by the combined influences of the westerlies in winter and the Indian monsoon in summer. The results suggested that the eastern Pamir Plateau is an ideal location to reconstruct past temperature variations and that the δ18O records preserved in ice cores from the region are a suitable and robust proxy for temperature.

B. Oueslati, S. Bony, C. Risi, and J.-L. Dufresne. Interpreting the inter-model spread in regional precipitation projections in the tropics: role of surface evaporation and cloud radiative effects. Climate Dynamics, 47:2801-2815, November 2016. [ bib | DOI | ADS link ]

In this study, we investigate and quantify different contributors to inter-model differences in regional precipitation projections among CMIP5 climate models. Contributors to the spread are very contrasted between land and ocean. While circulation changes dominate the spread over oceans and continental coasts, thermodynamic changes associated with water vapor increase dominate over inland regions. The inter-model spread in the dynamic component is associated with the change in atmospheric radiative cooling with warming, which largely relates to atmospheric cloud radiative effects. Differences in the thermodynamic component result from the differences in the change in surface evaporation that is explained by decreases in surface humidity and limited surface water availability over land. Secondary contributions to the inter-model spread in thermodynamic and dynamic components result respectively from present-day climatology (owing to the Clausius-Clapeyron scaling) and from the shape of the vertical velocity profile associated with changes in surface temperature gradients. Advancing the physical understanding of the cloud-circulation and precipitation-evaporation couplings and improving their representation in climate models may stand the best chance to reduce uncertainty in regional precipitation projections.

D. Konsta, J.-L. Dufresne, H. Chepfer, A. Idelkadi, and G. Cesana. Erratum: Erratum to: Use of A-train satellite observations (CALIPSO-PARASOL) to evaluate tropical cloud properties in the LMDZ5 GCM. Climate Dynamics, 47:3387-3387, November 2016. [ bib | DOI | ADS link ]

A. Cauquoin, P. Jean-Baptiste, C. Risi, É. Fourré, and A. Landais. Modeling the global bomb tritium transient signal with the AGCM LMDZ-iso: A method to evaluate aspects of the hydrological cycle. Journal of Geophysical Research (Atmospheres), 121:12, November 2016. [ bib | DOI | ADS link ]

Improving the representation of the hydrological cycle in atmospheric general circulation models (AGCMs) is one of the main challenges in modeling the Earth's climate system. One way to evaluate model performance is to simulate the transport of water isotopes. Among those available, tritium is an extremely valuable tracer, because its content in the different reservoirs involved in the water cycle (stratosphere, troposphere, and ocean) varies by order of magnitude. Previous work incorporated natural tritium into Laboratoire de Météorologie Dynamique Zoom (LMDZ)-iso, a version of the LMDZ general circulation model enhanced by water isotope diagnostics. Here for the first time, the anthropogenic tritium injected by each of the atmospheric nuclear bomb tests between 1945 and 1980 has been first estimated and further implemented in the model; it creates an opportunity to evaluate certain aspects of LDMZ over several decades by following the bomb tritium transient signal through the hydrological cycle. Simulations of tritium in water vapor and precipitation for the period 1950-2008, with both natural and anthropogenic components, are presented in this study. LMDZ-iso satisfactorily reproduces the general shape of the temporal evolution of tritium. However, LMDZ-iso simulates too high a bomb tritium peak followed by too strong a decrease of tritium in precipitation. The too diffusive vertical advection in AGCMs crucially affects the residence time of tritium in the stratosphere. This insight into model performance demonstrates that the implementation of tritium in an AGCM provides a new and valuable test of the modeled atmospheric transport, complementing water stable isotope modeling.

G. Rivière, L. Robert, and F. Codron. A Short-Term Negative Eddy Feedback on Midlatitude Jet Variability due to Planetary Wave Reflection. Journal of Atmospheric Sciences, 73:4311-4328, November 2016. [ bib | DOI | ADS link ]

P. Good, T. Andrews, R. Chadwick, J.-L. Dufresne, J. M. Gregory, J. A. Lowe, N. Schaller, and H. Shiogama. nonlinMIP contribution to CMIP6: model intercomparison project for non-linear mechanisms: physical basis, experimental design and analysis principles (v1.0). Geoscientific Model Development, 9:4019-4028, November 2016. [ bib | DOI | ADS link ]

nonlinMIP provides experiments that account for state-dependent regional and global climate responses. The experiments have two main applications: (1) to focus understanding of responses to CO2 forcing on states relevant to specific policy or scientific questions (e.g. change under low-forcing scenarios, the benefits of mitigation, or from past cold climates to the present day), or (2) to understand the state dependence (non-linearity) of climate change - i.e. why doubling the forcing may not double the response. State dependence (non-linearity) of responses can be large at regional scales, with important implications for understanding mechanisms and for general circulation model (GCM) emulation techniques (e.g. energy balance models and pattern-scaling methods). However, these processes are hard to explore using traditional experiments, which explains why they have had so little attention in previous studies. Some single model studies have established novel analysis principles and some physical mechanisms. There is now a need to explore robustness and uncertainty in such mechanisms across a range of models (point 2 above), and, more broadly, to focus work on understanding the response to CO2 on climate states relevant to specific policy/science questions (point 1).<BR /><BR /> nonlinMIP addresses this using a simple, small set of CO2-forced experiments that are able to separate linear and non-linear mechanisms cleanly, with a good signal-to-noise ratio - while being demonstrably traceable to realistic transient scenarios. The design builds on the CMIP5 (Coupled Model Intercomparison Project Phase 5) and CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) protocols, and is centred around a suite of instantaneous atmospheric CO2 change experiments, with a ramp-up-ramp-down experiment to test traceability to gradual forcing scenarios. In all cases the models are intended to be used with CO2 concentrations rather than CO2 emissions as the input. The understanding gained will help interpret the spread in policy-relevant scenario projections.<BR /><BR /> Here we outline the basic physical principles behind nonlinMIP, and the method of establishing traceability from abruptCO2 to gradual forcing experiments, before detailing the experimental design, and finally some analysis principles. The test of traceability from abruptCO2 to transient experiments is recommended as a standard analysis within the CMIP5 and CMIP6 DECK protocols.

B. Stevens, S. C. Sherwood, S. Bony, and M. J. Webb. Prospects for narrowing bounds on Earth's equilibrium climate sensitivity. Earth's Future, 4:512-522, November 2016. [ bib | DOI | ADS link ]

The concept of Earth's Equilibrium Climate Sensitivity (ECS) is reviewed. A particular problem in quantifying plausible bounds for ECS has been how to account for all of the diverse lines of relevant scientific evidence. It is argued that developing and refuting physical storylines (hypotheses) for values outside any proposed range has the potential to better constrain these bounds and to help articulate the science needed to narrow the range further. A careful reassessment of all important lines of evidence supporting these storylines, their limitations, and the assumptions required to combine them is therefore required urgently.

H. Yang, Z. Jiang, and L. Li. Biases and improvements in three dynamical downscaling climate simulations over China. Climate Dynamics, 47:3235-3251, November 2016. [ bib | DOI | ADS link ]

A dynamical downscaling is performed to improve the regional climate simulation in China. It consists of using a variable resolution model LMDZ4 nested into three global climate models (GCMs): BCC-csm1-1-m, FGOALS-g2 and IPSL-CM5A-MR, respectively. The regional climate from different simulations is assessed in terms of surface air temperature and rainfalls through a comparison to observations (both station data and gridded data). The comparison includes climatic trends during the last 40 years, statistical distribution of sub-regional climate, and the seasonal cycle. For surface air temperature, a significant part of the improvement provided by LMDZ4 is related to the effect of surface elevation which is more realistic in high-resolution simulations; the rest is related to changes in regional or local atmospheric general circulation. All GCMs and the downscaling model LMDZ4 are, more or less, able to describe the spatial distribution of surface air temperature and precipitation in China. LMDZ4 does show its superiority, compared to GCMs, in depicting a good regional terrain including the Tibetan Plateau, the Sichuan Basin and the Qilian Mountains.

B. Stenni, C. Scarchilli, V. Masson-Delmotte, E. Schlosser, V. Ciardini, G. Dreossi, P. Grigioni, M. Bonazza, A. Cagnati, D. Karlicek, C. Risi, R. Udisti, and M. Valt. Three-year monitoring of stable isotopes of precipitation at Concordia Station, East Antarctica. The Cryosphere, 10:2415-2428, October 2016. [ bib | DOI | ADS link ]

Past temperature reconstructions from Antarctic ice cores require a good quantification and understanding of the relationship between snow isotopic composition and 2 m air or inversion (condensation) temperature. Here, we focus on the French-Italian Concordia Station, central East Antarctic plateau, where the European Project for Ice Coring in Antarctica (EPICA) Dome C ice cores were drilled. We provide a multi-year record of daily precipitation types identified from crystal morphologies, daily precipitation amounts and isotopic composition. Our sampling period (2008-2010) encompasses a warmer year (2009, +1.2 degC with respect to 2 m air temperature long-term average 1996-2010), with larger total precipitation and snowfall amounts (14 and 76 % above sampling period average, respectively), and a colder and drier year (2010, -1.8 degC, 4 % below long-term and sampling period averages, respectively) with larger diamond dust amounts (49 % above sampling period average). Relationships between local meteorological data and precipitation isotopic composition are investigated at daily, monthly and inter-annual scale, and for the different types of precipitation. Water stable isotopes are more closely related to 2 m air temperature than to inversion temperature at all timescales (e.g. R2 = 0.63 and 0.44, respectively for daily values). The slope of the temporal relationship between daily δ18O and 2 m air temperature is approximately 2 times smaller (0.49 degC-1) than the average Antarctic spatial (0.8 degC-1) relationship initially used for the interpretation of EPICA Dome C records. In accordance with results from precipitation monitoring at Vostok and Dome F, deuterium excess is anti-correlated with δ18O at daily and monthly scales, reaching maximum values in winter. Hoar frost precipitation samples have a specific fingerprint with more depleted δ18O (about 5 below average) and higher deuterium excess (about 8 above average) values than other precipitation types. These datasets provide a basis for comparison with shallow ice core records, to investigate post-deposition effects. A preliminary comparison between observations and precipitation from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis and the simulated water stable isotopes from the Laboratoire de Météorologie Dynamique Zoom atmospheric general circulation model (LMDZiso) shows that models do correctly capture the amount of precipitation as well as more than 50 % of the variance of the observed δ18O, driven by large-scale weather patterns. Despite a warm bias and an underestimation of the variance in water stable isotopes, LMDZiso correctly captures these relationships between δ18O, 2 m air temperature and deuterium excess. Our dataset is therefore available for further in-depth model evaluation at the synoptic scale.

A. Berg, K. Findell, B. Lintner, A. Giannini, S. I. Seneviratne, B. van den Hurk, R. Lorenz, A. Pitman, S. Hagemann, A. Meier, F. Cheruy, A. Ducharne, S. Malyshev, and P. C. D. Milly. Land-atmosphere feedbacks amplify aridity increase over land under global warming. Nature Climate Change, 6:869-874, September 2016. [ bib | DOI | ADS link ]

The response of the terrestrial water cycle to global warming is central to issues including water resources, agriculture and ecosystem health. Recent studies indicate that aridity, defined in terms of atmospheric supply (precipitation, P) and demand (potential evapotranspiration, Ep) of water at the land surface, will increase globally in a warmer world. Recently proposed mechanisms for this response emphasize the driving role of oceanic warming and associated atmospheric processes. Here we show that the aridity response is substantially amplified by land-atmosphere feedbacks associated with the land surface's response to climate and CO2 change. Using simulations from the Global Land Atmosphere Coupling Experiment (GLACE)-CMIP5 experiment, we show that global aridity is enhanced by the feedbacks of projected soil moisture decrease on land surface temperature, relative humidity and precipitation. The physiological impact of increasing atmospheric CO2 on vegetation exerts a qualitatively similar control on aridity. We reconcile these findings with previously proposed mechanisms by showing that the moist enthalpy change over land is unaffected by the land hydrological response. Thus, although oceanic warming constrains the combined moisture and temperature changes over land, land hydrology modulates the partitioning of this enthalpy increase towards increased aridity.

D. Konsta, J.-L. Dufresne, H. Chepfer, A. Idelkadi, and G. Cesana. Use of A-train satellite observations (CALIPSO-PARASOL) to evaluate tropical cloud properties in the LMDZ5 GCM. Climate Dynamics, 47:1263-1284, August 2016. [ bib | DOI | ADS link ]

The evaluation of key cloud properties such as cloud cover, vertical profile and optical depth as well as the analysis of their intercorrelation lead to greater confidence in climate change projections. In addition, the comparison between observations and parameterizations of clouds in climate models is improved by using collocated and instantaneous data of cloud properties. Simultaneous and independent observations of the cloud cover and its three-dimensional structure at high spatial and temporal resolutions are made possible by the new space-borne multi-instruments observations collected with the A-train. The cloud cover and its vertical structure observed by CALIPSO and the visible directional reflectance (a surrogate for the cloud optical depth) observed by PARASOL, are used to evaluate the representation of cloudiness in two versions of the atmospheric component of the IPSL-CM5 climate model (LMDZ5). A model-to-satellite approach, applying the CFMIP Observation Simulation Package (COSP), is used to allow a quantitative comparison between model results and observations. The representation of clouds in the two model versions is first evaluated using monthly mean data. This classical approach reveals biases of different magnitudes in the two model versions. These biases consist of (1) an underestimation of cloud cover associated to an overestimation of cloud optical depth, (2) an underestimation of low- and mid-level tropical clouds and (3) an overestimation of high clouds. The difference in the magnitude of these biases between the two model versions clearly highlights the improvement of the amount of boundary layer clouds, the improvement of the properties of high-level clouds, and the improvement of the simulated mid-level clouds in the tropics in LMDZ5B compared to LMDZ5A, due to the new convective, boundary layer, and cloud parametrizations implemented in LMDZ5B. The correlation between instantaneous cloud properties allows for a process-oriented evaluation of tropical oceanic clouds. This process-oriented evaluation shows that the cloud population characterized by intermediate values of cloud cover and cloud reflectance can be split in two groups of clouds when using monthly mean values of cloud cover and cloud reflectance: one group with low to intermediate values of the cloud cover, and one group with cloud cover close to one. The precise determination of cloud height allows us to focus on specific types of clouds (i.e. boundary layer clouds, high clouds, low-level clouds with no clouds above). For low-level clouds over the tropical oceans, the relationship between instantaneous values of the cloud cover and of the cloud reflectance reveals a major bias in the simulated liquid water content for both model versions. The origin of this bias is identified and possible improvements, such as considering the sub-grid heterogeneity of cloud properties, are investigated using sensitivity experiments. In summary, the analysis of the relationship between different instantaneous and collocated variables allows for process-oriented evaluations. These evaluations may in turn help to improve model parameterizations, and may also help to bridge the gap between model evaluation and model development.

R. Krishnan, T. P. Sabin, R. Vellore, M. Mujumdar, J. Sanjay, B. N. Goswami, F. Hourdin, J.-L. Dufresne, and P. Terray. Deciphering the desiccation trend of the South Asian monsoon hydroclimate in a warming world. Climate Dynamics, 47:1007-1027, August 2016. [ bib | DOI | ADS link ]

Rising propensity of precipitation extremes and concomitant decline of summer-monsoon rains are amongst the most distinctive hydroclimatic signals that have emerged over South Asia since 1950s. A clear understanding of the underlying causes driving these monsoon hydroclimatic signals has remained elusive. Using a state-of-the-art global climate model with high-resolution zooming over South Asia, we demonstrate that a juxtaposition of regional land-use changes, anthropogenic-aerosol forcing and the rapid warming signal of the equatorial Indian Ocean is crucial to produce the observed monsoon weakening in recent decades. Our findings also show that this monsoonal weakening significantly enhances occurrence of localized intense precipitation events, as compared to the global-warming response. A 21st century climate projection using the same high-resolution model indicates persistent decrease of monsoonal rains and prolongation of soil drying. Critical value-additions from this study include (1) realistic simulation of the mean and long-term historical trends in the Indian monsoon rainfall (2) robust attributions of changes in moderate and heavy precipitation events over Central India (3) a 21st century projection of drying trend of the South Asian monsoon. The present findings have profound bearing on the regional water-security, which is already under severe hydrological-stress.

S. Bony, B. Stevens, D. Coppin, T. Becker, K. A. Reed, A. Voigt, and B. Medeiros. Thermodynamic control of anvil cloud amount. Proceedings of the National Academy of Science, 113:8927-8932, August 2016. [ bib | DOI | ADS link ]

General circulation models show that as the surface temperature increases, the convective anvil clouds shrink. By analyzing radiative-convective equilibrium simulations, we show that this behavior is rooted in basic energetic and thermodynamic properties of the atmosphere: As the climate warms, the clouds rise and remain at nearly the same temperature, but find themselves in a more stable atmosphere; this enhanced stability reduces the convective outflow in the upper troposphere and decreases the anvil cloud fraction. By warming the troposphere and increasing the upper-tropospheric stability, the clustering of deep convection also reduces the convective outflow and the anvil cloud fraction. When clouds are radiatively active, this robust coupling between temperature, high clouds, and circulation exerts a positive feedback on convective aggregation and favors the maintenance of strongly aggregated atmospheric states at high temperatures. This stability iris mechanism likely contributes to the narrowing of rainy areas as the climate warms. Whether or not it influences climate sensitivity requires further investigation.

J. M. de Moor, A. Aiuppa, G. Avard, H. Wehrmann, N. Dunbar, C. Muller, G. Tamburello, G. Giudice, M. Liuzzo, R. Moretti, V. Conde, and B. Galle. Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high-frequency gas monitoring. Journal of Geophysical Research (Solid Earth), 121:5761-5775, August 2016. [ bib | DOI | ADS link ]

Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high-frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2-rich gas (CO2/Stotal 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur-rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is 8-10 km deep, whereas the shallow magmatic gas source is at 3-5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (3000 T/d SO2 and H2S/SO2 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high-temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.

B. van den Hurk, H. Kim, G. Krinner, S. I. Seneviratne, C. Derksen, T. Oki, H. Douville, J. Colin, A. Ducharne, F. Cheruy, N. Viovy, M. J. Puma, Y. Wada, W. Li, B. Jia, A. Alessandri, D. M. Lawrence, G. P. Weedon, R. Ellis, S. Hagemann, J. Mao, M. G. Flanner, M. Zampieri, S. Materia, R. M. Law, and J. Sheffield. LS3MIP (v1.0) contribution to CMIP6: the Land Surface, Snow and Soil moisture Model Intercomparison Project - aims, setup and expected outcome. Geoscientific Model Development, 9:2809-2832, August 2016. [ bib | DOI | ADS link ]

The Land Surface, Snow and Soil Moisture Model Intercomparison Project (LS3MIP) is designed to provide a comprehensive assessment of land surface, snow and soil moisture feedbacks on climate variability and climate change, and to diagnose systematic biases in the land modules of current Earth system models (ESMs). The solid and liquid water stored at the land surface has a large influence on the regional climate, its variability and predictability, including effects on the energy, water and carbon cycles. Notably, snow and soil moisture affect surface radiation and flux partitioning properties, moisture storage and land surface memory. They both strongly affect atmospheric conditions, in particular surface air temperature and precipitation, but also large-scale circulation patterns. However, models show divergent responses and representations of these feedbacks as well as systematic biases in the underlying processes. LS3MIP will provide the means to quantify the associated uncertainties and better constrain climate change projections, which is of particular interest for highly vulnerable regions (densely populated areas, agricultural regions, the Arctic, semi-arid and other sensitive terrestrial ecosystems). <BR /><BR /> The experiments are subdivided in two components, the first addressing systematic land biases in offline mode (“LMIP”, building upon the 3rd phase of Global Soil Wetness Project; GSWP3) and the second addressing land feedbacks attributed to soil moisture and snow in an integrated framework (“LFMIP”, building upon the GLACE-CMIP blueprint).

T. Bolliet, P. Brockmann, V. Masson-Delmotte, F. Bassinot, V. Daux, D. Genty, A. Landais, M. Lavrieux, E. Michel, P. Ortega, C. Risi, D. M. Roche, F. Vimeux, and C. Waelbroeck. Water and carbon stable isotope records from natural archives: a new database and interactive online platform for data browsing, visualizing and downloading. Climate of the Past, 12:1693-1719, August 2016. [ bib | DOI | ADS link ]

Past climate is an important benchmark to assess the ability of climate models to simulate key processes and feedbacks. Numerous proxy records exist for stable isotopes of water and/or carbon, which are also implemented inside the components of a growing number of Earth system model. Model-data comparisons can help to constrain the uncertainties associated with transfer functions. This motivates the need of producing a comprehensive compilation of different proxy sources. We have put together a global database of proxy records of oxygen (δ18O), hydrogen (δD) and carbon (δ13C) stable isotopes from different archives: ocean and lake sediments, corals, ice cores, speleothems and tree-ring cellulose. Source records were obtained from the georeferenced open access PANGAEA and NOAA libraries, complemented by additional data obtained from a literature survey. About 3000 source records were screened for chronological information and temporal resolution of proxy records. Altogether, this database consists of hundreds of dated δ18O, δ13C and δD records in a standardized simple text format, complemented with a metadata Excel catalog. A quality control flag was implemented to describe age markers and inform on chronological uncertainty. This compilation effort highlights the need to homogenize and structure the format of datasets and chronological information as well as enhance the distribution of published datasets that are currently highly fragmented and scattered. We also provide an online portal based on the records included in this database with an intuitive and interactive platform (a href=“http://climateproxiesfinder.ipsl.fr/” target=“_blank”http://climateproxiesfinder.ipsl.fr//a), allowing one to easily select, visualize and download subsets of the homogeneously formatted records that constitute this database, following a choice of search criteria, and to upload new datasets. In the last part, we illustrate the type of application allowed by our database by comparing several key periods highly investigated by the paleoclimate community. For coherency with the Paleoclimate Modelling Intercomparison Project (PMIP), we focus on records spanning the past 200 years, the mid-Holocene (MH, 5.5-6.5 ka; calendar kiloyears before 1950), the Last Glacial Maximum (LGM, 19-23 ka), and those spanning the last interglacial period (LIG, 115-130 ka). Basic statistics have been applied to characterize anomalies between these different periods. Most changes from the MH to present day and from LIG to MH appear statistically insignificant. Significant global differences are reported from LGM to MH with regional discrepancies in signals from different archives and complex patterns.

Y. Chavaillaz, S. Joussaume, S. Bony, and P. Braconnot. Spatial stabilization and intensification of moistening and drying rate patterns under future climate change. Climate Dynamics, 47:951-965, August 2016. [ bib | DOI | ADS link ]

Precipitation projections are usually presented as the change in precipitation between a fixed current baseline and a particular time in the future. However, upcoming generations will be affected in a way probably more related to the moving trend in precipitation patterns, i.e. to the rate and the persistence of regional precipitation changes from one generation to the next, than to changes relative to a fixed current baseline. In this perspective, we propose an alternative characterization of the future precipitation changes predicted by general circulation models, focusing on the precipitation difference between two subsequent 20-year periods. We show that in a business-as-usual emission pathway, the moistening and drying rates increase by 30-40 %, both over land and ocean. As we move further over the twenty-first century, more regions exhibit a significant rate of precipitation change, while the patterns become geographically stationary and the trends persistent. The stabilization of the geographical rate patterns that occurs despite the acceleration of global warming can be physically explained: it results from the increasing contribution of thermodynamic processes compared to dynamic processes in the control of precipitation change. We show that such an evolution is already noticeable over the last decades, and that it could be reversed if strong mitigation policies were quickly implemented. The combination of intensification and increasing persistence of precipitation rate patterns may affect the way human societies and natural ecosystems adapt to climate change, especially in the Mediterranean basin, in Central America, in South Asia and in the Arctic.

J.-F. Rysman, A. Lahellec, E. Vignon, C. Genthon, and S. Verrier. Characterization of Atmospheric Ekman Spirals at Dome C, Antarctica. Boundary-Layer Meteorology, 160:363-373, August 2016. [ bib | DOI | ADS link ]

We use wind speed and temperature measurements taken along a 45-m meteorological tower located at Dome C, Antarctica (75.06degS, 123.19degE) to highlight and characterize the Ekman spiral. Firstly, temperature records reveal that the atmospheric boundary layer at Dome C is stable during winter and summer nights (i.e., 85 % of the time). The wind vector, in both speed and direction, also shows a strong dependence with elevation. An Ekman model was then fitted to the measurements. Results show that the wind vector follows the Ekman spiral structure for more than 20 % of the year (2009). Most Ekman spirals have been detected during summer nights, that is, when the boundary layer is slightly stratified. During these episodes, the boundary-layer height ranged from 25 to 100 m, the eddy viscosity from 0.004 to 0.06 m^2 s^{-1}, and the Richardson number from zero to 1.6.

R. A. Scheepmaker, J. aan de Brugh, H. Hu, T. Borsdorff, C. Frankenberg, C. Risi, O. Hasekamp, I. Aben, and J. Landgraf. HDO and H2O total column retrievals from TROPOMI shortwave infrared measurements. Atmospheric Measurement Techniques, 9:3921-3937, August 2016. [ bib | DOI | ADS link ]

The TROPOspheric Monitoring Instrument (TROPOMI) on board the European Space Agency Sentinel-5 Precursor mission is scheduled for launch in the last quarter of 2016. As part of its operational processing the mission will provide CH4 and CO total columns using backscattered sunlight in the shortwave infrared band (2.3 m). By adapting the CO retrieval algorithm, we have developed a non-scattering algorithm to retrieve total column HDO and H2O from the same measurements under clear-sky conditions. The isotopologue ratio HDO / H2O is a powerful diagnostic in the efforts to improve our understanding of the hydrological cycle and its role in climate change, as it provides an insight into the source and transport history of water vapour, nature's strongest greenhouse gas. Due to the weak reflectivity over water surfaces, we need to restrict the retrieval to cloud-free scenes over land. We exploit a novel 2-band filter technique, using strong vs. weak water or methane absorption bands, to prefilter scenes with medium-to-high-level clouds, cirrus or aerosol and to significantly reduce processing time. Scenes with cloud top heights 1 km, very low fractions of high-level clouds or an aerosol layer above a high surface albedo are not filtered out. We use an ensemble of realistic measurement simulations for various conditions to show the efficiency of the cloud filter and to quantify the performance of the retrieval. The single-measurement precision in terms of δD is better than 15-25 for even the lowest surface albedo (2-4 for high albedos), while a small bias remains possible of up to 20 due to remaining aerosol or up to 70 due to remaining cloud contamination. We also present an analysis of the sensitivity towards prior assumptions, which shows that the retrieval has a small but significant sensitivity to the a priori assumption of the atmospheric trace gas profiles. Averaging multiple measurements over time and space, however, will reduce these errors, due to the quasi-random nature of the profile uncertainties. The sensitivity of the retrieval with respect to instrumental parameters within the expected instrument performance is 3 , which represents only a small contribution to the overall error budget. Spectroscopic uncertainties of the water lines, however, can have a larger and more systematic impact on the performance of the retrieval and warrant further reassessment of the water line parameters. With TROPOMI's high radiometric sensitivity, wide swath (resulting in daily global coverage) and efficient cloud filtering, in combination with a spatial resolution of 7 × 7 km2, we will greatly increase the amount of useful data on HDO, H2O and their ratio HDO / H2O. We showcase the overall performance of the retrieval algorithm and cloud filter with an accurate simulation of TROPOMI measurements from a single overpass over parts of the USA and Mexico, based on MODIS satellite data and realistic conditions for the surface, atmosphere and chemistry (including isotopologues). This shows that TROPOMI will pave the way for new studies of the hydrological cycle, both globally and locally, on timescales of mere days and weeks instead of seasons and years and will greatly extend the HDO / H2O datasets from the SCIAMACHY and GOSAT missions.

F. Ritter, H. C. Steen-Larsen, M. Werner, V. Masson-Delmotte, A. Orsi, M. Behrens, G. Birnbaum, J. Freitag, C. Risi, and S. Kipfstuhl. Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica. The Cryosphere, 10:1647-1663, July 2016. [ bib | DOI | ADS link ]

Quantifying the magnitude of post-depositional processes affecting the isotopic composition of surface snow is essential for a more accurate interpretation of ice core data. To achieve this, high temporal resolution measurements of both lower atmospheric water vapor and surface snow isotopic composition are required. This study presents continuous measurements of water vapor isotopes performed in East Antarctica (Kohnen station) from December 2013 to January 2014 using a laser spectrometer. Observations have been compared with the outputs of two atmospheric general circulation models (AGCMs) equipped with water vapor isotopes: ECHAM5-wiso and LMDZ5Aiso. During our monitoring period, the signals in the 2 m air temperature T, humidity mixing ratio q and both water vapor isotopes δD and δ18O are dominated by the presence of diurnal cycles. Both AGCMs simulate similar diurnal cycles with a mean amplitude 30 to 70 % lower than observed, possibly due to an incorrect simulation of the surface energy balance and the boundary layer dynamics. In parallel, snow surface samples were collected each hour over 35 h, with a sampling depth of 2-5 mm. A diurnal cycle in the isotopic composition of the snow surface is observed in phase with the water vapor, reaching a peak-to-peak amplitude of 3 for δD over 24 h (compared to 36 for δD in the water vapor). A simple box model treated as a closed system has been developed to study the exchange of water molecules between an air and a snow reservoir. In the vapor, the box model simulations show too much isotopic depletion compared to the observations. Mixing with other sources (advection, free troposphere) has to be included in order to fit the observations. At the snow surface, the simulated isotopic values are close to the observations with a snow reservoir of thinsp;5 mm depth (range of the snow sample depth). Our analysis suggests that fractionation occurs during sublimation and that vapor-snow exchanges can no longer be considered insignificant for the isotopic composition of near-surface snow in polar regions.

J. Escribano, O. Boucher, F. Chevallier, and N. Huneeus. Subregional inversion of North African dust sources. Journal of Geophysical Research (Atmospheres), 121:8549-8566, July 2016. [ bib | DOI | ADS link ]

The emission of mineral dust aerosols in arid and semiarid regions is a complex process whose representation in atmospheric models remains crude, due to insufficient knowledge about the aerosol lifting process itself, the lack of global data on soil characteristics, and the impossibility for the models to resolve the fine-scale variability in the wind field that drives some of the dust events. As a result, there are large uncertainties in the total emission flux of mineral dust, its natural variability at various timescales, and the possible contribution from anthropogenic land use changes. This work aims for estimating dust emissions and reduces their uncertainty over the Sahara desert and the Arabian Peninsulathe largest dust source region of the globe. We use a data assimilation approach to constrain dust emission fluxes at a monthly resolution for 18 subregions. The Moderate Resolution Imaging Spectroradiometer satellite-derived aerosol optical depth is assimilated in a regional configuration of a general circulation model coupled to an aerosol model. We describe this data assimilation system and apply it for 1 year, resulting in a total mineral dust emissions flux estimate of 2900 Tg yr-1 over the Sahara desert and the Arabian Peninsula for the year 2006. The analysis field of aerosol optical depth shows an improved fit relative to independent Aerosol Robotic Network measurements as compared to the model prior field.

F. Brient, T. Schneider, Z. Tan, S. Bony, X. Qu, and A. Hall. Shallowness of tropical low clouds as a predictor of climate models' response to warming. Climate Dynamics, 47:433-449, July 2016. [ bib | DOI | ADS link ]

How tropical low clouds change with climate remains the dominant source of uncertainty in global warming projections. An analysis of an ensemble of CMIP5 climate models reveals that a significant part of the spread in the models' climate sensitivity can be accounted by differences in the climatological shallowness of tropical low clouds in weak-subsidence regimes: models with shallower low clouds in weak-subsidence regimes tend to have a higher climate sensitivity than models with deeper low clouds. The dynamical mechanisms responsible for the model differences are analyzed. Competing effects of parameterized boundary-layer turbulence and shallow convection are found to be essential. Boundary-layer turbulence and shallow convection are typically represented by distinct parameterization schemes in current modelsparameterization schemes that often produce opposing effects on low clouds. Convective drying of the boundary layer tends to deepen low clouds and reduce the cloud fraction at the lowest levels; turbulent moistening tends to make low clouds more shallow but affects the low-cloud fraction less. The relative importance different models assign to these opposing mechanisms contributes to the spread of the climatological shallowness of low clouds and thus to the spread of low-cloud changes under global warming.

S. Botsyun, P. Sepulchre, C. Risi, and Y. Donnadieu. Impacts of Tibetan Plateau uplift on atmospheric dynamics and associated precipitation δ18O. Climate of the Past, 12:1401-1420, June 2016. [ bib | DOI | ADS link ]

Palaeoelevation reconstructions of mountain belts have become a focus of modern science since surface elevation provides crucial information for understanding both geodynamic mechanisms of Earth's interior and the influence of mountain growth on climate. Stable oxygen isotopes palaeoaltimetry is one of the most popular techniques nowadays, and relies on the difference between δ18O of palaeo-precipitation reconstructed using the natural archives, and modern measured values for the point of interest. Our goal is to understand where and how complex climatic changes linked with the growth of mountains affect δ18O in precipitation. For this purpose, we develop a theoretical expression for the precipitation composition based on the Rayleigh distillation and the isotope-equipped atmospheric general circulation model LMDZ-iso outputs. Experiments with reduced height over the Tibetan Plateau and the Himalayas have been designed. Our results show that the isotopic composition of precipitation is very sensitive to climate changes related to the growth of the Himalayas and Tibetan Plateau. Specifically our simulations suggest that only 40 % of sampled sites for palaeoaltimetry depict a full topographic signal, and that uplift-related changes in relative humidity (northern region) and precipitation amount (southern region) could explain absolute deviations of up to 2.5 of the isotopic signal, thereby creating biases in palaeoelevation reconstructions.

P. Sellitto, A. di Sarra, S. Corradini, M. Boichu, H. Herbin, P. Dubuisson, G. Sèze, D. Meloni, F. Monteleone, L. Merucci, J. Rusalem, G. Salerno, P. Briole, and B. Legras. Synergistic use of Lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the Mount Etna eruption of 25-27 October 2013. Atmospheric Chemistry & Physics, 16:6841-6861, June 2016. [ bib | DOI | ADS link ]

In this paper we combine SO2 and ash plume dispersion modelling with satellite and surface remote sensing observations to study the regional influence of a relatively weak volcanic eruption from Mount Etna on the optical and micro-physical properties of Mediterranean aerosols. We analyse the Mount Etna eruption episode of 25-27 October 2013. The evolution of the plume along the trajectory is investigated by means of the FLEXible PARTicle Lagrangian dispersion (FLEXPART) model. The satellite data set includes true colour images, retrieved values of volcanic SO2 and ash, estimates of SO2 and ash emission rates derived from MODIS (MODerate resolution Imaging Spectroradiometer) observations and estimates of cloud top pressure from SEVIRI (Spinning Enhanced Visible and InfraRed Imager). Surface remote sensing measurements of aerosol and SO2 made at the ENEA Station for Climate Observations (35.52deg N, 12.63deg E; 50 m a.s.l.) on the island of Lampedusa are used in the analysis. The combination of these different data sets suggests that SO2 and ash, despite the initial injection at about 7.0 km altitude, reached altitudes around 10-12 km and influenced the column average aerosol particle size distribution at a distance of more than 350 km downwind. This study indicates that even a relatively weak volcanic eruption may produce an observable effect on the aerosol properties at the regional scale. The impact of secondary sulfate particles on the aerosol size distribution at Lampedusa is discussed and estimates of the clear-sky direct aerosol radiative forcing are derived. Daily shortwave radiative forcing efficiencies, i.e. radiative forcing per unit AOD (aerosol optical depth), are calculated with the LibRadtran model. They are estimated between -39 and -48 W m-2 AOD-1 at the top of the atmosphere and between -66 and -49 W m-2 AOD-1 at the surface, with the variability in the estimates mainly depending on the aerosol single scattering albedo. These results suggest that sulfate particles played a large role in the transported plume composition and radiative forcing, while the contribution by ash particles was small in the volcanic plume arriving at Lampedusa during this event.

G. Gastineau, B. L'Hévéder, F. Codron, and C. Frankignoul. Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation. Journal of Climate, 29:3767-3785, May 2016. [ bib | DOI | ADS link ]

I. Gómez-Leal, F. Codron, and F. Selsis. Thermal light curves of Earth-like planets: 1. Varying surface and rotation on planets in a terrestrial orbit. Icarus, 269:98-110, May 2016. [ bib | DOI | ADS link ]

The integrated thermal emission of an exoplanet and its variations along the orbital motion can carry information about the climatic conditions and the rotation of the planet. In this study, we use the LMDZ 3D Global Climate Model (GCM) to simulate the climate of a synthetic Earth and three quasi-Earth configurations: a slowly rotating Earth, an ocean-covered Earth and its snowball counterpart. We also generate the time-dependent broadband thermal emission of the planet from these simulations. In a first step, we validate the model by comparing the synthetic Earth emission with the actual emission of our planet as constrained by observations. Then, we determine the main properties of the climate and emission of the three Earth-like planets and compare them to those of the Earth. We show that planets with an uneven distribution of continents exhibit a maximum of emission during the summer of the hemisphere with larger continental masses, and they may exhibit a maximum of emission at apastron. Large convective clouds might form over the continents of slow rotating planets, having an important effect over their climate and their emission. We also show that, in all the modeled cases, the equilibrium temperature, the Bond albedo and the rotation period can in theory be retrieved from the light curve by a distant observer. The values obtained at transiting geometries have a low deviation from the global values for cases with an axis tilt similar to that of the Earth, and we are able to distinguish between the four planets presented here by the data obtained from their light curves. However, this might not be the case under different conditions.

V. Eyring, S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor. Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization. Geoscientific Model Development, 9:1937-1958, May 2016. [ bib | DOI | ADS link ]

By coordinating the design and distribution of global climate model simulations of the past, current, and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the foundational elements of climate science. However, the need to address an ever-expanding range of scientific questions arising from more and more research communities has made it necessary to revise the organization of CMIP. After a long and wide community consultation, a new and more federated structure has been put in place. It consists of three major elements: (1) a handful of common experiments, the DECK (Diagnostic, Evaluation and Characterization of Klima) and CMIP historical simulations (1850-near present) that will maintain continuity and help document basic characteristics of models across different phases of CMIP; (2) common standards, coordination, infrastructure, and documentation that will facilitate the distribution of model outputs and the characterization of the model ensemble; and (3) an ensemble of CMIP-Endorsed Model Intercomparison Projects (MIPs) that will be specific to a particular phase of CMIP (now CMIP6) and that will build on the DECK and CMIP historical simulations to address a large range of specific questions and fill the scientific gaps of the previous CMIP phases. The DECK and CMIP historical simulations, together with the use of CMIP data standards, will be the entry cards for models participating in CMIP. Participation in CMIP6-Endorsed MIPs by individual modelling groups will be at their own discretion and will depend on their scientific interests and priorities. With the Grand Science Challenges of the World Climate Research Programme (WCRP) as its scientific backdrop, CMIP6 will address three broad questions: <BR /><BR /> - How does the Earth system respond to forcing?<BR /><BR /> - What are the origins and consequences of systematic model biases? <BR /><BR /> - How can we assess future climate changes given internal climate variability, predictability, and uncertainties in scenarios?<BR /><BR /> This CMIP6 overview paper presents the background and rationale for the new structure of CMIP, provides a detailed description of the DECK and CMIP6 historical simulations, and includes a brief introduction to the 21 CMIP6-Endorsed MIPs.

V. Eyring, M. Righi, A. Lauer, M. Evaldsson, S. Wenzel, C. Jones, A. Anav, O. Andrews, I. Cionni, E. L. Davin, C. Deser, C. Ehbrecht, P. Friedlingstein, P. Gleckler, K.-D. Gottschaldt, S. Hagemann, M. Juckes, S. Kindermann, J. Krasting, D. Kunert, R. Levine, A. Loew, J. Mäkelä, G. Martin, E. Mason, A. S. Phillips, S. Read, C. Rio, R. Roehrig, D. Senftleben, A. Sterl, L. H. van Ulft, J. Walton, S. Wang, and K. D. Williams. ESMValTool (v1.0) - a community diagnostic and performance metrics tool for routine evaluation of Earth system models in CMIP. Geoscientific Model Development, 9:1747-1802, May 2016. [ bib | DOI | ADS link ]

A community diagnostics and performance metrics tool for the evaluation of Earth system models (ESMs) has been developed that allows for routine comparison of single or multiple models, either against predecessor versions or against observations. The priority of the effort so far has been to target specific scientific themes focusing on selected essential climate variables (ECVs), a range of known systematic biases common to ESMs, such as coupled tropical climate variability, monsoons, Southern Ocean processes, continental dry biases, and soil hydrology-climate interactions, as well as atmospheric CO2 budgets, tropospheric and stratospheric ozone, and tropospheric aerosols. The tool is being developed in such a way that additional analyses can easily be added. A set of standard namelists for each scientific topic reproduces specific sets of diagnostics or performance metrics that have demonstrated their importance in ESM evaluation in the peer-reviewed literature. The Earth System Model Evaluation Tool (ESMValTool) is a community effort open to both users and developers encouraging open exchange of diagnostic source code and evaluation results from the Coupled Model Intercomparison Project (CMIP) ensemble. This will facilitate and improve ESM evaluation beyond the state-of-the-art and aims at supporting such activities within CMIP and at individual modelling centres. Ultimately, we envisage running the ESMValTool alongside the Earth System Grid Federation (ESGF) as part of a more routine evaluation of CMIP model simulations while utilizing observations available in standard formats (obs4MIPs) or provided by the user.

J.-L. Dufresne and M. Saint-Lu. Positive Feedback in Climate: Stabilization or Runaway, Illustrated by a Simple Experiment. Bulletin of the American Meteorological Society, 97:755-765, May 2016. [ bib | DOI | ADS link ]

A. Touzeau, A. Landais, B. Stenni, R. Uemura, K. Fukui, S. Fujita, S. Guilbaud, A. Ekaykin, M. Casado, E. Barkan, B. Luz, O. Magand, G. Teste, E. Le Meur, M. Baroni, J. Savarino, I. Bourgeois, and C. Risi. Acquisition of isotopic composition for surface snow in East Antarctica and the links to climatic parameters. The Cryosphere, 10:837-852, April 2016. [ bib | DOI | ADS link ]

The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica. First, we look at the relationship between isotopes and temperature from a geographical point of view, using results from three traverses across Antarctica, to see how the relationship is built up through the distillation process. We also take advantage of these measures to see how second-order parameters (d-excess and 17O-excess) are related to δ18O and how they are controlled. d-excess increases in the interior of the continent (i.e., when δ18O decreases), due to the distillation process, whereas 17O-excess decreases in remote areas, due to kinetic fractionation at low temperature. In both cases, these changes are associated with the loss of original information regarding the source. Then, we look at the same relationships in precipitation samples collected over 1 year at Dome C and Vostok, as well as in surface snow at Dome C. We note that the slope of the δ18O vs. temperature (T) relationship decreases in these samples compared to those from the traverses, and thus caution is advocated when using spatial slopes for past climate reconstruction. The second-order parameters behave in the same way in the precipitation as in the surface snow from traverses, indicating that similar processes are active and that their interpretation in terms of source climatic parameters is strongly complicated by local temperature effects in East Antarctica. Finally we check if the same relationships between δ18O and second-order parameters are also found in the snow from four snow pits. While the d-excess remains opposed to δ18O in most snow pits, the 17O-excess is no longer positively correlated to δ18O and even shows anti-correlation to δ18O at Vostok. This may be due to a stratospheric influence at this site and/or to post-deposition processes.

P. Drobinski, B. Alonzo, S. Bastin, N. D. Silva, and C. Muller. Scaling of precipitation extremes with temperature in the French Mediterranean region: What explains the hook shape? Journal of Geophysical Research (Atmospheres), 121:3100-3119, April 2016. [ bib | DOI | ADS link ]

Expected changes to future extreme precipitation remain a key uncertainty associated with anthropogenic climate change. Extreme precipitation has been proposed to scale with the precipitable water content in the atmosphere. Assuming constant relative humidity, this implies an increase of precipitation extremes at a rate of about 7% degC-1 globally as indicated by the Clausius-Clapeyron relationship. Increases faster and slower than Clausius-Clapeyron have also been reported. In this work, we examine the scaling between precipitation extremes and temperature in the present climate using simulations and measurements from surface weather stations collected in the frame of the HyMeX and MED-CORDEX programs in Southern France. Of particular interest are departures from the Clausius-Clapeyron thermodynamic expectation, their spatial and temporal distribution, and their origin. Looking at the scaling of precipitation extreme with temperature, two regimes emerge which form a hook shape: one at low temperatures (cooler than around 15degC) with rates of increase close to the Clausius-Clapeyron rate and one at high temperatures (warmer than about 15degC) with sub-Clausius-Clapeyron rates and most often negative rates. On average, the region of focus does not seem to exhibit super Clausius-Clapeyron behavior except at some stations, in contrast to earlier studies. Many factors can contribute to departure from Clausius-Clapeyron scaling: time and spatial averaging, choice of scaling temperature (surface versus condensation level), and precipitation efficiency and vertical velocity in updrafts that are not necessarily constant with temperature. But most importantly, the dynamical contribution of orography to precipitation in the fall over this area during the so-called “Cevenoles” events, explains the hook shape of the scaling of precipitation extremes.

F. Wang, F. Cheruy, and J.-L. Dufresne. The improvement of soil thermodynamics and its effects on land surface meteorology in the IPSL climate model. Geoscientific Model Development, 9:363-381, January 2016. [ bib | DOI | ADS link ]

This paper describes the implementation of an improved soil thermodynamics in the hydrological module of Earth system model (ESM) developed at the Institut Pierre Simon Laplace (IPSL) and its effects on land surface meteorology in the IPSL climate model. A common vertical discretization scheme for the soil moisture and for the soil temperature is adopted. In addition to the heat conduction process, the heat transported by liquid water into the soil is modeled. The thermal conductivity and the heat capacity are parameterized as a function of the soil moisture and the texture. Preliminary tests are performed in an idealized 1-D (one-dimensional) framework and the full model is then evaluated in the coupled land-atmospheric module of the IPSL ESM. A nudging approach is used in order to avoid the time-consuming long-term simulations required to account for the natural variability of the climate. Thanks to this nudging approach, the effects of the modified parameterizations can be modeled. The dependence of the soil thermal properties on moisture and texture lead to the most significant changes in the surface energy budget and in the surface temperature, with the strongest effects on the surface energy budget taking place over dry areas and during the night. This has important consequences on the mean surface temperature over dry areas and during the night and on its short-term variability. The parameterization of the soil thermal properties could therefore explain some of the temperature biases and part of the dispersion over dry areas in simulations of extreme events such as heat waves in state-of-the-art climate models.

R. Lorenz, D. Argüeso, M. G. Donat, A. J. Pitman, B. van den Hurk, A. Berg, D. M. Lawrence, F. Chéruy, A. Ducharne, S. Hagemann, A. Meier, P. C. D. Milly, and S. I. Seneviratne. Influence of land-atmosphere feedbacks on temperature and precipitation extremes in the GLACE-CMIP5 ensemble. Journal of Geophysical Research (Atmospheres), 121:607-623, January 2016. [ bib | DOI | ADS link ]

We examine how soil moisture variability and trends affect the simulation of temperature and precipitation extremes in six global climate models using the experimental protocol of the Global Land-Atmosphere Coupling Experiment of the Coupled Model Intercomparison Project, Phase 5 (GLACE-CMIP5). This protocol enables separate examinations of the influences of soil moisture variability and trends on the intensity, frequency, and duration of climate extremes by the end of the 21st century under a business-as-usual (Representative Concentration Pathway 8.5) emission scenario. Removing soil moisture variability significantly reduces temperature extremes over most continental surfaces, while wet precipitation extremes are enhanced in the tropics. Projected drying trends in soil moisture lead to increases in intensity, frequency, and duration of temperature extremes by the end of the 21st century. Wet precipitation extremes are decreased in the tropics with soil moisture trends in the simulations, while dry extremes are enhanced in some regions, in particular the Mediterranean and Australia. However, the ensemble results mask considerable differences in the soil moisture trends simulated by the six climate models. We find that the large differences between the models in soil moisture trends, which are related to an unknown combination of differences in atmospheric forcing (precipitation, net radiation), flux partitioning at the land surface, and how soil moisture is parameterized, imply considerable uncertainty in future changes in climate extremes.

D. P. Mulholland, S. R. Lewis, P. L. Read, J.-B. Madeleine, and F. Forget. The solsticial pause on Mars: 2 modelling and investigation of causes. Icarus, 264:465-477, January 2016. [ bib | DOI | ADS link ]

The martian solsticial pause, presented in a companion paper (Lewis et al., 2016), was investigated further through a series of model runs using the UK version of the LMD/UK Mars Global Climate Model. It was found that the pause could not be adequately reproduced if radiatively active water ice clouds were omitted from the model. When clouds were used, along with a realistic time-dependent dust opacity distribution, a substantial minimum in near-surface transient eddy activity formed around solstice in both hemispheres. The net effect of the clouds in the model is, by altering the thermal structure of the atmosphere, to decrease the vertical shear of the westerly jet near the surface around solstice, and thus reduce baroclinic growth rates. A similar effect was seen under conditions of large dust loading, implying that northern midlatitude eddy activity will tend to become suppressed after a period of intense flushing storm formation around the northern cap edge. Suppression of baroclinic eddy generation by the barotropic component of the flow and via diabatic eddy dissipation were also investigated as possible mechanisms leading to the formation of the solsticial pause but were found not to make major contributions. Zonal variations in topography were found to be important, as their presence results in weakened transient eddies around winter solstice in both hemispheres, through modification of the near-surface flow. The zonal topographic asymmetry appears to be the primary reason for the weakness of eddy activity in the southern hemisphere relative to the northern hemisphere, and the ultimate cause of the solsticial pause in both hemispheres. The meridional topographic gradient was found to exert a much weaker influence on near-surface transient eddies.

J. Gao, C. Risi, V. Masson-Delmotte, Y. He, and B. Xu. Southern Tibetan Plateau ice core δ18O reflects abrupt shifts in atmospheric circulation in the late 1970s. Climate Dynamics, 46:291-302, January 2016. [ bib | DOI | ADS link ]

Ice cores from the Tibetan Plateau provide high-resolution records of changes in the snow and ice isotopic composition. In the monsoon sector of southern Tibetan Plateau, their climatic interpretation has been controversial. Here, we present a new high-resolution δ18O record obtained from 2206 measurements performed at 2-3 cm depth resolution along a 55.1 m depth ice core retrieved from the Noijinkansang glacier (NK, 5950 m a.s.l.) that spans the period from 1864 to 2006 AD. The data are characterized by high δ18O values in the nineteenth century, 1910s and 1960s, followed by a drop in the late 1970s and a recent increasing trend. The comparison with regional meteorological data and with a simulation performed with the LMDZiso general circulation model leads to the attribution of the abrupt shift in the late 1970s predominantly to changes in regional atmospheric circulation, together with the impact of atmospheric temperature change. Correlation analyses suggest that the large-scale modes of variability (PDO and ENSO, i.e. Pacific Decadal Oscillation and El Nino-Southern Oscillation) play important roles in modulating NK δ18O changes. The NK δ18O minimum at the end of the 1970s coincides with a PDO phase shift, an inflexion point of the zonal index (representing the overall intensity of the surface westerly anomalies over middle latitudes) as well as ENSO, implying interdecadal modulation of the influence of the PDO/ENSO on the Indian monsoon on southern TP precipitation δ18O. While convective activity above North India controls the intra-seasonal variability of precipitation δ18O in southern TP, other processes associated with changes in large-scale atmospheric circulation act at the inter-annual scale.