2014 .

(37 publications)

N. Gedney, C. Huntingford, G. P. Weedon, N. Bellouin, O. Boucher, and P. M. Cox. Detection of solar dimming and brightening effects on Northern Hemisphere river flow. Nature Geoscience, 7:796-800, November 2014. [ bib | DOI | ADS link ]

Anthropogenic aerosols in the atmosphere have the potential to affect regional-scale land hydrology through solar dimming. Increased aerosol loading may have reduced historical surface evaporation over some locations, but the magnitude and extent of this effect is uncertain. Any reduction in evaporation due to historical solar dimming may have resulted in an increase in river flow. Here we formally detect and quantify the historical effect of changing aerosol concentrations, via solar radiation, on observed river flow over the heavily industrialized, northern extra-tropics. We use a state-of-the-art estimate of twentieth century surface meteorology as input data for a detailed land surface model, and show that the simulations capture the observed strong inter-annual variability in runoff in response to climatic fluctuations. Using statistical techniques, we identify a detectable aerosol signal in the observed river flow both over the combined region, and over individual river basins in Europe and North America. We estimate that solar dimming due to rising aerosol concentrations in the atmosphere around 1980 led to an increase in river runoff by up to 25% in the most heavily polluted regions in Europe. We propose that, conversely, these regions may experience reduced freshwater availability in the future, as air quality improvements are set to lower aerosol loading and solar dimming.

Y. Blanchard, J. Pelon, E. W. Eloranta, K. P. Moran, J. Delanoë, and G. Sèze. A Synergistic Analysis of Cloud Cover and Vertical Distribution from A-Train and Ground-Based Sensors over the High Arctic Station Eureka from 2006 to 2010. Journal of Applied Meteorology and Climatology, 53:2553-2570, November 2014. [ bib | DOI | ADS link ]

D.-D. Rousseau, C. Chauvel, A. Sima, C. Hatté, F. Lagroix, P. Antoine, Y. Balkanski, M. Fuchs, C. Mellett, M. Kageyama, G. Ramstein, and A. Lang. European glacial dust deposits: Geochemical constraints on atmospheric dust cycle modeling. Geophysical Research Letters, 41:7666-7674, November 2014. [ bib | DOI | ADS link ]

For a long time global paleodust numerical simulations have greatly underestimated dust sources other than modern deserts. Recent modeling experiments incorporating glaciogenic sources of dust have positively improved the agreement between model and paleodust data. This highlights the importance of accurately representing all areas potentially subjected to deflation during an investigated interval. Geochemical results, obtained from European loess sequences collected along a 50degN transect, combined with dust emission simulations reveal the geographical distribution of the most important European dust sources between 34 ka and 18 ka. We demonstrate that most European dust traveled only a few hundred kilometers or less within the boundary layer from its source before deposition. We conclude that our results encourage acquisition of similar geochemical data for other relevant areas in the world. Further, they could provide critical constraints to benchmark atmospheric models, contributing to improve their performance in simulating dust cycle and associated climate feedbacks.

P. Ortega, D. Swingedouw, V. Masson-Delmotte, C. Risi, B. Vinther, P. Yiou, R. Vautard, and K. Yoshimura. Characterizing atmospheric circulation signals in Greenland ice cores: insights from a weather regime approach. Climate Dynamics, 43:2585-2605, November 2014. [ bib | DOI | ADS link ]

Greenland ice cores offer seasonal to annual records of δ18O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ18O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ18O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO- regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ18O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ18O variance explained of similar magnitude for the whole set of ice core records. Greenland δ18O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region.

A. Belmadani, V. Echevin, F. Codron, K. Takahashi, and C. Junquas. What dynamics drive future wind scenarios for coastal upwelling off Peru and Chile? Climate Dynamics, 43:1893-1914, October 2014. [ bib | DOI | ADS link ]

The dynamics of the Peru-Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35degS (at 30degS-35degS) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land-sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall.

M. Schröder, R. Roca, L. Picon, A. Kniffka, and H. Brogniez. Climatology of free-tropospheric humidity: extension into the SEVIRI era, evaluation and exemplary analysis. Atmospheric Chemistry & Physics, 14:11129-11148, October 2014. [ bib | DOI | ADS link ]

A new free-tropospheric humidity (FTH) data record is presented. It is based on observations from the Meteosat Visible and Infrared Imager (MVIRI) onboard Meteosat-2-Meteosat-5, as well as Meteosat-7, and the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat-8 and Meteosat-9 at the water absorption band near 6.3 μm. The data set is available under clear-sky and low-level cloud conditions. With the extension to SEVIRI observations, the data record covers the period 1983-2009 with a spatial resolution of 0.625deg × 0.625deg and a temporal resolution of 3 h. <BR /><BR /> The FTH is the mean relative humidity (RH) in a broad layer in the free troposphere. The relation between the observed brightness temperature (BT) and the FTH is well established. Previous retrievals are refined by taking into account the relative humidity Jacobians in the training process of the statistical retrieval. The temporal coverage is extended into the SEVIRI period, the homogenization of the BT record is improved, and the full archive is reprocessed using updated regression coefficients. <BR /><BR /> The FTH estimated from the Meteosat observations is compared to the FTH computed from the RH profiles of the Analyzed RadioSoundings Archive (ARSA). An average relative bias of -3.2% and a relative root-mean-square difference (RMSD) of 16.8% are observed. This relative RMSD agrees with the outcome of an analysis of the total uncertainty of the FTH product. The decadal stability of the FTH data record is 0.5 0.45% per decade. <BR /><BR /> As exemplary applications, the interannual standard deviation, the differences on decadal scales, and the linear trend in the FTH data record and in the frequency of occurrence of FTH 10% (FTHp10) are analyzed per season. Interannual standard deviation maxima and maxima in absolute decadal differences are featured in gradient areas between dry and wet regions, as well as in areas where FTH reaches minima and FTHp10 reaches maxima. An analysis of the FTH linear trends and of the associated uncertainty estimates is achieved to identify possible problems with the data record. Positive trends in FTHp10 are featured in gradient areas between wet and dry regions, in regions where the FTH is minimum, in regions where FTHp10 is maximum, and in regions where differences between FTHp10 averaged over the 2000s and 1990s are negative. However, these positive trends in FTHp10 are associated with maximum standard deviation and are thus hardly significant. This analysis and intercomparisons with other humidity data records are part of the Global Energy and Water Cycle Experiment (GEWEX) Water Vapor Assessment (G-VAP).

F. Cheruy, J. L. Dufresne, F. Hourdin, and A. Ducharne. Role of clouds and land-atmosphere coupling in midlatitude continental summer warm biases and climate change amplification in CMIP5 simulations. Geophysical Research Letters, 41:6493-6500, September 2014. [ bib | DOI | ADS link ]

Over land, most state-of-the-art climate models contributing to Coupled Model Intercomparison Project Phase 5 (CMIP5) share a strong summertime warm bias in midlatitude areas, especially in regions where the coupling between soil moisture and atmosphere is effective. The most biased models overestimate solar incoming radiation, because of cloud deficit and have difficulty to sustain evaporation. These deficiencies are also involved in the spread of the summer temperature projections among models in the midlatitude; the models which simulate a higher-than-average warming overestimate the present climate net shortwave radiation which increases more-than-average in the future, in link with a decrease of cloudiness. They also show a higher-than-average reduction of evaporative fraction in areas with soil moisture-limited evaporation regimes. Over these areas, the most biased models in the present climate simulate a larger warming in response to climate change which is likely to be overestimated.

L. D. Rotstayn, E. L. Plymin, M. A. Collier, O. Boucher, J.-L. Dufresne, J.-J. Luo, K. von Salzen, S. J. Jeffrey, M.-A. Foujols, Y. Ming, and L. W. Horowitz. Declining Aerosols in CMIP5 Projections: Effects on Atmospheric Temperature Structure and Midlatitude Jets. Journal of Climate, 27:6960-6977, September 2014. [ bib | DOI | ADS link ]

A. Lahellec and J.-L. Dufresne. A Formal Analysis of the Feedback Concept in Climate Models. Part II: Tangent Linear Systems in GCMs. Journal of Atmospheric Sciences, 71:3350-3375, September 2014. [ bib | DOI | ADS link ]

A. Harzallah, M. Alioua, and L. Li. Mass exchange at the Strait of Gibraltar in response to tidal and lower frequency forcing as simulated by a Mediterranean Sea model. Tellus Series A, 66:23871, September 2014. [ bib | DOI | ADS link ]

The exchange between the Atlantic and the Mediterranean at the Strait of Gibraltar is studied based on numerical simulations of the Mediterranean Sea compared to two sets of observations. The model used has a varying horizontal resolution, highest at the Strait of Gibraltar. Numerical simulations forced by tide, by the subinertial variability, by both and by increasing the diffusion at the Strait are performed and compared to each other. The model successfully reproduces the main observed features of the variability at the tidal and at the lower frequency time scales including the phasing between the barotropic and baroclinic flow components and density variations. The model also simulates the strong mixing at the strait by tide and the resulting fortnightly modulation of the flow, with exchange reduction during spring tides and outflowing waters and acceleration during neap tides and inflowing waters. It is shown that tidal oscillations reduce the two-way exchange by interaction with the subinertial variability. The effects of tide on the Mediterranean Sea thermohaline circulation are also examined using multi-decadal simulations. It is shown that the model reproduces the cooling and saltening of waters crossing the strait in the upper layer and the warming and freshening of waters crossing the strait in the deeper layer, as previously shown by high resolution models of the Strait of Gibraltar. These changes are shown to cool and increase the salinity of the Mediterranean waters especially in the upper and intermediate layers. The water-cooling is shown to lead to a reduction of the heat loss at the sea surface. Based on model results, it is concluded that tide may have an effect on the Mediterranean Sea heat budget and hence on the atmosphere above. A validation of this conclusion is however needed, in particular using higher resolution models.

S. Fermepin and S. Bony. Influence of low-cloud radiative effects on tropical circulation and precipitation. Journal of Advances in Modeling Earth Systems, 6:513-526, September 2014. [ bib | DOI | ADS link ]

Low-level clouds, which constitute the most prevalent cloud type over tropical oceans, exert a radiative cooling within the planetary boundary layer. By using an atmospheric general circulation model, we investigate the role that this cloud radiative cooling plays in the present-day climate. Low-cloud radiative effects are found to increase the tropics-wide precipitation, to strengthen the winds at the surface of the tropical oceans, and to amplify the atmospheric overturning circulation. An analysis of the water and energy budgets of the atmosphere reveals that most of these effects arises from the strong coupling of cloud-radiative cooling with turbulent fluxes at the ocean surface. The impact of cloud-radiative effects on atmospheric dynamics and precipitation is shown to occur on very short time scales (a few days). Therefore, short-term atmospheric forecasts constitute a valuable framework for evaluating the interactions between cloud processes and atmospheric dynamics, and for assessing their dependence on model physics.

N. L. Pessacg, S. A. Solman, P. Samuelsson, E. Sanchez, J. Marengo, L. Li, A. R. C. Remedio, R. P. da Rocha, C. Mourão, and D. Jacob. The surface radiation budget over South America in a set of regional climate models from the CLARIS-LPB project. Climate Dynamics, 43:1221-1239, September 2014. [ bib | DOI | ADS link ]

The performance of seven regional climate models in simulating the radiation and heat fluxes at the surface over South America (SA) is evaluated. Sources of uncertainty and errors are identified. All simulations have been performed in the context of the CLARIS-LPB Project for the period 1990-2008 and are compared with the GEWEX-SRB, CRU, and GLDAS2 dataset and NCEP-NOAA reanalysis. Results showed that most of the models overestimate the net surface short-wave radiation over tropical SA and La Plata Basin and underestimate it over oceanic regions. Errors in the short-wave radiation are mainly associated with uncertainties in the representation of surface albedo and cloud fraction. For the net surface long-wave radiation, model biases are diverse. However, the ensemble mean showed a good agreement with the GEWEX-SRB dataset due to the compensation of individual model biases. Errors in the net surface long-wave radiation can be explained, in a large proportion, by errors in cloud fraction. For some particular models, errors in temperature also contribute to errors in the net long-wave radiation. Analysis of the annual cycle of each component of the energy budget indicates that the RCMs reproduce generally well the main characteristics of the short- and long-wave radiations in terms of timing and amplitude. However, a large spread among models over tropical SA is apparent. The annual cycle of the sensible heat flux showed a strong overestimation in comparison with the reanalysis and GLDAS2 dataset. For the latent heat flux, strong differences between the reanalysis and GLDAS2 are calculated particularly over tropical SA.

V. Gryazin, C. Risi, J. Jouzel, N. Kurita, J. Worden, C. Frankenberg, V. Bastrikov, K. Gribanov, and O. Stukova. To what extent could water isotopic measurements help us understand model biases in the water cycle over Western Siberia. Atmospheric Chemistry & Physics, 14:9807-9830, September 2014. [ bib | DOI | ADS link ]

We evaluate the isotopic composition of water vapor and precipitation simulated by the LMDZ (Laboratoire de Météorologie Dynamique-Zoom) GCM (General Circulation Model) over Siberia using several data sets: TES (Tropospheric Emission Spectrometer) and GOSAT (Greenhouse gases Observing SATellite) satellite observations of tropospheric water vapor, GNIP (Global Network for Isotopes in Precipitation) and SNIP (Siberian Network for Isotopes in Precipitation) precipitation networks, and daily, in situ measurements of water vapor and precipitation at the Kourovka site in Western Siberia. LMDZ captures the spatial, seasonal and daily variations reasonably well, but it underestimates humidity (q) in summer and overestimates δD in the vapor and precipitation in all seasons. The performance of LMDZ is put in the context of other isotopic models from the SWING2 (Stable Water Intercomparison Group phase 2) models. There is significant spread among models in the simulation of δD, and of the δD-q relationship. This confirms that δD brings additional information compared to q only. We specifically investigate the added value of water isotopic measurements to interpret the warm and dry bias featured by most GCMs over mid and high latitude continents in summer. The analysis of the slopes in δD-q diagrams and of processes controlling δD and q variations suggests that the cause of the dry bias could be either a problem in the large-scale advection transporting too much dry and warm air from the south, or too strong boundary-layer mixing. However, δD-q diagrams using the available data do not tell the full story. Additional measurements would be needed, or a more sophisticated theoretical framework would need to be developed.

S. Bony, G. Bellon, D. Klocke, S. Sherwood, S. Fermepin, and S. Denvil. Addendum: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation. Nature Geoscience, 7:547, July 2014. [ bib | DOI | ADS link ]

T. Navarro, J.-B. Madeleine, F. Forget, A. Spiga, E. Millour, F. Montmessin, and A. Määttänen. Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds. Journal of Geophysical Research (Planets), 119:1479-1495, July 2014. [ bib | DOI | arXiv | ADS link ]

Water ice clouds play a key role in the radiative transfer of the Martian atmosphere, impacting its thermal structure, its circulation, and, in turn, the water cycle. Recent studies including the radiative effects of clouds in global climate models (GCMs) have found that the corresponding feedbacks amplify the model defaults. In particular, it prevents models with simple microphysics from reproducing even the basic characteristics of the water cycle. Within that context, we propose a new implementation of the water cycle in GCMs, including a detailed cloud microphysics taking into account nucleation on dust particles, ice particle growth, and scavenging of dust particles due to the condensation of ice. We implement these new methods in the Laboratoire de Météorologie Dynamique GCM and find satisfying agreement with the Thermal Emission Spectrometer observations of both water vapor and cloud opacities, with a significant improvement when compared to GCMs taking into account radiative effects of water ice clouds without this implementation. However, a lack of water vapor in the tropics after Ls = 180deg is persistent in simulations compared to observations, as a consequence of aphelion cloud radiative effects strengthening the Hadley cell. Our improvements also allow us to explore questions raised by recent observations of the Martian atmosphere. Supersaturation above the hygropause is predicted in line with Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars observations. The model also suggests for the first time that the scavenging of dust by water ice clouds alone fails to fully account for the detached dust layers observed by the Mars Climate Sounder.

P. J. Irvine, O. Boucher, B. Kravitz, K. Alterskjær, J. N. S. Cole, D. Ji, A. Jones, D. J. Lunt, J. C. Moore, H. Muri, U. Niemeier, A. Robock, B. Singh, S. Tilmes, S. Watanabe, S. Yang, and J.-H. Yoon. Key factors governing uncertainty in the response to sunshade geoengineering from a comparison of the GeoMIP ensemble and a perturbed parameter ensemble. Journal of Geophysical Research (Atmospheres), 119:7946-7962, July 2014. [ bib | DOI | ADS link ]

Climate model studies of the consequences of solar geoengineering are central to evaluating whether such approaches may help to reduce the harmful impacts of global warming. In this study we compare the sunshade solar geoengineering response of a perturbed parameter ensemble (PPE) of the Hadley Centre Coupled Model version 3 (HadCM3) with a multimodel ensemble (MME) by analyzing the G1 experiment from the Geoengineering Model Intercomparison Project (GeoMIP). The PPE only perturbed a small number of parameters and shares a common structure with the unperturbed HadCM3 model, and so the additional weight the PPE adds to the robustness of the common climate response features in the MME is minor. However, analysis of the PPE indicates some of the factors that drive the spread within the MME. We isolate the role of global mean temperature biases for both ensembles and find that these biases have little effect on the ensemble spread in the hydrological response but do reduce the spread in surface air temperature response, particularly at high latitudes. We investigate the role of the preindustrial climatology and find that biases here are likely a key source of ensemble spread at the zonal and grid cell level. The role of vegetation, and its response to elevated CO2 concentrations through the CO2 physiological effect and changes in plant productivity, is also investigated and proves to have a substantial effect on the terrestrial hydrological response to solar geoengineering and to be a major source of variation within the GeoMIP ensemble.

J.-B. Madeleine, J. W. Head, F. Forget, T. Navarro, E. Millour, A. Spiga, A. Colaïtis, A. Määttänen, F. Montmessin, and J. L. Dickson. Recent Ice Ages on Mars: The role of radiatively active clouds and cloud microphysics. Geophysical Research Letters, 41:4873-4879, July 2014. [ bib | DOI | ADS link ]

Global climate models (GCMs) have been successfully employed to explain the origin of many glacial deposits on Mars. However, the latitude-dependent mantle (LDM), a dust-ice mantling deposit that is thought to represent a recent “Ice Age,” remains poorly explained by GCMs. We reexamine this question by considering the effect of radiatively active water-ice clouds (RACs) and cloud microphysics. We find that when obliquity is set to 35deg, as often occurred in the past 2 million years, warming of the atmosphere and polar caps by clouds modifies the water cycle and leads to the formation of a several centimeter-thick ice mantle poleward of 30deg in each hemisphere during winter. This mantle can be preserved over the summer if increased atmospheric dust content obscures the surface and provides dust nuclei to low-altitude clouds. We outline a scenario for its deposition and preservation that compares favorably with the characteristics of the LDM.

A. Voigt, S. Bony, J.-L. Dufresne, and B. Stevens. The radiative impact of clouds on the shift of the Intertropical Convergence Zone. Geophysical Research Letters, 41:4308-4315, June 2014. [ bib | DOI | ADS link ]

Whereas it is well established that clouds are important to changes in Earth's surface temperature, their impact on changes of the large-scale atmospheric circulation is less well understood. Here we study the radiative impact of clouds on the shift of the Intertropical Convergence Zone (ITCZ) in response to hemispheric surface albedo forcings. The problem is approached using aquaplanet simulations with four comprehensive atmosphere models. The radiative impact of clouds on the ITCZ shift differs in sign and magnitude across models and is responsible for half of the model spread in the ITCZ shift. The model spread is dominated by tropical clouds whose radiative impact is linked to the dependence of their cloud radiative properties on the circulation. The simulations not only demonstrate the importance of clouds for circulation changes but also propose a way to reduce the model uncertainty in ITCZ shifts.

M. Pommier, J.-L. Lacour, C. Risi, F. M. Bréon, C. Clerbaux, P.-F. Coheur, K. Gribanov, D. Hurtmans, J. Jouzel, and V. Zakharov. Observation of tropospheric δD by IASI over western Siberia: comparison with a general circulation model. Atmospheric Measurement Techniques, 7:1581-1595, June 2014. [ bib | DOI | ADS link ]

This study presents the joint H216O and HDO retrieval from Infrared Atmospheric Sounding Interferometer (IASI) spectra over western Siberia. IASI is an instrument on board the MetOp-A European satellite. The global coverage of the instrument and the good signal-to-noise ratio allow us to provide information on δD over this remote region. We show that IASI measurements may be used to estimate integrated δD between the surface and 3 km altitude or from 1 to 5 km depending on the thermal contrast, with observational errors lower than 4% and 7%, respectively. The retrieved data are compared to simulations from an isotopic general circulation model, LMDZ-iso for 2011. The satellite measurements and the model agree well and they reproduce well the seasonal and day-to-day variations for δD, presenting a good correlation (r up to 0.8 with the smoothed data in summer). The IASI-based retrievals also show the seasonal variation of the specific humidity in both altitude ranges.

M. Butzin, M. Werner, V. Masson-Delmotte, C. Risi, C. Frankenberg, K. Gribanov, J. Jouzel, and V. I. Zakharov. Variations of oxygen-18 in West Siberian precipitation during the last 50 years. Atmospheric Chemistry & Physics, 14:5853-5869, June 2014. [ bib | DOI | ADS link ]

Global warming is associated with large increases in surface air temperature in Siberia. Here, we apply the isotope-enabled atmospheric general circulation model ECHAM5-wiso to explore the potential of water isotope measurements at a recently opened monitoring station in Kourovka (57.04deg N, 59.55deg E) in order to successfully trace climate change in western Siberia. Our model is constrained to atmospheric reanalysis fields for the period 1957-2013 to facilitate the comparison with observations of δD in total column water vapour from the GOSAT satellite, and with precipitation δ18O measurements from 15 Russian stations of the Global Network of Isotopes in Precipitation. The model captures the observed Russian climate within reasonable error margins, and displays the observed isotopic gradients associated with increasing continentality and decreasing meridional temperatures. The model also reproduces the observed seasonal cycle of δ18O, which parallels the seasonal cycle of temperature and ranges from -25 in winter to -5 in summer. Investigating West Siberian climate and precipitation δ18O variability during the last 50 years, we find long-term increasing trends in temperature and δ18O, while precipitation trends are uncertain. During the last 50 years, winter temperatures have increased by 1.7 degC. The simulated long-term increase of precipitation δ18O is at the detection limit (1 per 50 years) but significant. West Siberian climate is characterized by strong interannual variability, which in winter is strongly related to the North Atlantic Oscillation. In winter, regional temperature is the predominant factor controlling δ18O variations on interannual to decadal timescales with a slope of about 0.5 degC-1. In summer, the interannual variability of δ18O can be attributed to short-term, regional-scale processes such as evaporation and convective precipitation. This finding suggests that precipitation δ18O has the potential to reveal hydrometeorological regime shifts in western Siberia which are otherwise difficult to identify. Focusing on Kourovka, the simulated evolution of temperature, δ18O and, to a smaller extent, precipitation during the last 50 years is synchronous with model results averaged over all of western Siberia, suggesting that this site will be representative to monitor future isotopic changes in the entire region.

N. Huneeus, O. Boucher, K. Alterskjær, J. N. S. Cole, C. L. Curry, D. Ji, A. Jones, B. Kravitz, J. E. Kristjánsson, J. C. Moore, H. Muri, U. Niemeier, P. Rasch, A. Robock, B. Singh, H. Schmidt, M. Schulz, S. Tilmes, S. Watanabe, and J.-H. Yoon. Forcings and feedbacks in the GeoMIP ensemble for a reduction in solar irradiance and increase in CO2. Journal of Geophysical Research (Atmospheres), 119:5226-5239, May 2014. [ bib | DOI | ADS link ]

The effective radiative forcings (including rapid adjustments) and feedbacks associated with an instantaneous quadrupling of the preindustrial CO2 concentration and a counterbalancing reduction of the solar constant are investigated in the context of the Geoengineering Model Intercomparison Project (GeoMIP). The forcing and feedback parameters of the net energy flux, as well as its different components at the top-of-atmosphere (TOA) and surface, were examined in 10 Earth System Models to better understand the impact of solar radiation management on the energy budget. In spite of their very different nature, the feedback parameter and its components at the TOA and surface are almost identical for the two forcing mechanisms, not only in the global mean but also in their geographical distributions. This conclusion holds for each of the individual models despite intermodel differences in how feedbacks affect the energy budget. This indicates that the climate sensitivity parameter is independent of the forcing (when measured as an effective radiative forcing). We also show the existence of a large contribution of the cloudy-sky component to the shortwave effective radiative forcing at the TOA suggesting rapid cloud adjustments to a change in solar irradiance. In addition, the models present significant diversity in the spatial distribution of the shortwave feedback parameter in cloudy regions, indicating persistent uncertainties in cloud feedback mechanisms.

G. Tremoy, F. Vimeux, S. Soumana, I. Souley, C. Risi, G. Favreau, and M. Oï. Clustering mesoscale convective systems with laser-based water vapor δ18O monitoring in Niamey (Niger). Journal of Geophysical Research (Atmospheres), 119:5079-5103, May 2014. [ bib | DOI | ADS link ]

The isotopic composition of surface water vapor (δv) has been measured continuously in Niamey along with the isotopic composition of event-based precipitation (δp) since 2010. We investigate the evolution of water vapor and precipitation isotope ratios during rain events of the 2010, 2011, and 2012 monsoon periods. We establish a classification of rain systems into three types based on the δv temporal evolution. We find that 51% of rain events (class A) exhibit a sharp decrease in δ18Ov in phase with the surface air temperature drop, leading to a depletion of water vapor by -1.9 on average during rainfall. Twenty-nine percent of rain events (class B) show a similar decrease in δ18Ov in phase with the temperature drop but are characterized by a progressive enrichment of the vapor in the stratiform region, resulting in a depletion of water vapor by -1.2 on average during rainfall. The last 20% of the rain events (class C) are associated with a progressive increase in δ18Ov during rainfall (+0.8). We also examine the temporal evolution of water vapor deuterium excess (dv) which shows a sharp increase as δ18Ov decreases, followed by a progressive decrease in the stratiform part for classes A and B. Using a basic box model, we examine for each class the respective roles that mesoscale subsidence and rain evaporation play on the evolution of δ18Ov. We show that those two processes are dominant for class A, whereas other processes may exert a major role on δ18Ov for classes B and C.

J.-L. Bonne, V. Masson-Delmotte, O. Cattani, M. Delmotte, C. Risi, H. Sodemann, and H. C. Steen-Larsen. The isotopic composition of water vapour and precipitation in Ivittuut, southern Greenland. Atmospheric Chemistry & Physics, 14:4419-4439, May 2014. [ bib | DOI | ADS link ]

Since September 2011, a wavelength-scanned cavity ring-down spectroscopy analyser has been remotely operated in Ivittuut, southern Greenland, providing the first record of surface water vapour isotopic composition based on continuous measurements in South Greenland and the first record including the winter season in Greenland. The comparison of vapour data with measurements of precipitation isotopic composition suggest an equilibrium between surface vapour and precipitation. <BR /><BR /> δ18O and deuterium excess are generally anti-correlated and show important seasonal variations, with respective amplitudes of ˜10 and ˜20, as well as large synoptic variations. The data depict small summer diurnal variations. At the seasonal scale, δ18O has a minimum in November-December and a maximum in June-July, while deuterium excess has a minimum in May-June and a maximum in November. The approach of low-pressure systems towards South Greenland leads to δ18O increase (typically +5) and deuterium excess decrease (typically -15). <BR /><BR /> Seasonal and synoptic variations coincide with shifts in the moisture sources, estimated using a quantitative moisture source diagnostic based on a Lagrangian back-trajectory model. The atmospheric general circulation model LMDZiso correctly captures the seasonal and synoptic variability of δ18O, but does not capture the observed magnitude of deuterium excess variability. <BR /><BR /> Covariations of water vapour isotopic composition with local and moisture source meteorological parameters have been evaluated. δ18O is strongly correlated with the logarithm of local surface humidity, consistent with Rayleigh distillation processes, and with local surface air temperature, associated with a slope of ˜0.4 degC-1. Deuterium excess correlates with local surface relative humidity as well as surface relative humidity from the dominant moisture source area located in the North Atlantic, south of Greenland and Iceland.

M.-S. Deroche, M. Choux, F. Codron, and P. Yiou. Three variables are better than one: detection of european winter windstorms causing important damages. Natural Hazards and Earth System Sciences, 14:981-993, April 2014. [ bib | DOI | ADS link ]

In this paper, we present a new approach for detecting potentially damaging European winter windstorms from a multi-variable perspective. European winter windstorms being usually associated with extra-tropical cyclones (ETCs), there is a coupling between the intensity of the surface wind speeds and other meso-scale and large-scale features characteristic of ETCs. Here we focus on the relative vorticity at 850 hPa and the sea level pressure anomaly, which are also used in ETC detection studies, along with the ratio of the 10 m wind speed to its 98th percentile. When analysing 10 events known by the insurance industry to have caused extreme damages, we find that they share an intense signature in each of the 3 fields. This shows that the relative vorticity and the mean sea level pressure have a predictive value of the intensity of the generated windstorms. The 10 major events are not the most intense in any of the 3 variables considered separately, but we show that the combination of the 3 variables is an efficient way of extracting these events from a reanalysis data set.

Z. Liu, K. Yoshimura, G. J. Bowen, N. H. Buenning, C. Risi, J. M. Welker, and F. Yuan. Paired oxygen isotope records reveal modern North American atmospheric dynamics during the Holocene. Nature Communications, 5:3701, April 2014. [ bib | DOI | ADS link ]

The Pacific North American (PNA) teleconnection has a strong influence on North American climate. Instrumental records and century-scale reconstructions indicate an accelerating tendency towards the positive PNA state since the mid-1850s, but much less is known about long-term PNA variability. Here we reconstruct PNA-like climate variability during the mid- and late Holocene using paired oxygen isotope records from two regions in North America with robust, anticorrelated isotopic response to the modern PNA. We identify mean states of more negative and positive PNA-like climate during the mid- and late Holocene, respectively. Superimposed on the secular change between states is a robust, quasi-200-year oscillation, which we associate with the de Vries solar cycle. These findings suggest the persistence of PNA-like climate variability throughout the mid- and late Holocene, provide evidence for modulation of PNA over multiple timescales and may help researchers de-convolve PNA pattern variation from other factors reflected in palaeorecords.

Y. Li, D. W. J. Thompson, G. L. Stephens, and S. Bony. A global survey of the instantaneous linkages between cloud vertical structure and large-scale climate. Journal of Geophysical Research (Atmospheres), 119:3770-3792, April 2014. [ bib | DOI | ADS link ]

The instantaneous linkages between cloud vertical structure and various large-scale meteorological parameters are investigated using 5 years of data from the CloudSat/CALIPSO instruments. The linkages are systemically explored and quantified at all vertical levels and throughout the global ocean in both the long-term mean and on month-to-month timescales. A number of novel large-scale meteorological parameters are used in the analysis, including tropopause temperatures, upper tropospheric stability, and storm track activity. The results provide a baseline for evaluating physical parameterizations of clouds in GCMs and a reference for interpreting the signatures of large-scale atmospheric phenomena in cloud vertical structure. In the long-term mean, upper tropospheric cloud incidence throughout the globe increases with (1) decreasing tropopause temperature (at a rate of 2-4% K-1), (2) decreasing upper tropospheric stability (5-10% per K km-1), and (3) increasing large-scale vertical motion (1-4% per 10 hPa d-1). In contrast, lower tropospheric cloud incidence increases with (1) increasing lower tropospheric stability (10% per K km-1) and descending motion (1% per 10 hPa d-1) in regions of subtropical regime but (2) decreasing lower tropospheric stability (4% per K km-1) and ascending motion (2% per 10 hPa d-1) over the Arctic region. Variations in static stability and vertical motion account for 20-35% of the month-to-month variance in upper tropospheric cloudiness but less than 10% of the variance in lower tropospheric clouds. Upper tropospheric cloud incidence in the storm track regions is strongly linked to the variance of large-scale vertical motion and thus the amplitude of baroclinic waves.

M. Ménégoz, G. Krinner, Y. Balkanski, O. Boucher, A. Cozic, S. Lim, P. Ginot, P. Laj, H. Gallée, P. Wagnon, A. Marinoni, and H. W. Jacobi. Snow cover sensitivity to black carbon deposition in the Himalayas: from atmospheric and ice core measurements to regional climate simulations. Atmospheric Chemistry & Physics, 14:4237-4249, April 2014. [ bib | DOI | ADS link ]

We applied a climate-chemistry global model to evaluate the impact of black carbon (BC) deposition on the Himalayan snow cover from 1998 to 2008. Using a stretched grid with a resolution of 50 km over this complex topography, the model reproduces reasonably well the remotely sensed observations of the snow cover duration. Similar to observations, modelled atmospheric BC concentrations in the central Himalayas reach a minimum during the monsoon and a maximum during the post- and pre-monsoon periods. Comparing the simulated BC concentrations in the snow with observations is more challenging because of their high spatial variability and complex vertical distribution. We simulated spring BC concentrations in surface snow varying from tens to hundreds of μg kg-1, higher by one to two orders of magnitude than those observed in ice cores extracted from central Himalayan glaciers at high elevations (6000 m a.s.l.), but typical for seasonal snow cover sampled in middle elevation regions (6000 m a.s.l.). In these areas, we estimate that both wet and dry BC depositions affect the Himalayan snow cover reducing its annual duration by 1 to 8 days. In our simulations, the effect of anthropogenic BC deposition on snow is quite low over the Tibetan Plateau because this area is only sparsely snow covered. However, the impact becomes larger along the entire Hindu-Kush, Karakorum and Himalayan mountain ranges. In these regions, BC in snow induces an increase of the net short-wave radiation at the surface with an annual mean of 1 to 3 W m-2 leading to a localised warming between 0.05 and 0.3 degC.

N. Rochetin, J.-Y. Grandpeix, C. Rio, and F. Couvreux. Deep Convection Triggering by Boundary Layer Thermals. Part II: Stochastic Triggering Parameterization for the LMDZ GCM. Journal of Atmospheric Sciences, 71:515-538, February 2014. [ bib | DOI | ADS link ]

N. Rochetin, F. Couvreux, J.-Y. Grandpeix, and C. Rio. Deep Convection Triggering by Boundary Layer Thermals. Part I: LES Analysis and Stochastic Triggering Formulation. Journal of Atmospheric Sciences, 71:496-514, February 2014. [ bib | DOI | ADS link ]

H.-Y. Ma, S. Xie, S. A. Klein, K. D. Williams, J. S. Boyle, S. Bony, H. Douville, S. Fermepin, B. Medeiros, S. Tyteca, M. Watanabe, and D. Williamson. On the Correspondence between Mean Forecast Errors and Climate Errors in CMIP5 Models. Journal of Climate, 27:1781-1798, February 2014. [ bib | DOI | ADS link ]

H. C. Steen-Larsen, V. Masson-Delmotte, M. Hirabayashi, R. Winkler, K. Satow, F. Prié, N. Bayou, E. Brun, K. M. Cuffey, D. Dahl-Jensen, M. Dumont, M. Guillevic, S. Kipfstuhl, A. Landais, T. Popp, C. Risi, K. Steffen, B. Stenni, and A. E. Sveinbjörnsdottír. What controls the isotopic composition of Greenland surface snow? Climate of the Past, 10:377-392, February 2014. [ bib | DOI | ADS link ]

Water stable isotopes in Greenland ice core data provide key paleoclimatic information, and have been compared with precipitation isotopic composition simulated by isotopically enabled atmospheric models. However, post-depositional processes linked with snow metamorphism remain poorly documented. For this purpose, monitoring of the isotopic composition (δ18O, δD) of near-surface water vapor, precipitation and samples of the top (0.5 cm) snow surface has been conducted during two summers (2011-2012) at NEEM, NW Greenland. The samples also include a subset of 17O-excess measurements over 4 days, and the measurements span the 2012 Greenland heat wave. Our observations are consistent with calculations assuming isotopic equilibrium between surface snow and water vapor. We observe a strong correlation between near-surface vapor δ18O and air temperature (0.85 0.11 degC-1 (R = 0.76) for 2012). The correlation with air temperature is not observed in precipitation data or surface snow data. Deuterium excess (d-excess) is strongly anti-correlated with δ18O with a stronger slope for vapor than for precipitation and snow surface data. During nine 1-5-day periods between precipitation events, our data demonstrate parallel changes of δ18O and d-excess in surface snow and near-surface vapor. The changes in δ18O of the vapor are similar or larger than those of the snow δ18O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. In our data, the sign of surface snow isotopic changes is not related to the sign or magnitude of sublimation or deposition. Comparisons with atmospheric models show that day-to-day variations in near-surface vapor isotopic composition are driven by synoptic variations and changes in air mass trajectories and distillation histories. We suggest that, in between precipitation events, changes in the surface snow isotopic composition are driven by these changes in near-surface vapor isotopic composition. This is consistent with an estimated 60% mass turnover of surface snow per day driven by snow recrystallization processes under NEEM summer surface snow temperature gradients. Our findings have implications for ice core data interpretation and model-data comparisons, and call for further process studies.

G. A. Schmidt, J. D. Annan, P. J. Bartlein, B. I. Cook, E. Guilyardi, J. C. Hargreaves, S. P. Harrison, M. Kageyama, A. N. LeGrande, B. Konecky, S. Lovejoy, M. E. Mann, V. Masson-Delmotte, C. Risi, D. Thompson, A. Timmermann, L.-B. Tremblay, and P. Yiou. Using palaeo-climate comparisons to constrain future projections in CMIP5. Climate of the Past, 10:221-250, February 2014. [ bib | DOI | ADS link ]

We present a selection of methodologies for using the palaeo-climate model component of the Coupled Model Intercomparison Project (Phase 5) (CMIP5) to attempt to constrain future climate projections using the same models. The constraints arise from measures of skill in hindcasting palaeo-climate changes from the present over three periods: the Last Glacial Maximum (LGM) (21 000 yr before present, ka), the mid-Holocene (MH) (6 ka) and the Last Millennium (LM) (850-1850 CE). The skill measures may be used to validate robust patterns of climate change across scenarios or to distinguish between models that have differing outcomes in future scenarios. We find that the multi-model ensemble of palaeo-simulations is adequate for addressing at least some of these issues. For example, selected benchmarks for the LGM and MH are correlated to the rank of future projections of precipitation/temperature or sea ice extent to indicate that models that produce the best agreement with palaeo-climate information give demonstrably different future results than the rest of the models. We also explore cases where comparisons are strongly dependent on uncertain forcing time series or show important non-stationarity, making direct inferences for the future problematic. Overall, we demonstrate that there is a strong potential for the palaeo-climate simulations to help inform the future projections and urge all the modelling groups to complete this subset of the CMIP5 runs.

S. C. Sherwood, S. Bony, and J.-L. Dufresne. Spread in model climate sensitivity traced to atmospheric convective mixing. Nature, 505:37-42, January 2014. [ bib | DOI | ADS link ]

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

O. Boucher, P. M. Forster, N. Gruber, H.-D. Minh, M. G. Lawrence, T. M. Lenton, A. Maas, and N. E. Vaughan. Rethinking climate engineering categorization in the context of climate change mitigation and adaptation. Wiley Interdisciplinary Reviews: Climate Change, Volume 5, Issue 1, pages 23â#128#14735, 5, January 2014. [ bib | DOI | ADS link ]

M. Benetti, G. Reverdin, C. Pierre, L. Merlivat, C. Risi, H. C. Steen-Larsen, and F. Vimeux. Deuterium excess in marine water vapor: Dependency on relative humidity and surface wind speed during evaporation. Journal of Geophysical Research (Atmospheres), 119:584-593, January 2014. [ bib | DOI | ADS link ]

We provide the first continuous measurements of isotopic composition (δD and δ18O) of water vapor over the subtropical Eastern North Atlantic Ocean from mid-August to mid-September 2012. The ship was located mostly around 26degN, 35degW where evaporation exceeded by far precipitation and water vapor at 20 m largely originated from surface evaporation. The only large deviations from that occurred during a 2 day period in the vicinity of a weak low-pressure system. The continuous measurements were used to investigate deuterium excess (d-excess) relation to evaporation. During 25 days d-excess was negatively correlated with relative humidity (r2 = 0.89). Moreover, d-excess estimated in an evaporative model with a closure assumption reproduced most of the observed variability. From these observations, the d-excess parameter seems to be a good indicator of evaporative conditions. We also conclude that in this region, d-excess into the marine boundary layer is less affected by mixing with the free troposphere than the isotopic composition. From our data, the transition from smooth to rough regime at the ocean surface is associated with a d-excess decrease of 5, which suggests the importance of the ocean surface roughness in controlling d-excess in this region.

J. C. Moore, A. Rinke, X. Yu, D. Ji, X. Cui, Y. Li, K. Alterskjær, J. E. Kristjánsson, H. Muri, O. Boucher, N. Huneeus, B. Kravitz, A. Robock, U. Niemeier, M. Schulz, S. Tilmes, S. Watanabe, and S. Yang. Arctic sea ice and atmospheric circulation under the GeoMIP G1 scenario. Journal of Geophysical Research (Atmospheres), 119:567-583, January 2014. [ bib | DOI | ADS link ]

We analyze simulated sea ice changes in eight different Earth System Models that have conducted experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP). The simulated response of balancing abrupt quadrupling of CO2 (abrupt4xCO2) with reduced shortwave radiation successfully moderates annually averaged Arctic temperature rise to about 1degC, with modest changes in seasonal sea ice cycle compared with the preindustrial control simulations (piControl). Changes in summer and autumn sea ice extent are spatially correlated with temperature patterns but much less in winter and spring seasons. However, there are changes of 20% in sea ice concentration in all seasons, and these will induce changes in atmospheric circulation patterns. In summer and autumn, the models consistently simulate less sea ice relative to preindustrial simulations in the Beaufort, Chukchi, East Siberian, and Laptev Seas, and some models show increased sea ice in the Barents/Kara Seas region. Sea ice extent increases in the Greenland Sea, particularly in winter and spring and is to some extent associated with changed sea ice drift. Decreased sea ice cover in winter and spring in the Barents Sea is associated with increased cyclonic activity entering this area under G1. In comparison, the abrupt4xCO2 experiment shows almost total sea ice loss in September and strong correlation with regional temperatures in all seasons consistent with open ocean conditions. The tropospheric circulation displays a Pacific North America pattern-like anomaly with negative phase in G1-piControl and positive phase under abrupt4xCO2-piControl.

A. Bodas-Salcedo, K. D. Williams, M. A. Ringer, I. Beau, J. N. S. Cole, J.-L. Dufresne, T. Koshiro, B. Stevens, Z. Wang, and T. Yokohata. Origins of the Solar Radiation Biases over the Southern Ocean in CFMIP2 Models*. Journal of Climate, 27:41-56, January 2014. [ bib | DOI | ADS link ]