2015 .

(54 publications)

F. Hourdin, A. Gainusa-Bogdan, P. Braconnot, J.-L. Dufresne, A.-K. Traore, and C. Rio. Air moisture control on ocean surface temperature, hidden key to the warm bias enigma. Geophysical Research Letters, 42:10, December 2015. [ bib | DOI | ADS link ]

The systematic overestimation by climate models of the surface temperature over the eastern tropical oceans is generally attributed to an insufficient oceanic cooling or to an underestimation of stratocumulus clouds. We show that surface evaporation contributes as much as clouds to the dispersion of the warm bias intensity in a multimodel simulations ensemble. The models with the largest warm biases are those with the highest surface heating by radiation and lowest evaporative cooling in atmospheric simulations with prescribed sea surface temperatures. Surface evaporation also controls the amplitude of the surface temperature response to this overestimated heating, when the atmosphere is coupled to an ocean. Evaporation increases with temperature both because of increasing saturation humidity and of an unexpected drying of the near-surface air. Both the origin of the bias and this temperature adjustment point to the key role of near-surface relative humidity and its control by the atmospheric model.

D. Coppin and S. Bony. Physical mechanisms controlling the initiation of convective self-aggregation in a General Circulation Model. Journal of Advances in Modeling Earth Systems, 7:2060-2078, December 2015. [ bib | DOI | ADS link ]

Cloud-resolving models have shown that under certain conditions, the Radiative-Convective Equilibrium (RCE) could become unstable and lead to the spontaneous organization of the atmosphere into dry and wet areas, and the aggregation of convection. In this study, we show that this “self-aggregation” behavior also occurs in nonrotating RCE simulations performed with the IPSL-CM5A-LR General Circulation Model (GCM), and that it exhibits a strong dependence on sea surface temperature (SST). We investigate the physical mechanisms that control the initiation of self-aggregation in this model, and their dependence on temperature. At low SSTs, the onset of self-aggregation is primarily controlled by the coupling between low-cloud radiative effects and shallow circulations and the formation of “radiatively driven cold pools” in areas devoid of deep convection, while at high SSTs it is primarily controlled by the coupling between surface fluxes and circulation within convective areas. At intermediate temperatures, the occurrence of self-aggregation is less spontaneous and depends on initial conditions, but it can arise through a combination of both mechanisms. Through their coupling to circulation and surface fluxes, the radiative effects of low-level clouds play a critical role in both initiation mechanisms, and the sensitivity of boundary layer clouds to surface temperature explains to a large extent the temperature dependence of convective self-aggregation. At any SST, the presence of cloud-radiative effects in the free troposphere is necessary to the initiation, growth, and maintenance of convective aggregation.

J. Zhang, L. Li, Z. Wu, and X. Li. Prolonged dry spells in recent decades over north-central China and their association with a northward shift in planetary waves. International Journal of Climatology, 35:4829-4842, December 2015. [ bib | DOI | ADS link ]

R. Wang, Y. Balkanski, L. Bopp, O. Aumont, O. Boucher, P. Ciais, M. Gehlen, J. Peñuelas, C. Ethé, D. Hauglustaine, B. Li, J. Liu, F. Zhou, and S. Tao. Influence of anthropogenic aerosol deposition on the relationship between oceanic productivity and warming. Geophysical Research Letters, 42:10, December 2015. [ bib | DOI | ADS link ]

Satellite data and models suggest that oceanic productivity is reduced in response to less nutrient supply under warming. In contrast, anthropogenic aerosols provide nutrients and exert a fertilizing effect, but its contribution to evolution of oceanic productivity is unknown. We simulate the response of oceanic biogeochemistry to anthropogenic aerosols deposition under varying climate from 1850 to 2010. We find a positive response of observed chlorophyll to deposition of anthropogenic aerosols. Our results suggest that anthropogenic aerosols reduce the sensitivity of oceanic productivity to warming from -15.2 1.8 to -13.3 1.6 Pg C yr-1 degC-1 in global stratified oceans during 1948-2007. The reducing percentage over the North Atlantic, North Pacific, and Indian Oceans reaches 40, 24, and 25%, respectively. We hypothesize that inevitable reduction of aerosol emissions in response to higher air quality standards in the future might accelerate the decline of oceanic productivity per unit warming.

W. May, A. Meier, M. Rummukainen, A. Berg, F. Chéruy, and S. Hagemann. Contributions of soil moisture interactions to climate change in the tropics in the GLACE-CMIP5 experiment. Climate Dynamics, 45:3275-3297, December 2015. [ bib | DOI | ADS link ]

Contributions of changes in soil moisture to the projected climate change in the tropics at the end of the twenty first century are quantified using the simulations from five different global climate models, which contributed to the GLACE-CMIP5 experiment. “GLACE” refers to the Global Land Atmosphere Coupling Experiment and “CMIP5” to the fifth phase of the Coupled Model Intercomparison Project. This is done by relating the overall projected changes in climate to those changes in climate that are related to the projected changes in soil moisture. The study focusses on two particular aspects of the interactions of the soil moisture with climate, the soil moisture-temperature coupling and the soil moisture-precipitation coupling. The simulations show distinct future changes in soil moisture content in the tropics, with a general tendency of increases in the central parts of the tropics and decreases in the subtropics. These changes are associated with corresponding changes in precipitation, with an overall tendency of an approximate 5 % change in soil moisture in response to a precipitation change of 1 mm/day. All five individual models are characterized by the same qualitative behaviour, despite differences in the strength and in the robustness of the coupling between soil moisture and precipitation. The changes in soil moisture content are found to give important contributions to the overall climate change in the tropics. This is in particularly the case for latent and sensible heat flux, for which about 80 % of the overall changes are related to soil moisture changes. Similarly, about 80 % of the overall near-surface temperature changes (with the mean temperature changes in the tropics removed) are associated with soil moisture changes. For precipitation, on the other hand, about 30-40 % of the overall change can be attributed to soil moisture changes. The robustness of the contributions of the soil moisture changes to the overall climate change varies between the different meteorological variables, with a high degree of robustness for the surface energy fluxes, a fair degree for near-surface temperature and a low degree for precipitation. Similar to the coupling between soil moisture and precipitation, the five individual models are characterized by the same qualitative behaviour, albeit differences in the strength and the robustness of the contributions of the soil moisture change. This suggests that despite the regional differences in the projected climate changes between the individual models, the basic physical mechanisms governing the soil moisture-temperature coupling and the soil moisture-precipitation coupling work similarly in these models. The experiment confirms the conceptual models of the soil moisture-temperature coupling and the soil moisture-precipitation coupling described Seneviratne et al. (Earth-Sci Rev 99:125-161, 2010). For the soil moisture-temperature coupling, decreases (increases) in soil moisture lead to increasing (decreasing) sensible heat fluxes and near-surface temperatures. The soil moisture-precipitation coupling is part of a positive feedback loop, where increases (decreases) in precipitation cause increases (decreases) in soil moisture content, which, in turn, lead to increasing (decreasing) latent heat fluxes and precipitation.

Z. Jiang, W. Li, J. Xu, and L. Li. Extreme Precipitation Indices over China in CMIP5 Models. Part I: Model Evaluation. Journal of Climate, 28:8603-8619, November 2015. [ bib | DOI | ADS link ]

H. Eskes, V. Huijnen, A. Arola, A. Benedictow, A.-M. Blechschmidt, E. Botek, O. Boucher, I. Bouarar, S. Chabrillat, E. Cuevas, R. Engelen, H. Flentje, A. Gaudel, J. Griesfeller, L. Jones, J. Kapsomenakis, E. Katragkou, S. Kinne, B. Langerock, M. Razinger, A. Richter, M. Schultz, M. Schulz, N. Sudarchikova, V. Thouret, M. Vrekoussis, A. Wagner, and C. Zerefos. Validation of reactive gases and aerosols in the MACC global analysis and forecast system. Geoscientific Model Development, 8:3523-3543, November 2015. [ bib | DOI | ADS link ]

The European MACC (Monitoring Atmospheric Composition and Climate) project is preparing the operational Copernicus Atmosphere Monitoring Service (CAMS), one of the services of the European Copernicus Programme on Earth observation and environmental services. MACC uses data assimilation to combine in situ and remote sensing observations with global and regional models of atmospheric reactive gases, aerosols, and greenhouse gases, and is based on the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts (ECMWF). The global component of the MACC service has a dedicated validation activity to document the quality of the atmospheric composition products. In this paper we discuss the approach to validation that has been developed over the past 3 years. Topics discussed are the validation requirements, the operational aspects, the measurement data sets used, the structure of the validation reports, the models and assimilation systems validated, the procedure to introduce new upgrades, and the scoring methods. One specific target of the MACC system concerns forecasting special events with high-pollution concentrations. Such events receive extra attention in the validation process. Finally, a summary is provided of the results from the validation of the latest set of daily global analysis and forecast products from the MACC system reported in November 2014.

C. Reutenauer, A. Landais, T. Blunier, C. Bréant, M. Kageyama, M.-N. Woillez, C. Risi, V. Mariotti, and P. Braconnot. Quantifying molecular oxygen isotope variations during a Heinrich stadial. Climate of the Past, 11:1527-1551, November 2015. [ bib | DOI | ADS link ]

δ18O of atmospheric oxygen (δ18Oatm) undergoes millennial-scale variations during the last glacial period, and systematically increases during Heinrich stadials (HSs). Changes in δ18Oatm combine variations in biospheric and water cycle processes. The identification of the main driver of the millennial variability in δ18Oatm is thus not straightforward. Here, we quantify the response of δ18Oatm to such millennial events using a freshwater hosing simulation performed under glacial boundary conditions. Our global approach takes into account the latest estimates of isotope fractionation factor for respiratory and photosynthetic processes and make use of atmospheric water isotope and vegetation changes. Our modeling approach allows to reproduce the main observed features of a HS in terms of climatic conditions, vegetation distribution and δ18O of precipitation. We use it to decipher the relative importance of the different processes behind the observed changes in δ18Oatm. The results highlight the dominant role of hydrology on δ18Oatm and confirm that δ18Oatm can be seen as a global integrator of hydrological changes over vegetated areas.

O. A. Tuinenburg, C. Risi, J. L. Lacour, M. Schneider, A. Wiegele, J. Worden, N. Kurita, J. P. Duvel, N. Deutscher, S. Bony, P. F. Coheur, and C. Clerbaux. Moist processes during MJO events as diagnosed from water isotopic measurements from the IASI satellite. Journal of Geophysical Research (Atmospheres), 120:10, October 2015. [ bib | DOI | ADS link ]

This study aims to investigate some characteristics of the moist processes of the Madden-Julian oscillation (MJO), by making use of joint HDO (or δD) and H2O vapor measurements. The MJO is the main intraseasonal mode of the tropical climate but is hard to properly simulate in global atmospheric models. The joint use of δD-H2O diagnostics yields additional information compared to sole humidity measurements. We use midtropospheric Infrared Atmospheric Sounding Interferometer (IASI) satellite δD and H2O measurements to determine the mean MJO humidity and δD evolution. Moreover, by making use of high temporal resolution data, we determine the variability in this evolution during about eight MJO events from 2010 to 2012 (including those monitored during the DYNAMO (the Dynamics of the MJO), CINDY (Cooperative Indian Ocean Experiment in Y2011) campaign). These data have a higher spatiotemporal coverage than previous δD measurements, enabling the sampling of individual MJO events. IASI measurements over the Indian Ocean confirm earlier findings that the moistening before the precipitation peak of an MJO event is due to water vapor slightly enriched in HDO. There is then a HDO depletion around the precipitation peak that also corresponds to the moister environment. Most interevent variability determined in the current study occurs 5 to 10 days after the MJO event. In 75% of the events, humidity decreases while the atmosphere remains depleted. In a quarter of the events, humidity increases simultaneously with an increase in δD. After this, the advection of relatively dry and enriched air brings back the state to the mean. Over the maritime continent, δD-H2O cycles are more variable on time scales shorter than the MJO and the interevent variability is larger than over the Indian Ocean. The sequence of moistening and drying processes as revealed by the q-δD cycles can be used as a benchmark to evaluate the representation of moist processes in models. This is done here by comparing observations to simulations of the isotope enabled LMDZ (Laboratoire de Météorologie Dynamique Zoom) global climate model nudged with reanalysis wind fields. These simulations also give information to investigate possible physical origins of the observed q-δD cycles.

B. Kravitz, A. Robock, S. Tilmes, O. Boucher, J. M. English, P. J. Irvine, A. Jones, M. G. Lawrence, M. MacCracken, H. Muri, J. C. Moore, U. Niemeier, S. J. Phipps, J. Sillmann, T. Storelvmo, H. Wang, and S. Watanabe. The Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6): simulation design and preliminary results. Geoscientific Model Development, 8:3379-3392, October 2015. [ bib | DOI | ADS link ]

We present a suite of new climate model experiment designs for the Geoengineering Model Intercomparison Project (GeoMIP). This set of experiments, named GeoMIP6 (to be consistent with the Coupled Model Intercomparison Project Phase 6), builds on the previous GeoMIP project simulations, and has been expanded to address several further important topics, including key uncertainties in extreme events, the use of geoengineering as part of a portfolio of responses to climate change, and the relatively new idea of cirrus cloud thinning to allow more longwave radiation to escape to space. We discuss experiment designs, as well as the rationale for those designs, showing preliminary results from individual models when available. We also introduce a new feature, called the GeoMIP Testbed, which provides a platform for simulations that will be performed with a few models and subsequently assessed to determine whether the proposed experiment designs will be adopted as core (Tier 1) GeoMIP experiments. This is meant to encourage various stakeholders to propose new targeted experiments that address their key open science questions, with the goal of making GeoMIP more relevant to a broader set of communities.

T. Dubos, S. Dubey, M. Tort, R. Mittal, Y. Meurdesoif, and F. Hourdin. DYNAMICO-1.0, an icosahedral hydrostatic dynamical core designed for consistency and versatility. Geoscientific Model Development, 8:3131-3150, October 2015. [ bib | DOI | ADS link ]

The design of the icosahedral dynamical core DYNAMICO is presented. DYNAMICO solves the multi-layer rotating shallow-water equations, a compressible variant of the same equivalent to a discretization of the hydrostatic primitive equations in a Lagrangian vertical coordinate, and the primitive equations in a hybrid mass-based vertical coordinate. The common Hamiltonian structure of these sets of equations is exploited to formulate energy-conserving spatial discretizations in a unified way. <BR /><BR /> The horizontal mesh is a quasi-uniform icosahedral C-grid obtained by subdivision of a regular icosahedron. Control volumes for mass, tracers and entropy/potential temperature are the hexagonal cells of the Voronoi mesh to avoid the fast numerical modes of the triangular C-grid. The horizontal discretization is that of Ringler et al. (2010), whose discrete quasi-Hamiltonian structure is identified. The prognostic variables are arranged vertically on a Lorenz grid with all thermodynamical variables collocated with mass. The vertical discretization is obtained from the three-dimensional Hamiltonian formulation. Tracers are transported using a second-order finite-volume scheme with slope limiting for positivity. Explicit Runge-Kutta time integration is used for dynamics, and forward-in-time integration with horizontal/vertical splitting is used for tracers. Most of the model code is common to the three sets of equations solved, making it easier to develop and validate each piece of the model separately. <BR /><BR /> Representative three-dimensional test cases are run and analyzed, showing correctness of the model. The design permits to consider several extensions in the near future, from higher-order transport to more general dynamics, especially deep-atmosphere and non-hydrostatic equations.

A. Cauquoin, P. Jean-Baptiste, C. Risi, É. Fourré, B. Stenni, and A. Landais. The global distribution of natural tritium in precipitation simulated with an Atmospheric General Circulation Model and comparison with observations. Earth and Planetary Science Letters, 427:160-170, October 2015. [ bib | DOI | ADS link ]

The description of the hydrological cycle in Atmospheric General Circulation Models (GCMs) can be validated using water isotopes as tracers. Many GCMs now simulate the movement of the stable isotopes of water, but here we present the first GCM simulations modelling the content of natural tritium in water. These simulations were obtained using a version of the LMDZ General Circulation Model enhanced by water isotopes diagnostics, LMDZ-iso. To avoid tritium generated by nuclear bomb testing, the simulations have been evaluated against a compilation of published tritium datasets dating from before 1950, or measured recently. LMDZ-iso correctly captures the observed tritium enrichment in precipitation as oceanic air moves inland (the so-called continental effect) and the observed north-south variations due to the latitudinal dependency of the cosmogenic tritium production rate. The seasonal variability, linked to the stratospheric intrusions of air masses with higher tritium content into the troposphere, is correctly reproduced for Antarctica with a maximum in winter. LMDZ-iso reproduces the spring maximum of tritium over Europe, but underestimates it and produces a peak in winter that is not apparent in the data. This implementation of tritium in a GCM promises to provide a better constraint on: (1) the intrusions and transport of air masses from the stratosphere, and (2) the dynamics of the modelled water cycle. The method complements the existing approach of using stable water isotopes.

S. Jasechko, A. Lechler, F. S. R. Pausata, P. J. Fawcett, T. Gleeson, D. I. Cendón, J. Galewsky, A. N. LeGrande, C. Risi, Z. D. Sharp, J. M. Welker, M. Werner, and K. Yoshimura. Late-glacial to late-Holocene shifts in global precipitation δ18O. Climate of the Past, 11:1375-1393, October 2015. [ bib | DOI | ADS link ]

Reconstructions of Quaternary climate are often based on the isotopic content of paleo-precipitation preserved in proxy records. While many paleo-precipitation isotope records are available, few studies have synthesized these dispersed records to explore spatial patterns of late-glacial precipitation δ18O. Here we present a synthesis of 86 globally distributed groundwater (n = 59), cave calcite (n = 15) and ice core (n = 12) isotope records spanning the late-glacial (defined as ˜ 50 000 to ˜ 20 000 years ago) to the late-Holocene (within the past ˜ 5000 years). We show that precipitation δ18O changes from the late-glacial to the late-Holocene range from -7.1 (δ18Olate-Holocene δ18Olate-glacial) to +1.7 (δ18Olate-glacial δ18Olate-Holocene), with the majority (77 %) of records having lower late-glacial δ18O than late-Holocene δ18O values. High-magnitude, negative precipitation δ18O shifts are common at high latitudes, high altitudes and continental interiors (δ18Olate-Holocene δ18Olate-glacial by more than 3 ). Conversely, low-magnitude, positive precipitation δ18O shifts are concentrated along tropical and subtropical coasts (δ18Olate-glacial δ18Olate-Holocene by less than 2 ). Broad, global patterns of late-glacial to late-Holocene precipitation δ18O shifts suggest that stronger-than-modern isotopic distillation of air masses prevailed during the late-glacial, likely impacted by larger global temperature differences between the tropics and the poles. Further, to test how well general circulation models reproduce global precipitation δ18O shifts, we compiled simulated precipitation δ18O shifts from five isotope-enabled general circulation models simulated under recent and last glacial maximum climate states. Climate simulations generally show better inter-model and model-measurement agreement in temperate regions than in the tropics, highlighting a need for further research to better understand how inter-model spread in convective rainout, seawater δ18O and glacial topography parameterizations impact simulated precipitation δ18O. Future research on paleo-precipitation δ18O records can use the global maps of measured and simulated late-glacial precipitation isotope compositions to target and prioritize field sites.

S. Aït-Mesbah, J. L. Dufresne, F. Cheruy, and F. Hourdin. The role of thermal inertia in the representation of mean and diurnal range of surface temperature in semiarid and arid regions. Geophysical Research Letters, 42:7572-7580, September 2015. [ bib | DOI | ADS link ]

In this article we show the possible role of thermal inertia in the large spread of the simulated surface temperature over arid and semiarid regions among climate models. The surface-atmosphere interactions are investigated based on single-column simulations of the DIurnal land-atmosphere Coupling Experiment and on global simulations. Low values of the surface thermal inertia increase the diurnal temperature range (DTR) and the daytime turbulent heat fluxes. The diurnal response of surface temperature to the thermal inertia is asymmetric between daytime and nighttime, inducing a change in the daily mean surface temperature that can reach several degrees over large areas. This asymmetrical response to thermal inertia is shown to be due to the diurnal contrast of the stability state of the atmospheric boundary layer. Thermal inertia in dry regions plays a critical role on both DTR and surface mean temperature and needs to be carefully represented in land surface models.

G. Asrar, S. Bony, O. Boucher, A. Busalacchi, A. Cazenave, M. Dowell, G. Flato, G. Hegerl, E. Källén, T. Nakajima, A. Ratier, R. Saunders, J. Slingo, B.-J. Sohn, J. Schmetz, B. Stevens, P. Zhang, and F. Zwiers. Climate Symposium 2014: Findings and Recommendations. Bulletin of the American Meteorological Society, 96:ES145-ES147, September 2015. [ bib | DOI | ADS link ]

Y. Li, D. W. J. Thompson, and S. Bony. The Influence of Atmospheric Cloud Radiative Effects on the Large-Scale Atmospheric Circulation. Journal of Climate, 28:7263-7278, September 2015. [ bib | DOI | ADS link ]

L. D. Rotstayn, M. A. Collier, D. T. Shindell, and O. Boucher. Why Does Aerosol Forcing Control Historical Global-Mean Surface Temperature Change in CMIP5 Models? Journal of Climate, 28:6608-6625, September 2015. [ bib | DOI | ADS link ]

A. Lahellec and K. Reimer. A Formal Analysis of the Feedback Concept in Climate Models. Part III: Feedback Dynamics and the Seasonal Cycle in a Floquet Analysis. Journal of Atmospheric Sciences, 72:3574-3596, September 2015. [ bib | DOI | ADS link ]

A. Stohl, B. Aamaas, M. Amann, L. H. Baker, N. Bellouin, T. K. Berntsen, O. Boucher, R. Cherian, W. Collins, N. Daskalakis, M. Dusinska, S. Eckhardt, J. S. Fuglestvedt, M. Harju, C. Heyes, Ø. Hodnebrog, J. Hao, U. Im, M. Kanakidou, Z. Klimont, K. Kupiainen, K. S. Law, M. T. Lund, R. Maas, C. R. MacIntosh, G. Myhre, S. Myriokefalitakis, D. Olivié, J. Quaas, B. Quennehen, J.-C. Raut, S. T. Rumbold, B. H. Samset, M. Schulz, Ø. Seland, K. P. Shine, R. B. Skeie, S. Wang, K. E. Yttri, and T. Zhu. Evaluating the climate and air quality impacts of short-lived pollutants. Atmospheric Chemistry & Physics, 15:10529-10566, September 2015. [ bib | DOI | ADS link ]

This paper presents a summary of the work done within the European Union's Seventh Framework Programme project ECLIPSE (Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants). ECLIPSE had a unique systematic concept for designing a realistic and effective mitigation scenario for short-lived climate pollutants (SLCPs; methane, aerosols and ozone, and their precursor species) and quantifying its climate and air quality impacts, and this paper presents the results in the context of this overarching strategy. The first step in ECLIPSE was to create a new emission inventory based on current legislation (CLE) for the recent past and until 2050. Substantial progress compared to previous work was made by including previously unaccounted types of sources such as flaring of gas associated with oil production, and wick lamps. These emission data were used for present-day reference simulations with four advanced Earth system models (ESMs) and six chemistry transport models (CTMs). The model simulations were compared with a variety of ground-based and satellite observational data sets from Asia, Europe and the Arctic. It was found that the models still underestimate the measured seasonality of aerosols in the Arctic but to a lesser extent than in previous studies. Problems likely related to the emissions were identified for northern Russia and India, in particular. To estimate the climate impacts of SLCPs, ECLIPSE followed two paths of research: the first path calculated radiative forcing (RF) values for a large matrix of SLCP species emissions, for different seasons and regions independently. Based on these RF calculations, the Global Temperature change Potential metric for a time horizon of 20 years (GTP20) was calculated for each SLCP emission type. This climate metric was then used in an integrated assessment model to identify all emission mitigation measures with a beneficial air quality and short-term (20-year) climate impact. These measures together defined a SLCP mitigation (MIT) scenario. Compared to CLE, the MIT scenario would reduce global methane (CH4) and black carbon (BC) emissions by about 50 and 80 %, respectively. For CH4, measures on shale gas production, waste management and coal mines were most important. For non-CH4 SLCPs, elimination of high-emitting vehicles and wick lamps, as well as reducing emissions from gas flaring, coal and biomass stoves, agricultural waste, solvents and diesel engines were most important. These measures lead to large reductions in calculated surface concentrations of ozone and particulate matter. We estimate that in the EU, the loss of statistical life expectancy due to air pollution was 7.5 months in 2010, which will be reduced to 5.2 months by 2030 in the CLE scenario. The MIT scenario would reduce this value by another 0.9 to 4.3 months. Substantially larger reductions due to the mitigation are found for China (1.8 months) and India (11-12 months). The climate metrics cannot fully quantify the climate response. Therefore, a second research path was taken. Transient climate ensemble simulations with the four ESMs were run for the CLE and MIT scenarios, to determine the climate impacts of the mitigation. In these simulations, the CLE scenario resulted in a surface temperature increase of 0.70 0.14 K between the years 2006 and 2050. For the decade 2041-2050, the warming was reduced by 0.22 0.07 K in the MIT scenario, and this result was in almost exact agreement with the response calculated based on the emission metrics (reduced warming of 0.22 0.09 K). The metrics calculations suggest that non-CH4 SLCPs contribute ˜ 22 % to this response and CH4 78 %. This could not be fully confirmed by the transient simulations, which attributed about 90 % of the temperature response to CH4 reductions. Attribution of the observed temperature response to non-CH4 SLCP emission reductions and BC specifically is hampered in the transient simulations by small forcing and co-emitted species of the emission basket chosen. Nevertheless, an important conclusion is that our mitigation basket as a whole would lead to clear benefits for both air quality and climate. The climate response from BC reductions in our study is smaller than reported previously, possibly because our study is one of the first to use fully coupled climate models, where unforced variability and sea ice responses cause relatively strong temperature fluctuations that may counteract (and, thus, mask) the impacts of small emission reductions. The temperature responses to the mitigation were generally stronger over the continents than over the oceans, and with a warming reduction of 0.44 K (0.39-0.49) K the largest over the Arctic. Our calculations suggest particularly beneficial climate responses in southern Europe, where surface warming was reduced by about 0.3 K and precipitation rates were increased by about 15 (6-21) mm yr-1 (more than 4 % of total precipitation) from spring to autumn. Thus, the mitigation could help to alleviate expected future drought and water shortages in the Mediterranean area. We also report other important results of the ECLIPSE project.

V. Masson-Delmotte, H. C. Steen-Larsen, P. Ortega, D. Swingedouw, T. Popp, B. M. Vinther, H. Oerter, A. E. Sveinbjornsdottir, H. Gudlaugsdottir, J. E. Box, S. Falourd, X. Fettweis, H. Gallée, E. Garnier, V. Gkinis, J. Jouzel, A. Landais, B. Minster, N. Paradis, A. Orsi, C. Risi, M. Werner, and J. W. C. White. Recent changes in north-west Greenland climate documented by NEEM shallow ice core data and simulations, and implications for past-temperature reconstructions. The Cryosphere, 9:1481-1504, August 2015. [ bib | DOI | ADS link ]

Combined records of snow accumulation rate, δ18O and deuterium excess were produced from several shallow ice cores and snow pits at NEEM (North Greenland Eemian Ice Drilling), covering the period from 1724 to 2007. They are used to investigate recent climate variability and characterise the isotope-temperature relationship. We find that NEEM records are only weakly affected by inter-annual changes in the North Atlantic Oscillation. Decadal δ18O and accumulation variability is related to North Atlantic sea surface temperature and is enhanced at the beginning of the 19th century. No long-term trend is observed in the accumulation record. By contrast, NEEM δ18O shows multidecadal increasing trends in the late 19th century and since the 1980s. The strongest annual positive δ18O values are recorded at NEEM in 1928 and 2010, while maximum accumulation occurs in 1933. The last decade is the most enriched in δ18O (warmest), while the 11-year periods with the strongest depletion (coldest) are depicted at NEEM in 1815-1825 and 1836-1846, which are also the driest 11-year periods. The NEEM accumulation and δ18O records are strongly correlated with outputs from atmospheric models, nudged to atmospheric reanalyses. Best performance is observed for ERA reanalyses. Gridded temperature reconstructions, instrumental data and model outputs at NEEM are used to estimate the multidecadal accumulation-temperature and δ18O-temperature relationships for the strong warming period in 1979-2007. The accumulation sensitivity to temperature is estimated at 11 2 % degC-1 and the δ18O-temperature slope at 1.1 0.2 degC-1, about twice as large as previously used to estimate last interglacial temperature change from the bottom part of the NEEM deep ice core.

J. Zhang, L. Li, D. Li, and W. Deng. Summer droughts in the northern Yellow River basin in association with recent Arctic ice loss. International Journal of Climatology, 35:2849-2859, August 2015. [ bib | DOI | ADS link ]

V. N. Aswathy, O. Boucher, M. Quaas, U. Niemeier, H. Muri, J. Mülmenstädt, and J. Quaas. Climate extremes in multi-model simulations of stratospheric aerosol and marine cloud brightening climate engineering. Atmospheric Chemistry & Physics, 15:9593-9610, August 2015. [ bib | DOI | ADS link ]

Simulations from a multi-model ensemble for the RCP4.5 climate change scenario for the 21st century, and for two solar radiation management (SRM) schemes (stratospheric sulfate injection (G3), SULF and marine cloud brightening by sea salt emission SALT) have been analysed in terms of changes in the mean and extremes of surface air temperature and precipitation. The climate engineering and termination periods are investigated. During the climate engineering period, both schemes, as intended, offset temperature increases by about 60 % globally, but are more effective in the low latitudes and exhibit some residual warming in the Arctic (especially in the case of SALT which is only applied in the low latitudes). In both climate engineering scenarios, extreme temperature changes are similar to the mean temperature changes over much of the globe. The exceptions are the mid- and high latitudes in the Northern Hemisphere, where high temperatures (90th percentile of the distribution) of the climate engineering period compared to RCP4.5 control period rise less than the mean, and cold temperatures (10th percentile), much more than the mean. This aspect of the SRM schemes is also reflected in simulated reduction in the frost day frequency of occurrence for both schemes. However, summer day frequency of occurrence increases less in the SALT experiment than the SULF experiment, especially over the tropics. Precipitation extremes in the two SRM scenarios act differently - the SULF experiment more effectively mitigates extreme precipitation increases over land compared to the SALT experiment. A reduction in dry spell occurrence over land is observed in the SALT experiment. The SULF experiment has a slight increase in the length of dry spells. A strong termination effect is found for the two climate engineering schemes, with large temperature increases especially in the Arctic. Globally, SULF is more effective in reducing extreme temperature increases over land than SALT. Extreme precipitation increases over land is also more reduced in SULF than the SALT experiment. However, globally SALT decreases the frequency of dry spell length and reduces the occurrence of hot days compared to SULF.

F. Couvreux, R. Roehrig, C. Rio, M.-P. Lefebvre, M. Caian, T. Komori, S. Derbyshire, F. Guichard, F. Favot, F. D'Andrea, P. Bechtold, and P. Gentine. Representation of daytime moist convection over the semi-arid Tropics by parametrizations used in climate and meteorological models. Quarterly Journal of the Royal Meteorological Society, 141:2220-2236, July 2015. [ bib | DOI | ADS link ]

C. Muller and S. Bony. What favors convective aggregation and why? Geophysical Research Letters, 42:5626-5634, July 2015. [ bib | DOI | ADS link ]

The organization of convection is ubiquitous, but its physical understanding remains limited. One particular type of organization is the spatial self-aggregation of convection, taking the form of cloud clusters, or tropical cyclones in the presence of rotation. We show that several physical processes can give rise to self-aggregation and highlight the key features responsible for it, using idealized simulations. Longwave radiative feedbacks yield a “radiative aggregation.” In that case, sufficient spatial variability of radiative cooling rates yields a low-level circulation, which induces the upgradient energy transport and radiative-convective instability. Not only do vertically integrated radiative budgets matter but the vertical profile of cooling is also crucial. Convective aggregation is facilitated when downdrafts below clouds are weak (“moisture-memory aggregation”), and this is sufficient to trigger aggregation in the absence of longwave radiative feedbacks. These results shed some light on the sensitivity of self-aggregation to various parameters, including resolution or domain size.

A. Lefauve, C. Muller, and A. Melet. A three-dimensional map of tidal dissipation over abyssal hills. Journal of Geophysical Research (Oceans), 120:4760-4777, July 2015. [ bib | DOI | ADS link ]

The breaking of internal tides is believed to provide a large part of the power needed to mix the abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale abyssal hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale abyssal hill bathymetry, stratification, and tidal data. We show that a large fraction of abyssal-hill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to abyssal hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved abyssal hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.

R. Pincus, E. J. Mlawer, L. Oreopoulos, A. S. Ackerman, S. Baek, M. Brath, S. A. Buehler, K. E. Cady-Pereira, J. N. S. Cole, J.-L. Dufresne, M. Kelley, J. Li, J. Manners, D. J. Paynter, R. Roehrig, M. Sekiguchi, and D. M. Schwarzkopf. Radiative flux and forcing parameterization error in aerosol-free clear skies. Geophysical Research Letters, 42:5485-5492, July 2015. [ bib | DOI | ADS link ]

This article reports on the accuracy in aerosol- and cloud-free conditions of the radiation parameterizations used in climate models. Accuracy is assessed relative to observationally validated reference models for fluxes under present-day conditions and forcing (flux changes) from quadrupled concentrations of carbon dioxide. Agreement among reference models is typically within 1 W/m2, while parameterized calculations are roughly half as accurate in the longwave and even less accurate, and more variable, in the shortwave. Absorption of shortwave radiation is underestimated by most parameterizations in the present day and has relatively large errors in forcing. Error in present-day conditions is essentially unrelated to error in forcing calculations. Recent revisions to parameterizations have reduced error in most cases. A dependence on atmospheric conditions, including integrated water vapor, means that global estimates of parameterization error relevant for the radiative forcing of climate change will require much more ambitious calculations.

J.-F. Rysman, S. Verrier, A. Lahellec, and C. Genthon. Analysis of Boundary-Layer Statistical Properties at Dome C, Antarctica. Boundary-Layer Meteorology, 156:145-155, July 2015. [ bib | DOI | ADS link ]

The atmospheric boundary layer over the Antarctic Plateau is unique on account of its isolated location and extreme stability. Here we investigate the characteristics of the boundary layer using wind and temperature measurements from a 45-m high tower located at Dome C. First, spectral analysis reveals that both fields have a scaling behaviour from 30 min to 10 days (spectral slope ). Wind and temperature time series also show a multifractal behaviour. Therefore, it is possible to fit the moment-scaling function to the universal multifractal model and obtain multifractal parameters for temperature () and wind speed (). The same analysis is repeated separately in winter and summer at six different heights. The parameter shows a strong stratification with height especially in summer, implying that properties of turbulence change surprisingly rapidly from the ground to the top of the tower.

F. Hourdin, M. Gueye, B. Diallo, J.-L. Dufresne, J. Escribano, L. Menut, B. Marticoréna, G. Siour, and F. Guichard. Parameterization of convective transport in the boundary layer and its impact on the representation of the diurnal cycle of wind and dust emissions. Atmospheric Chemistry & Physics, 15:6775-6788, June 2015. [ bib | DOI | ADS link ]

We investigate how the representation of the boundary layer in a climate model impacts the representation of the near-surface wind and dust emission, with a focus on the Sahel/Sahara region. We show that the combination of vertical turbulent diffusion with a representation of the thermal cells of the convective boundary layer by a mass flux scheme leads to realistic representation of the diurnal cycle of wind in spring, with a maximum near-surface wind in the morning. This maximum occurs when the thermal plumes reach the low-level jet that forms during the night at a few hundred meters above surface. The horizontal momentum in the jet is transported downward to the surface by compensating subsidence around thermal plumes in typically less than 1 h. This leads to a rapid increase of wind speed at surface and therefore of dust emissions owing to the strong nonlinearity of emission laws. The numerical experiments are performed with a zoomed and nudged configuration of the LMDZ general circulation model coupled to the emission module of the CHIMERE chemistry transport model, in which winds are relaxed toward that of the ERA-Interim reanalyses. The new set of parameterizations leads to a strong improvement of the representation of the diurnal cycle of wind when compared to a previous version of LMDZ as well as to the reanalyses used for nudging themselves. It also generates dust emissions in better agreement with current estimates, but the aerosol optical thickness is still significantly underestimated.

J. A. Day, I. Fung, and C. Risi. Coupling of South and East Asian Monsoon Precipitation in July-August*. Journal of Climate, 28:4330-4356, June 2015. [ bib | DOI | ADS link ]

R. Wang, Y. Balkanski, O. Boucher, L. Bopp, A. Chappell, P. Ciais, D. Hauglustaine, J. Peñuelas, and S. Tao. Sources, transport and deposition of iron in the global atmosphere. Atmospheric Chemistry & Physics, 15:6247-6270, June 2015. [ bib | DOI | ADS link ]

Atmospheric deposition of iron (Fe) plays an important role in controlling oceanic primary productivity. However, the sources of Fe in the atmosphere are not well understood. In particular, the combustion sources of Fe and the subsequent deposition to the oceans have been accounted for in only few ocean biogeochemical models of the carbon cycle. Here we used a mass-balance method to estimate the emissions of Fe from the combustion of fossil fuels and biomass by accounting for the Fe contents in fuel and the partitioning of Fe during combustion. The emissions of Fe attached to aerosols from combustion sources were estimated by particle size, and their uncertainties were quantified by a Monte Carlo simulation. The emissions of Fe from mineral sources were estimated using the latest soil mineralogical database to date. As a result, the total Fe emissions from combustion averaged for 1960-2007 were estimated to be 5.3 Tg yr-1 (90% confidence of 2.3 to 12.1). Of these emissions, 1, 27 and 72% were emitted in particles 1 μm (PM1), 1-10 μm (PM1-10), and 10 μm (PM> 10), respectively, compared to a total Fe emission from mineral dust of 41.0 Tg yr-1 in a log-normal distribution with a mass median diameter of 2.5 μm and a geometric standard deviation of 2. For combustion sources, different temporal trends were found in fine and medium-to-coarse particles, with a notable increase in Fe emissions in PM1 since 2000 due to an increase in Fe emission from motor vehicles (from 0.008 to 0.0103 Tg yr-1 in 2000 and 2007, respectively). These emissions have been introduced in a global 3-D transport model run at a spatial resolution of 0.94deg latitude by 1.28deg longitude to evaluate our estimation of Fe emissions. The modelled Fe concentrations as monthly means were compared with the monthly (57 sites) or daily (768 sites) measured concentrations at a total of 825 sampling stations. The deviation between modelled and observed Fe concentrations attached to aerosols at the surface was within a factor of 2 at most sampling stations, and the deviation was within a factor of 1.5 at sampling stations dominated by combustion sources. We analysed the relative contribution of combustion sources to total Fe concentrations over different regions of the world. The new mineralogical database led to a modest improvement in the simulation relative to station data even in dust-dominated regions, but could provide useful information on the chemical forms of Fe in dust for coupling with ocean biota models. We estimated a total Fe deposition sink of 8.4 Tg yr-1 over global oceans, 7% of which originated from the combustion sources. Our central estimates of Fe emissions from fossil fuel combustion (mainly from coal) are generally higher than those in previous studies, although they are within the uncertainty range of our estimates. In particular, the higher than previously estimated Fe emission from coal combustion implies a larger atmospheric anthropogenic input of soluble Fe to the northern Atlantic and northern Pacific Oceans, which is expected to enhance the biological carbon pump in those regions.

J. Gao, S. S. P. Shen, T. Yao, N. Tafolla, C. Risi, and Y. He. Reconstruction of precipitation δ18O over the Tibetan Plateau since 1910. Journal of Geophysical Research (Atmospheres), 120:4878-4888, May 2015. [ bib | DOI | ADS link ]

An accurate representation of the spatiotemporal variation of stable isotopes in precipitation over the Tibetan Plateau (TP) is critical information for hydrological and ecological applications for the region. This paper reconstructs annual δ18O data with 2.5deg × 3.75deg latitude-longitude resolutions over the TP since 1910 using the spectral optimal gridding (SOG) method. The SOG calculates empirical orthogonal functions (EOFs) from the Laboratoire de Météorologie Dynamique isotopic version general circulation model over the TP from 1978 to 2007 and regresses the δ18O data of 10 ice cores over TP against the EOF basis functions. The reconstructed data can effectively demonstrate spatiotemporal characteristics of TP precipitation δ18O. The spatial average interannual δ18O anomalies agree well with the time series of Guliya ice core δ18O, implying Guliya δ18O's typical representation of the TP δ18O temporal variation.

Y. He, C. Risi, J. Gao, V. Masson-Delmotte, T. Yao, C.-T. Lai, Y. Ding, J. Worden, C. Frankenberg, H. Chepfer, and G. Cesana. Impact of atmospheric convection on south Tibet summer precipitation isotopologue composition using a combination of in situ measurements, satellite data, and atmospheric general circulation modeling. Journal of Geophysical Research (Atmospheres), 120:3852-3871, May 2015. [ bib | DOI | ADS link ]

Precipitation isotopologues recorded in natural archives from the southern Tibetan Plateau may document past variations of Indian monsoon intensity. The exact processes controlling the variability of precipitation isotopologue composition must therefore first be deciphered and understood. This study investigates how atmospheric convection affects the summer variability of δ18O in precipitation (δ18Op) and δD in water vapor (δDv) at the daily scale. This is achieved using isotopic data from precipitation samples at Lhasa, isotopic measurements of water vapor retrieved from satellites (Tropospheric Emission Spectrometer (TES), GOSAT) and atmospheric general circulation modeling. We reveal that both δ18Op and δDv at Lhasa are well correlated with upstream convective activity, especially above northern India. First, during days of strong convection, northern India surface air contains large amounts of vapor with relatively low δDv. Second, when this low-δDv moisture is uplifted toward southern Tibet, this initial depletion in HDO is further amplified by Rayleigh distillation as the vapor moves over the Himalayan. The intraseasonal variability of the isotopologue composition of vapor and precipitation over the southern Tibetan Plateau results from these processes occurring during air mass transportation.

R. Pilon, J.-Y. Grandpeix, and P. Heinrich. Representation of transport and scavenging of trace particlesin the Emanuel moist convection scheme. Quarterly Journal of the Royal Meteorological Society, 141:1244-1258, April 2015. [ bib | DOI | ADS link ]

G. Sèze, J. Pelon, M. Derrien, H. Le Gléau, and B. Six. Evaluation against CALIPSO lidar observations of the multi-geostationary cloud cover and type dataset assembled in the framework of the Megha-Tropiques mission. Quarterly Journal of the Royal Meteorological Society, 141:774-797, April 2015. [ bib | DOI | ADS link ]

S. Bony, B. Stevens, D. M. W. Frierson, C. Jakob, M. Kageyama, R. Pincus, T. G. Shepherd, S. C. Sherwood, A. P. Siebesma, A. H. Sobel, M. Watanabe, and M. J. Webb. Clouds, circulation and climate sensitivity. Nature Geoscience, 8:261-268, April 2015. [ bib | DOI | ADS link ]

Fundamental puzzles of climate science remain unsolved because of our limited understanding of how clouds, circulation and climate interact. One example is our inability to provide robust assessments of future global and regional climate changes. However, ongoing advances in our capacity to observe, simulate and conceptualize the climate system now make it possible to fill gaps in our knowledge. We argue that progress can be accelerated by focusing research on a handful of important scientific questions that have become tractable as a result of recent advances. We propose four such questions below; they involve understanding the role of cloud feedbacks and convective organization in climate, and the factors that control the position, the strength and the variability of the tropical rain belts and the extratropical storm tracks.

J.-L. Bonne, H. C. Steen-Larsen, C. Risi, M. Werner, H. Sodemann, J.-L. Lacour, X. Fettweis, G. Cesana, M. Delmotte, O. Cattani, P. Vallelonga, H. A. Kjær, C. Clerbaux, Á. E. Sveinbjörnsdóttir, and V. Masson-Delmotte. The summer 2012 Greenland heat wave: In situ and remote sensing observations of water vapor isotopic composition during an atmospheric river event. Journal of Geophysical Research (Atmospheres), 120:2970-2989, April 2015. [ bib | DOI | ADS link ]

During 7-12 July 2012, extreme moist and warm conditions occurred over Greenland, leading to widespread surface melt. To investigate the physical processes during the atmospheric moisture transport of this event, we study the water vapor isotopic composition using surface in situ observations in Bermuda Island, South Greenland coast (Ivittuut), and northwest Greenland ice sheet (NEEM), as well as remote sensing observations (Infrared Atmospheric Sounding Interferometer (IASI) instrument on board MetOp-A), depicting propagation of similar surface and midtropospheric humidity and δD signals. Simulations using Lagrangian moisture source diagnostic and water tagging in a regional model showed that Greenland was affected by an atmospheric river transporting moisture from the western subtropical North Atlantic Ocean, which is coherent with observations of snow pit impurities deposited at NEEM. At Ivittuut, surface air temperature, humidity, and δD increases are observed. At NEEM, similar temperature increase is associated with a large and long-lasting 100δD enrichment and 15 deuterium excess decrease, thereby reaching Ivittuut level. We assess the simulation of this event in two isotope-enabled atmospheric general circulation models (LMDz-iso and ECHAM5-wiso). LMDz-iso correctly captures the timing of propagation for this event identified in IASI data but depict too gradual variations when compared to surface data. Both models reproduce the surface meteorological and isotopic values during the event but underestimate the background deuterium excess at NEEM. Cloud liquid water content parametrization in LMDz-iso poorly impacts the vapor isotopic composition. Our data demonstrate that during this atmospheric river event the deuterium excess signal is conserved from the moisture source to northwest Greenland.

B. L'Hévéder, F. Codron, and M. Ghil. Impact of Anomalous Northward Oceanic Heat Transport on Global Climate in a Slab Ocean Setting. Journal of Climate, 28:2650-2664, April 2015. [ bib | DOI | ADS link ]

H. Bellenger, M. Katsumata, and K. Yoneyama. Turbulent mixing and its impact on lower tropospheric moisture over tropical ocean. Geophysical Research Letters, 42:3030-3037, April 2015. [ bib | DOI | ADS link ]

The variability of lower tropospheric humidity is a crucial feature of the tropical climate. Among the processes that impact moisture budget, the vertical transport by turbulent mixing is generally overlooked. Using observations from Cooperative Indian Ocean experiment on intraseasonal variability/Dynamics of the Madden-Julian Oscillation (MJO), CINDY/DYNAMO, campaign, this is a first attempt to quantify it over the tropical ocean. Turbulent patches of ˜100 m depth are observed in relation with large vertical gradients of specific humidity. Intense mixing is diagnosed within these intermittent patches. Three approaches are used to diagnose the overall effect of this intermittent turbulence. Large uncertainties on the corresponding eddy diffusivity coefficient arise from parameters hard to experimentally constrain. However, dry conditions are associated with steep moisture vertical gradients above the boundary layers. Owing to the uncertainties on the eddy diffusivity, these gradients can correspond to negligible or to significant moisture tendencies (˜0.5-1 g kg-1 d-1) during the recovery following a dry intrusion or the preconditioning stage of an MJO.

B. Medeiros, B. Stevens, and S. Bony. Using aquaplanets to understand the robust responses of comprehensive climate models to forcing. Climate Dynamics, 44:1957-1977, April 2015. [ bib | DOI | ADS link ]

Idealized climate change experiments using fixed sea-surface temperature are investigated to determine whether zonally symmetric aquaplanet configurations are useful for understanding climate feedbacks in more realistic configurations. The aquaplanets capture many of the robust responses of the large-scale circulation and hydrologic cycle to both warming the sea-surface temperature and quadrupling atmospheric CO2. The cloud response to both perturbations varies across models in both Earth-like and aquaplanet configurations, and this spread arises primarily from regions of large-scale subsidence. Most models produce a consistent cloud change across the subsidence regimes, and the feedback in trade-wind cumulus regions dominates the tropical response. It is shown that these trade-wind regions have similar cloud feedback in Earth-like and aquaplanet warming experiments. The tropical average cloud feedback of the Earth-like experiment is captured by five of eight aquaplanets, and the three outliers are investigated to understand the discrepancy. In two models, the discrepancy is due to warming induced dissipation of stratocumulus decks in the Earth-like configuration which are not represented in the aquaplanet. One model shows a circulation response in the aquaplanet experiment accompanied by a cloud response that differs from the Earth-like configuration. Quadrupling atmospheric CO2 in aquaplanets produces slightly greater adjusted forcing than in Earth-like configurations, showing that land-surface effects dampen the adjusted forcing. The analysis demonstrates how aquaplanets, as part of a model hierarchy, help elucidate robust aspects of climate change and develop understanding of the processes underlying them.

M. Benetti, G. Aloisi, G. Reverdin, C. Risi, and G. Sèze. Importance of boundary layer mixing for the isotopic composition of surface vapor over the subtropical North Atlantic Ocean. Journal of Geophysical Research (Atmospheres), 120:2190-2209, March 2015. [ bib | DOI | ADS link ]

During the summer 2012, we carried out continuous measurements of the isotopic composition (δ) of water vapor over the near-surface subtropical North Atlantic Ocean (STRASSE cruise). In this region of excess evaporation, we investigate the control of evaporation and mixing with a lower troposphere-derived, isotopically depleted air mass on the near-surface δ. We use a simple model to simulate the near-surface δ as the result of a two end-member mixing of the evaporative flux with free tropospheric air. The evaporative flux δ was estimated by the Craig and Gordon equation while the δ of the lower troposphere was taken from the LMDZ-iso global atmospheric circulation model. This simulation considers instantaneous mixing of lower tropospheric air with the evaporated flux and neglects lateral advection. Despite these simplifications, the simulations allow to identify the controls on the near-surface δ. The d-excess variability is largely a consequence of varying kinetic effects during evaporation, even during a convection event when the input of tropospheric vapor was strong. Kinetic effects and mixing processes affect simultaneously the near-surface δ and result in the vapor occupying distinct domains in the δ18O-δD space. The relative humidity-d-excess relationship shows that the closure assumption overestimates the d-excess variability at short time scales (less than a day). We interpret this as due to an effect of the residence time of the near-surface water vapor on the d-excess. Finally, we highlight the importance of reproducing mixing processes in models simulating isotopes over the subtropical North Atlantic Ocean and propose an extension of the closure assumption for use in initial conditions of distillation calculations.

G. Myhre, O. Boucher, F.-M. Bréon, P. Forster, and D. Shindell. Declining uncertainty in transient climate response as CO2 forcing dominates future climate change. Nature Geoscience, 8:181-185, March 2015. [ bib | DOI | ADS link ]

Carbon dioxide has exerted the largest portion of radiative forcing and surface temperature change over the industrial era, but other anthropogenic influences have also contributed. However, large uncertainties in total forcing make it difficult to derive climate sensitivity from historical observations. Anthropogenic forcing has increased between the Fourth and Fifth Assessment Reports of the Intergovernmental Panel of Climate Change (IPCC; refs , ), although its relative uncertainty has decreased. Here we show, based on data from the two reports, that this evolution towards lower uncertainty can be expected to continue into the future. Because it is easier to reduce air pollution than carbon dioxide emissions and because of the long lifetime of carbon dioxide, the less uncertain carbon dioxide forcing is expected to become increasingly dominant. Using a statistical model, we estimate that the relative uncertainty in anthropogenic forcing of more than 40% quoted in the latest IPCC report for 2011 will be almost halved by 2030, even without better scientific understanding. Absolute forcing uncertainty will also decline for the first time, provided projected decreases in aerosols occur. Other factors being equal, this stronger constraint on forcing will bring a significant reduction in the uncertainty of observation-based estimates of the transient climate response, with a 50% reduction in its uncertainty range expected by 2030.

P. Good, J. A. Lowe, T. Andrews, A. Wiltshire, R. Chadwick, J.-K. Ridley, M. B. Menary, N. Bouttes, J. L. Dufresne, J. M. Gregory, N. Schaller, and H. Shiogama. Corrigendum: Nonlinear regional warming with increasing CO2 concentrations. Nature Climate Change, 5:280, March 2015. [ bib | DOI | ADS link ]

J. H. Jiang, H. Su, C. Zhai, T. Janice Shen, T. Wu, J. Zhang, J. N. S. Cole, K. von Salzen, L. J. Donner, C. Seman, A. Del Genio, L. S. Nazarenko, J.-L. Dufresne, M. Watanabe, C. Morcrette, T. Koshiro, H. Kawai, A. Gettelman, L. Millán, W. G. Read, N. J. Livesey, Y. Kasai, and M. Shiotani. Evaluating the Diurnal Cycle of Upper-Tropospheric Ice Clouds in Climate Models Using SMILES Observations. Journal of Atmospheric Sciences, 72:1022-1044, March 2015. [ bib | DOI | ADS link ]

H. Pang, S. Hou, A. Landais, V. Masson-Delmotte, F. Prie, H. C. Steen-Larsen, C. Risi, Y. Li, J. Jouzel, Y. Wang, J. He, B. Minster, and S. Falourd. Spatial distribution of 17O-excess in surface snow along a traverse from Zhongshan station to Dome A, East Antarctica. Earth and Planetary Science Letters, 414:126-133, March 2015. [ bib | DOI | ADS link ]

The influence of temperature on the triple isotopic composition of oxygen in water is still an open question and limits the interpretation of water isotopic profiles in Antarctic ice cores. The main limitation arises from the lack of 17O-excess measurements in surface snow and especially for remote regions characterized by low temperature and accumulation rate. In this study, we present new 17O-excess measurements of surface snow along an East Antarctic traverse, from the coastal Zhongshan station to the highest point of the Antarctic ice sheet at Dome A. The 17O-excess data significantly decrease inland, with a latitudinal gradient of - 1.33 0.41 per meg/degree, an altitudinal gradient of - 0.48 0.17 permeg / 100 m, and a temperature gradient of 0.35 0.11 permeg /degC. Theoretical calculations performed using a Rayleigh model attribute this inland decrease to kinetic isotopic fractionation occurring during condensation from vapor to ice under supersaturation conditions at low temperatures. However, large heterogeneity of 17O-excess in Antarctic precipitation cannot only be explained by temperature at condensation and/or influences of relative humidity in the moisture source region.

M. J. Webb, A. P. Lock, A. Bodas-Salcedo, S. Bony, J. N. S. Cole, T. Koshiro, H. Kawai, C. Lacagnina, F. M. Selten, R. Roehrig, and B. Stevens. The diurnal cycle of marine cloud feedback in climate models. Climate Dynamics, 44:1419-1436, March 2015. [ bib | DOI | ADS link ]

We examine the diurnal cycle of marine cloud feedback using high frequency outputs in CFMIP-2 idealised uniform +4 K SST perturbation experiments from seven CMIP5 models. Most of the inter-model spread in the diurnal mean marine shortwave cloud feedback can be explained by low cloud responses, although these do not explain the model responses at the neutral/weakly negative end of the feedback range, where changes in mid and high level cloud properties are more important. All of the models show reductions in marine low cloud fraction in the warmer climate, and these are in almost all cases largest in the mornings when more cloud is present in the control simulations. This results in shortwave cloud feedbacks being slightly stronger and having the largest inter-model spread at this time of day. The diurnal amplitudes of the responses of marine cloud properties to the warming climate are however small compared to the inter-model differences in their diurnally meaned responses. This indicates that the diurnal cycle of cloud feedback is not strongly relevant to understanding inter-model spread in overall cloud feedback and climate sensitivity. A number of unusual behaviours in individual models are highlighted for future investigation.

J.-L. Lacour, L. Clarisse, J. Worden, M. Schneider, S. Barthlott, F. Hase, C. Risi, C. Clerbaux, D. Hurtmans, and P.-F. Coheur. Cross-validation of IASI/MetOp derived tropospheric δD with TES and ground-based FTIR observations. Atmospheric Measurement Techniques, 8:1447-1466, March 2015. [ bib | DOI | ADS link ]

The Infrared Atmospheric Sounding Interferometer (IASI) flying onboard MetOpA and MetOpB is able to capture fine isotopic variations of the HDO to H2O ratio (δD) in the troposphere. Such observations at the high spatio-temporal resolution of the sounder are of great interest to improve our understanding of the mechanisms controlling humidity in the troposphere. In this study we aim to empirically assess the validity of our error estimation previously evaluated theoretically. To achieve this, we compare IASI δD retrieved profiles with other available profiles of δD, from the TES infrared sounder onboard AURA and from three ground-based FTIR stations produced within the MUSICA project: the NDACC (Network for the Detection of Atmospheric Composition Change) sites Kiruna and Izaña, and the TCCON site Karlsruhe, which in addition to near-infrared TCCON spectra also records mid-infrared spectra. We describe the achievable level of agreement between the different retrievals and show that these theoretical errors are in good agreement with empirical differences. The comparisons are made at different locations from tropical to Arctic latitudes, above sea and above land. Generally IASI and TES are similarly sensitive to δD in the free troposphere which allows one to compare their measurements directly. At tropical latitudes where IASI's sensitivity is lower than that of TES, we show that the agreement improves when taking into account the sensitivity of IASI in the TES retrieval. For the comparison IASI-FTIR only direct comparisons are performed because the sensitivity profiles of the two observing systems do not allow to take into account their differences of sensitivity. We identify a quasi negligible bias in the free troposphere (-3) between IASI retrieved δD with the TES, which are bias corrected, but important with the ground-based FTIR reaching -47. We also suggest that model-satellite observation comparisons could be optimized with IASI thanks to its high spatial and temporal sampling.

R. Locatelli, P. Bousquet, F. Hourdin, M. Saunois, A. Cozic, F. Couvreux, J.-Y. Grandpeix, M.-P. Lefebvre, C. Rio, P. Bergamaschi, S. D. Chambers, U. Karstens, V. Kazan, S. van der Laan, H. A. J. Meijer, J. Moncrieff, M. Ramonet, H. A. Scheeren, C. Schlosser, M. Schmidt, A. Vermeulen, and A. G. Williams. Atmospheric transport and chemistry of trace gases in LMDz5B: evaluation and implications for inverse modelling. Geoscientific Model Development, 8:129-150, February 2015. [ bib | DOI | ADS link ]

Representation of atmospheric transport is a major source of error in the estimation of greenhouse gas sources and sinks by inverse modelling. Here we assess the impact on trace gas mole fractions of the new physical parameterizations recently implemented in the atmospheric global climate model LMDz to improve vertical diffusion, mesoscale mixing by thermal plumes in the planetary boundary layer (PBL), and deep convection in the troposphere. At the same time, the horizontal and vertical resolution of the model used in the inverse system has been increased. The aim of this paper is to evaluate the impact of these developments on the representation of trace gas transport and chemistry, and to anticipate the implications for inversions of greenhouse gas emissions using such an updated model. <BR /><BR /> Comparison of a one-dimensional version of LMDz with large eddy simulations shows that the thermal scheme simulates shallow convective tracer transport in the PBL over land very efficiently, and much better than previous versions of the model. This result is confirmed in three-dimensional simulations, by a much improved reproduction of the radon-222 diurnal cycle. However, the enhanced dynamics of tracer concentrations induces a stronger sensitivity of the new LMDz configuration to external meteorological forcings. At larger scales, the inter-hemispheric exchange is slightly slower when using the new version of the model, bringing them closer to observations. The increase in the vertical resolution (from 19 to 39 layers) significantly improves the representation of stratosphere/troposphere exchange. Furthermore, changes in atmospheric thermodynamic variables, such as temperature, due to changes in the PBL mixing modify chemical reaction rates, which perturb chemical equilibriums of reactive trace gases. <BR /><BR /> One implication of LMDz model developments for future inversions of greenhouse gas emissions is the ability of the updated system to assimilate a larger amount of high-frequency data sampled at high-variability stations. Others implications are discussed at the end of the paper.

S. C. Sherwood, S. Bony, O. Boucher, C. Bretherton, P. M. Forster, J. M. Gregory, and B. Stevens. Adjustments in the Forcing-Feedback Framework for Understanding Climate Change. Bulletin of the American Meteorological Society, 96:217-228, February 2015. [ bib | DOI | ADS link ]

P. Good, J. A. Lowe, T. Andrews, A. Wiltshire, R. Chadwick, J. K. Ridley, M. B. Menary, N. Bouttes, J. L. Dufresne, J. M. Gregory, N. Schaller, and H. Shiogama. Nonlinear regional warming with increasing CO2 concentrations. Nature Climate Change, 5:138-142, February 2015. [ bib | DOI | ADS link ]

When considering adaptation measures and global climate mitigation goals, stakeholders need regional-scale climate projections, including the range of plausible warming rates. To assist these stakeholders, it is important to understand whether some locations may see disproportionately high or low warming from additional forcing above targets such as 2 K (ref. ). There is a need to narrow uncertainty in this nonlinear warming, which requires understanding how climate changes as forcings increase from medium to high levels. However, quantifying and understanding regional nonlinear processes is challenging. Here we show that regional-scale warming can be strongly superlinear to successive CO2 doublings, using five different climate models. Ensemble-mean warming is superlinear over most land locations. Further, the inter-model spread tends to be amplified at higher forcing levels, as nonlinearities grow-especially when considering changes per kelvin of global warming. Regional nonlinearities in surface warming arise from nonlinearities in global-mean radiative balance, the Atlantic meridional overturning circulation, surface snow/ice cover and evapotranspiration. For robust adaptation and mitigation advice, therefore, potentially avoidable climate change (the difference between business-as-usual and mitigation scenarios) and unavoidable climate change (change under strong mitigation scenarios) may need different analysis methods.

V. Oerder, F. Colas, V. Echevin, F. Codron, J. Tam, and A. Belmadani. Peru-Chile upwelling dynamics under climate change. Journal of Geophysical Research (Oceans), 120:1152-1172, February 2015. [ bib | DOI | ADS link ]

The consequences of global warming on the Peru-Chile Current System (PCCS) ocean circulation are examined with a high-resolution, eddy-resolving regional oceanic model. We performed a dynamical downscaling of climate scenarios from the IPSL-CM4 Coupled General Circulation Model (CGCM), corresponding to various levels of CO2 concentrations in the atmosphere. High-resolution atmospheric forcing for the regional ocean model are obtained from the IPSL atmospheric model run on a stretched grid with increased horizontal resolution in the PCCS region. When comparing future scenarios to preindustrial (PI) conditions, the circulation along the Peru and Chile coasts is strongly modified by changes in surface winds and increased stratification caused by the regional warming. While the coastal poleward undercurrent is intensified, the surface equatorial coastal jet shoals and the nearshore mesoscale activity are reinforced. Reduction in alongshore wind stress and nearshore wind stress curl drive a year-round reduction in upwelling intensity off Peru. Modifications in geostrophic circulation mitigate this upwelling decrease in late austral summer. The depth of the upwelling source waters becomes shallower in warmer conditions, which may have a major impact on the system's biological productivity.

A. Berg, B. R. Lintner, K. Findell, S. I. Seneviratne, B. van den Hurk, A. Ducharne, F. Chéruy, S. Hagemann, D. M. Lawrence, S. Malyshev, A. Meier, and P. Gentine. Interannual Coupling between Summertime Surface Temperature and Precipitation over Land: Processes and Implications for Climate Change*. Journal of Climate, 28:1308-1328, February 2015. [ bib | DOI | ADS link ]

H. Bellenger, K. Yoneyama, M. Katsumata, T. Nishizawa, K. Yasunaga, and R. Shirooka. Observation of Moisture Tendencies Related to Shallow Convection. Journal of Atmospheric Sciences, 72:641-659, February 2015. [ bib | DOI | ADS link ]

K. Drushka, H. Bellenger, E. Guilyardi, M. Lengaigne, J. Vialard, and G. Madec. Processes driving intraseasonal displacements of the eastern edge of the warm pool: the contribution of westerly wind events. Climate Dynamics, 44:735-755, February 2015. [ bib | DOI | ADS link ]

We investigate the processes responsible for the intraseasonal displacements of the eastern edge of the western Pacific warm pool (WPEE), which appear to play a role in the onset and development of El Niño events. We use 25 years of output from an ocean general circulation model experiment that is able to accurately capture the observed displacements of the WPEE, sea level anomalies, and upper ocean zonal currents at intraseasonal time scales in the western and central Pacific Ocean. Our results confirm that WPEE displacements driven by westerly wind events (WWEs) are largely controlled by zonal advection. This paper has also two novel findings: first, the zonal current anomalies responsible for the WPEE advection are driven primarily by local wind stress anomalies and not by intraseasonal wind-forced Kelvin waves as has been shown in most previous studies. Second, we find that intraseasonal WPEE fluctuations that are not related to WWEs are generally caused by intraseasonal variations in net heat flux, in contrast to interannual WPEE displacements that are largely driven by zonal advection. This study hence raises an interesting question: can surface heat flux-induced zonal WPEE motions contribute to El Niño-Southern Oscillation evolution, as WWEs have been shown to be able to do?

R. Wang, Y. Balkanski, O. Boucher, P. Ciais, J. Peñuelas, and S. Tao. Significant contribution of combustion-related emissions to the atmospheric phosphorus budget. Nature Geoscience, 8:48-54, January 2015. [ bib | DOI | ADS link ]

Atmospheric phosphorus fertilizes plants and contributes to Earth's biogeochemical phosphorus cycle. However, calculations of the global budget of atmospheric phosphorus have been unbalanced, with global deposition exceeding estimated emissions from dust and sea-salt transport, volcanic eruptions, biogenic sources and combustion of fossil fuels, biofuels and biomass, the latter of which thought to contribute about 5% of total emissions. Here we use measurements of the phosphorus content of various fuels and estimates of the partitioning of phosphorus during combustion to calculate phosphorus emissions to the atmosphere from all combustion sources. We estimate combustion-related emissions of 1.8 Tg P yr-1, which represent over 50% of global atmospheric sources of phosphorus. Using these estimates in atmospheric transport model simulations, we find that the total global emissions of atmospheric phosphorus (3.5 Tg P yr-1) translate to a depositional sink of 2.7 Tg P yr-1 over land and 0.8 Tg P yr-1 over the oceans. The modelled spatial patterns of phosphorus deposition agree with observations from globally distributed measurement stations, and indicate a near balance of the phosphorus budget. Our finding suggests that the perturbation of the global phosphorus cycle by anthropogenic emissions is larger thanpreviously thought.