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2014 .

(10 publications)

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. 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.

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.

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.

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 ]

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 ]

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