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2014 .

(6 publications)

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. 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.

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.

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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