CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

CLIMATE VARIABILITY AND GLOBAL CHANGE
Soil moisture-precipitation coupling has been shown to be highly dependent on large scale circulation,
even simply in terms of sign. If dry soil conditions always lead to an increase of the lifted condensation
level, which is unfavorable to precipitation, the sign of the effect of previous soil moisture conditions
on atmospheric stability depends on large scale circulation. In the end, for two weather regimes, dry
soil conditions lead to less precipitation. However, this should not be interpreted as the sign of a
positive soil moisture-precipitation feedback, because drier soils also lead to a decrease of subsequent
evapotranspiration. As soil moisture effect on evapotranspiration is larger than its effect on precipitation
independently of large scale circulation, the total soil moisture-precipitation feedback over France has
therefore been found to be negative.
2.1.2 Ocean-atmosphere coupling
Within the ANR-IRCAAM project, a dedicated study of the 2003 climate dynamics has been carried out
using a suite of GCM configurations based on the ARPEGE AGCM. Tropical-limited atmospheric nudging
towards observed circulation in ARPEGE confirms that tropical-extratropical connections are important to
explain the 2003 NAE anomalous atmospheric circulation (Cassou et al 2005). We demonstrated that the
anomalous convective activity over a broad tropical Atlantic basin is at the origin of a Rossby wave train
which propagates northeastward towards Europe and alters in fine the occurrence/strength statistics of the
NAE weather regimes. We then set up a simplified coupled model configuration where ARPEGE is coupled
to NEMIX, acronym for the NEMO model transformed into independant single column ocean models.
NEMIX has been developed at Cerfacs in 2010 : the full vertical ocean physics of NEMO is retained while
horizontal exchanges are cast off and compensated by a flux correction term. Targeted simulations for 2003
using the ARPEGE-NEMIX configuration highlight that the ocean-atmosphere extratropical coupling is
important to reinforce the primary tropical-extratropical atmospheric signal. The mechanism is based on the
so-called water wapor feedback at work over the continent due to the advection of moister air from the ocean
when the coupling is activated. This is associated with a strong positive feedback on the anomalous NAE
atmospheric circulation leading to enhanced northward advection of warm air from North Africa towards
Europe, as well as local enhanced subsidence over Europe and excessive solar radiation at the surface. All
together, the retroaction when the ocean-atmosphere coupling activated, is such that the warmer conditions
over Europe are amplified by a factor of two.
2.2 Decadal prediction (C. Cassou, E. Sanchez, Y. Ruprich-Robert,
E. Maisonnave, E. Fernandez, P. Rogel, L. Terray, C. Pagé,
S. Valcke, M.-P. Moine, L. Coquart )
2010-2011 has been almost entirely devoted to the setup, the realization and the basic evaluation of the socalled
decadal or “near term” coupled experiments performed within the Coupled Model Intercomparison
Project (CMIP5) further used for AR5. A new version of the general circulation model CNRM-CM
(Voldoire et al 2011) has been developed jointly by Météo-France (CNRM-GMGEC) and the Cerfacs
Global Change team. The exact same model, named CNRM-CM5, is used in both centennial (carried out
by CNRM) and decadal exercises that consequently differ only by the initialization step (same binaries
and same namelists), . The use of the exact same code in the two centers is the concrete result of strong
colloborations that have been reinforced to pool the limited resources devoted to the development of the
new Earth system for CMIP5. This very demanding project absorbed most of our human ressources over the
2010-2011 period and required a full investment from all the team members, leaving little room, compared
to previous years, for other fundamental research activities.
The CORE experiments of CMIP5 considered as the “entry ticket” for model intercompararison studies,
have been completed and are available on the local CNRM-Cerfacs Earth System Grid (ESG) node.
CERFACS ACTIVITY REPORT 117