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

2 Climate variability and predictability
The weather/climate system exhibits considerable variability on a wide range of spatial and temporal scales.
Those are characterized by strong interactions that must be taken into account to better understand and
predict the atmosphere from daily variability to climate change. Interest in weather/climate fluctuations
have been rapidly growing over the last few years following the recent recurrence of extremes and the
more pressing evidence for anthropogenic influences on climate. In such a context covering a large
spatio-temporal spectrum, demand for predictability of weather/climate variability over the next decades
is strongly emerging.
The activities of the “Climate variability and predictability” project directly take place within this
framework. Based on a process-oriented approach, our goal is to better understand the physical origin
of the low-frequency variability of the climate system and its interaction with weather-type events, with
the underlying objective to eventually improve their prediction over a wide range of timescale. Our grand
challenge is to tackle the issue of decadal variability and predictability using a hierarchy of numerical tools.
This activity has been at the forefront of our research in 2010-2011 and will be so for many years due to the
numerous complex scientific and technical issues remaining to be solved. In line with these objectives, the
key step was our participation to the current international decadal prediction exercise which is coordinated
by the World Climate Research Program (WCRP) to fulfill the IPCC requirements for their next assessment
report (AR5) to be delivered in 2013.
Fundamental research activities, in line with traditional and historical expertises, are first presented,
followed by a more detailled description of the decadal forecast initiative.
2.1 Interaction between the North Atlantic-Europe atmospheric
dynamics and the local surface conditions (J. Boé, C. Cassou,
E. Maisonnave, S. Bielli)
Previous studies realized at Cerfacs have shown that weather regimes can be considered as elementary
bricks for North Atlantic-Europe (NAE) atmospheric dynamics whose seasonal-to-decadal variability can
be interpreted to the first order as the time integration of their anomalous occurrence. In addition to
occurrence, transitions between regimes also appear a relevant criterion especially to link large-scale
variability and extreme events. Finally, evidence has been provided that the internal characteristics of the
regime (strength, spatial departure to mean etc.) are also crucial to interpret climate variability accross
timescales. Interaction between the atmosphere and the continent on one hand, and the ocean on the other
hand, have been investigated in 2010-2011.
2.1.1 Soil-moisture-precipitation interactions
How soil moisture affects precipitation is an important question – with far reaching consequences, from
weather prediction to centennial climate change –, albeit a poorly understood one. Thanks to observations
and the results of an high resolution hydro-meteorological analysis over France, we have studied how
previous soil moisture conditions affect precipitation and surface water budget over France during summer,
for different weather regimes (Boé 2011).
116 Jan. 2010 – Dec. 2011