How far can global models predict extreme hydrological events Hervé Le Treut 28/06/2015

Plan of the talk: - French interest on the subject - Remarks on AR4 models - One test case: South-East South-America 28/06/2015

Two French zoomed climate models LMDZ-Mediterranean (IPSL, Paris) Arpege-Mediterranean (Météo-France, Toulouse)

Global Regional buffer zone Schematic of the quardruple coupling in IPSL Global O-A coupled model: LMDZ-global / ORCA2 Regional O-A coupled model: LMDZ-regional / MED8 Two atmospheric models are coupled through buffer zones Two oceanic models are also coupled through buffer zones

Applications to: Europe HYmex (Hydrology in the Mediterranean area) AMMA (West Africa) China South America Wüst, 1961

A downscaling study for France (Li et al): Three versions: Global / Europe / France Two-way nesting between Global/Europe One-way nesting from Europe to France

Annual-mean precipitation (mm) in three LMDZ models: Globe (top), Europe (middle) and France (bottom)

Intense precipitations (mm/day), return level at 50 years MarseilleObs300km100km20km 1961/ / ParisObs300km100km20km 1961/ / StrasbourgObs300km100km20km 1961/ / Observations and three versions LMDZ

Future evolution of extremes Future evolution of extremes SON DJFMAMJJA Precipitation (mm/day) averages 30-year return levels 2070/2099 minus 1970/1999 Goubanova et al. 2007, Planetary and Global Change

(a) Normalized frequency and (b) amount of precipitation as a function of daily intensity for observation and the reference simulations of LMDZ-Global 、 LMDZ/CTRL and LMDZ/CTRL2. Added values of LMDZ-regional: extremes Chen et al. 2010, Cli Dyn

Hadley mass transport (DJF, winter cell) and its dependence on CO2 doubling AR4 models

Hadley cell latitudinal extension (JJA, Southern limit) and its dependence on CO2 doubling AR4 models Extension vers les pôles des branches subsidentes, S JJA (en ° latitude) Gastineau et al, 2008

Main features of tropical response to CO2 increase in current GCMs Hadley cell intensity diminished (consistent with changes in vertical stability of the atmosphere, with radiative forcing) Precipitation rates are increased (increase in water vapour, in convective precipitation) The position of the main tropical system undergoes only moderate changes SENTIVITY EXPERIMENTS in reponse to prescribed SSTS show that these agreements between models reflect a response to globally average SST changess, or zonally averaged SST changes. The impact of zonal asymmetries is much more complex and involves competition between Walker and Hadley cells.

A useful paradigm to understand tropical circulations: the two-column model.

The dynamic link between two cells determines specific scales (Bellon, Ghil, Le Treut, GRL, 2006) R -2 R -1

Coupling two adjacent cells sharing a common subsidence zone (through a modification the low level heat transport) clearly shows the possibility of non-linear behaviour (Le Treut and Bellon, 2005)

Differences of simulated seasonal mean precipitation between and periods from IPCC-AR4 models (see list in table 1). Color scale interval is 0.2 mm day-1. The black contour indicates the 0 level. (Vera, Junquas, Li and Le Treut)

EOF1 of DJF ( ) rainfall from IPCC-AR4 model. Color scale interval is 0.2 non- dimensional units. The black contour indicates the 0 level.

a. b. Figure 14: Composite differences of mean DJF geopotential height (zg) at 500hPa between the positive and the negative EOF1- SE for ( ) (a) and ( ) (b) from 8 of the 9 selected models. Color scale and contour interval is 10 m. The black contour indicates the 0 level.

Conclusions: - Inceasing Model resolution is a necessity - But the scale interactions are complex, and non-linearity can arise from the large-scale synoptic circulations - Using a GCM for regional studies, and more so for the deterination of extreme events, requires a combination of approaches, and careful process studies 28/06/2015