Promise - News

6-monthly progress report on PROMISE from CNRM Subject: Impact of land surface processes on tropical climate variability (WP1300)

Recent experiments at CNRM have suggested that, besides sea surface temperature (SST), soil moisture (SM) exerts a significant influence on the interannual climate variability (Douville and Chauvin 2000a). In the framework of the Global Soil Wetness Project (GSWP), the ISBA land- surface scheme of the ARPEGE atmospheric GCM has been forced with meteorological observations and analyses in order to produce a two-year (1987-1988) high resolution (1 degree) SM climatology. This is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. Therefore, it can be used for relaxing the total soil water content simulated by ARPEGE and thereby investigate if the use of a more realistic SM boundary conditions improves the climate simulated by ARPEGE. Ensembles of 7-month integrations from March to September have been performed, with free- running or relaxed SM, and using climatological or observed SSTs. The results averaged from June to September (JJAS) indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability (differences between 1987 and 1988), not only in the Northern Hemisphere mid-latitudes but also in the tropics.

In order to focus more particularly on the Asian and African monsoon climates, other experiments have been designed with a SM relaxation prescribed only over south Asia or Sudan and Sahel (Douville et al. 2000b). More extreme sensitivity experiments (dry or wet soils) have been also performed. The results show a different sensitivity of the Asian and African monsoons to the land surface hydrology. Whereas African rainfall increases with increasing SM, such a clear and homogeneous response is not found over the Indian continent. Precipitation does increase over northern India as a consequence of wetter surface conditions, but the increased evaporation is counterbalanced by a reduced moisture convergence when averaging the results over the whole Indian peninsula. This contrasted behaviour is partly related to the more chaotic nature of the Asian monsoon, where moisture convergence is about twice that found over Sudan and Sahel so that water recycling has a weaker influence on seasonal rainfall. It is also due to a different response of the frequency distribution of daily precipitation, and particularly to an increased number of strong convective events with decreasing SM over India. These experiments will be further analysed to understand how SM also affects the interannual variability of the monsoons, but still over the limited period (1987-1988) available in GSWP.

Moreover, a new set of ensembles has been launched over a 15-year period and with a new version (semi-Lagrangian advection scheme and improved physical package) of the ARPEGE model. Since the focus is mainly on the Asian and African monsoons, all experiments are ensembles of 4-month integration from June to September (JJAS). Each experiment is made up of 10-member ensembles for each of the 15 JJAS seasons. In the 1st set, the ISBA land surface scheme is fully interactive and SM is running free. In the 2nd set, SM is relaxed towards the GSWP climatology (average of 1987 and 1988 monthly means). Therefore, the SM climatology is presumably improved but there is nearly no interannual variability in SM. In the 3rd set, SM is relaxed towards a pseudoclimatology which has been obtained by superimposing the interannual anomalies derived from the 1st set of simulations to the GSWP climatology. The analysis of these experiments is in progress. Further experiments should be relaxed towards the ERA40 SM climatology when available (hopefully around mid-2001).

References:

Douville H., Chauvin F. (2000a): Relevance of soil moisture for seasonal climate predictions: a preliminary study. Climate Dynamics (in press).

Douville H, Chauvin F, Broqua H (2000b): Influence of soil moisture on the Asian and African monsoons. Part I: Mean monsoon and daily precipitation. J Climate (accepted)

Subject: Climate change scenarios (WP2000)

During the first semester of the project efforts at CNRM have been concentrated on the development of a new climate scenario for the 21-rst century using the latest version of the coupled atmosphere-ocean-sea ice model developed at CNRM and CERFACS. This latest version is based on the most recent release of Arpege-Climat version 3 (cycle 22a) with a T63 spectral truncation and a linear-grid of 64x128 points, and 45 levels over the vertical so as to allow a detailed resolution of the stratosphere for taking into account the transport of ozone and a simplified photochemistry. The atmospheric GCM is coupled by means of the OASIS coupler (version 2.3)to the OPA-8.0 version of the ocean general circulation model developed at LODYC (Terray et al, 1995), and has been interfaced with the new dynamic-thermodynamic multi-layer sea-ice model GELATO with a visco-plastic rheology (Salas y Melia, 2000).

Two simulations starting in 1950 have been performed: a control simulation in which the greenhouse gas concentrations are kept at their 1950 values, and a scenario simulation in which the concentrations are updated each year according to their observed values until 1999, and according to scenario SRES-b2 of IPCC thereafter. The greenhouse gases taken into account are CO2, CH4, N20, CFC-11 and CFC-12, and O3. The anthropogenic sulfate aerosols are taken into account both for their direct and indirect effects using the parameterisation of Boucher et al. (1995), and their specified spatial distribution is updated every 10 years.

The control simulation gives a realistic representation of the current climate with only a very small drift. The tropical SST exhibit interannual variability with the El Nino pattern in the equatorial Pacific. Sea-ice distribution is realistic over the Arctic, but is underestimated in the southern hemisphere due to an underestimation of the short-wave radiative forcing of clouds around the Antarctic. The scenario simulation has just been completed for the period 1950-2070, and is in the process of being analysed in detail. We plan to provide data from this scenario simulation for the Promise database over the two selected periods. The next step will be to use the computed SSTs from this coupled scenario as boundary conditions for a time-slice simulation with increased resolution over Africa using the variable resolution version of Arpege- Climat.

References:

Boucher,O; Le Treut,H; Baker,MB (1995): Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes. J. Geophys. Res. - Atmos. 100(D8, 20 Aug), 16395-16414.

Salas y Melia, D (2000): Dveloppement et validation d'un modle coupl ocan-glace de mer pour l'tude du climat des hautes latitudes. Thse de doctorat de l'Universit Paul Sabatier de Toulouse, 10 July 2000.

Terray,L; Thual,O; Belamari,S; Dqu,M; Dandin,P; Delecluse,P; Lvy,C (1995): Climatology and interannual variability simulated by the ARPEGE-OPA coupled model. Climate Dyn. 11(8, Oct), 487-505.