lmd_Laval2005_abstracts.html

2005 .

T. Ngo-Duc, K. Laval, J. Polcher, and A. Cazenave. Contribution of continental water to sea level variations during the 1997-1998 El Niño-Southern Oscillation event: Comparison between Atmospheric Model Intercomparison Project simulations and TOPEX/Poseidon satellite data. Journal of Geophysical Research (Atmospheres), 110:9103, May 2005. [ bib | DOI | ADS link ]

Satellite altimetry from TOPEX/Poseidon (T/P) is used to estimate the variation of the global sea level. This signal, once corrected for steric effects, reflects water mass exchange with the atmosphere and land reservoirs (mainly ice caps, soils and snowpack). It can thus be used to test the capacity of general circulations models (GCMs) to estimate change in land water storage. In this study, we compare the land hydrology contribution to global mean sea level variations during the major 1997-1998 El Niño-Southern Oscillation event from two data sets: (1) the results of the Organizing Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface scheme, developed at the Institute Pierre Simon Laplace, coupled to the Laboratoire de Météorologie Dynamique Atmospheric General Circulation Model (LMD AGCM) and (2) the T/P-based estimates. We show that the seasonal variation of the continental water storage is well represented in the model. The drastic amplitude change between the two contrasted years, 1997 and 1998, observed from satellite altimetry, is also simulated. We analyze the role of each component of simulated water fluxes (precipitation, evaporation, and runoff) in determining the range of annual continental water mass variation and its interannual variability. The difference between the two years, 1997 and 1998, is, for an essential part, due to land precipitation in the 20degN-20degS domain. This analysis emphasizes the important role of tropical regions in interannual variability of climate.

T. Ngo-Duc, K. Laval, J. Polcher, A. Lombard, and A. Cazenave. Effects of land water storage on global mean sea level over the past half century. Geophysical Research Letters, 32:9704, May 2005. [ bib | DOI | ADS link ]

The output of the ORCHIDEE Land Surface Model, driven by a 53-yr (1948-2000) atmospheric forcing data set, was used to estimate the effects of land water storage on global mean sea level. Over the past half century, no significant trend was detected but there is a strong decadal variability in the land water storage, driven by precipitation and originating principally in the tropics. The land water contribution to sea level change over the past 50 yr appears highly anti-correlated with thermal expansion of the oceans. This result suggests that change in ocean heat content influences the global water cycle. It also shows that, at decadal time scale, there is partial compensation in sea level changes between thermal expansion and ocean water mass change due to changes in land water storage.

T. Ngo-Duc, J. Polcher, and K. Laval. A 53-year forcing data set for land surface models. Journal of Geophysical Research (Atmospheres), 110:6116, March 2005. [ bib | DOI | ADS link ]

As most variables describing the state of the surface are not directly observable, we have to use land surface models in order to reconstruct an estimate of their evolution. These large-scale land surface models often require high-quality forcing data with a subdiurnal sampling. Building these data sets is a major challenge but an essential step for estimating the land surface water budget, which is a crucial part of climate change prediction. To study the interannual variability of surface conditions over the last half century, we have built a 53-year forcing data set, named NCC. NCC has a 6-hourly time step from 1948 to 2000 and a spatial resolution of 1deg × 1deg. It is based on the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project and a number of independent in situ observations. In this study we show the adjustments which need to be applied to the reanalysis and how they impact the simulated continental water balance. The model outputs are validated with the observed discharges of the world's 10 largest rivers to estimate the combined errors of the forcing data and the land surface model. The seasonal and interannual variations of these discharges are used for this validation. Five numerical experiments have been carried out. They used the forcing data sets obtained after each step of data adjustment and the forcing of the Global Soil Wetness Project 2 as inputs for the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model. The quality of forcing data is improved after each adjustment. The precipitation correction gives the most important improvement in the simulated river discharges, while the temperature correction has a significant effect only at high latitudes. The radiation correction also improves the forcing quality, especially in term of discharge amplitude. The NCC forcing data set can be used to study the water budget over many areas and catchment basins that have not been yet analyzed in this study. With its period of 53 years, NCC can also be used to evaluate the trends of terrestrial water storage in particular regions.