Abstract for the 5th International Scientific Conference on the GLOBAL ENERGY and WATER CYCLE: Observing and Predicting the Earth's Water and Energy Cycle: current state of knowledge and future research requirements June 20-24, 2005, Orange County, California, USA

Construction, validation and applications of the 53 year atmospheric forcing NCC

T. NGO-DUC (1), K. LAVAL (1), J. POLCHER (1)
A. LOMBARD (2), A. CAZENAVE (2)
1.LMD/CNRS, Univ. of Paris VI, Paris, France
2.LEGOS-CNES, Toulouse, France
contact: thanh.ngo-duc@lmd.jussieu.fr/Fax:+33-144276272

To study the inter-annual variability of surface conditions over the last half past century, a 53 year forcing data set was built based on the NCEP/NCAR reanalysis project and a number of independent in-situ observations. It has a 6-hourly time step from 1948 to 2000 and a spatial resolution of 1°x1°. We will call it NCC (NCEP Corrected by CRU).

To validate the NCC forcing, the outputs or the ORCHIDEE land-surface model are compared to 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 inter-annual variations of these discharges are validated. The quality of forcing data is improved with the integration of the various observations. The precipitation correction has the most important impact on the simulated river discharges while the temperature correction has a significant effect only in high latitudes. The radiation correction improves in particular the amplitude of the simulated discharge.

The continental water budget simulated by ORCHIDEE are used to analyse trends. Over the last 53 years, no significant trends are detected but there is a strong low frequency variability in the land water storage. The low frequency variability originates principally in the tropics. In the northern tropic, the continents lose water to the benefit of the oceans during the last half century. The contribution of land water to sea level, simulated by ORCHIDEE, is highly anti-correlated with the thermal expansion of the oceans, obtained via global ocean temperature data set. This result indicates that a warming of the oceans accelerates the water cycle and thus contributes to a reduction in the sea-level partly compensating the thermal expansion.