Seasonal Cycle and Interannual Variability in the Amazon Hydrologic Cycle

Ning Zeng
Department of Atmospheric Sciences and
Institute of Geophysics and Planetary Physics
University of California, Los Angeles

An analysis of the Amazon basin hydrologic cycle has been carried out using the NASA/GEOS-1 atmospheric reanalysis, observed rainfall of Xie and Arkin, and historical Amazon river discharge. Over a seasonal cycle, the precipitation is found to vary by 5 mm day$^{-1}$, the runoff by 2 mm day$^{-1}$ while the evaporation largely remains constant.

On interannual time scales, the hydrologic variability both in the atmosphere and at the land-surface is found to be closely related to ENSO. The correlation between the Southern Oscillation Index and Xie-Arkin precipitation is 0.8 for the period 1985-1993 and 0.56 for the period 1979-1996. The precipitation lags behind the Southern Oscillation Index by 3-4 months while the Amazon river discharge lags behind the precipitation by another 3 months. The lagged relationship suggests interesting dynamic mechanisms. The reanalysis moisture convergence and observed discharge are used to diagnose basin average soil water storage. The year to year variation in the annual mean soil water storage is about 200 mm, comparable to the change within a climatological seasonal cycle. In one case, the basin soil water storage increases by 462 mm from September 1987 to March 1989, suggesting the remarkable ability of the tropical rainforest environment to store and take up water.