The Amazon is the world's largest river, contributing about 13 percent of global land runoff to the oceans. A large part of its basin is among the most important rain forests in the world. The importance of this basin has attracted many studies on its natural systems, including its hydroclimatology. However, beyond estimates of precipitation recycling, a study on source-sink pairs of precipitation supply has never been done. Using new approaches of backward- and forward-trajectory analysis for water vapor, this study analyzes spatio-temporal source-sink patterns of precipitation supply in the Amazon River basin. The backward-trajectory analysis identifies the most recent evaporative sources of precipitation falling in the Amazon basin. The forward-trajectory analysis locates the precipitation sinks of evaporated moisture from the basin.

Using the NCEP reanalysis dynamic fields and COLA's global hybrid precipitation data, this study provides seasonal maps of evaporated moisture sources for precipitation in the basin and precipitation sinks of evaporated moisture from the basin over a 19-year period, from September 1979 to August 1998. The basin is divided into two climatic zones, North Amazon (NA) and South Amazon (SA), at 5°S. The seasonal swing of the inter-tropical convergence zone helps to establish the climatic zones. On an annual basis, precipitation supply to NA is dominated by evaporated moisture from the tropical North Atlantic Ocean (31%) and recycled precipitation (21%). Terrestrial sources (61%) dominate precipitation in SA. Recycled precipitation for the entire Amazon River basin is 39% on an annual basis.

Results from EOF analysis on the sources for precipitation in NA and SA suggest the importance of the contributions from the basin itself and tropical North Atlantic Ocean (for NA) and the basin itself (for SA) to the interannual variability of precipitation in both subbasins. Results from EOF analysis on the sinks of evaporated moisture from NA and SA show a switch between internal (recycled precipitation) and external sinks, suggesting the importance of the interannual variability of local land-atmosphere interactions in determining the variability of the sinks.

Higher (lower) contributions from the tropical North Atlantic Ocean and recycling to precipitation in the Amazon River basin are shown to be statistically associated with El Niņo (La Niņa) events. Detected statistically significant negative trends during JJA for source contributions from the North Amazon River sub-basin and the Tropical North Atlantic Ocean over the 19-year period without statistically significant trends in total precipitation in the Amazon or recycling ratio during the same period suggest a possibility of the weakening of the Amazon-Atlantic atmospheric circulation.