Dynamic identification of moisture sources in the Orinoco basin in equatorial South America

The main areas of net moisture uptake are examined in air masses over the Orinoco River basin, located in equatorial South America, north of the Amazon basin. Although the Orinoco River has the third largest average annual discharge in the world (with 5.4 x 10(11) m(3) year(-1) draining into the Atlantic Ocean), the sources of moisture that feed it have not previously been Studied in any detail. The results are presented from analyses of back-tracking of all the air masses over the Orinoco basin over a period of five years (2000-2004) using the diagnostic Lagrangian tool FLEXPART. The input data for the model were obtained from the European Centre for Medium-range Weather Forecasts (ECMWF). Air transported into the Orinoco basin experiences a large uptake of water over the tropical North Atlantic within the three days prior to its arrival over the basin. The Tropical South Atlantic and the eastern coast of the Pacific become significant moisture sources for about 5-10 days before arriving over the Orinoco basin. Contrary to what might be expected, large areas of the Amazon basin, along with the Gulf of Mexico, do not provide significant moisture to the study area. Interestingly, over these zones the air experiences net moisture loss. Preliminary analysis of the processes that occur leads to the conclusion that most of the water observed over the Orinoco basin derives from advective fluxes into the area, while recycling of moisture is negligible.