1. Precipitation source/sink connections between the Amazon and La Plata River basins A Sudradjat*, K L Brubaker**, P A Dirmeyer*** Abstract This study examines the hydro-climatological connection of the Amazon River basin with the adjacent La Plata River basin. The Amazon is the world’s largest river, and the La Plata the sixth largest, in terms of discharge. The most recent evaporative sources of precipitation falling in the two basins are mapped on a seasonal basis, using backward-trajectory analysis for water vapor. The precipitation sinks within the La Plata basin of evaporated moisture from the Amazon basin are mapped using forward-trajectory analysis. The analysis covers a 19-year period, from September 1979 to August 1998. The trajectory analysis uses atmospheric dynamics and evapotranspiration from the NCEP reanalyses and COLA's global hybrid precipitation data. Precipitated water vapor of local evaporative origin (recycling precipitation) dominates annual precipitation in the La Plata basin with a contribution of 32% of the total during the 19-year period. Due to South American orography and atmospheric general circulation over the region, the Amazon basin is the most important non-local evaporative source of precipitation in the La Plata basin, with an annual contribution of about 19% of total. The controlling factors of the mean moisture transport over the La Plata and the fact that the Tropical Atlantic Ocean contributes about 32% of annual precipitation in the Amazon basin suggest a "leap-frogging" transport, in which the moisture from upwind sources is precipitated in the Amazon basin, then evaporated to fall as precipitation in the La Plata River basin. The important source regions for the interannual variability of precipitation in the La Plata River basin are identified using Empirical Orthogonal Functions. These findings reinforce the hydro- climatological importance of the Amazon River basin on a continental scale. Introduction Precipitation in a sink region depends on the fate of moisture evaporated from a source region. Hence, a study on the fate of moisture evaporated from a source region may improve the understanding of precipitation in a sink region.The lack of the study of source-sink pairs of precipitation-evaporation between the Amazon and La Plata River basins motivates this study. The Amazon and La Plata River basins are the world’s largest and the sixth largest river in terms of discharge (Dingman, 1994), respectively. This study investigates the source-sink connections between the Amazon (A) and La Plata (LP) River basins. Backward- (BT) (see Dirmeyer and Brubaker, 1999) and forward-trajectory (FT) analyses for parcels of water vapor are used as tools. Methods Backward-trajectory analysis identifies and quantifies probabilistic contributions of evaporative moisture from source regions to precipitation in a sink region. The analysis tracks backward in time the paths of parcels of air from a sink region and tallies the contributions of evaporation events in source regions along the paths starting from precipitation events during a pentad (a 5-day period) (see Figure 1 for a schematic representation of the analysis). The analysis identifies and quantifies the sources of all evaporated moisture that falls as precipitation in a sink region. Details of backward-trajectory analysis are available in Dirmeyer and Brubaker (1999). Forward-trajectory analysis (FT) tracks forward in time trajectories of parcels of air launched from a source region during a pentad to locations of precipitation contributed by the evaporated moisture from the parcels (see Figure 1 for a schematic representation of the analysis). Hence, locations of precipitation for evaporated moisture from the source region can then be identified. The precipitation can also be quantified. This study covers a 19-year period, from September 1979 through August 1998. Figure 2. The spatial domain. References Dingman, S. L., 1994: Physical Hydrology. Prentice Hall, 575 pp. Dirmeyer, P. A., and K. L. Brubaker, 1999: Contrasting evaporative moisture sources during the drought of 1988 and the flood of 1993, J. Geophys. Res., 104, D16, 19,383-19,397. Dirmeyer, P. A., and L. Tan, 2001: A multi-decadal global land-surface data set of state variables and fluxes. COLA Technical Report 102 [Available from the Center for Ocean-Land-Atmosphere Studies, 4041 Powder Mill Road, Suite 302, Calverton, MD 20705 USA], 43 pp. Higgins, R. W., J. E. Janowiak, and Y. P. Yao, 1996: A gridded hourly precipitation database for the United States. ATLAS 1, NCEP/Clim. Predict. Cent., Camp Springs, Md., 47 pp. Kalnay, E., et al., 1996: The NCEP/NCAR 40-year reanalysis project, Bull. Am. Meteorol. Soc., 77, 437-471. Acknowledgements This study is supported under the NOAA/NASA GCIP Program Grant NA96-GP0268. Access to the NCEP reanalyses data is supported under the NCAR/SCD Grant 35161040. Figure 3. Seasonal variation of the mean of vertically integrated moisture transport averaged over the 19-year period shown as vectors in kg m-1 s-1. Tropical North Atlantic Ocean (TRNA) and the Amazon (A) and La Plata (LP) River basins are outlined. Also shown are contours of annual mean surface pressure, in mb, over the same period as proxies for surface elevations. * Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park, MD 20742; ph. 301-405-9211; fax 301-314-1876; e-mail: arief@essic.umd.edu ** Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742; ph. 301-405-1965; fax 301-405-2585; e-mail: klbrubak@eng.umd.edu ***Center for Ocean-Land-Atmosphere Studies, Calverton, MD 20705-3106; ph. 301-902-1254; fax 301-595-9793; e-mail: dirmeyer@cola.iges.org DJF JJA MAM SON EQ Dynamics and ET: NCEP reanalyses (Kalnay et al. 1996). Precipitation (gridded-observed): Higgins et al. (1996) for M and Dirmeyer and Tan (2001) for the rest of the domain. Figure 1. Backward v. forward trajectory analyses. P ET (Dirmeyer and Brubaker, 1999) (developed in this study) Backward trajectory analysis (backward in time) Forward trajectory analysis (forward in time) time ( t ) through solutions of eq.s of mass, momentum, and energy (NCEP reanalyses) Figure 5. Seasonal variation of sinks of evaporated moisture from the Amazon River basin (A) averaged over the 19-year period. A (and subbasins) and La Plata River basin (LP) are outlined. Contours are at 0.05, 0.2, 0.4, 0.8, 1.2, 2.4, and 3.6 kg m-2 day-1 (or equivalent to mm day-1). Figure 4. Seasonal variation of sources for precipitation in La Plata River basin (LP) averaged over the 19-year period. The Amazon (A) (and subbasins) and La Plata (LP) River basins and Tropical North Atlantic Ocean (TRNA) are outlined. Contours are at 0.05, 0.2, 0.4, 0.8, 1.2, 2.4, and 3.6 kg m-2 day-1. Table 1. The top 10 source/sink regions for the Amazon (A) and La Plata River basins. Figure 6. EOF-1 and EOF-2 of source anomalies for precipitation in La Plata River basin during the 19-year period. TRNA A LP TRNA A LP TRNA A LP TRNA A LP EQ DJF JJA MAM SON Figure 3 shows that the mean of vertically integrated moisture transport averaged over the 19-year period allows moisture from A to LP throughout the year. This suggests the role of transient eddies in the contribution from LP to precipitation in A as shown in Table 1. Figure 3 also shows the role of the orography of the Andes in directing the low- level moisture transport vectors over the region. The moisture transport varies seasonally throughout the year due to seasonality of the general circulation and the inter-tropical covergence zone (ITCZ). Figure 4 shows the dominance of recycling precipitation, precipitated moisture of local origin, in LP during the 19-year period. EQ DJF JJA MAM SON Figure 5 shows the role of the ITCZ in directing the sink of evaporated moisture from A. The lowest sink of the moisture in LP is in JJA when the ITCZ reaches its northernmost position. EOF-1 of source anomalies for precipitation in La Plata River basin during the 19-year period in Figure 6 reflects the dominance of recycling precipitation in the interannual variability of the sources for precipitation in LP. EOF-2 may reflect the relationship between the directed moisture transport by the Andes and precipitation recycling. Higher precipitation recycling is expected if the moisture transport is reduced. EQ EOF-1 (EV= 26%)EOF-2 (EV= 11%) EQ