The role of atmospheric circulation patterns in controlling the regional distribution of stable isotope contents in precipitation: Preliminary results from two transects in the Ecuadorian Andes

Spatial and temporal variability of oxygen-18 and deuterium contents in precipitation in the equatorial zone of the Pacific coast of South America is reviewed based on a network of 13 meteorological stations collecting composite monthly rainfall samples. The meteorological stations are situated along two latitudinal transects (1 deg. S and 3 deg. S) from the Pacific coast of Ecuador to the Amazon Basin. The Andes, blocking the free transport of water vapour from the Amazon basin to the Pacific Ocean, and the seasonal displacement of the Intertropical Convergence Zone (ITCZ) are the two main factors controlling the precipitation regime and the isotope composition of precipitation in the region. Annual mean δ18O values range from about -2 per mille in coastal stations in the southern part of Ecuador to about -11 to -14 per mille in stations located above 3000 m a.s.l. The seasonal displacement of the ITCZ is responsible for the observed single or bimodal distribution of precipitation in some parts of the country, and its passage is usually associated with depleted isotope contents in precipitation. Despite the long distance to the Atlantic coast and the high precipitation rates measured in the low altitude stations of the Amazon Basin, the typical annual mean δ18O values in these stations are around -6 to -7 per mille. The unusually low continental gradient in δ18O and δ2H is explained by the intense recycling of water vapour in the Amazon rainforest. (author)