The impact of the El Niņo-Southern Oscillation (ENSO) phenomenon on the annual cycle of Colombia´s hydro-climatology is quantified. Results of the NCEP/NCAR Climatic Reanalysis Project are used to compare the annual cycle of diverse variables and to evaluate their behavior during the extreme phases of ENSO. The main circulation mechanisms interacting to reduce rainfall during El Niņo are: (1) the weakening of the low-level westerly jet that penetrates from the Pacific Ocean inland Colombia at 5°N; (2) the reduction of the 700 hPa equatorial easterly jet; (3) the reduction of moisture advection from the Caribbean Sea; (4) the reduction in number and intesity of easterly waves along the tropical North Atlantic; (5) the displacement of the ITCZ to the south-west of its normal position, due to an anomalous Hadley cell over tropical South America, and (6 amosphere feedbacks. Correlation analyses confirm the strong
influence of ENSO on rainfall and river discharges anomalies of Colombia, in particular during SON and DJF. Multiple linear regression analysis are used to predict seasonal river discharges, including the influence of ENSO, the influence of the North Atlantic Oscillation, and other large scale phenomena, as well as the hydrological persistence. We discuss the societal, economical and environmental impacts of El Niņo in Colombia.