One of the most basic questions asked to hydrologists is that of the quantification of catchment response to climatic variations, i.e. that of the variations around the average annual flow given the climatic anomaly of a given year. This paper presents a large sample analysis based on 4122 catchments from four continents, where we investigate how annual streamflow variability depends on climate variables – rainfall and potential evaporation – and on the season when precipitation occurs, i.e. on the synchronicity between precipitation and potential evaporation. We use catchment data to verify the existence of this link, and show that, in all countries and under the main climates represented, synchronicity anomalies come as the second most important factor to explain annual streamflow anomalies: after precipitation but before potential evaporation. Introducing the synchronicity between precipitation and potential evaporation as an independent variable improves the prediction of annual streamflow variability significantly.