Coastal sea level, current, satellite altimeter data and an equatorial Kelvin wave model are used to study the dynamics of seasonal and interannual fluctuations of the Peru-Chile Undercurrent. Satellite altimetry shows that low frequency, sea level anomalies near 30¿S propagate offshore as expected from Rossby wave theory. Those anomalies are related to disturbances of equatorial origin observed near the coast of South America. With a wind-forced equatorial Kelvin wave model and simple Rossby wave dynamics we show that much of the sea level anomalies, and more than 50% of the observed undercurrent variability near 30¿S, at seasonal and interannual periods can be explained by long Rossby waves forced by the equatorial Kelvin waves. The amplitude of the equatorial Kelvin wave can be used to estimate the amplitude of the Rossby wave near the South American coast, and thus, to monitor low frequency changes in the Peru-Chile Undercurrent.