Local and remote forcing of subinertial fluctuations in the ocean off the west coast of South America

al Chile. Several years of current records from two stations over the continental slope at 30°S and 35°S off Chile and sea level along the west coast of South America are used together with a linear model for wind-driven frictional flow over the continental shelf and slope to examine and evaluate the relative importance of the equatorial and local wind forcing along this coast from 7°S and 37°S. The model is forced with both coastal and satellite-derived wind data and includes an appropriate northern boundary condition that accounts for poleward-propagating coastal trapped waves (CTW) of equatorial origin.

Our results confirm early observations off Peru and northern Chile showing that alongshore wind stress only explains a small fraction of synoptic period (2-15 days) sea level fluctuations. Synoptic wind fluctuations are relatively weak in that region with a minimum near 19°S and they barely contribute to the sea-level variability. South of about 24°S wind-stress fluctuations increase and wind-driven disturbances are more energetic. However remote-generated oscillations are still largely significant as far south as 37°S. Sea-level perturbations propagate southward with a mean observed phase speed of 3.0 m s-1. Model results show phase speeds between 2.33 m s-1 and 3.92 m s-1 depending on the local topography and stratification. Most of the seasonal variations of the stratification are restricted to the upper 200 m such that they only slightly affect the propagation speed of sea level fluctuations.

Alongshore current disturbances over the slope off central Chile between about 30°S and 35°S propagate at about 2.6 m s-1 consistent with the theoretical phase speed of the first CTW mode. Model simulations show that the wind-forced contribution to the alongshore flow variability in this region only explains about one third of the observed current variance at the synoptic band. In conclusion, comparison of simulations with observations shows that much of the synoptic disturbances of the coastal flow and sea level along the west coast of South America can be explained using linear, wind-forced CTW theory with an initial amplitude at the northern boundary given by waves of equatorial origin. The large remote signal overwhelms any possible local coherence between alongshore flow over the slope and the regional alongshore wind stress. Observations show large seasonal and interannual modulations in the amplitudes of the remotely-generated waves over the slope off cent