Propagation and forcing of high‐frequency sea level variability along the west coast of South America

Abstract

Tide and wind data from coastal and island stations from Buenaventura, Colombia (4°N), to Callao, Peru (12°S), have been analyzed for the 1979–1984 time period to determine the propagation and forcing characteristics of coastal sea level variability at periods of days to weeks, as well as how they vary either with season or between the 1982–1983 El Niño‐Southern Oscillation (ENSO) period and non‐ENSO years. During four non‐ENSO years, the ensemble averaged cross spectra between coastal sea level height (SLH) and local winds show weak evidence of local forcing during the whole year without significant differences between the austral summer and winter seasons, other than a greater energy in the wind fluctuations at Talara during summer. Cross spectra between SLH series from neighboring stations show evidence of poleward phase propagation during winter seasons at speeds of about 2.0 m s−1.between La Libertad and Talara at periods of a week or more, and about 2.7 m s−1 between Talara and Callao at periods of 5–11 days, but no propagation is found during summers. During the 1982–1983 ENSO there is a large increase in SLH energy at most frequencies at all coastal stations, but especially in the 8‐to‐11‐day band, where energies are enhanced by as much as an order of magnitude above non‐ENSO levels. The cross spectra between adjacent SLH stations indicate a nondispersive poleward propagation of events during the 1982–1983 ENSO with phase speeds of 2.2–3.5 m s−1 from La Libertad to Talara (periods of a week or more) and 3.4–3.6 m s−1 from Talara to Callao (3.5 days or more). As with the SLH energy, the coherence and phase propagation were much stronger along the Peru coast in 1982–1983 than during non‐ENSO periods, especially in the 8‐to‐11‐day band. The one‐third increase in phase speeds during the ENSO over the non‐ENSO speeds is found to be consistent with the anomalous depression of the density structure during El Niño. Comparisons between coastal SLH and the local alongshore wind suggest that locally forced SLH variability was obscured during the 1982–1983 ENSO by the noncoastally forced, but energetic propagating fluctuations, which probably originated in the equatorial waveguide.