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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 characteristicso f coastals eal evel variability at periodso f days to weeks,a s well as how they vary either with season or between the 1982-1983 E1 Nifio-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 significantd ifferencesb etweent he australs ummera nd winter seasonso, ther than a greatere nergyi n 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-x between La Libertad and Talara at periods of a week or more, and about 2.7 m s-x 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 increasei n SLH energy at most frequenciesa t all coastals tations,b ut especiallyi n 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 propagationo f eventsd uringt he 1982-1983E NSO with phases peedso f 2.2-3.5 m s-x from La Libertad to Talara (periods of a week or more) and 3.4-3.6 m s-x 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-l 1-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 E1 Nifio. Comparisons between coastal SLH and the local alongshorew ind suggestth at locallyf orcedS LH variabilityw aso bscuredd uringt he 1982-1983E NSO by the noncoastally forced, but energetic propagating fluctuations, which probably originated in the equatorial waveguide.