El Niño Sea Level and Currents along the South American Coast: Comparison of Observations with Theory

Abstract

Interannual sea level fluctuations along the South American coast and 7.3-year-long current records at 30°S off central Chile were examined with a low-frequency linear numerical model. The model includes stratification and shelf and slope bottom topography, which varies alongshore. The coastal flow is driven by interannual alongshore wind stress and an interannual alongshore pressure gradient resulting from El Niño/La Niña pressure fluctuations at the equator. Calculations suggest that along the South American coast the coastal interannual variability is primarily due to the remote equatorial forcing rather than interannual coastal alongshore wind stress. Past work using this model has shown that, due to the scattering of lower vertical modes to higher ones by bottom friction, the model predicts a low-frequency jet trapped near the bottom along the continental slope. Until now adequate interannual current time series were unavailable to check this prediction. Analysis of the observations shows that at interannual frequencies the observed alongshore current fluctuated with an amplitude of about 5 cm s?1 at 220 m and 4 cm s?1 at 750 m depth; that is, the observed interannual flow does not have the bottom-trapped continental slope jet predicted by the model. If, however, one takes into account the reduction in bottom friction over the continental slope due to buoyancy forces in the bottom boundary layer, then both model and observed currents at 750 m are small. Although bottom friction is negligible on the continental slope, it should remain strong close to the coast where surface and bottom boundary layers overlap. Inclusion of this bottom friction causes the model interannual sea level to propagate poleward with speeds that vary from 0.5 m s?1 near the equator to 2.9 m s?1 at 37°S. At interannual timescales the phase differences associated with such propagation are small. Model and observed interannual sea levels at six locations along the coast are in basic agreement in the sense that for both the equatorial signal extends all along the South American coast and is nearly in phase. However, the phase differences were too small to check the poleward propagation with the available data. Because model sea level propagates offshore approximately like a Rossby wave at interannual frequencies, the model predicts that coastal sea level should lead the near-surface geostrophic equatorward flow by about 7 months. A lead time of about this size is consistent with coastal sea level and the 7.3-year-long 220-m depth coastal current observations off central Chile. Since the mean flow at 220-m depth is poleward, at about 7 months after the peak of an El Niño (high sea level) the poleward undercurrent is weaker and about 7 months after the peak of a La Niña (low sea level) it is stronger.