Dynamics of Remotely Forced Intraseasonal Oscillations off the Western Coast of South America

Abstract

Six years of sea level observations at six locations along the South American coast from Peru to Chile were analyzed at intraseasonal frequencies (periodicity 1?2 months). A high-resolution, low-frequency numerical coastal model, having realistic shelf and slope topography and bottom friction and forced by an equatorial signal, was used to analyze the intraseasonal coastal dynamics at these latitudes. The following results were obtained. Lagged correlations between sea level stations imply a sea level poleward propagation of 250 km day?1. The frictionless vertical wall theory poleward phase speed was considerably less at 216 km day?1. Model runs with realistic bottom friction and shelf and slope bottom topography gave poleward phase speeds much closer to that observed. For typical parameters the phase speed was 253 km day?1. Both bottom friction and topography significantly affect the propagation speed. Past work has shown that the sea level amplitude should grow alongshore like |f| (f = Coriolis parameter). The observed amplitude is much more uniform alongshore, similar to the frictional model with shelf and slope bottom topography. The modeled intraseasonal alongshore velocity field with bottom friction and shelf and slope bottom topography differs considerably from that of the inviscid vertical wall case. Although both have negligible amplitude in the deep sea away from the coast, the velocity amplitude in the more realistic case is trapped along the upper slope with its core at about 1000?1200 m and with decreasing amplitude toward the coast and above the core. Velocity amplitudes in the core are typically about 15 cm s?1.