Abstract

Results are reported from the first long, recording current meter observations over the slope off Chile. These observations, at 30°S during the 1991-92 El Niño event, are analyzed together with observations of currents at a local deep sea site; local wind and sea level; sea level from the Peru and Chile coasts; and wind, temperature, and currents from the equatorial Pacific. Mean poleward flow of 12 cm s-1 was observed within the Peru-Chile Undercurrent over the slope. Mean flow in the depth range of Antarctic Intermediate Water was not distinguishable from zero in the presence of strong, low-frequency (LF) variability, which dominated slope currents at all depths. The strongest LF fluctuations had periods of about 50 days, but periods of 10 and about 5 days were also observed. Significant, local wind forcing of slope currents was only found in the period band 6-10 days and may be related to coastal-trapped waves in the atmosphere. Our analysis shows that free, coastal-trapped waves in the ocean, arriving from the north, dominated the LF variability over the shelf and slope off northern and central Chile during the 1991-92 El Niño event. Strong 50-day period fluctuations there started their journey about two months earlier - and 15 000 km farther up the coastal-equatorial waveguide - near the date line in the equatorial Pacific as equatorial Kelvin waves forced by westerly wind events of similar period. Upon reaching the South American coast, these waves forced coastal-trapped waves, which propagated along the Peru coast into the study region. Likewise, a scenario of equatorial-trapped waves forcing coastal-trapped waves may explain 10-day as well as 6-day and 4.5-day period coastal-trapped waves off Chile stemming from mixed Rossby-gravity and inertia-gravity waves trapped at the equator. Since the large, 50-day period, coastal-trapped waves may strongly modify coastal upwelling source water, such remotely forced waves may have a significant influence on the pelagic ecosystem off Chile, at least during El Niño events.