CIRCULATION AND LOW-FREQUENCY VARIABILITY NEAR THE CHILEAN COAST - REMOTELY FORCED FLUCTUATIONS DURING THE 1991-92 EL-NINO

Results are reported from the first long, recording current meter obse rvations over the slope off Chile. These observations, at 30 degrees S during the 1991-92 El Nino event, are analyzed together with observat ions of currents at a local deep sea sire; local wind and sea level; s ea level from the Peru and Chile coasts; and wind, temperature, and cu rrents from the equatorial Pacific. Mean poleward flow of 12 cm s(-1) was observed within the Peru-Chile Undercurrent over the slope. Mean f low in the depth range of Antarctic Intermediate Water was not disting uishable from zero in the presence of strong, low-frequency (LF) varia bility, which dominated slope currents at all depths. The strongest LF fluctuations had periods of about 50 days, but periods of 10 and abou t 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 relate d to coastal-trapped waves in the atmosphere. Our analysis shows that free, coastal-trapped waves in the ocean, arriving from the north, dom inated the LF variability over the shelf and slope off northern and ce ntral Chile during the 1991-92 El Nino event. Strong 50-day period flu ctuations there started their journey about two months earlier-and 15 000 km farther up the coastal-equatorial waveguide-near the dateline i n 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 equ ator. Since the large, 50-day period, coastal-trapped waves may strong ly modify coastal upwelling source water, such remotely forced waves m ay have a significant influence on the pelagic ecosystem off Chile, at least during El Nino events.