Circulation and Low-Frequency Variability near the Chilean Coast: Remotely Forced Fluctuations during the 1991–92 El Niño

Abstract

Results are reported from the first long, recording current meter observations over the slope off Chile. These observations, at 30ЊS during the 1991-92 El Nin ˜o 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 Ϫ1 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 Nin ˜o 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 coastaltrapped waves, which propagated along the Peru coast into the study region. Likewise, a scenario of equatorialtrapped waves forcing coastal-trapped waves may explain 10-day as well as 6-day and 4.5-day period coastaltrapped 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 Nin ˜o events.

ENSO modulation of the upwelling season off southern‐central Chile

“…However, there is an essential difference about the physical setting forced by ENSO in both upwelling areas of Chile. The main physical processes connecting ENSO with the northern upwelling region are related to thermocline perturbations and changes in the alongshore currents forced by equatorial Kelvin waves [e.g., Shaffer et al, 1997]. Moreover, changes in equatorward-favorable upwelling winds in northern Chile (north of about 25°S) are not clearly modulated by ENSO [Montecinos, 1991].…”

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part I: hypothetical boundaries

“…Theoretical studies (Moore, 1968;Anderson and Rowlands, 1976;McCreary, 1976;Clarke, 1983Clarke, , 1992En®eld et al, 1987;Shaer et al, 1997) have shown that the eastern boundary is an extension of the equatorial wave guide. They also point out that the re¯ection of low-frequency waves, incident on the boundary at the equator, should be visible at higher latitudes along the coast.…”

Barotropic and baroclinic semidiurnal tidal currents in two contrasting coastal upwelling zones of Chile

“…Here, the internal waves are generated at shallow thermoclines [Baines, 1982] and not with local changes in the stratification near the ADCPs [e.g., Holloway, 1984]. In general, summer thermal stratification is formed in the north when upwelling-favorable winds diminish and solar radiation dominates [Strub et al, 1998;Shaffer et al, 1997;Yannicelli et al, 2012]. The south is characterized by strong stratification in winter due to the freshwater input from precipitation and river discharge of the Itata and Biobío rivers [Sobarzo et al, 2007;Saldías et al, 2012].…”

“…In upwelling systems such as the coasts of central Chile, stratification shows a marked synoptic and seasonal variability that may play a role in the modulation of semidiurnal internal waves [Rosenfeld, 1990;Aguirre et al, 2010]. In north and central Chile, stratification during summer is caused mainly by solar radiation, which competes with coastal upwelling that homogenizes the water column [Strub et al, 1998;Shaffer et al, 1997;Yannicelli et al, 2012]. During winter in south-central Chile, freshwater input is the main factor responsible for the stratification of the water column on the continental shelf [Sobarzo et al, 2007;Saldías et al, 2012].…”