Seasonal and interannual variability of currents and temperature off central Chile

Abstract

Results are reported for continuous, 6 year records of current and temperature from a continental slope station at 30°S off Chile. These observations for November 1991 to November 1997 span the moderate, weak, and strong El Niño events of 1991–1992, 1994, and 1997 and the La Niña event of 1995–1996. Results for current and temperature are analyzed together with observations of wind along the South American coast from the ERS‐1 and ‐2 satellite scatterometers, wind from a local coastal site, sea level from northern and central Chile, sea surface temperature anomalies from the South Pacific region, and traditional El Niño indices. Mean poleward flow of 12.8 cm s−1 was observed for the 6 year period in the core of the Peru‐Chile Undercurrent (220 m). Mean flow in the depth range of Antarctic Intermediate Water (750 m) was equatorward at 1.1 cm s−1. Intraseasonal‐scale variability associated with coastal trapped waves was weaker during the austral winter: greatest during El Niño events and weakest during the La Niña event. Winds adjacent to the coast showed a strong annual cycle with a southward progression of maximum equatorward (upwelling favorable) winds. Alongshore flow over the slope exhibited strong semiannual and weaker annual variations. Poleward flow in the undercurrent was strongest in the austral spring and fall and weakest in the winter. Near the bottom (750 m), seasonal‐scale variability was less intense but led that higher up in the water column (220 m) by ∼1 month. Strong interannual variability of alongshore current and temperature was found at all depths over the slope, whereby changes at depth led changes higher up. A rapid warming event was observed at the end of 1996‐beginning of 1997 over the slope. This event, also seen in sea level and sea surface temperature off Chile, led El Niño indices by several months and may be best explained by local wind anomalies coupled to atmospheric teleconnections from the western tropical Pacific at the onset of El Niño. Some aspects of our current and temperature observations are consistent with results from eastern boundary current models. Our results indicate that both ocean and atmosphere pathways from the equatorial Pacific contribute significantly to interannual‐scale variability of alongshore flow and thermocline depth off Chile.