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Investigaciones marinas

On-line version ISSN 0717-7178

Investig. mar. vol.30 no.1 suppl.Symp Valparaíso Aug. 2002

http://dx.doi.org/10.4067/S0717-71782002030100013 

Intraseasonal Oscillations along the
West Coast of South America

Samuel Hormazabal1, Gary Shaffer1, Oscar Pizarro2

1Danish Center for Earth System Science, Niels Bohr
Institute for Astronomy, Physics and Geophysics,
University of Copenhagen, Copenhagen Denmark,
E-mail: sam@dcess.ku.dk
2Programa Regional de Oceanografía Física y Clima,
University of Concepción, Concepción, Chile.

Introduction

Ten years (1991-2001) of continuous current meter records over the continental slope at 30°S off Chile, coastal sea level along the Peru-Chile coast, satellite wind data (ERS1 and ERS2), and a simple model of wind-forced, low frequency waves are used to study the origins of strong, observed intraseasonal variability in the ocean and atmosphere off Chile.

Intraseasonal Variability of Current and Sea Level

The current measurements at 220, 485 and 750 m depth over the slope at 30°S as well as sea level data off South America exhibit significant variance in the intraseasonal band (30-90 days) with energy enhancement during the warm, El Niño events in 1991-92 and 1997-98. There is less such variability during the cold, La Niña event in 1995-96, particularly at the deeper current meter depths and at the more northern sea level positions. There is also enhanced intraseasonal variability during the (austral) summer.

Lagged-correlations among the coastal sea level records yield a tightly-constrained, poleward propagation speed of 266 km/d. This speed agrees very well with theoretical phase speeds of first mode, coastal trapped waves along the South American coast.

Intraseasonal Wind Variability

Along the equator in the Pacific, intraseasonal variability in zonal winds increases west of 110-130°W while such variability in alongshore winds along the West Coast of South America increases abruptly south of 20-25°S.

Intraseasonal winds on the equator tend to be strongest in the western tropical Pacific during the austral summer and in the central tropical Pacific during the austral winter. This pattern broke down during the 1995 La Niña event.

Off the South American coast, the strongest intraseasonal winds are stronger and extend farther northward during winter. The largest intraseasonal wind event along much of the equator was found at the onset of the (moderate) 1991-92 El Niño event. In contrast, intraseasonal winds off Chile were strongest during the (strong) 1997-98 event.

Intraseasonal wind fluctuations off Chile are correlated to wind fluctuations in the equatorial Pacific. During summer, such downwelling (upwelling) favorable winds off Chile are related to downwelling (upwelling) favorable winds in the western tropical Pacific. During winter, a similar relationship is found but with winds from the central tropical Pacific.

The structure of the results may be associated with two atmospheric teleconnection pathways:

1. Eastward propagation of atmospheric equatorial Kelvin waves which generate atmospheric, poleward-propagating waves, trapped by the Andes mountains, and

2. Rossby wave trains from the western tropical Pacific to subtropical latitudes off South America.

Model Results

Simulations with a very simple, linear, wind-driven, ocean model of equatorial Kelvin waves and coastal trapped waves, forced by satellite winds from the equator and the South American coast compare well with observed fluctuations of currents and sea level off central Chile.

Intraseasonal variability, particularly during summer and during El Niño events, arrives as free, oceanic, coastal trapped waves via oceanic, equatorial Kelvin waves, forced by tropical Pacific winds. South of 20°S, some oceanic intraseasonal variability is forced by local winds associated with the atmospheric teleconnections.

Conclusion

Intraseasonal winds along the equator in the Pacific strongly influence atmospheric and oceanic conditions off the West Coast of South America by way of long waves in the atmosphere and ocean.

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