Intraseasonal variability of wind-forced coastal upwelling off central Chile (30°S)

Abstract

Satellite-derived along-shore windstress variability off subtropical Chile at intraseasonal periods has been recently associated with the corresponding wind variability over the tropical Pacific. Along the coastal strip in which satellite winds can not be resolved, wind-driven upwelling might be in principle modulated through large-scale intraseasonal changes in the strength of the subtropical anticyclone and by intraseasonal ocean coastal-trapped waves (CTWs) through associated changes in coastal sea-surface temperatures. Daily anomalies of band-passed outgoing long-wave radiation over the central equatorial Pacific have been selected to represent Madden–Julian Oscillations, as the dominant mode of equatorially sourced, intraseasonal variability in the atmosphere and coastal ocean. Afternoon alongshore pseudo-windstresses at a representative coastal site in central Chile (30°S) describe the upwelling-favourable wind field over a 30–40 km coastal strip. The subtropical anticyclone strength is expressed as a 900-km offshore sea-level pressure (San Felix Island), and the sea-level pressure difference with the coast. CTWs are represented by the smoothed sea-level at 27°S. The intermittent nature of intraseasonal variability led to statistical procedures as Wavelet and Singular-Spectrum (SSA) analyses to search for associations among those series. Cross-correlation analysis of the SSA-reconstructed series with the two principal oscillatory modes (40–100-day, and 28–50-day periods, respectively), shows that intraseasonal variability in the subtropical anticyclone strength, partially associated with the Pacific South America teleconnection pattern, has the best association with coastal windstress variability. CTWs appeared to be important in their association with coastal winds only during strong MJO activity in connection with El Niño in the low-frequency intraseasonal band, while in the high-frequency one a fraction of the ASL variability appears to be locally wind-forced.