Relationship between the Southeast Pacific Anticyclone and the upwelling favorable winds along the west coast of South America

The Eastern Boundary Upwelling Systems maintains high productive fisheries and marine ecosystems. In particular, the influence of the Southeast Pacific Anticyclone (SPA) results in predominantly southerly winds that sustain the Humboldt Upwelling System (HUS) along the west coast of South America. Under this classical picture, paleoclimatic reconstructions of winds using coastal marine sediments have often assumed that an increase in upwelling favorable winds equals to an intensified SPA through geostrophic adjustment. On the other hand, climate projections under warming scenarios show an increase of the upwelling intensity at the HUS south of 35ºS. Nevertheless, connection between changes in the SPA and upwelling favorable winds has not been considered in detail and Bakun´s hypothesis (i.e. alongshore winds intensified by strengthening the land-sea thermal difference in upwelling regions) has concentrated more attention. We use reanalysis data (CFSR), as well as historical and future simulations of 27 models of the CMIP5 project to investigate the relationship between the SPA and the upwelling favorable winds at several latitudes along the HUS during the upwelling season (Dec-Jan-Feb). Results show a significant relationship between the intensity of the SPA and the upwelling favorable winds south of 35ºS, but not further north. No significant relationship at lower latitudes is probably due to the frequent occurrence of a synoptic low-level jet, which is forced by an alongshore pressure gradient. Composite of the reanalysis data reveal that an increase of upwelling favorable winds at 30ºS is accompanied by an intensified SPA and an increment of the synoptic variability at the coast. In contrast, an increase of upwelling favorable winds at higher latitudes (south of 35ºS) is mainly due to a displacement of the SPA and a decrease of the synoptic variability at the coast is observed. The CMIP5 models show a positive tendency of the SPA intensity which produces an increment of the zonal pressure gradient south of 35ºS. This suggests that geostrophic balance is playing a role in the projected increase of upwelling favorable winds at the HUS.