Ocean Science Trends in coastal upwelling intensity during the late 20th century

Abstract. This study presents linear trends of coastal up-welling intensity in the later part of the 20th century (1960– 2001) employing various indices of upwelling, derived from meridional wind stress and sea surface temperature. The analysis was conducted in the four major coastal upwelling regions in the world, which are off North-West Africa, Lüderitz, California and Peru. The trends in meridional wind stress showed a steady increase of intensity from 1960–2001, which was also reflected in the SST index calculated for the same time period. The steady cooling observed in the instru-mental records of SST off California substantiated this ob-servation further. It was also noted that the trends in merid-ional wind stress obtained from different datasets differ sub-stantially from each other. Correlation analysis showed that basin-scale oscillations like the Atlantic Multidecadal Os-cillation (AMO) and the Pacific Decadal Oscillation (PDO) could not be directly linked to the observed increase of up-welling intensity off NW Africa and California respectively. The relationship of the North Atlantic Oscillation (NAO) with coastal upwelling off NW Africa turned out to be am-biguous due to a negative correlation between the NAO index and the meridional wind stress and a lack of correlation with the SST index. Our results give additional support to the hy-pothesis that the coastal upwelling intensity increases glob-ally because of raising greenhouse gas concentrations in the atmosphere and an associated increase of the land-sea pres-sure gradient and meridional wind stress