Secular Variability of the Upwelling at the Canaries Latitude: An Instrumental Approach.

In this research we make use of historical wind direction observations to assemble an instrumental upwelling intensity index (the so‐called Directional Upwelling Index [DUI]) for the coast of Northwest Africa between 26° and 33°N and from 1825 to 2014. The DUI is defined as the persistence of the alongshore winds at the coast and unlike other upwelling indices, it relies on observed wind direction solely, avoiding the suspected bias toward increasing wind speed of historical wind observations documented in previous research. We have found that between June and October, when the upwelling intensity in the area is at its seasonal maximum, the persistence of the north‐easterlies measured by the DUI is significantly related to the alongshore wind stress and subsequently with Sea Surface Temperature anomalies at the coast of NW Africa. The analysis of the DUI record does not display a consistent long‐term trend but an oscillatory behavior. At interannual time scales this variability can be linked to the changes in the strength and location of the subtropical north Atlantic high‐pressure center and at multidecadal scales, the upwelling seems mainly driven by the Atlantic Multidecadal Variability through the modulation exerted by this climatic pattern on the intensity of the Saharan low. Plain Language Summary: Upwelling is a process in which sea water from intermediate depths rises toward the surface as a response to the wind friction along the west coast of continents. Upwelled water is rich in nutrients, creating areas of paramount importance for fisheries. A long‐standing hypothesis contends that upwelling might be intensified because of global warming, but due to the impediments to quantify the upwelling intensity for long periods, the scientific community still debates whether the upwelling is changing. We have used historical wind observations taken aboard ships sailing along the coast of Northwest Africa to show that there, during the last two centuries, upwelling has not increased but it has oscillated synchronically with the temperature of the North Atlantic. Key Points: Historical wind direction observations are used to build a centennial‐long upwelling intensity series for the CanariesUpwelling at the Canaries shows trends highly dependent upon the period consideredNorth Atlantic Sea Surface Temperatures drive the upwelling intensity at the Canaries at multidecadal scales [ABSTRACT FROM AUTHOR]