A Shift in the Wind Regime of the Southern End of the Canary Upwelling System at the Turn of the 20th Century

Abstract

Along the coasts of Northwestern Africa, friction of the predominant winds results in an upward motion of sea water from intermediate depths toward the ocean surface. This phenomenon is known as "coastal upwelling" and it has a huge economic and social relevance. Cold upwelled water is rich in nutrients and yields very productive marine ecosystems. Therefore, knowing the changes of the upwelling intensity is of great importance. Unfortunately, prior to the 1950s, the scarcity of wind observations along these coasts difficulties the estimation of the coastal upwelling intensity, making the climatic history of this system uncertain. Since the late 18th century, a lot of ships have circumnavigated the African continent. Most of them took observations of wind direction that have come to the present day in form of records preserved in the ships' logbooks. In this study, we make use of these historical observations to calculate the intensity of the coastal upwelling. We have found that coastal upwelling in Northwest Africa is highly variable at decadal scale and we provide a strong evidence of a large change in the upwelling intensity that occurred around 1900.