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

Autor: Gallego, D.1 dgalpuy@upo.es, García‐Herrera, R.2,3, Losada, T.2, Mohino, E.2, Rodríguez de Fonseca, B.2,3
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Zdroj: Journal of Geophysical Research. Oceans. May2021, Vol. 126 Issue 5, p1-17. 17p.
Abstrakt: In this study, we make use of historical wind direction observations to assemble an instrumental upwelling index (DUI) at the southern end of the Canary Current Upwelling System. The DUI covers the period between 1825 and 2014 and, unlike other upwelling indices, it does not rely neither in wind speed nor in reanalyzed data. In this sense, the DUI can be regarded as an instrumental index. Additionally, it avoids the suspected bias toward increasing wind speed of historical wind observations documented in previous research. Our results indicate that the frequency of the alongshore winds at the west coast of Africa between 10°N and 20°N measured by the DUI is significantly related with the wind stress and therefore the upwelling intensity in this region. The DUI presents a significant variability both at interannual and decadal timescales. We have not found any significant trend for the 20th century. However, when the entire length of the series is considered, a large shift toward more frequent alongshore winds is evidenced as a result of several decade‐long fluctuations which took place between the late 19th century and the beginning of the 20th century. This fact would imply that a significant change in the upwelling intensity at the southern end of the Canary Current Upwelling System should have occurred at the turn of the 20th century. Plain Language Summary: 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. Key Points: Historical wind observations can be used to calculate the upwelling intensity at the Senegal‐Mauritanian coasts since the mid‐19th centuryWinds over the Senegal‐Mauritania coast may have experienced a shift around the year 1900 [ABSTRACT FROM AUTHOR]
Databáze: GreenFILE
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