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An instrumental index of the West African Monsoon back to the nineteenth century

A new index, namely the African Southwesterly Index (ASWI), based on the persistence of the low‐level southwesterly winds in the region (29°–17°W; 7–13°N) is developed to characterise the strength of the West African Monsoon. The ASWI is significantly correlated with the monsoonal precipitation in the Sahel in the 1900–2013 period (r = + 0.57, r = + 0.66 and r = + 0.53 for July, August and September respectively). By using historical wind direction measurements it has been possible to compute the ASWI back to 1790 for July and to 1839 for August and September, thus providing the first instrumental record of the West African Monsoon strength for a large portion of the nineteenth century.