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

Abstract

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. Since the 1970s, the Sahel has experienced a strong and persistent drought. During this period the West African Monsoon has been weaker than normal and its correlation with different climatic patterns such as the ENSO has changed. The analysis of our new series indicates that this anomalous behaviour of the West African Monsoon has no precedent in the last 170 years. Our results also show that the period 1839-1890 was characterised by a stronger than average monsoon, resulting in relatively wet conditions in the Sahel. Notwithstanding, two of the few dry years within this period were concurrent with large volcanic eruptions in the Northern Hemisphere. This latter result supports the recently described relationship between major volcanic eruptions and the occurrence of isolated drought episodes in the Sahel.