INCITE: West African Summer Monsoon
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An instrumental index of the West African Monsoon back to the nineteenth century
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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 theperiod 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.
Key Words: monsoon; Sahel; precipitation; Africa; climate; reconstruction; logbook
Reference: Gallego, D., P. Ordóñez, P. Ribera, C. Peña-Ortiz and R. García-Herrera, 2015:
An instrumental index of the West African Monsoon back to the nineteenth century. Q.J.R. Meteorol. Soc.
141, 3166-3176.
doi:10.1002/qj.2601
Figure 1: Standardised seasonal (JAS) ASWI between 1840 and 2013 and seasonal average.
Shaded curve is computed as a robust locally weighted regression with a 31-year window.
The ASWI series can be downloaded HERE.
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