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Oceanic influence on the sub-seasonal to interannual timing and frequency of extreme dry spells over the West African Sahel Seyni

Seyni Salack, Alessandra Giannini, Moussa Diakhaté, Amadou. T. Gaye & Bertrand Muller

Abstract Intra-seasonal drought episodes (extreme dry
spells) are strongly linked to crop yield loss in the West
African Sahel, especially when they occur at crop critical
stages such as juvenile or flowering stage. This paper seeks
to expose potentially predictable features in the sub-seasonal
to inter-annual occurrence of ‘‘extreme dry spells’’
(extDS) through their links to sea surface temperature
anomalies (SSTAs). We consider two kinds of extreme dry
spells : more than 2 weeks of consecutive dry days following
a rain event (often found at the beginning of the
rainy season, after the first rain events) and more than a
week (observed towards the end of the rainy season, before
the last rain events). We extract dry spells from daily rainfall data at 43 stations (31 stations in Senegal over
1950–2010 and 12 stations in Niger over 1960–2000) to
identify the intra-seasonal distribution of extDS and their
significant correlation with local rainfall deficits. Seasonality
of distribution and high spatial coherence are found in
the timing and the frequency of occurrence of extDS in
different rainfall regions over Niger and Senegal. The
correlation between the regional occurrence index (ROI),
necessary to capture the spatial extent of extDS, and
observed global sea surface temperature anomalies
(SSTAs) sheds light on the influence of the external factors
on the decadal, interannual and sub-seasonal variability of
extDS over the West African Sahel. When the global tropics
and the Atlantic are warmer than normal, more
coherent and delayed June–July extDS are observed after
onset of rainy season, as well as early cessation type in
August–September. When the Indo-Pacific is cooler and
the equatorial south Atlantic is warmer than normal little to
no extDS are found in the onset sub-period of the monsoon
season. Mostly late types of extDS occur in October as a
result of late cessation. These results show potential predictability
of extreme dry spells after onset and before
cessation of monsoonal rain based on global patterns of sea
surface temperature anomalies.

Keywords : Extreme dry spells - Seasonality - Spatial
coherence - Regional Occurrence Index - SST anomaly -
Niger and Senegal - West African Sahel