Weather Types and Rainfall over Senegal. Part I: Observational Analysis

A k-means cluster analysis is used to summarize unfiltered daily atmospheric variability at regional-scale over the western Sahel and eastern tropical North Atlantic during the boreal summer season (July—September, JAS) 1961—1998. It employs zonal and meridional regional wind fields at 925, 700 and 200 hPa from the European Centre for Medium Range Weather Forecast Reanalyses. An 8-cluster solution is shown to yield an integrated view of the complex regional circulation variability, without the need for explicit time filtering. Five of the weather types identified characterize typical phases of African easterly waves and persistent monsoon surges, while the three others describe different stages of the seasonal cycle. Their temporal sequencing describes a systematic monsoonal evolution, together with considerable variability at sub-seasonal and inter-annual time scales. Daily rainfall occurrence at 13 gauge stations in Senegal is found to be moderately wellconditioned by the 8 weather types, with positive rainfall anomalies usually associated with lowlevel southerly wind anomalies. Interannual variability of daily rainfall frequency is shown to depend substantially on the frequency of occurrence of weather types specific to the beginning and end of the JAS season, together with the number of persistent monsoon surges over the western Sahel. In contrast, year-to-year changes in the frequency of the weather types associated with African Easterly Waves are not found to influence seasonal rainfall substantially. The fraction of seasonal rainfall variability related to weather type frequency is found to have a strong relationship with tropical Pacific sea surface temperatures (SST): an El Niño (La Niña) event tends to be associated higher (lower) frequency of dry early and late JAS season weather