Abstract
AbstractA 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)] 1961–98. The analysis employs zonal and meridional regional wind fields at 925, 700, and 200 hPa from the European Centre for Medium-Range Weather Forecasts reanalyses. An eight-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 mostly typical phases of westward-moving wave disturbances, such as African easterly waves (AEWs), and persistent monsoon surges, while the three others describe mostly different stages of the seasonal cycle. Their temporal sequencing describes a systematic monsoonal evolution, together with considerable variability at subseasonal and interannual time scales.Daily rainfall occurrence at 13 gauge stations in Senegal is found to be moderately well conditioned by the eight weather types, with positive rainfall anomalies usually associated with southerly wind anomalies at 925 hPa. 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 mostly associated with westward-moving waves such as AEWs are not found to influence seasonal frequency of occurrence of daily 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 (SSTs): an El Niño (La Niña) event tends to be associated with a higher (lower) frequency of dry weather types during early and late JAS season with enhanced trade winds over the western Sahel, together with lower (higher) prevalence of persistent monsoon surges. The component of seasonal rainfall variability not related to weather-type frequency is characterized by changes in rainfall probability within each weather type, especially those occurring in the core of the JAS season; it exhibits a larger decadal component that is associated with an SST pattern previously identified with recent observed trends in Sahel rainfall.
Highlights
Results of the first canonical correlation analyses (CCA) between ni,k and Ni,j are presented in Fig. 11, in terms of (a) correlation between the “predicted” and observed rainfall occurrence frequency Ni,j at each station, (b) the loading of each weather type (WT)’s frequency ni,k, and (c) the cross-validated CCA time scores
WT 1 was found to be associated with dry spells during the core of the JAS season (Fig. 4)
WT 8 may be associated with late-season interactions with the extratropics (Fig. 8), leading to scattered rainfall mainly over northwest Senegal
Summary
Model (AGCM) experiments with historical SSTs prescribed (Rowell et al 1995; Moron et al 1998, 2004; Giannini et al 2003; Paeth and Friederichs 2004) Such experiments have been highly successful at simulating the multidecadal variability of Sahelian rainfall (Giannini et al 2003), though relatively few models are successful at interannual time scales, especially during the post-1967 dry period (Sperber and Palmer 1996; Moron et al 2003; Moron 2005). Rainfall over West Africa during the summer monsoon is mainly associated with mesoscale convective systems, especially over the Sahel (D’Amato and Lebel 1998; Laurent et al 1998; Lebel et al 2003). The subseasonal variability of rainfall in Sahel is composed of the interactions between interrelated processes with various time and space scales (Sultan et al 2003; Druyan et al 2006) It is little understood how these subseasonal processes are influenced by SST forcing
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
