Abstract
Abstract The onset of the West African monsoon (WAM) marks a vital time for local and regional stakeholders. While the seasonal progression of monsoon winds and the related migration of precipitation from the Guinea Coast toward the Sudan/Sahel is apparent, there exist contrasting man-made definitions of what the WAM onset means. Broadly speaking, onset can be analyzed regionally, locally, or over a designated intermediate scale. There are at least 18 distinct definitions of the WAM onset in publication, with little work done on comparing observed onset from different definitions or comparing onset realizations across different datasets and resolutions. Here, nine definitions have been calculated using multiple datasets of different metrics at different resolutions. It is found that mean regional onset dates are consistent across multiple datasets and different definitions. There is low interannual variability in regional onset, suggesting that regional seasonal forecasting of the onset provides few benefits over climatology. In contrast, local onsets show high spatial, interannual, and interdefinition variability. Furthermore, it is found that there is little correlation between local onset dates and regional onset dates across West Africa, implying a disharmony between regional measures of onset and the experience on a local scale. The results of this study show that evaluation of seasonal monsoon onset forecasts is far from straightforward. Given a seasonal forecasting model, it is possible to simultaneously have a good and a bad prediction of monsoon onset simply through selection of the onset definition and observational dataset used for comparison.
Highlights
The annual advance of the West African monsoon (WAM) is a pivotal period for the inhabitants of the region
The progression of the monsoon system into the Sahel marks a shift in regional climate dynamics signifying the beginning of an active period for the formation of westward-propagating mesoscale convective systems and African easterly waves (AEW; Fink and Reiner 2003), which can transform
Onset composites for Mart, Omo, and Yam using Global Precipitation Climatology Project (GPCP) show earlier onsets than for TRMM v7 for almost the entire West African region
Summary
The annual advance of the West African monsoon (WAM) is a pivotal period for the inhabitants of the region. The regional definitions employ a variety of data: precipitation (e.g., Sultan and Janicot 2003), outgoing longwave radiation (OLR, such as Fontaine et al 2008), moist static energy (MSE, as found in Vellinga et al 2012), zonal winds (Nguyen et al 2014), and daily rainfall event frequency (Le Barbé et al 2002), among others. Their work highlights an appreciable jump (or abrupt northward surge) of the time-smoothed maximum daily rain event frequency in composites from 1950 to 1990 from 58N to 108N during late June/early July This result allowed for formation of a regional onset definition, LeB_Reg, influenced by the original work. LeB_Loc has been constructed in the same way as LeB_Reg for 18 longitude ranges, as opposed to zonal averaging across 108W–108E This allows for examination of longitudinal variability across the monsoon region within a given onset definition and a direct comparison of one definition across two scales of observations. The comparison between this definition and that given by Mart provides an interesting contrast between a readily modifiable and agronomic definition WAM onset
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