Abstract
The biannual seasonal rainfall regime over the southern part of West Africa is characterised by two wet seasons, separated by the ‘Little Dry Season’ in July–August. Lower rainfall totals during this intervening dry season may be detrimental for crop yields over a region with a dense population that depends on agricultural output. Coupled Model Intercomparison Project Phase 5 (CMIP5) models do not correctly capture this seasonal regime, and instead generate a single wet season, peaking at the observed timing of the Little Dry Season. Hence, the realism of future climate projections over this region is questionable. Here, the representation of the Little Dry Season in coupled model simulations is investigated, to elucidate factors leading to this misrepresentation. The Global Ocean Mixed Layer configuration of the Met Office Unified Model is particularly useful for exploring this misrepresentation, as it enables separating the effects of coupled model ocean biases in different ocean basins while maintaining air–sea coupling. Atlantic Ocean SST biases cause the incorrect seasonal regime over southern West Africa. Upper level descent in August reduces ascent along the coastline, which is associated with the observed reduction in rainfall during the Little Dry Season. When coupled model Atlantic Ocean biases are introduced, ascent over the coastline is deeper and rainfall totals are higher during July–August. Hence, this study indicates detrimental impacts introduced by Atlantic Ocean biases, and highlights an area of model development required for production of meaningful climate change projections over the West Africa region.
Highlights
The southern part of West Africa is a highly populated region, with many people dependent upon seasonal rainfall for farming activities and domestic purposes
4.3 Southward Bias in the Tropical Rain Belt (TRB) position in May In May, simulations using coupled model ocean mean state (GC2–GC2, Global Ocean Mixed Layer (GOML)-GC2, and GOML-ATL-N96), which includes a warm bias over the southern tropical Atlantic, underestimate rainfall over the Little Dry Season’ (LDS) region (Fig. 6a) as part of a wider southward bias in the position of the tropical rain belt (Fig. 7a, c)
Several configurations of the Met Office Unified Model (MetUM) were used to explore factors that influence the representation of the seasonal cycle of precipitation over the southern part of West Africa, which is unrealistically represented in coupled climate model simulations (Dunning et al 2017)
Summary
The southern part of West Africa is a highly populated region, with many people dependent upon seasonal rainfall for farming activities and domestic purposes. Many studies have identified and explored deficiencies in the representation of the West African Monsoon in atmosphere-only climate model simulations (AGCMs) and CGCMs (Cook and Vizy 2006; Roehrig et al 2013; Flato et al 2013). Most models still have significant difficulties simulating African Easterly Waves, similar to the CMIP5 models (Martin and Thorncroft 2015) In their assessment of the representation of rainfall seasonality in AMIP and CMIP5 models across Africa, Dunning et al (2017) identified a further deficiency in the representation of the seasonal cycle of precipitation over the southern part of West Africa in CGCMs. While AGCMs, forced by observed SSTs, correctly produced wet seasons in April-June and September-October, separated by the LDS, the CGCMs generated a single wet season, with a single rainfall peak in July-August, coincident with the observed LDS.
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