The impact of air–sea coupling and ocean biases on the seasonal cycle of southern West African precipitation

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.

Weakened impact of the Atlantic Niño on the future equatorial Atlantic and Guinea Coast rainfall

“…The ERA5 dataset, a reanalysis product from the European Centre for Medium-Range Weather Forecasts (ECMWF, Hersbach et al, 2020), is used as a reference to evaluate the performance of the CMIP6 models in simulating the surface and atmospheric fields under present-day conditions. ERA5 has replaced ERA-Interim (Dee et al, 2011), which has been widely used to study the characteristics of the West African climate (Manzanas et al, 2014;Lavaysse et al, 2015;Kebe et al, 2016;Raj et al, 2018;Maranan et al, 2018;Diakhaté et al, 2019Diakhaté et al, , 2020Wainwright et al, 2019). ERA5, for instance, shows an improvement of the rainfall representation over Burkina Faso (Tall et al, 2019), which is a part of the Sahelian region.…”

“…As a consequence, during July and August, the atmospheric circulation above the Guinea Coast is characterized by a downward flow at upper levels. This leads to the so-called "little dry season" (Adejuwon and Odekunle, 2006;Fink et al, 2017;Wainwright et al, 2019), during which the Guinea coast rainfall decreases in magnitude compared to its mean over the April-May-June season. Over the 20th century, the interannual variability of the Guinea Coast rainfall was strongly influenced by the sea surface temperature (SST) fluctuations in the eastern equatorial Atlantic (Giannini, 2003;Polo et al, 2008;Suárez-Moreno et al, 2018).…”

Weakened impact of the Atlantic Niño on the future equatorial Atlantic and Guinean Coast rainfall

“…The CMIP5 models are also known to overestimate the rainfall in coastal areas of West Africa during the "little dry season" (Sow et al, 2020). Wainwright et al (2019) have demonstrated that the misrepresentation of the July-August Guinean Coast rainfall in these models comes from the positive SST biases in the Atlantic Ocean, which strengthen the rising motions over the Guinea Coast and increase the rainfall. Consistently, the CMIP6 ensemble mean exhibits a positive (negative) mean SST bias in the eastern (western) part of the tropical Atlantic, as depicted in Fig.…”

“…The ERA5 dataset, a reanalysis product from the European Centre for Medium-Range Weather Forecasts (ECMWF, Hersbach et al, 2020), is used as a reference to evaluate the performance of the CMIP6 models in simulating the surface and atmospheric fields under present-day conditions. ERA5 has replaced ERA-Interim (Dee et al, 2011) that has been widely used to study the characteristics of the West African climate (Manzanas et al, 2014;Lavaysse et al, 2015;Kebe et al, 2016;Raj et al, 2018;Maranan et al, 2018;Diakhaté et al, 2019Diakhaté et al, , 2020Wainwright et al, 2019). ERA5 for instance shows an improvement of the rainfall representation over Burkina Faso (Tall et al, 2019), which is a part of the Sahelian region.…”

Reply on RC1

“…The ERA5 dataset, a reanalysis product from the European Centre for Medium-Range Weather Forecasts (ECMWF, Hersbach et al, 2020), is used as a reference to evaluate the performance of the CMIP6 models in simulating the surface and atmospheric fields under present-day conditions. ERA5 has replaced ERA-Interim (Dee et al, 2011) that has been widely used to study the characteristics of the West African climate (Manzanas et al, 2014;Lavaysse et al, 2015;Kebe et al, 2016;Raj et al, 2018;Maranan et al, 2018;Diakhaté et al, 2019Diakhaté et al, , 2020Wainwright et al, 2019). ERA5 for instance shows an improvement of the rainfall representation over Burkina Faso (Tall et al, 2019), which is a part of the Sahelian region.…”