Capturing the Atlantic cold tongue and coastal upwelling in an intermediate-level ocean model coupled to a regional climate model

Coupled atmosphere–ocean general circulation models are known to have difficulties simulating the cold tongue in the equatorial Atlantic Ocean. Here a regional climate model coupled to an intermediate-level mixed layer ocean model with Ekman dynamics is developed and used to better understand the seasonal evolution of the equatorial Atlantic cold tongue and upwelling off western Africa. Parameterization improvements are made to an earlier version of the ocean model to account for the variations in temperature and shearing stress at the base of the mixed layer. 90-km resolution sensitivity tests demonstrate that the development of the equatorial Atlantic cold tongue in the boreal spring/summer is captured only if seasonal variations in the temperature at the base of the ocean mixed layer are included. The development of cold temperatures off the northwest African coast in the late boreal winter/spring is found to be primarily associated with the net radiation balance as shortwave warming of the mixed layer is relatively low while latent cooling is relatively high yielding a net cooling of mixed layer temperatures, consistent with other studies. The westward extension of the Atlantic cold tongue is primarily due to the horizontal advection of cool water from the South Atlantic African coast. This coastal cooling is associated with vertical diffusion and vertical entrainment, while the vertical entrainment has a secondary and more localized role over the equatorial Atlantic.     

Impact of climate change on mid-twenty-first century growing seasons in Africa

Changes in growing seasons for 2041–2060 across Africa are projected using a regional climate model at 90-km resolution, and confidence in the predictions is evaluated. The response is highly regional over West Africa, with decreases in growing season days up to 20% in the western Guinean coast and some regions to the east experiencing 5–10% increases. A longer growing season up to 30% in the central and eastern Sahel is predicted, with shorter seasons in parts of the western Sahel. In East Africa, the short rains (boreal fall) growing season is extended as the Indian Ocean warms, but anomalous mid-tropospheric moisture divergence and a northward shift of Sahel rainfall severely curtails the long rains (boreal spring) season. Enhanced rainfall in January and February increases the growing season in the Congo basin by 5–15% in association with enhanced southwesterly moisture transport from the tropical Atlantic. In Angola and the southern Congo basin, 40–80% reductions in austral spring growing season days are associated with reduced precipitation and increased evapotranspiration. Large simulated reductions in growing season over southeastern Africa are judged to be inaccurate because they occur due to a reduction in rainfall in winter which is over-produced in the model. Only small decreases in the actual growing season are simulated when evapotranspiration increases in the warmer climate. The continent-wide changes in growing season are primarily the result of increased evapotranspiration over the warmed land, changes in the intensity and seasonal cycle of the thermal low, and warming of the Indian Ocean.