The Potential to Enhance Tropical Atlantic Sector Rainfall Projections by Alleviating the Tropical Atlantic Sea Surface Temperature Bias

We investigate the influence of the Tropical Atlantic sea surface temperature (SST) bias, a common bias in state-art-of-the-art climate models, on the Tropical Atlantic Sector rainfall response to increasing atmospheric carbon dioxide (CO2) content. For this purpose, two sets of simulations with the Kiel Climate Model (KCM) are compared. Each set consists of a present-day control run and an ensemble of greenhouse warming experiments in which the atmospheric CO2-concentration is increasing by 1 %/year until CO2-doubling is reached. The first set of simulations is conducted with a KCM version that consists of a coarse-resolution ocean model (ORCA2 grid) coupled to a coarse-resolution atmosphere model (T42, L31). This version exhibits a large SST bias over the eastern Tropical Atlantic, similar to that observed in many CMIP5 models with respect to both magnitude and pattern. The second set of simulations is performed with a KCM version which employs the identical ocean model but an atmospheric component with much higher horizontal and vertical resolution (T255, L62). This KCM version depicts very little SST bias over the eastern Tropical Atlantic and a realistic warm pool-cold tongue structure.

We find a large sensitivity of the CO2-induced rainfall changes to the presence of the Tropical Atlantic SST bias. In the KCM version exhibiting a large SST bias, there is hardly any significant rainfall response over the Tropical Atlantic and the adjacent land areas. On the contrary, the KCM version exhibiting a much reduced SST bias depicts a highly significant rainfall response over the Tropical Atlantic sector in the form of a marked southward migration of the ITCZ that is associated, for example, with a major rainfall reduction over the Caribbean and a large rainfall increase over equatorial Africa. The lack of a significant rainfall response in the presence of a large SST bias in the KCM version employing a coarse-resolution atmosphere model can be traced back to the SST response in the Atlantic, which is rather uniform in the region 20°N-20°S. In the KCM version employing the high-resolution atmosphere model, exhibiting a relative small SST bias, the SST response in the Atlantic is “El Niño-like” which drives the southward migration of the ITCZ, in a way similar to the southward migration of the ITCZ over the tropical Pacific during classical El Niño events. This study suggests that climate model projections of Tropical Atlantic sector rainfall for the 21^st century could greatly benefit from alleviating the Tropical Atlantic SST bias.