Search

The upper-tropospheric Tropical Easterly Jet (TEJ) is an integral part of the summer-time West African Monsoon (WAM) circulation and is strongly correlated with Sahel rainfall changes on interannual and decadal time scales. To explain the covariability between the TEJ and Sahel rainfall, this study investigates whether the variability of the seasonal-mean TEJ over West Africa is mainly forced by local WAM rainfall changes or whether the TEJ variability is caused by remote atmospheric forcing that might also drive Sahel rainfall variability. For this, AGCM simulations with dry dynamics are driven by different atmospheric diabatic heating patterns derived from reanalyses and observations. To disentangle the effects of remote versus local forcing, either the remote diabatic forcing is varied from year to year while the diabatic forcing over Africa is kept constant to its climatological values or the local diabatic forcing is varied and the remote forcing is fixed. It is found that a major part of the interannual TEJ variability over West Africa can be explained by remote planetary-scale diabatic heating anomalies that are strongly related to well-known modes of SST variability, in particular ENSO. In many cases, remote heating anomalies leading to a more intense West-African TEJ are associated with SST anomaly pattern which were shown to enhance Sahel rainfall. The influence of WAM rainfall anomalies on the TEJ over West Africa is relatively weak on interannual time scales. On decadal time scales, this local forcing becomes more important, though. The increase of West-African TEJ intensity over the last two decades is strongly linked to the coincident recovery of Sahel rainfall. In synopsis, the model experiments strongly suggest that the high correlation between the TEJ and Sahel rainfall is mainly explained by the simultaneous influence of the remote diabatic forcing on the local WAM rainfall and the TEJ and less by the sole effect of the local WAM rainfall variability on the TEJ over West Africa. [ABSTRACT FROM AUTHOR]

The presence of large internal variability makes detecting a forced response of rainfall to global warming difficult—in particular for regional studies. Therefore, we use a large ensemble of a comprehensive climate model to disentangle the forced response from internal variability in Sahel rainfall. We use both historical simulations to understand the observed record and different future scenarios to investigate the response of Sahel rainfall to future warming in the Max Planck Institute Grand Ensemble (MPI-GE).Observed Sahel rainfall has been decreasing from the 1960s to the 1980s and recovering in the decades thereafter. Moreover, some studies have suggested that the interannual variability in Sahel rainfall has increased in the recent decades. Using the large ensemble, we are able to disentangle internal variability and the forced response and find an externally forced increase in mean Sahel rainfall in the end of the 20th century. In addition we find evidence for a forced increase in interannual rainfall variability. However, the presence of large internal variability makes it impossible to robustly detect and attribute these forced changes in a single realisation. If the simulated magnitude of change is realistic, this implies that even if we can identify these changes in the observed record, they cannot be clearly attributed to a change in the forcing. Only a large ensemble allows a clean separation of forced changes and internal variability.In a strong forcing scenario, we find a forced increase in Sahel rainfall and rainfall variability in the 21st century. For weaker forcing scenarios, we find no change or even a projected decrease in Sahel rainfall with no forced change in the variability. For future scenarios, a large ensemble not only enables us to identify forced changes, but also to estimate how likely it is that we will be able to detect these changes in the single realisation of the real world. [ABSTRACT FROM AUTHOR]