Understanding Rainfall Prediction Skill Over the Sahel in NMME Seasonal Forecast

Sahelian rainfall presents large interannual variability which is partly controlled by the sea surface temperature anomalies (SSTa) over the eastern Mediterranean, equatorial Pacific and Atlantic oceans, making seasonal prediction of rainfall changes in Sahel potentially possible. However, it is not clear whether seasonal forecast models present skill to predict the Sahelian rainfall anomalies. Here, we consider the set of models from the North American Multi-model ensemble (NMME) and analyze their skill in predicting the Sahelian precipitation and address the sources of this skill.Results show that though the skill in predicting the Sahelian rainfall is generally low, it and can be mostly explained by a combination of how well models predict the SSTa in the Mediterranean and in the equatorial Pacific regions, and how well they simulate the teleconnections of these SSTa with Sahelian rainfall. Our results suggest that Sahelian rainfall skill is improved for those models in which the Pacific SST - Sahel rainfall teleconnection is correctly simulated. On the other hand, models present a good ability to reproduce the sign of the Mediterranean SSTa – Sahel teleconnection, albeit with underestimated amplitude due to an underestimation of the variance of the SSTa over this oceanic region. However, they fail to correctly predict the SSTa over this basin, which is the main reason for the poor Sahel rainfall skill in models. Therefore, results suggest models need to improve their ability to reproduce the variability of the SSTa over the Mediterranean as well as the teleconnections of Sahelian rainfall with Pacific and Mediterranean SSTa.

Understanding Sahelian rainfall skill in the NMME seasonal forecast

<p>Sahelian rainfall presents variability from internannual to interdecadal timescales, which is influenced by the sea surface temperature anomalies (SSTa) in different basins. At interannual times scales it has been shown that this variability depends on the SSTa over the equatorial Pacific, Atlantic and eastern Mediterranean. In this work we consider the set of models from the North American Multi-model ensemble (NMME) in order to analyze their skill in reproducing the Sahelian precipitation variability and relate it to their skill in reproducing the variability of the SSTa over the equatorial Pacific, equatorial Atlantic and eastern Mediterranean as well as their ability to simulate their teleconnections with Sahel rainfall.</p><p>Results show that the skill in predicting Sahel rainfall is low, decreases rapidly with lead time and is highly model dependent. Skill is improved for those models that are able to correctly simulate the Pacific SST - Sahel rainfall teleconnection.  Models present a good ability to reproduce the Mediterranean SST – Sahel teleconnection, and skill in Sahel rainfall prediction is more dependent on the correct prediction of the Mediterranean SST anomalies. These results suggest a path to increase skill in Sahel rainfall prediction.</p>

On the decadal scale correlation between African dust and Sahel rainfall: The role of Saharan heat low–forced winds

A large body of work has shown that year-to-year variations in North African dust emission are inversely proportional to previous-year monsoon rainfall in the Sahel, implying that African dust emission is highly sensitive to vegetation changes in this narrow transitional zone. However, such a theory is not supported by field observations or modeling studies, as both suggest that interannual variability in dust is due to changes in wind speeds over the major emitting regions, which lie to the north of the Sahelian vegetated zone. We reconcile this contradiction showing that interannual variability in Sahelian rainfall and surface wind speeds over the Sahara are the result of changes in lower tropospheric air temperatures over the Saharan heat low (SHL). As the SHL warms, an anomalous tropospheric circulation develops that reduces wind speeds over the Sahara and displaces the monsoonal rainfall northward, thus simultaneously increasing Sahelian rainfall and reducing dust emission from the major dust “hotspots” in the Sahara. Our results shed light on why climate models are, to date, unable to reproduce observed historical variability in dust emission and transport from this region.