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.

Rainfall Recovery in North-East Arid Zone of Nigeria: Comparative Analysis of Drought and Post Drought Decades

This study used annual rainfall records from three stations within the North East Arid Zone of Nigeria for the period (1957-2017) to measure the extent of the rainfall recovery by comparing the drought decades and post drought decades rainfall patterns. Monthly rainfall records from Potiskum, Maiduguri and Nguru Stations were used. Descriptive and inferential statistical tools were employed in analysing the data. The findings of the study revealed a significant year-to-year variability in rainfall characteristics around 61 years (1957-2017) averages. The variability was large in 1970s up till 1990s, and lower in 1960s and from 2000 to 2018. Decreasing trend in annual rainfall amount was observed during the study period while a stability in onset and cessation dates were observed. The differences between 1957-1986 and 1987-2017 climatic season were found to be statistically insignificant. The study concluded that the reported rainfall recovery from drought is statistically insignificant and the observed long term mean trend revealed a decreasing trend. Therefore, the theory of Sahel rainfall recovery can be better termed as a ‘’break of the series of drought or decline in frequency and magnitude of occurrence of drought’’ The research recommended the continuations with the drought adaptation and mitigation strategies adopted by local population, decisions makers and organizations following the series Sahelian droughts of 1970s and 1980s.

Sahel droughts induced by large volcanic eruptions over the last millennium in IPSL-CM6A-LR model

<p><span>The Sahel region is extremely sensible to alterations in its characteristic precipitation regime, associated with the West African Monsoon (WAM). In fact, the WAM presents strong variability at several timescales which has focused the attention of many works that mainly attribute such changes to variations in the sea surface temperature, the emerging increase of greenhouse gases concentration and to alterations in land use. However, the impact of large volcanic eruptions has been just tentatively addressed. This work aims at shedding more light on the influence of large volcanic eruptions on Sahel rainfall relying on past1000 simulations, covering the last millennium, of the IPSL-CM6A-LR model. The results show the mechanisms involved and the differences between tropical and high-latitude eruptions.</span></p>

Anthropogenic aerosols modulated twentieth-century Sahel rainfall variability via impacts on North Atlantic sea surface temperature

<p>Sahel rainfall experienced significant multidecadal variability over the twentieth century. Previous work have proposed several drivers to explain the severe drought and the subsequent recovery of Sahel rainfall in the past century, including anthropogenic aerosols, GHGs, and internal variabilities. However, the attribution remained ambiguous. Sahel summertime monsoon has close teleconnections with North Atlantic sea surface temperature (NASST) variability, which has been proven to be affected by aerosols. Therefore, changes in regional aerosols emission can potentially drive multidecadal Sahel rainfall variability.</p><p>Here we use ensembles of state-of-the-art global climate models (the CESM-large ensemble and CMIP6 models) and observational datasets to demonstrate that anthropogenic aerosols have significant impacts on twentieth-century Sahel rainfall multidecadal variability through modifying NASST. Aerosol-induced multidecadal variations of downward solar fluxes over the North Atlantic Ocean cause NASST variability during the 20<sup>th</sup> century, altering the strength of the Hadley cell and the ITCZ position, therefore, dynamically linking aerosol effects to Sahel rainfall variability. While the observed linear trend of NASST might still be affected by a mix of various external and internal drivers, our results suggest that NASST variability is most likely caused by aerosol-induced changes in radiative fluxes rather than changes in ocean circulations, and that anthropogenic aerosols can explain most of the detrended Sahel rainfall variability. CMIP6 models further suggest that aerosol-cloud interactions contributed more to the variability than aerosol-radiation interactions. These findings highlight the importance of accurate representation of regional aerosol radiative effects for the simulation of Sahel rainfall variability.</p>