Sahel and Congo Basin Precipitation: Variability and Teleconnections

Future projections of African precipitation are still mired in uncertainty. Reducing this uncertainty requires a better understanding of the processes that drive variability in present day African precipitation. Using the Community Earth System Model (CESM), we examined the linkages between regional and local atmospheric circulation patterns and their impact on precipitation in the Sahel and Congo Basin, with a particular focus on the influence of the Indian Ocean on precipitation in these regions. The influence of the Indian Ocean on Sahel precipitation was investigated using a series of sea surface temperature (SST) sensitivity experiments. We identified two mechanisms by which the Indian Ocean can alter Sahel precipitation. The first mechanism is associated with perturbations on the equator that alter Sahel precipitation by modulating the Asian monsoon circulation and driving changes in descent in North Africa. The second mechanism is associated with SST perturbations that cover more of the basin and alter the overturning circulation between the Indian and Atlantic Oceans. These two mechanisms result in different precipitation responses in the Sahel. The first mechanism induces an increase in precipitation as a result of warming in the Indian Ocean, whereas the second mechanism produces a decrease in Sahel precipitation in response to warming. Congo Basin precipitation was examined with regional water tagging and stable isotope tracers in CESM. Using regionally defined water tags, the moisture contribution from different sources to Congo Basin precipitation was investigated. We found that the Indian Ocean and evaporation from the Congo Basin are the dominant moisture sources, and that the Atlantic Ocean is a comparatively small source of moisture. Variability in Congo Basin precipitation was found to be a result of changes in the available moisture and in the atmospheric circulation in the region. In wet years, more of the precipitation in the Congo Basin is derived from Indian Ocean moisture, although the spatial distributions of the dominant sources is shifted, reflecting changes in the mid-tropospheric circulation over the Indian Ocean. The isotopic signatures in precipitation were consistent with the dominant influence of moisture from local evaporation and the ocean sources.Ph.D