Moisture budget analysis of SST-driven decadal Sahel precipitation variability in the twentieth century
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It is well known that the Sahel region of Africa is impacted by decadal scale variability in precipitation, driven by global sea surface temperatures. This work demonstrates that the National Center for Atmospheric Research’s Community Atmosphere Model, version 4 is capable of reproducing relationships between Sahelian precipitation variability and Indian and Atlantic Ocean sea surface temperature variations on such timescales. Further analysis then constructs a moisture budget breakdown using model output and shows that the change in precipitation minus evaporation in the region is dominated by column integrated moisture convergence due to the mean flow, with the convergence of mass in the atmospheric column mainly responsible. It is concluded that the oceanic forcing of atmospheric mass convergence and divergence to a first order explains the moisture balance patterns in the region. In particular, the anomalous circulation patterns, including net moisture divergence by the mean and transient flows combined with negative moisture advection, together explain the drying of the Sahel during the second half of the twentieth century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation.
KeywordsSahel precipitation Sea surface temperatures Atmospheric moisture budget Monsoon
The authors would like to thank Dr. Jennifer Nakamura for providing assistance with data analysis as well as Drs. Naomi Henderson, Haibo Liu, and Richard Seager and the Global Decadal Hydroclimate Predictability, Variability, and Change (GloDecH) group with its support through the National Oceanic and Atmospheric Association (NOAA) for useful discussions and completion of the model runs. CP would also like to thank the National Science Foundation for funding through the Graduate Research Fellowship Program and Etienne Dunn-Sigouin for assistance with programming. Finally, we thank the editor and an anonymous reviewer for providing useful comments that improved this study.
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