14 February 2019

Using Information Theory to Evaluate Directional Precipitation Interactions Over The West Sahel Region In Observations and Models

A new approach to benchmark process relationships and underlying mechanisms of the coupled land-atmosphere system.

Science

Water availability has historically been one of the most significant threats to African regional social and economic well-being. Over the Sahel region, a megadrought during the 1960s and 1970s caused widespread famine and death. The ongoing post-drought recovery has been characterized by gradual increases in rainfall, but with large fluctuations. The large negative human impacts, slow recovery, and variability raise important questions regarding why and how rainfall dynamics evolve and interact with other components of the regional climate system.

Impact

Information theory is proposed as a powerful method to quantify controls on precipitation and develop benchmarks for fully coupled Earth System Models (ESMs). West Sahel precipitation seasonal variation is controlled by Sea Surface Temperature (SST) variation over the Gulf of Guinea. West Sahel precipitation variation is also affected by local vegetation dynamics. CMIP5 ESMs represented either the unidirectional control of SST on precipitation or the bidirectional interactions between vegetation and precipitation, but no ESM properly represented both controls.

Summary

Here, DOE scientists provide an observational assessment of rainfall interaction mechanisms (informed by directional transfer of information entropy) that regulate Sahel rainfall. This analysis quantitatively demonstrates that (1) Sea Surface Temperature (SST) over the Gulf of Guinea controls moisture advection and transport to the West Sahel region and (2) strong bidirectional interactions exist between local vegetation dynamics and rainfall patterns. The directional interaction patterns from nine state-of-the-art Earth System Models (ESMs) were assessed and show that most ESMs are able to represent either the unidirectional control of SST on precipitation or the bidirectional interaction between vegetation and precipitation. However, none of the ESMs represent both interactive patterns. The GFDL and IPSL-CM5A-LR models successfully reproduced observed patterns over ~50% of the West Sahel region but were not accurate in reproducing observed precipitation regional trends or inter-annual variation. Directional information transfer is a powerful mechanistic benchmark to assess coupled land-atmosphere-ocean model fidelity at the process level.

Contact
William J. Riley
Lawrence Berkeley National Laboratory
Publications
Liu, BY, Q Zhu, WJ Riley, L Zhao, H Ma, M Van Gordon, and L Larsen.  2019.  "Using Information Theory to Evaluate Directional Precipitation Interactions Over the West Sahel Region in Observations and Models."  Journal of Geophysical Research: Atmospheres 124.  https://doi.org/10.1029/2018JD029160.