09 November 2017

Observed Positive Vegetation-Rainfall Feedbacks in the Sahel Dominated by a Moisture Recycling Mechanism

Observed Positive Vegetation-Rainfall Feedbacks in the Sahel


Through the application of a multivariate statistical method to observational, remote sensing, and reanalysis data, the observed positive vegetation-precipitation feedback over the Sahel, as proposed by Charney, is demonstrated for the first time, although associated with moisture recycling rather than Charney's proposed albedo hypothesis.


The identified observational vegetation feedback benchmark will be used to evaluate state-of-the-art Earth System Models, leading to bias identification, future model development, and the formation of process-based metrics for weighting future climate change projections.


Classic, model-based theory of land-atmosphere interactions across the Sahel, originating with Jules Charney, has promoted positive vegetation-rainfall feedbacks, dominated by a surface albedo mechanism.  However, neither this proposed positive vegetation-rainfall feedback nor its underlying albedo mechanism has been convincingly demonstrated using observational data.  The current study applies a multivariate statistical method, the Generalized Equilibrium Feedback Assessment (GEFA), to observational and remotely sensed data to extract observed Sahel vegetation feedbacks and their associated mechanisms to test Charney’s original theory.

We present observational evidence for the Sahel’s proposed positive vegetation-rainfall feedback, although it is associated with a moisture recycling mechanism, rather than the classic albedo-based mechanism, on seasonal to interannual time scales. Positive anomalies of remotely-sensed vegetation greenness during the late and post-monsoon periods favor enhanced evapotranspiration, precipitable water, convection, and rainfall, indicative of amplified moisture recycling. The identified low-level cooling and anomalous subsidence in response to positive vegetation greenness anomalies are counter to the responses expected through the classic vegetation-albedo feedback mechanism.

In further studies, the GEFA-based assessment of the key observed oceanic and terrestrial drivers of North African regional climate will serve as an observational benchmark for evaluating the representation of ocean-land-atmosphere interactions in state-of-the-art climate models as applied by the Intergovernmental Panel on Climate Change. This approach will foster model evaluation and development, along with the formulation of process-based model performance metrics for weighting future climate projections for the Sahel and reducing associated uncertainty.

Michael Notaro
University of Wisconsin-Madison