Yan Yu, PhD Defense
Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison
Vegetation-climate feedbacks in the Sahel:a combined statistical and dynamical assessment
Room 811 AOSS, April 24, 2017, 1:45 PM
The Sahel is characterized by substantial interannual to decadal variability in rainfall and high socio-economic vulnerability to hydrologic extremes. Attribution of this pronounced rainfall variability to either oceanic or terrestrial drivers has proven to be an elusive challenge. Classic, model-based theory of land-atmosphere interactions across Sahel has promoted positive vegetation-rainfall feedbacks associated with a dominant surface albedo mechanism. However, neither the proposed positive vegetation-rainfall feedback nor its underlying albedo mechanism has been convincingly demonstrated in observation. Here, the multivariate Generalized Equilibrium Feedback Assessment (GEFA) is applied to identify the observed terrestrial impacts on Sahel climate and explore the underlying mechanisms.
The reliability of GEFA is first evaluated against dynamical experiments with modified leaf area index (LAI) and soil moisture across the Sahel within the Community Earth System Model (CESM). In order to reduce the sampling error caused by short data records, the traditional GEFA approach is refined through stepwise GEFA, in which unimportant forcings are dropped through stepwise selection. Consistent statistically- and dynamically-assessed atmospheric responses to Sahel LAI anomalies demonstrate GEFA's capability of isolating terrestrial impacts on Sahel climate. Furthermore, stepwise GEFA provides reliable estimates of the terrestrial impacts with the typical length of observational datasets, thereby enhancing the method's applicability.
After successful validation of stepwise GEFA, the observed terrestrial impacts on the Sahel climate are examined by applying stepwise GEFA to multiple gridded observations, remote sensing products, and reanalyses. The observational analysis confirms the proposed positive vegetation-rainfall feedback across the Sahel, which is confined to the post-monsoon season, although it is associated with a moisture recycling mechanism, rather than the classic albedo-based mechanism, on the seasonal to interannual time scale. Positive anomalies of vegetation greenness across the Sahel during the late and post-monsoon periods favor enhanced evapotranspiration, precipitable water, convective activity, and rainfall, indicative of amplified moisture recycling.