Global and Sahel Regional Biophysical Processes, Vegetation Dynamics, and Climate Interactions

Author(s): Song, Guoqiong | Advisor(s): Xue, Yongkang | Abstract: The objective of this research is to gain a better understanding of the biophysical processes, vegetation dynamics, and climate interactions on global and regional scales. Sahel is the major focus region for this research. Our methodology is based on the analysis of observations, as well as simulations by the National Centers for Environmental Prediction (NCEP) Atmospheric General Circulation Model (AGCM) coupled with the Simplified Simple Biosphere model version 2 (SSiB2) and an offline version of the SSiB4 coupled with a Dynamic Global Vegetation Model (DGVM) Top-down Representation of Interactive Foliage and Flora Including Dynamics (TRIFFID). We first examine the impact of vegetation biophysical processes (VBP) on climate from a long temporal scale point of view, specifically, the climate variability on inter-annual and inter-decadal time scales in the past six decades by using the NCEP AGCM coupled with two different land surface parameterizations: SSiB2 and the two-layer soil model. At the inter-annual time scale, the simulation with VBP decreases the root mean squared error by about 65%. Moreover, on inter-decadal time scale, VBP corrects the wet or dry biases over West Africa, South Africa, Amazon, and East Asia, through changing surface energy balances and the partitioning of surface latent and sensible heat fluxes, as well as changing atmospheric circulation and moisture flux convergence.In the second part, we systematically investigate the climate impact of large-scale land use land cover change (LULCC), and identify the mechanisms that control the response of climate to LULCC, by using the most recent LULCC data in an "idealized but realistic" way for the past six decades. LULCC leads to an increase in albedo and decrease in evaporation. The albedo effect (cooling) and evaporation effect (warming) compete with each other, resulting in warmer surface temperatures at tropics, and cooler surface temperatures at middle latitude. Over global land, the LULCC amplifies surface warming (0.11K over global land and 0.43K over degraded area respectively). LULCC cause a precipitation reduction globally (-0.15mm/day over global land and -0.35mm/day over degraded areas), with strongest signals over monsoon regions, resulting from evaporation reduction and less convergence from monsoon convergence zones. Finally, I systematically investigate how climate variability and anomalies in West Africa affect the regional terrestrial ecosystem, including spatial distribution and temporal variations of plant functional types' (PFT) and other vegetation characteristics, though biophysical and photosynthesis processes at different scales. The offline SSiB4/TRIFFID model is used in this study. The results show that the simulated PFT's spatial distribution and total leaf area index (LAI) correspond well to climate variability and are consistent with satellite derived vegetation conditions. The simulated inter-decadal variability in vegetation conditions is consistent with the Sahel drought in the 1970s and the 1980s and partial recovery in the 1990s and the 2000s. The vegetation characteristics simulated by SSiB4/TRIFFID responds primarily to air temperature, soil moisture and radiative fluxes.