External Climate Forcings and Vegetation Interannual and Decadal Variability in West Africa and North America

Yongkang Xue, UCLA, yxue@geog.ucla.edu (Presenter)
Guoqiong Song, UCLA, song@atmos.ucla.edu
Zhengqiu Zhang, UCLA, zhangzq@geog.ucla.edu
Peter Cox, University of Exeter, p.m.cox@exeter.ac.uk
Stephen D Prince, University of Maryland, sprince@umd.edu
Glen MacDonald, UCLA, macdonal@geog.ucla.edu
George James Collatz, NASA GSFC, jim.collatz@nasa.gov

Regional climate and ecosystem in West Africa and North America show strong decadal variation during the past 60-years. We apply an off-line biophysical/dynamic vegetation model, SSiB4/TRIFFID, over West Africa and North America to investigate the relationship between vegetation dynamics and climate variability. The observed precipitation, atmospheric carbon, and reanalysis-based meteorological forcing from 1948 through 2006 with 1 degree horizontal resolution was used to drive the offline models.

The simulation results indicate that the model with realistic forcing simulated strong interannual and decadal variabilities of leaf area index (LAI) over West Africa, consistent with the Sahel drought in the 1970s and the 1980s and partial recovery in the 1990s and the 2000s. Meanwhile, the model simulated strong forest recovery in southeast U.S. since the 1980s and drought impact on West U.S. after the year 2000. Remote sensing products are applied to evaluate the simulation results. These simulated variabilities are consistent with remote sensed LAI variabilities.

To further understand the cause of such decadal and interannual variability, a set of experiments has been design to further isolate the effect of global warming and atmospheric carbon elevation. In one experiment, atmospheric carbon dioxide was fixed at the 1948 level, i.e., 310 ppmv. The LAI difference between the elevated CO2 and fixed CO2 increases linearly with time, and show a clear fertilization effect. Most increases in LAI over Sahel occurred during the winter.

We also conducted an experiment, in which the temperature warming trend was removed. The simulation shows that after 1980, the LAI reduction due to the warming was evident, but it was not as strong as the carbon fertilization effects. High temperature created stress on vegetation over the West African transition zone. By and large, the fertilization effect seems to dominate the global warming effect.

Presentation Type:  Poster

Session:  Coupled Processes at Land-Atmosphere-Ocean Interfaces   (Mon 4:00 PM)

Associated Project(s): 

• Prince, Steve: Vegetation dynamics in drylands and implications for regional climate: analysis of two decades of observations in the African Sahel ...details

Poster Location ID: 97