Bimodality in a Monostable Climate-Ecosystem: The Role of Climate Variability and Soil Moisture Memory

The probabilistic modal response of vegetation to stochastic precipitation variability is studied in a conceptual climate-ecosystem model. It is found that vegetation can exhibit bimodality in a monostable climate-ecosystem under strong rainfall variability and with soil moisture memory comparable with that of the vegetation. The bimodality of vegetation is generated by a convolution of a nonlinear vegetation response and a colored stochastic noise. The nonlinear vegetation response is such that vegetation becomes insensitive to precipitation variability near either end state (green or desert), providing the potential for two preferred modes. The long memory of soil moisture allows the vegetation to respond to a slow stochastic forcing such that the vegetation tends to grow toward its equilibrium states. The implication of the noise-induced bimodality to abrupt changes in the climate-ecosystem is also discussed.