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Persistent and prolonged periods of dry or moist conditions are often evident in the interannual variability of continental-type climates This variability appears as fluctuations around several distinct and preferred moisture states. These fluctuations and transitions between the preferred states are commonly attributed to large-scale changes in atmospheric circulation patterns possibly caused by oceanic influence. This paper argues that a major contributing factor to the persistent dry or moist behavior could be due to feedback and nonlinear interaction between the components of the hydrologic cycle in both the land and the atmosphere. A model that couples the water balance of continental landmasses and the overlying atmosphere is presented. The large-scale variabilities in atmospheric circulation are introduced by way of simple randomness in key forcing parameters. The result is a multiplicative-noise stochastic differential equation for the water balance dynamics of continental-type climates that includes land surface-atmosphere interaction. The solution to this differential equation exhibits a bimodal probability distribution function for soil moisture and precipitation. Extended periods of anomalous dry conditions (drought) or alternatively wet conditions (pluvial), with abrupt transitions between them, are present in the model. The statistics of persistent anomalous conditions are analyzed for two climatic classifications. The probability distribution function for transitions out of droughts are developed for the modeled climates.
Entekhabi et al. (Sat,) studied this question.