Hot and cool summers: multiple equilibria of the continental water cycle

Large variations in summer temperature and soil water reserves, such as the heat wave and drought of August 2003 in Europe, are known to be linked to a positive feedback between precipitation and soil moisture. This positive feedback has been suggested as the source of the bimodality in the probability distribution of soil moisture that is observed in North America. Here, we propose that the bimodality can be the consequence of multiple equilibria of water balance in the coupled system including the upper soil layer and the atmospheric planetary boundary layer. This mechanism is described in the framework of an idealized box model, which has two equilibrium states. This model represents the water cycle over an average continental location at midlatitudes during summer. It includes convergence of moisture fluxes in the lower atmosphere, their divergence in the upper troposphere and water recycling by convection, precipitation and evapotranspiration. The existence of two equilibria hinges on the variation of precipitation efficiency, i.e. the amount of water that is locally recycled through precipitation. This in turn depends on convection intensity. The two regimes correspond to realistic values of physical variables associated with mean wet or dry summers, and are able to persist beyond the seasonal time scale even when forced by a stochastic moisture flux convergence. Spring continental soil water content is thus seen as the key factor for droughts or wet condition in the following summer.