The time scale of land surface hydrology in response to initial soil moisture anomalies: a case study

The sensitivity of a general circulation model to changes in the initial soil moisture distribution is illustrated through the detailed analysis of a case study. Two summer climate simulations are compared where initial soil moisture was obtained from a climatological distribution or from a previous integration of the model. In a third experiment, the climatological distribution of soil moisture is used as a fixed boundary condition throughout the integration. At low latitudes, the initial difference in the zonal mean soil moisture causes a significant difference in the soil temperature or land-surface evapotranspiration, but persists only up to 15 days. At high latitudes, the initial difference is more persistent, lasting through the end of the integration (50 days). In particular, large soil moisture differences persist over Siberia or North America during the whole integration and cause persistent temperature differences of a few degrees in the same regions. At high or middle latitudes, the soil-moisture values at the end of spring may therefore significantly influence the climate of the subsequent summer. Potential evapotranspiration determines to a large extent the characteristic time scale of the ground hydrology.