Impact of land-atmosphere fluxes on the spring precipitation regime of the Iberian Peninsula

Abstract

In this thesis, we investigate the physical processes underlying the spring maximum of precipitation observed throughout the interior of the Iberian Peninsula, with a specific incidence in the inland regions to the east and northeast. This upturn in the rainfall totals occurs mostly in May, having a critical impact on human activities, and in particular on agriculture over these interior areas, most of them suffering from water scarcity. The present thesis adds valuable information to better characterize the precipitation regime of these regions. In a context of climate change, this study may be useful for the hydrological planning of these regions and for their long-term sustainability. Apart from being a direct source of moisture, evapotranspiration fluxes are also found to have an indirect impact on the Iberian spring rainfall by increasing the low level instability of the atmospheric column. In this thesis, we develop an original method to separate and quantify both direct (recycling) and indirect (amplification) contributions to rainfall totals, based on experimental model simulations where land evapotranspiration over the Iberian Peninsula is suppressed. These experiments confirm that the spring maximum of precipitation occurring in inland Iberia cannot be explained without considering the crucial role of land-atmosphere interactions, and in particular evapotranspiration fluxes. Actually, suppressing land evapotranspiration in May would lead to an average reduction of precipitation by 37\%. We therefore state that the pluviometric upturn observed in large interior regions of the Iberian Peninsula in May is directly related to the intensification of the hydrological cycle and to the impact of evapotranspiration fluxes on rainfall. Furthermore, the fraction of spring precipitation directly and indirectly related to land-atmosphere interactions is found to be of the same order as the reduction in precipitation totals expected as a consequence of climate change in the Iberian Peninsula. As a result, a better understanding of land-air interactions in the large water-limited regions of the interior of Iberia is very relevant to assess the impact of future climate change scenarios in the regional hydrological cycle.