Local drivers of aridity in the Middle East and beyond

This dissertation explores the hypothesis that land-atmosphere interactions exert a significant control on aridity at the regional scale. Interactions related to topography, drought, and land use are considered, with a geographic focus on the Middle East.; In the first study it is hypothesized that aridity in the Middle East Plain (MEP) is enforced by a heat-driven circulation associated with the Zagros Plateau of Iran. Simulations with a regional climate model (RCM) indicate that heat flux on the Zagros causes atmospheric subsidence and reduced precipitation in the MEP. These effects are seasonal: the forcing on subsidence is greatest in summer, while effects on precipitation peak in spring. Zagros-based heating also acts to cool the MEP by strengthening prevailing northwesterly winds.; The second study of the dissertation investigates land-atmosphere feedbacks on drought. The hypothesis that surface albedo increases during drought is confirmed for the MEP but rejected for the Zagros. This increase in MEP albedo is a negative forcing on air temperature and convection. In the Zagros, meanwhile, sensible heat flux increases during drought. This is a positive forcing on temperature and a negative forcing on convection. RCM simulations indicate that precipitation feedbacks are marginal during moderate drought, though the direction of the surface forcing is consistent with local inhibition of precipitation.; In a third study the RCM is combined with satellite imagery to quantify evapotranspiration in irrigated agriculture and to test the hypothesis that irrigation enhances precipitation. This modeling system provides spatially-distributed estimates of water consumption that are consistent with conventional, spatially aggregated figures. Simulations indicate that irrigation alters wind patterns and air temperature in the MEP but does not impact precipitation.; A final study offers a regional comparison for drought feedbacks. Energy fluxes during a drought in France are investigated using satellite data. In contrast to the MEP, albedo of the French landscape did not increase during drought. There was, however, a substantial increase in sensible and radiative heat flux, leading to increased air temperature. This drought/temperature interaction is consistent in magnitude and geography with a feedback predicted to be significant in the future climate of Europe.