RAPID: Mechanisms of 20th Century Mediteranean Drying: The Role of External Forcing Versus Natural Variability

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This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.The project considers the drying of the Mediterranean region over the second half of the 20th century as portrayed in the ensemble of 20th century climate simulations prepared for the AR5. The central question motivating the project is whether this drying occurred due to natural variability or anthropogenic greenhouse warming. The PIs note that continued drying of the Mediterranean is a robust result across the ensemble of climate model projections of greenhouse gas-induced warming over the 21st century, so there is reason to believe that the region is susceptible to drying as a consequence of global warming. On the other hand, much of the observed drying of the Mediterranean can be linked to the prolonged period in the late 20th century during which the North Atlantic Oscillation (NAO) was in its high phase, as the high phase is associated with a northward shift of the Atlantic stormtrack that provides the Mediterranean with much of its wintertime precipitation. The extent to which the NAO is influenced by global warming is not known, and several mechanisms of influence have been proposed. Work performed in this project uses statistical analysis, primarily in the form of signal-to-noise maximizing empirical orthogonal functions (EOFs), to determine the relative importance of natural variability and greenhouse gas-induced global warming in producing Mediterranean drying in 20th century climate simulations. Additional analysis using a decomposition of the ensemble-mean moisture budget is used to discriminate between possible mechanisms for global warming-induced drying.The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. In particular, the work will provide guidance for decision makers concerned with Mediterranean drying, as the drying of recent decades may or may not reverse depending on whether it is part of a natural fluctuation or a consequence of secular global warming. The mechanisms of natural variability and forced change explored in this project may also be consequential for climate change occuring in other parts of the world including the semiarid subtropical regions of the United States.