Mechanisms of Mediterranean Region Hydroclimate Variability and Change

DESCRIPTION: Drought has long been a concern for the countries surrounding the Mediterranean, from the Iberian Peninsula to Turkey on the north and from Morocco to Syria along the southern shore. Much of the region experienced prolonged drying in the second half of the 20th century, and the recent turmoil in Syria follows a particularly acute drought in the Fertile Crescent. Moreover, climate models are remarkably consistent in projecting a strong drying trend around the Mediterranean over the 21st century.

This project seeks to understand the dynamical mechanisms that drive natural variability and forced change in precipitation and evaporation over the Mediterranean region. The observational record is extended through the use of tree ring - based climate reconstructions to give long-term context to the drying of recent decades. Other work considers the role of Mediterranean cyclones in regional precipitation variability, along with changes in cyclone behavior that may contribute to the future drying seen in climate models. The role of jet stream shifts in future drying is examined using a linear stationary wave model and other diagnostic techniques. A thorough moisture budget analysis is carried out using both observationally-based reanalysis products and climate model output. Previous work shows that much of the hydroclimate variability in the Mediterranean is linked to the North Atlantic Oscillation, which involves a north-south shift of the Atlantic jet stream and storm track. Preliminary work by the PIs suggests that this variability can be distinguished from aridity occurring as a consequence of a warming climate. The work also includes an effort to develop probabilistic projections of Mediterranean hydroclimate that take into account the skill of models in simulating past variability.

As noted above, the work has great societal relevance due to the human impacts of widespread drying and the somewhat dire model projections of future Mediterranean aridity. The projections are plausible given their consistency from one model to another, but an examination of the underlying the mechanisms, in the context of a long-term climate record, can do much to assess their validity. The PIs engage with the Mediterranean climate and impacts communities through MedCLIVAR, a research coordination network which conducts activities including workshops, schools, and exchange programs focused on Mediterranean climate. Closer to home, the project engages with undergraduate students through the Lamont Summer Intern Program and conducts outreach through the biannual Lamont Open House. The project also provides support and training for a postdoctoral research scholar, thereby developing the workforce in this research area.

OUTCOMES: Simpson, Isla R and Hitchcock, Peter and Seager, Richard and Wu, Yutian and Callaghan, Patrick. "The Downward Influence of Uncertainty in the Northern Hemisphere Stratospheric Polar Vortex Response to Climate Change," Journal of Climate, v.31, 2018. doi:10.1175/JCLI-D-18-0041.1