Insolation Gradients and Eastern Mediterranean Aridity: Impacts on Winter Storms and Implications for Climate Projections

Simulations of the climatic effect of greenhouse gas increases are in remarkable agreement that the Mediterranean Basin will dry out substantially over the 21st century. The projected future drying is consistent with precipitation declines since the mid-20th century, which have already caused hardship in the region. A prime example is the 2007-2010 Syrian drought, which was a contributing factor in the civil war and refugee crisis that began the following year. The multiple lines of evidence pointing to future drying, together with its potential for severe societal disruptions, motivate a concerted effort to understand the dynamics of Mediterranean rainfall change. An overarching question in this effort is how radiative forcing acting on the planetary scale, in this case from greenhouse gas increases, produces such a strong precipitation response over this particular region.Work performed here considers the insights to be gained from the Mediterranean precipitation response to another kind of planetary-scale forcing: the change in sunlight received by the earth over the orbital cycles that produce the ice ages. The project takes advantage of sediment cores and speleothems from the Eastern Mediterranean (EM) that record wet and dry periods during the Last Interglacial (LIG, also called the Eemian), from 135 to 110 thousand years ago (kya), and the Holocene, from 15kya to the present. The changes in aridity during these periods are thought to be due to changes in the seasonality of insolation, with wetter conditions when summer insolation is at its peak and dry periods when fall insolation peaks. The Principal Investigators (PIs) argue that stronger fall insolation leads to a stronger latitudinal surface temperature contrast over the North Atlantic during winter, which leads to a northward shift of the Atlantic jet stream and the paths of winter weather systems that move along it. The northward shift in weather systems causes substantial annual rainfall reductions as the Mediterranean Basin receives most of its rainfall in winter. The insolation changes due to orbital cycles are of course quite distinct from greenhouse warming, but the PIs note that greenhouse gas increases cause a similar latitudinal temperature contrast since the West African landmass heats up more than the adjacent Atlantic Ocean, and the northern North Atlantic features a "warming hole" which further enhances the north-south temperature contrast.The work involves a combination of analysis of the present-day observational record, paleoclimate proxy data, and model simulations of past, present, and projected future climate. One issue to be addressed is the conflation in the proxy record between the effects of the temperature contrast, which are relevant to current climate change, and incursions of the North African monsoon, which are not. A further issue is the abruptness of the aridity changes in the paleoclimate record compared to the orbital forcing, which suggests a role for the dynamics of oceans and ice sheets. These issues are examined using specialized simulations performed with the Community Atmosphere Model, the atmospheric component of the Community Earth System Model (CESM).The work is of societal as well as scientific interest given the profound impacts of aridification in the EM as noted above. The PIs enhance the real-world impact of their work through organizations in the region including the Columbia University Global Center in Amman, Jordan, the Eco-Peace Middle East non-governmental organization, the Weizmann Institute, and Hebrew University. The PIs also participate in outreach activities at their home institution, including the Lamont Open House and visits to local high schools. The project also provides support and training to a postdoc, thereby providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.