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

Lead PI: Dr. Michela Biasutti , Steven L. Goldstein , Dr. Yochanan Kushnir , Patrick Alexander

Unit Affiliation: Ocean and Climate Physics, Lamont-Doherty Earth Observatory (LDEO)

September 2023 - August 2026
North America ; United States
Project Type: Research

DESCRIPTION: 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 from the Eastern Mediterranean (EM) that record wet and dry periods during the last 220 thousand years. The changes in aridity during interglacial intervals 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 occurrence of aridity episodes in the paleoclimate record linked not to the orbital forcing, but to ocean circulation changes that occurred at glacial-to-interglacial transition periods and that could occur again if the climate system reaches a tipping point in the future. These issues are examined using specialized simulations performed with the Community Atmosphere Model, the atmospheric component of the Community Earth System Model (CESM).

BROADER IMPACTS: As the Levant experiences water shortages and prepares for worsening conditions, we will re-establish contacts with the C.U. Global Center in Amman and the Middle East Eco-Peace project to renew meetings that occurred pre-pandemic with regional stakeholders interested in addressing the reliability of climate projections and discussing the implications of paleoclimate studies for addressing present day hydrological change. Future meetings will update the stakeholders with new insights on the cause and magnitude of persistent EM droughts drawing from this proposal and a complementary project supported by the Israel- US Binational Science Foundation (BSF) to continue geochemical analysis of the DSDDP core. In synergy, our modeling study will inform the direction of that research and vice versa. The proposed project will also have a positive impact on the scientific workforce, supporting a post-doc and a mid-career female soft- money scientist. All PIs have a strong track record in mentoring and in leading and supporting DEI activities.