This is a special OCP Seminar, celebrating the 75th anniversary of Lamont Doherty Earth Observatory.
Title: Recent and near-term hydroclimate change in the world’s Mediterranean climate regions with a diversion into mechanisms of aridification in southwest North America
Abstract:
Mediterranean Climate Regions (MCRs) exist in Europe/Northwest Africa, North and South America, southwestern Africa and southwest Australia. They are intermediate between the subtropical arid regions and the humid extratropics and are vulnerable to hydroclimate variability and change. I will show how all of the southern hemisphere MCRs have been drying year-round over past decades, in agreement with CMIP6 model simulations, as a consequence of a radiatively-forced poleward shift of the eddy-driven jet and expansion of subtropical subsidence. This drying will continue over coming decades as CO2 concentrations rise, even as O3 recovers. The northern hemisphere is more complex. The west coast of North America has had both drying and wetting trends in various seasons and latitudes that are not consistent with either radiatively-forced or SST-forced models. The western Mediterranean has been drying strongly in winter and spring but this has been dynamically forced by the positive North Atlantic Oscillation trend. Since this is outside the range of CMIP6 historical simulations, its’ cause is unclear. Although not an MCR, the interior southwest of North America has been drying under the influence of the Pacific Decadal Oscillation. A Large Boundary Value Ensemble generated at Lamont projects the southwest will never return to the levels of surface moisture availability familiar in the late 20th century. Southwest aridification is driven by declining cool season precipitation and spring warming such that drier summer soils go along with higher vapor pressure deficit, a relatively drier atmosphere and reduced evapotranspiration.
