Drying of regions with Mediterranean-type climates
Mediterranean-type climates have mild, wet winters and dry summers that are warm or hot. They occur in Mediterranean Europe but also on the western edges of four other continents.
New research by Columbia University and UEA’s Climatic Research Unit shows that the winters of all Mediterranean-type climate regions (MCRs), except in North America, have dried and that climate models project further drying over coming decades.
The study looked at similarities and differences in climate variability, changes and mechanisms across regions with Mediterranean-type climates. These are the regions shown in brown (hot) and orange (warm) in the map.
All MCRs except those in W USA show long-term drying trends in winter (the most important season for precipitation in MCRs).
Driven by historical forcings, CMIP5 models also simulate long-term drying trends in all MCRs except W USA, in qualitative agreement with observations
Quantitative comparisons (observed vs. modelled) of historical winter precipitation trends show good agreement in some MCRs (Pacific NW, the Mediterranean, SW Africa) but observed drying is greater than CMIP5 models in others (Chile, SW Australia).
Models project that this winter drying (in all MCRs except W USA) will continue, but different mechanisms seem to be responsible between regions in the northern and southern hemispheres (NH and SH).
Atmospheric circulation changes appear to dominate in NH regions: increasing high pressure suppresses Mediterranean precipitation, lower pressure off W USA coast shifts Pacific storms to bring more precipitation into W USA.
Mixed circulation and humidity changes cause the winter drying in SH MCRs: Hadley Cell expansion and poleward shift of westerlies reduces moisture convergence over MCRs, enhanced by coastal minima in specific humidity changes that increases dry advection into the MCRs.
The paper is Seager R, Osborn TJ, Kushnir Y, Simpson IR, Nakamura J and Liu H (2019) Climate variability and change of Mediterranean-type climates. Journal of Climate,32, 2887-2916 and is available here: https://doi.org/10.1175/JCLI-D-18-0472.1