2019 Articles

Climate variability and change of Mediterranean-type climates

Seager, Richard; Osborn, Timothy J.; Kushnir, Yochanan; Simpson, Isla R.; Nakamura, Jennifer A.; Liu, Haibo

Mediterranean-type climates are defined by temperate, wet winters and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter storm tracks and summer subtropical anticyclones. Here the climatology, variability and long term changes in winter precipitation in Mediterranean-type climates are analyzed and the mechanisms for model-projected change in the near-term future. Despite commonalities in terms of location in the context of planetary scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Only in California and Chile is ocean forcing of variability a notable influence. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in Coupled Model Intercomparison Project Five. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the northern hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific stormtrack equatorward. In the southern hemisphere, mixed dynamic-thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.