DATE2018-05-16 03:03:19
AUTHORSR Seager (1), TJ Osborn (2), Y Kushnir (1), H Liu (1), J Nakamura (1)
  1. Lamont Doherty Earth Observatory of Columbia U., Palisades, USA
  2. Climatic Research Unit, UEA, Norwich , UK
ABSTRACTMediterranean-type climates are defined by temperate wet winter 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 as well as the mechanisms for model-projected change in the near-term future. Despite their commonalities in terms of location in the context of planetary scale dynamics, the causes of variability are distinct across the regions. Only in California and Chile is ocean-forcing of variability a notable influence. Instead in all Mediterranean-type climate regions, internal atmosphere variability is the dominant source of winter precipitation variability but only the the Mediterranean is this clearly related to annular mode variability. As a consequence potential predictability of winter precipitation variability in the the regions is low. In all regions, the trend in winter precipitation since 1901 is markedly 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 California are drying 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 thermodynamic changes are important that enhance zonal dry advection into Mediterranean-type climate regions due to a minimum in vertically integrated water vapor change at the coast of the MCRs.