DATE2016-05-30 13:37:50
AUTHORSHadas Saaroni (1), Baruch Ziv (2), Tzvi Harpaz (1,2), Judith Lempert (1)
  1. Tel Aviv University Tel Aviv (Israel)
  2. The Open University Of Israel Ra'anana (Israel)
ABSTRACTProlonged dry spells (PDSs) which last a week or longer, during the rainy season, have been classified into three types; the 'subtropical', associated with an expansion of the subtropical high over most of the Mediterranean. The 'baroclinic', induced by a pronounced stagnant ridge, part of a Rossby wave, over the Eastern Mediterranean, is characterized by dry lower-level flow and subsidence. The 'polar', associated with a blocking high over Eastern Europe, which pushes lower-level dry polar air toward the Levant. The types' identification is based on quantitative indices developed by Saaroni et al. (2015). The evolution is studied through composite maps of 500-hPa GPH anomalies, derived for the days preceding the onset of the PDSs and their first days. Prior to the onset of the 'baroclinic' PDS, meridional regime dominates the Mediterranean and south Europe, disconnected from the higher-latitudes. During its first days the meridional regime intensifies, extending toward higher latitudes. Prior to the onset of the 'subtropical' PDS, zonal pattern covers Europe and the mid-latitudinal part of Asia, but, unexpected meridional pattern is discerned over the Mediterranean. The zonal pattern expands gradually over the Mediterranean, extending to the northeast toward central and northern Asia, but from the third day a meridional pattern starts to dominate the Mediterranean. Prior to the onset of the 'polar' PDS, a blocking high covers Northwest Europe and expands eastward during the PDS. Maps of cyclone tracks, derived for days belonging to each type, show distinct distributions; for 'subtropical' days the European cyclone track is densely populated with dominantly eastward tracks, and the Mediterranean is relatively empty of tracks. For 'baroclinic' days, the western and central Mediterranean are rich with cyclones, the eastern part is nearly empty and fewer tracks appear over Europe compared to the 'subtropical' days with dominant meridional northward component. During 'polar' days, minimal tracks appear over Europe and the eastern Mediterranean, whereas the Atlantic is densely populated, presumably because cyclones are blocked from entering Europe. Our results indicate a tendency for stagnant upper-level ridges to develop over the Levant during PDSs, accompanied by lower-level easterly continental flow. The analysis also reveals the dominance of 'baroclinic' PDSs and the tendency of 'subtropical' and 'polar' types to transform and become 'baroclinic'.