Semi-objective classification for daily synoptic systems: Application to the eastern Mediterranean climate change

Semi-objective classification of daily 1200 UTC synoptic systems was carried out by a discriminant-like analysis of the National Center for Environmental Prediction-National Center for Atmospheric Research reanalysis data over the eastern Mediterranean (EM) for 1948-2000. An example for a climate change application is given by the analysis of trends in the annual frequencies of the synoptic systems. The frequencies of the mostly dry Red Sea trough (RST) systems nearly doubled since the 1960s from 50 to about 100 days per year. This explains a dominant decreasing trend of rainfall in most of the EM, along with an increase in the southern part of the EM region, when the RST is deep enough to bring tropical moisture over this area. Also, the increasing tendency towards heavier daily rainfall in spite of the general decrease in the totals may be explained by the increase in the active and stormy types of RST situations. The annual frequency of the Cyprus lows was noticed to drop slightly in 1983-98 to 26, compared with about 30 during 1967-82. The high positive correlation between the recent increase in the North Atlantic oscillation index and the pressure over most of the EM countries is linked to these tendencies in the synoptic systems.