Circulation Types and Their Influence on the Interannual Variability and Precipitation Changes in Greece

Abstract

The data used in this study are daily precipitation amounts of 20 stations equally distributed over Greece and daily (12 TU) 500hPa geopotential heights, (20{\textdegree} to 65{\textdegree}N and 20{\textdegree} to 50{\textdegree}E) for the period 1958--1997. A decrease of annual, winter and autumn precipitation over the whole country was found, significant over several parts of Greece. An automatic classification scheme of the atmospheric circulation affecting Greece, between 1958--1997, is presented using spatial methods of topology and geometry. A short description of the synoptic characteristics of the 14 classified circulation types is given as well as the probability and the amount of precipitation associated with each type. It is found that the six anticyclonic types presented, during the rainy period (October to March), a cumulative frequency equal to 39.2{\%} and give a rather small (8.2{\%}) contribution to the ``rainy'' period precipitation amounts. On the other hand the four more ``rainy'' circulation types, namely the C, Csw, CNW and Cw together represent 37.8{\%} of all the rainy period days and account more than 72{\%} of the observed daily precipitation. The results obtained accentuate the existence of the strong links between the interannual variability of seasonal precipitation and interannual variability of seasonal frequency of the circulation types. The anomalous low or high precipitation amount during the hydrological year show a strong relationship with the anomalous low or high frequency of ``rainy'' circulation types. Multiple regression models developed for the simulation of the annual cycle of rain days and the corresponding precipitation, as well as selected extreme precipitation events for Greece stations. It is concluded that there is an agreement between the simulated annual cycle as well as the extreme rainfall events with the observations, both in terms of rainy days and precipitation amounts. It suggests that the precipitation regime in Greece including interannual variability, trend and extremes can be explained in terms of variability of a small number of circulation types patterns.