Connection between the large-scale 500 hPa geopotential height fields and precipitation over Greece during wintertime

Abstract

The spatial distribution of the winter (December to February) precipitation over Greece was related to the eastern North Atlantic-European scale mid-tropospheric circulation fields by means of empirical orthogonal functions (EOFs) and canonical correlation analysis (CCA). The data used in this study are winter precipitation totals, of 23 stations, equally distributed over Greece, and winter mean 500 hPa geopotential heights (30 to 70°N, 30°W to 50°E) for the period 1958 to 1994. The Greek precipitation data were found to be homogeneous (Alexandersson test). A decrease of winter precipitation over the whole country was found, although significant (Mann-Kendall trend test) only over the northern and eastern parts and in the western mountainous regions. Three CCA patterns represent links that are very reasonable from a physical point of view. It is supposed that stronger westerlies over the eastern North Atlantic and the raising of the 500 hPa geopotential height (and also the sea level pressure) over continental Europe during the last few decades were connected with enhanced atmospheric stabilization and anomalous advection of cold and dry air from northerly directions. This led to the winter dryness over the eastern Mediterranean. The probable mechanisms and processes in the complex atmosphere-ocean system, leading to the regional anomalous climatic conditions, are discussed.

Simulation of seasonal precipitation and raindays over Greece: a statistical downscaling technique based on artificial neural networks (ANNs)

“…It is noteworthy that the evaluation of precipitation data is quite complicated owing to the fact that the precipitation regime in Greece presents highly irregular behavior on both the spatial and temporal scale (Maheras and Anagnostopoulou, 2003). It is well accepted that the main physical and physicogeographical factors controlling the spatial distribution of precipitation over Greece are: the atmospheric circulation, mountains in the west and east, the Mediterranean Sea-surface temperature distribution, rehumidification of the air masses crossing the Aegean Sea and land and sea interactions (Xoplaki et al, 2000). Furthermore, the complex orography, the valleys along which the air masses are canalized and the large number of islands result in an even lower predictability of precipitation from the direct output of a coarse-scale GCM and the application of a downscaling technique is considered to be essential for the estimation of the changes of the aforementioned rainfall parameters in future climate conditions.…”

Wintertime surface temperature in Egypt in relation to the associated atmospheric circulation

“…Therefore, advective processes exerted by the atmospheric circulation are a crucial factor controlling regional air temperature changes (e.g. Trenberth 1990Trenberth , 1995Xu, 1993;Hurrell 1995;Hurrell and van Loon, 1997;Slonosky et al, 2001;Xoplaki et al, 2000Xoplaki et al, , 2002Jacobeit et al, 2001;Pozo-Vazquez et al, 2001;Slonosky and Yiou, 2002;Xoplaki, 2002).…”

Relation between sensible and latent heat fluxes in the Mediterranean and precipitation in the Greek area during winter

“…The most striking feature of winter precipitation variability in Greece is the broad minimum in the late 1980s-early 1990s and the maximum around 1970 in northwestern Greece (factor 1), revealed by many researchers (e.g. Xoplaki et al, 2000, Bartzokas et al, 2003 and attributed to the effects of the North Atlantic oscillation (NAO). The NAO index presented maximum values in the late 1980s-early 1990s and minimum values around 1970 (Philips, 1995;Nichols, 1998), indicating enhanced anticyclonic and cyclonic (respectively) circulation in the central Mediterranean.…”

“…Many researchers have followed this process, i.e. the reduction of the dimensionality of the initial data sets before the application of CCA (Zorita et al, 1992;Knappenberger and Michaels, 1993;Corte-Real et al, 1995;Diaz et al, 1998;Xoplaki et al, 2000). This is considered necessary in order to avoid, on the one hand, quasi-degeneracy of the autocovariance matrices of the data sets and, on the other hand, to filter the data by eliminating the noise (Barnett and Preisendorfer, 1987;Bretherton et al, 1992).…”