Plenary Lecture

Climatic Variability in Eastern Mediterranean: Dynamic Mechanisms and Impact on Regional Climate

 
Assistant Professor Helena A. Flocas
Department of Environmental Physics-Meteorology
Faculty of Physics
University of Athens, Greece
E-mail: efloca@phys.uoa.gr

Abstract: Teleconnection patterns appear as preferred modes of natural variability of low frequency atmospheric circulation, with fixed centres of action, that influence the regional climate and the occurrence of extreme weather events.
The existence of teleconnection patterns centered in Eastern Mediterranean, a region with intense topographic features and limited climatological research, was examined through the climatological data analysis of the Northern Hemisphere geopotential heights, for the period 1958-2006. This analysis leaded to the identification of a teleconnection pattern, in the winter fields of 300 and 500 hPa geopotential heights, with its poles located over Eastern Mediterranean and Northeastern Atlantic, respectively, and it will be referred to as Eastern Mediterranean teleconnection Pattern (EMP). An index, the EMP Index, was defined in order to discriminate the two phases of the pattern, the positive and the negative and that could be applied in the dynamical study of EMP and its impact on regional climate.
During positive (negative) phase of EMP, o strong anomalous anticyclonic (cyclonic) circulation prevails over northeastern Atlantic, while a weaker anomalous cyclonic (anticyclonic) exists over Eastern Atlantic, leading to an increased meridional (zonal) circulation over Europe and Mediterranean.
The frequency analysis revealed that the prevailing frequencies of EMP classify the pattern in the intermate scale of variability (10-30 days). The instant frequencies analysis of the Hilbert-Huang tramformation was applied, in order to find the temproral period of each winter, during which the EMP signal is maximized.
The theoretically proposed processes and mechanisms that lead to the formation of teleconnection patterns were examined: a) in planetary scale, the Rossby wave propagation from orographic or thermal forcing, b) in synoptic scale, the influence of transients (eddies defined as departures from time mean zonal circulation) and c) interaction between stratosphere and troposhere.
Studying the aforementioned processes, it was found that the positive phase of EMP forms when a forcing of tropical heating appears in tropical Pacific, along with an anomalous southern flow over the region of north Pacific. As a consequence, a Rossby wave-train propagates initially to the northeast and then it obtains a zonal structure over Atlantic and Europe, forming a ridge over Atlantic and a trough over eastern Mediterranean. The polar frontal jet shifts northward, having an anticyclonic circulation that creates o blocking system and so the storm track moves to the north. As the jet moves across Scandinavia, the transient eddies activity becomes enhanced taking energy from the mean flow, while in the mid-latitude Atlantic the existence of low-frequency eddies decelerates the mean flow, enhancing the blocking system.
During the negative phase, the existence of a tropical heating does not seem to force a wave-train propagation to the mid-latitudes. The Rossby wave-train has a zonal structure, while the Rossby waves are not clearly formed on the westerly flow. This fact is confirmed by the polar jet, which has zonal structure and comes charging into Europe. The northern pole of the pattern forms at the poleward side of the jet, while the southern pole forms at the equatorward side of the jet, due to the anticyclonic shear at this side of the jet and the topography of this region. The role of transient eddies in this case is to enhance the westerly flow.
The study of the interaction between stratosphere and troposphere, through tropopause dynamics, showed that the Atlantic storm track follows a different life cycle during each phase of the pattern. In the positive phase, the wave is dominated by a NE-SW shear then the trough becomes thinner, producing finally a cut-off cyclone. In the negative phase, there is less equatorward movement of the trough and the wave has a broader zonal structure.
The impact of each phase of EMP on mean regime of temperature and precipitation along with their extreme events was examined applying the Regularised Canonical Correlation Analysis (RCCA) and the analysis of the composite anomalies on the respective parametres. It was found that EMP indeed affects the mean winter patterns of temperature, precipitation and their extreme events inversely between the two phases. In particular, the positive (negative) phase of EMP is related to a decrease (increase) of temperatures and an increase (decrease) of precipitation. The extreme events of precipitation increase (decrease), the dry spells decrease (increase) and the extreme cold (warm) events intensify, during positive (negative) phase.

Brief Biography of the Speaker:
Flocas Helena is Assistant Professor at the University of Athens. She received her B.Sc. in Physics from University of Thessaloniki, Greece in 1987, her M.Sc. in Meterology from University of Reading, UK, in 1990, and her PhD in Meteorology from University of Thessaloniki in 1993. She worked as research assistant since 1990 in the Universities of Thessaloniki, Reading and Athens. She elected Lecturer in the University of Athens in 2000 and Assistant Professor in 2004. The fields of her scientific activities are Synoptic - Dynamic Meteorology, Climate Dynamics, Climatic changes, Bioclimatology, Atmospheric Boundary Layer and Air quality. She has participated in 50 research projects as research assistant (including 14 European projects). Coordinator of 2 research projects, funded by EU and private and/or national funds. She has published 58 papers in international journals, 68 in international conference proceedings and 16 in national conferences. She is reviewer in 14 journals. She has granted Scholarship from the National Institution of Scholarships during her undergraduate studies, Scholarship from NATO, through the Greek Ministry of Economics, during her postgraduate studies in UK, Scholarship from British Council for postdoctoral research at the University of Reading. She is member of the American Meteorological Society and Royal Meteorological Society.