D. A. Metaxas

Pressure covariability over the Atlantic, Europe and N. Africa. application: Centers of action for temperature, winter precipitation and summer winds in Athens, Greece

SummaryThis paper deals with the surface pressure covariability over the Altantic/European sector of the Northern Hemisphere, using monthly grid point data for the 100 year period 1890–1989. Factor analysis is applied to 90 grid point time series for January, February, July, and August. The initial 90 pressure variables can be reduced to 7–8 factors in winter and 10 in summer. A winter teleconnection was identified, known as the seesaw phenomenon, between the Icelandic low and the Azores subtropical anticyclone. In order to define the centers of action for temperature, winter precipitation and summer northerly wind frequency (etesian days) in Athens and in the Aegean sea, the variability of the factor scores and of these weather elements is compared. It is shown that the center of action for temperature in Athens is found to be in north and northwest Europe (centered over southern Scandinavia). For winter precipitation, the center of action is located in the west and southwest Mediterranean and northwest Africa. Finally, for the etesian winds frequency variability, this center of action is found over the northern Adriatic and northern former Yugoslavia, while there is no evidence of influence by the southwest Asia thermal low.

Spatial covariability of the climatic parameters in the Greek area

In this work, 12 monthly climatic parameters of 49 Meteorological stations in Greece are classified according to their spatial covariability, in order to define groups of climatic elements having the same, characteristic, spatial distribution. The grouping of the parameters is achieved objectively by using the method of factor analysis. The analysis is applied to the winter months of January and February and to the summer months of July and August. In winter, the parameters are classified into four groups as follows: (i) maximum, mean and minimum temperature and, negatively, pressure and number of frost days; (ii) wind and, negatively, diurnal temperature range; (iii) precipitation; and (iv) cloudiness and relative humidity. In summer, the parameters are classified into three groups, namely: (i) precipitation, cloudiness and, negatively, minimum temperature; (ii) maximum and mean temperature, relative humidity; and (iii) wind. From the above groups the most interesting is group (i) of summer, as its parameters belong to different groups in winter. Finally, the study of the spatial distribution of the above groups showed that some depend on latitude, others on the continentality and others on the leeward or windward character of the various areas. Copyright

Covariability and climatic changes of the lower-troposphere temperatures over the Northern Hemisphere

SummaryThe covariability and the long-term fluctuation of the lowertroposphere temperatures is studied, applying factor analysis on grid point (1000÷500) hPa thickness data for the Northern Hemisphere. About 30 factors were retained and rotated for each season and year, but only about half of them were found to be significant. The grouping of the grid points according to the factor loadings has been mainly influenced by the land and sea distribution. Three teleconnections were found only, one in winter and two in summer. The winter one is a see-saw phenomenon between West Asia and North-West Scandinavia. In summer, the British Isles were found to covary with East Canada and the same is the case for almost all the subtropical deserts. The study of the factor scores revealed that the recent warming of the Earth’s surface in the lower troposphere appears over the continents only.

Factor Analysis of Some Climatological Elements in Athens, 1931--1992: Covariability and Climatic Change

SummaryThe climatological parameters of a station are, in general, intercorrelated to various degrees (e.g. cloudiness with sunshine duration or possibly rainfall with temperature). In this work, 15 climatological parameters of the National Observatory of Athens are distributed in groups where the contained parameters are highly covariant. This classification is achieved using P-mode Factor Analysis, based on the monthly values for the period 1931–1992. Using several known criteria, we found up to four significant factors. The first two are the most important ones including the “temperature” and the “precipitation” parameters, respectively, with loading values above 0.85. The other two are weaker and variable, depending on the month. Finally, climatic trends of the grouped parameters (factors) are examined and the result compared with those of studies using classical methods.

A STATISTICAL STUDY OF PRECIPITATION IN NORTHWEST GREECE

In this paper, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for the time and space distribution of precipitation in northwest Greece is at first described. Precipitation varies from about 1000 mm at the coast to 2500 mm on the mountains, with a large variability. Then, the intra-annual variation is studied, using Fourier analysis. This variation is described well by two harmonics, explaining about 90% of the total variance. The first harmonic, over 80% of the variance, shows a maximum from late December (coastal areas) to early January (continental areas) and is caused by the northwest Mediterranean depression activity. The second harmonic exhibits maxima in May and November, also delayed in the continental areas, and is mainly caused by upper air troughs and instability. The spatial distribution of precipitation is then studied by using factor analysis. The 12 months of a year are classified in two parts corresponding to two statistically significant factors, explaining 88.5% of the total variance. Finally, the combination of positive and negative standardized scores of the two factors showed that northwest Greece is characterized by four precipitation regimes, depending on the combination of depression activity, the effects of the sea and of topography. Copyright

Intra-annual variation of atmospheric static stability in the Mediterranean region: a 60-year climatology

The seasonal characteristics of atmospheric static stability in the Mediterranean region are examined, for the 60-year period 1948–2007 and for the four 15-year sub-periods 1948–1962, 1963–1977, 1978–1992 and 1993–2007. S-Mode and T-Mode Factor Analysis are applied to the mean 5-day values of K static stability index over the Mediterranean region. Three dominant modes are revealed for both, the intra-annual variation and the spatial distribution of K-index. It is found that these modes are connected to the seasonal characteristics of the main atmospheric circulation systems affecting the region and the thermal properties of the Earth’s surface (land or sea). The differences among the results of the four sub-periods partially reflect the inter-decadal variations of the strength of the above factors.

Temporal Variations of Atmospheric Tides over Athens, Greece

SummaryThe mean daily march of pressure (hourly values) for each month of the 96 year period 1894–1989 in Athens, Greece is studied using Fourier analysis. The annual variation of the harmonic parameters (amplitude, time of maximum,x andy-components) is examined. The globally excited 2nd and 3rd harmonics show in general the same behaviour as in other places of the Earth. Similar features show the corresponding parameters of the daily solar radiation march. The first harmonic displays some local characteristics. The long period fluctuations study revealed statistically significant trends in most parameters of the 2nd harmonic, most strikingly the 0.5 h shift in the time of its maximum during the 20th century. Power spectrum analysis showed the existence of a biennial oscillation inB1, thex-component of the first harmonic, but this oscillation is mainly confined to the second half of the entire period. A periodicity of about 6 years which could be associated to the Southern Oscillation was also found inB1 while inA1, they-component, a periodicity close to the 11-year solar cycle and another one close to the nutation of the Earths axis are present. The lunar tides are too small to be detected by the same methods. However the semilunar tide was revealed to be 10–20 times smaller than the semisolar.

Large warm advection over athens: a climatological and synoptical study

SummaryIn the present paper, warm air advection in the Greek area as a precipitation causing factor, is examined.A total of 125 large warm advection (L.W.A.) cases, were selected. From the mean synoptic charts it is observed that L.W.A.s are a result of the eastward movement of a system of two depressions, separated by a high. The first depression is located over the eastern Mediterranean and the second over the eastern Atlantic.A superposed epoch analysis was applied to the precipitation amounts as well as to precipitable water and to the saturation deficit of the 1000/500 mb layer, during L.W.A. events. No appreciable precipitation amounts were found during the 48 hours of L.W.A. From the above analysis as well as from the calculations of the Showalter stability index it is concluded that the scarcity of precipitation in the south Greek area during L.W.As should be attributed to diabatic heating and static stability, counteracting warm advection and caused by a field of downward vertical velocities.ZusammenfassungIn der vorliegenden Arbeit wird die Warmluftadvektion über Griechenland als ein niederschlagsverursachender Faktor untersucht. Insgesamt wurden 125 Fälle mit bedeutender Warmluftadvektion ausgewählt. Aus synoptischen Mittelkarten ist ersichtlich, daß diese Fälle durch die Ostwärtsverlagerung zweier, durch ein Hochdruckbebiet getrennter Depressionen verursacht werden. Die erste dieser Depressionen liegt über dem östlichen Mittelmeer, die zweite über dem Ostatlantik.Die Methode überlagerter Epochen wurde auf Niederschlag, niederschlagbares Wasser und Sättigungsdefizit in der 1000–500mb-Schicht während der Fälle mit bedeutender Warmluftadvektion angewendet. Während der 48 Stunden Zeitspanne solcher Fälle wurden keine nennenswerten Niederschlagsbeträge beobachtet. Aus dieser Analyse, sowie aus dem Verhalten des Showalter Index wird geschlossen, daß der Niederschlagsmangel während starker Warmluftadvektion über Südgriechenland auf diabatische Erwärmung und vermehrte statische Stabilität zurückzuführen ist. Diese Effekte werden durch eine abwärts gerichtete Vertikalbewegung verursacht.