A. Bartzokas

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

Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration

Lake Pamvotis is a moderately sized (22 km2) shallow (zavg=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P l-1, TP=0.11 mg P l-1, NH4+=0.25 mg N l-1, NO3−=0.56 mg N l-1). FRP and NH4+ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (July–August green algae, August–September blue-green algae), autumn (October blue-green algae and diatoms), and winter (February diatoms), but not in the spring (March–June). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60–80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a decrease in FRP. A phosphorus mass balance identified further reductions in external loading from the predominately agricultural catchment will decrease FRP levels further. The commercial fishery and lake hatchery also provides opportunities to control algal biomass through biomanipulation measures.

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.

Sky luminance distribution in Central Europe and the Mediterranean area during the winter period

Five-minute data of global and diffuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of 5 years are used in order to define the prevailing sky luminance distributions at the two sites in the winter period. The classification into 15 sky types is mainly based on the value of the ratio of zenith luminance to diffuse illuminance with solar altitude as parameter (SSLD method). This is applied for the first time in a research work. It is found that the most frequent sky type in Bratislava is the overcast (with steep luminance gradation and slight brightening towards the sun); while in Athens it is the cloudless polluted sky (with a broad solar corona). That is, the winter daylight climate is quite different in Central Europe in comparison to the Mediterranean. An analysis for various solar altitudes reveals the frequency distribution of the 15 sky types suitable for the application in daylight climate studies.

An objective definition of air mass types affecting athens, greece; The corresponding atmospheric pressure patterns and air pollution levels

Abstract This work aims at defining characteristic air mass types that dominate in the region of Athens, Greece during the cold (November‐March) and the warm (May‐September) period of the year and also at evaluating the corresponding concentration levels of the main air pollutants. For each air mass type, the mean atmospheric pressure distribution (composite maps) over Europe and the Mediterranean is estimated in order to reveal the association of atmospheric circulation with air pollution levels in Athens. The data basis for this work consists of daily values of thirteen meteorological and six pollutant parameters covering the period 1993–97. The definition of the characteristic air mass types is attempted objectively by using the methods of Factor Analysis and Cluster Analysis. The results show that during the cold period of the year there are six prevailing air mass types (at least 3% of the total number of days) and six infrequent ones. The examination of the corresponding air pollution concentration levels shows that the primary air pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over the central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. During the warm period of the year, the southwestern Asia thermal low and the subtropical anticyclone of the Atlantic Ocean affect Greece. Though these synoptic systems cause almost stagnant conditions, four main air mass types are dominant and ten others, associated with extreme weather, are infrequent. Despite the large amounts of total solar radiation characterizing this period, ozone concentrations remain at low levels in central Athens because of its destruction by nitric oxide.

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.

Effect of meteorological parameters on acute laryngitis in adults.

The aim of this study was to investigate the influence of weather conditions on the frequency of acute laryngitis in adults. The medical records of 825 patients with a diagnosis of acute laryngitis obtained over a 5-year period were reviewed. Meteorological data reviewed included the daily values of 13 parameters. The monthly distribution of the disease was assessed. In addition, a detailed 2-part statistical analysis was performed as follows: ( i ) the relationship between each meteorological parameter and the frequency of the disease was investigated, using contingency tables; and ( ii ) the 1825-day period was divided into 8 groups characterized by similar weather, using factor and cluster analysis, and the disease frequency in each group was assessed. Our observations were as follows: 1. Meteorological parameters, such as low temperature, low diurnal temperature range, low atmospheric pressure and low mean water vapor pressure, were associated with an increased occurrence of acute laryngitis. 2. Significant changes in maximum temperature or a significant drop in atmospheric pressure, as compared to the previous days conditions, favored new cases of the disease. 3. The cold period of the year was associated with an almost two-fold increase in the frequency of disease cases, which peaked in March and reaching a minimum during August. 4. The peak frequency of cases was associated with winter weather conditions, which were characterized by high humidity readings. In conclusion, meteorological parameters, and their variation and covariation, are strongly associated with acute laryngitis in adults.

Large-Scale Atmospheric Circulation Favoring Deep- and Intermediate-Water Formation in the Mediterranean Sea

Atmospheric circulation patterns that are conducive to extreme ocean heat loss are investigated at four sites of special interest in the Mediterranean Sea. The Gulf of Lions, the South Adriatic Sea, the Cretan Sea, and the Levantine Sea are areas where anomalously high winter heat loss may lead to deep- or intermediate-water formation. At each of the above sites, the atmospheric circulation during such events is derived by averaging the sea level pressure (SLP) fields during the lower decile of the wintertime series of the net heat exchange. A relatively simple SLP pattern dominated by an anticyclone over northwestern Europe with a weaker cyclone to the southeast is found to be associated with strong heat loss in the selected sites with minor variations in pattern structure depending on the site. The SLP composite pattern reflects the combined effect of different atmospheric modes of variability and the authors consider the impacts on heat loss of a number of these modes (North Atlantic Oscillation, east Atlantic pattern, east Atlantic–west Russia pattern, and Scandinavian pattern), together with the North Sea–Caspian pattern and the Mediterranean index. The extremes in heat loss are strongly connected with the intensity and the positions of the poles of these patterns that modulate, through the necessary SLP gradient and associated northerlies, the transfer of cold and dry air over the areas of dense-water formation. Analysis of air–sea temperature difference, specific humidity, and evaporation anomalies indicates that the extremes of the net heat fluxes are primarily due to the latent and sensible heat flux components.

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