C. Varotsos

The southern hemisphere ozone hole split in 2002

Among the most important aspects of the atmospheric pollution problem are the anthropogenic impacts on the stratospheric ozone layer, the related trends of the total ozone content drop and the solar ultraviolet radiation enhancement at the Earth’s surface level.During September 2002, the ozone hole over the Antarctic was much smaller than in the previous six years. It has split into two separate holes, due to the appearance of sudden stratospheric warming that has never been observed before in the southern hemisphere.The analysis of this unprecedented event is attempted, regarding both the meteorological and photochemical aspects, in terms of the unusual thermal field patterns and the induced polar vortex disturbances.

Power‐law correlations in column ozone over Antarctica

Detrended fluctuation analysis is applied on springtime daily column ozone at the edge and into the Antarctic ozone hole, by using observations deduced from ground‐based (1972–2003) and satellite‐borne instrumentation (1979–2003). First, extreme column ozone fluctuations obey a power‐law with exponents, implying that large fluctuations are more likely to occur into the ozone hole than at its edge. Secondly, for time‐scales longer than one year, persistent long‐range power‐law correlations in the column ozone fluctuations were more pronounced during 1979–1992. However, by eliminating the long‐term trend, antipersistence (persistence) for time lags more (less) than ten days is detected for the entire data record. The latter crossover illustrates the role of planetary waves in the scaling characteristics of the spatio‐temporal variability of the Antarctic ozone hole. Finally, since 1996 the intrinsic dynamics in column ozone at the edge of Antarctica have differed; this does not hold for the Antarctic ozone hole.

Atmospheric greenhouse effect in the context of global climate change

SummaryGreat interest in the problem of the atmospheric greenhouse effect (not only in scientific publications, but also in mass media), on the one hand, and the undoubtfully overemphasised contribution of the greenhouse effect to the global climate change, on the other hand, motivate a necessity to analyse the role which the greenhouse effect plays as a factor of climate change. Significant progress in the analysis of existing observational data as well as succesful development of numerical climate modelling which have been achieved during the recent few years create a basis for a new survey of the atmospheric greenhouse effect in the context of global climate change. Such a survey is the principal purpose of this paper. After discussing a notion of the greenhouse effect, the detailed analysis of the present-day and paleoclimatic observational data has been conducted with subsequent consideration of numerical modelling results. A special attention has been paid to assessments of the greenhouse warmingvs. aerosol cooling. Then possibilities of the early detection of a greenhouse climate signal have been analysed and a few comments on the global climate observing system have been made with the general conclusion that more observations and further numerical modelling efforts are necessary to more reliably assess the contributions of various mechanisms to the observed global climate changes. It is only in the context of a coupled totality of significant climate forming factors and processes that the contribution of the greenhouse effect may be estimated.

Long-range persistence in global Aerosol Index dynamics

Detrended fluctuation analysis (DFA) was applied to zonal mean daily Aerosol Index (AI) values derived from satellite observations during 1979–2003 to search for self‐similarity properties. The results show that the detrended and deseasonalized AI fluctuations in both hemispheres and globally obey persistent long‐range power‐law correlations for time scales longer than about 4 days and shorter than about 2 years. This suggests that the AI fluctuations in small time intervals are related to the AI fluctuations in longer time intervals in a power‐law fashion (when the time intervals vary from about 4 days to about 2 years). In other words, an anomaly in AI in one time frame continues into the next, exhibiting a power‐law evolution. The influence of the annual and semiannual cycles on the scaling behaviour of the AI time series in both hemispheres is discussed. A plausible mechanism for the time scale of about 2 years in AI time series could be the modulation of the Brewer–Dobson cell by the quasi‐biennial oscillation at the equatorial stratosphere in the zonal wind. The synoptic‐scale meteorological systems probably give rise to the time scale of about 4 days. These findings could prove useful in testing the results of existing models, which should be examined to determine if they demonstrate the scaling behaviour mentioned above.

What is the lesson from the unprecedented event over antarctica in 2002

Varotsos (2002a,b), suggested that both the smaller-sized ozone hole over Antarctica and its splitting in two holes in September 2002 occurred due to an unprecedented major sudden stratospheric warming caused by very strong planetary waves propagated in the southern hemisphere. Subsequently, a NASA press release of December 6, 2002, also reported the prevalence of very strong planetary waves in Antarctica.The aim of this Letter is to further discuss the morphology of the Antarctic ozone hole, to detect the causes that allowed the Antarctic stratosphere to exhibit this exceptional warming and to examine what it denotes about its mechanisms.Concerning the morphology, among the principal findings is that the ozone hole split occurred not only in the stratosphere but extended in the lower altitudes (upper troposphere).As to the causes of the major sudden stratospheric warming of 2002, a comparison with the previous warmings in Antarctica since 1964 is made. The smaller-sized Antarctic ozone hole of 2002 is approximately equal to that of 1988 when a strong sudden stratospheric warming occurred. If only the destruction of ozone by chlorofluorocarbons resulted in the delayed sudden stratospheric warmings in Antarctica, then the early sudden stratospheric warmings of 1988 and 2002 would not have occurred, since chlorofluorocarbon loading of the stratosphere has remained relatively stable in recent years. Furthermore, it appears that the El Nino characteristics in 1988 and 2002 are not similar.

In-situ measurements of stratospheric ozone depletion rates in the Arctic winter 1991/1992: A Lagrangian approach

A Lagrangian approach has been used to assess the degree of chemically induced ozone loss in the Arctic lower stratosphere in winter 1991/1992. Trajectory calculations are used to identify air parcels probed by two ozonesondes at different points along the trajectories. A statistical analysis of the measured differences in ozone mixing ratio and the time the air parcel spent in sunlight between the measurements provides the chemical ozone loss. Initial results were first described by von der Gathen et al. [1995]. Here we present a more detailed description of the technique and a more comprehensive discussion of the results. Ozone loss rates of up to 10 ppbv per sunlit hour (or 54 ppbv per day) were found inside the polar vortex on the 475 K potential temperature surface (about 19.5 km in altitude) at the end of January. The period of rapid ozone loss coincides and slightly lags a period when temperatures were cold enough for type I polar stratospheric clouds to form. It is shown that the ozone loss occurs exclusively during the sunlit portions of the trajectories. The time evolution and vertical distribution of the ozone loss rates are discussed.

Global tropospheric ozone dynamics

An overview of the tropospheric ozone changes is presented focussing mainly on the tropospheric ozone precursors. The complexity of the problem is shown through the consideration of a great number of relevant substances, like nitrogen compounds, volatile organic compounds, peroxyacetyl nitrate, hydroxyl radical, carbon monoxide, alkyl nitrates. The up-to-date knowledge on the relevant numerical modelling is presented in Part II.

Further evidence of the role of air pollution on solar ultraviolet radiation reaching the ground

Abstract The influence of photochemical pollution on the ultraviolet radiation reaching the ground is examined. For this purpose, a series of UV-A and UV-B measurements as well as the results of a simple parametric model are compared. It was found that the hypothesis of UV-B depletion is significant at an almost 95 per cent confidence level. It is also indicated that the effect of photochemical pollution on UV-B levels reaching the ground is roughly three times the same effect on UV-A levels.

Tropospheric aerosol forcing of climate: a case study for the greater area of Greece

Although greenhouse gas forcing has global significance, the aerosol forcing is regional and seasonal, associated with the much shorter aerosol residence times in the atmosphere, and could become dominant on a regional scale. Several studies indicate that aerosol radiative forcing is among the highest in the world over the Mediterranean in the summer. In this study, the aerosol impact (forcing) on the short‐wave and long‐wave fluxes, as well as the radiative heating rate due to aerosols for different altitudes in the atmosphere over Athens, Greece, was estimated using satellite data and SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model. The short‐wave aerosols radiative forcing at the surface in cloud‐free conditions during the period 2000–2001 ranged from 10.8 to 20.1 W m−2 in the winter and from 15.2 to 16.6 W m−2 in the summer. The radiative heating rates near the surface due to aerosols were found to be in the range of 0.2–0.5 K day−1 during the winter period and 0.4 K day−1 during the summer period simultaneous with enhanced heating in the lower troposphere (below 5 km). The long‐wave radiative forcing (clear sky) at the top of the atmosphere induced by aerosols during night‐time was estimated to be only 0.02–0.04 W m−2 and 0.04–0.05 W m−2 for the winter and summer months, respectively.

The Long-Term Coupling between Column Ozone and Tropopause Properties

Abstract The observational data of the vertical temperature distribution and column ozone, obtained from 10 main stations in the Northern Hemisphere, are analyzed in order to explore the tropopause variations in conjunction with the dynamical variability in column ozone. From the analysis presented, it is evident that the summer distribution of the frequency of occurrence of the tropopause over Greece, apart from its main maximum (around 12 km), is also characterized by a secondary one around 16 km. It is proposed that this elevated maximum possibly originates from the height variation of the tropopause from 12 to 16 km depending on whether the Athens station is located below the cyclonic shear side or below the anticyclonic shear side of the subtropical jet stream. It is also suggested that the transport in the upper troposphere and lower stratosphere that originated in the equatorial region forces the appearance of the multiple tropopauses above Greece. Furthermore, the observational analysis of the ver...