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Atmospheric circulation in the southern hemisphere.
The latitudinal temperature gradient existing all year round in the whole troposphere above Antarctica is responsible for the formation of the circumpolar vortex in the free atmosphere which is characterized by lower pressure in its center and clockwise rotation. As a result, air is descending over the most of the continent [Voskresensky, Lysakov, 1976], which is one of the main factors in forming the anticyclonic type of weather. The predominance of clear sky typical for such weather is favorable for the radiative cooling of the surface, while low temperature causes an extreme dryness of the air which leads to further cooling. Cold air is flowing down along the glacier slope, while in the free atmosphere this flux is counterbalanced by inflow of moist and warm air from ocean. The change of the direction of the meridional component of the air flux takes place at the altitude of 3.8–5 km above sea level. This circulation develops most intensively in winter when the gradient between the pole and the low latitudes is the strongest [Averianov, 1990; Schwerdtfeger, 1987]). The above picture is often disturbed by meridional invasions into the high-latitude region of the cyclones formed at polar or, less often, Antarctic fronts [Dydina et al., 1976; Savitsky, 1976]. The latter are usually smaller and less developed in height. They are formed at the latitudes of 60–65 °S and the zonal component is dominant in their movement: they move around Antarctica parallel to the main stream, i.e., from west to east. Polar cyclones are generally deeper and larger than Antarctic ones. Possessing considerable meridional component in their movement, they sometimes penetrate far into the Antarctic plateau and thus play an important role in the inter-latitudinal exchange of heat and moisture of the southern hemisphere. Approaching the boundary between comparatively warm waters of the Southern ocean and cold Antarctic coast the polar cyclones can become stronger and, provided the presence of the blocking ridges of high pressure, stationary. On the maps of long-term average cyclone system density the areas of most frequent cyclone occurrence can be clearly seen (Fig. 3): Weddell, Ross, Bellingshausen and Commonwealth seas. These areas play important role in formation of climatic regime of Antarctica [Averianov, 1990; Schverdtfeger, 1987].
As a whole, the atmospheric circulation in the middle and high latitudes of the southern hemisphere is governed by the following basic regimes (see the review in [Simmonds, 2003]).
First of all, this is the so-called Southern Annular Mode [Thompson, Wallace, 2000] that is characterized by opposite air pressure variations in the middle and high latitudes of the southern hemisphere. The index of its intensity is Antarctic Oscillation Index (AOI) representing the difference of mean latitudinal near-surface air pressure at 40 and 65° S [Gong, Wang, 1999].
Higher index means stronger gradient of pressure and temperature between high and middle latitudes, stronger westerly and weaker inter-latitudinal exchange, which causes cooling in Antarctic. This annular mode is related to the tropical circulation (ENSO), which is confirmed by the fact that El-Nino years are often correspond to the lower values of AOI [Maslennikov, 2002a,b]. Another important regime is the Antarctic Circumpolar Wave, which characterizes the drift of anomalies of meteorological and oceanographical parameters around Antarctica from west to east with a period of about 8–10 years [White, Peterson, 1996]. Anomalies of temperature and pressure, being born in the subtropical zone of the Pacific in relation to El-Nino, are then transferred by Antarctic circumpolar current to the east. This phenomenon is specific to the southern hemisphere, because in the northern one there is no continuous circumpolar current [Peterson, White, 1998]. The period of oscillations related to this wave is 4–5 years.
Disturbance to the two previous regimes is brought by Antarctic Dipole Mode (ADM) that is opposite oscillations of temperature, pressure and sea ice cover in east part of Pacific sector and in Atlantic sector of Antarctic [Yuan, Martinson, 2001]. The Antarctic Dipole is related to tropical circulation, too, the El-Nino years being characterized by positive anomalies of temperature in the Pacific sector and negative ones in Atlantic sector. ADM is actually one of the strongest mechanisms responsible for the transmission of the climatic signal from low to high latitudes [Liu et al., 2002]. Despite relatively weak degree of investigation of the above circulation regime, their role in forming climatic variability of interior parts of Antarctica is in general beyond doubt. In years of anomalous development of meridional processes more cyclones invade into the continent, which causes warming and increasing of precipitation. On the contrary, when zonal processes are stronger, air temperature and pressure are lower in high latitudes [Dydina et al., 1976; Zhukova, 1986; Savitsky, 1976]. In particular, in years with higher AOI index increased temperature is observed over Antarctic Peninsula and decreased over the rest of the continent, especially in East Antarctica. The influence of the tropical circulation on the Antarctic climate mainly reveals itself in reduced sea ice cover in Amundsen and Bellingshausen seas in the El-Nino years (which corresponds to negative Southern Oscillation Index) and to lesser degree in cooling of the interior part of the Antarctic [Kwok, Comiso, 2002]. Thus, cooling observed during the last 10– 20 years over the most of the continent with simultaneous warming over the Peninsula [Doran et al., 2002] is consistent with stronger Southern Annular Mode and El-Nino during the same period. Rapid warming in the area of Antarctic Peninsula is related to stronger westerly and thus to more intensive advection of warm oceanic air, as well as with destruction of sea ice in th surrounding seas [Kwok, Comiso, 2002]. Intensification of the annular mode (increasing of AOI index) is accompanied by an increased air pressure to the north of 40° S and its decrease in the high latitudes. At the same time smaller amount of cyclones are observed in the southern hemisphere. This apparent contradiction is explained by the fact that though the number of cyclones is less, they became deeper and more intense [Simmonds, Keay, 2000].
Surface temperature inversion at Vostok and wind regime.
Since the mean values of main meteorological parameters at the near-surface level were discussed in the Introduction, below we will consider the meteorological regime of troposphere using the published data of balloon-sounding observations.
The most typical feature of tropospheric structure in central Antarctica is a stable thick layer of surface inversion of mixed radiation and dynamic origin [Connolley, 1996; Phillpot, Zillman, 1970; Tsigel’nitsky, 1982; Voskresensky, Tsigel’nitsky, 1985]. The mean thickness of inversion in winter is about 800 m with a temperature difference between upper and lower boundary of about 25 °C and occurrence of nearly 100 % (Fig. 4). These values are twice of those in central Greenland. Monthly means of the main inversion characteristics are listed in Table 1.
Isotope composition of precipitation and its relation to the conditions of formation: Theoretical considerations and empirical data
The term « isotope » was firstly suggested by English physicist F. Soddy in 1910, although the idea about elements that have the same charge but different atomic mass had appeared long before. Already 8–9 years after this event the first measurements of concentration of stable isotopes were carried out. In 1929 and 1932 heavy isotopes of oxygen and hydrogen were discovered by Giauque and Johnston (1929) in Great Britain and Urey with others (1932) in the United States. During the following 35 years an intensive work had been undertaken to study the processes governing the natural distribution of isotope composition of these two elements. The results of these efforts were summarized in a paper « Stable isotopes in precipitation » of a Danish scientist W. Dansgaard that came out in 1964 [Dansgaard, 1964].
The first idea of using isotopes as a natural paleothermometer belongs to Urey who found a small difference in the isotopic composition of carbonates depending on the temperature of calcite formation. The first relationship between isotope composition of water precipitation and temperature of condensation was described by Dansgaard. These discoveries set up a basis for the method of paleotemperature reconstructions by isotope analyses of fossil precipitation samples. It was soon realized that one of the best application for the water stable isotope geochemistry was the study of the isotope composition of past snow precipitation successively accumulated during many millennia in polar ice sheets. The ice cores obtained by deep drilling of glaciers represent unique archives of climatic information in which the data on past changes of temperature, snow accumulation rate, wind speed, chemical and gas composition of atmosphere are stored. The first deep drilling of polar ice was completed in 1964 at Camp Century site in Greenland [Dansgaard et al., 1971]. The 1390-m deep borehole reached bedrock, and the analysis of about 1600 ice samples allowed reconstructing climate in this area over 100 ka. In Antarctica the first deep drilling project was fulfilled in 1966 at American Byrd Station [Epstein et al., 1970]. In 1974 and 1978 the first boreholes were finished at Vostok Station [Barkov, 1970; Barkov et al., 1975] and French Dome C Station [Lorius et al., 1979]. At present, deep drilling projects are carried out in various sites in Arctic and Antarctica by specialists from Europe, the United States, Russia, Japan and other countries. In February 2003 the hole at Dome Concordia site drilled in the frame of EPICA (European Project of Ice Coring in Antarctica) reached the depth of 3200 m. The age of the deepest ice obtained from this borehole can be as old as 800 ka (personal communication of Jean Robert Petit, 2003). In July 2003 the head of the drill reached bedrock at NorthGRIP site (central Greenland) at the depth of 3085 m thus making this borehole the longest one ever drilled on this island. The studies of the obtained ice cores including the measurements of their isotope composition have substantially broadened our knowledge on the climatic changes in polar regions and the whole Earth during the last half million of years.
Theoretical basis of the relationship between isotope composition of precipitation and air temperature: Simple isotope models and GCMs
The relationship between isotope composition of precipitation and temperature of its formation is based on the « isotope depletion » of moisture in the precipitating air mass due to isotope fractionation at each phase change. Since saturation water vapor pressure is less for heavy water molecules (HD16O and H218O) than for light molecules (H216O), the concentration of heavy isotopes in the liquid phase is higher than in the vapor phase equilibrated with this liquid. So, the isotope composition of water vapor contained in an air mass formed over the ocean is negative (if expressed in δ notation† – see equation (2)). As the cooling of the air mass proceeds, the water vapor condenses and new portions of precipitation are enriched in heavy isotopes in relation to the vapor remaining in the air mass thus making the vapor more and more isotopically depleted (Fig. 6). Obviously, in the course of further cooling both vapor and condensate become isotopically lighter due to the washing out of heavy water molecules during precipitation formation.
Empirical estimations of relationship between isotope composition of precipitation and temperature
The use of the isotope signal as paleo-thermometer is based on the assumption that the present-day geographic (spatial) slope between δ in precipitation and TS is equal to the corresponding temporal slope. This assumption needs additional empirical validation because the isotope composition of precipitation in the past could be influenced by other factors than condensation temperature, e.g., changes of evaporation conditions in moisture source and seasonality of precipitation (see review in [Jouzel et al., 1997, 2003]). Moreover, independent estimates of past ice sheet surface temperature based on borehole thermometry show that the above described isotope approach underestimates the amplitude of temperature changes in Greenland by factor of two [Cuffey et al., 1995; Johnsen et al., 1995] and in Antarctica by about 30 % [Salamatin et al., 1998a, b].
Direct comparison of isotope composition and air temperature. In terms of experimental validation of δ/TS relationship the most attractive approach is direct comparison of recent snow isotope composition with instrumental temperature measurements carried out at the same site during a sufficiently long period of time. Only two polar stations meet these conditions: South Pole and Vostok. At South Pole a significant linear relationship was found between mean annual snow isotope composition (δD) values as measured on samples from pits and mean annual surface air temperature, the coefficient of regression being 20 ‰ °C-1 [Jouzel et al., 1983]. Such a large value of C (three times larger than corresponding present-day geographical slope) is still not very well understood, but could be explained either by difference between precipitation-weighted mean and simple mean air temperature, or by difference between temperature variability at the surface level and at the level of condensation.
As for Vostok Station, prior to the beginning of this study three papers have been published concerning isotope composition of precipitation and deposited snow. Gordienko et al. (1976) presented data on seasonal changes of isotope composition (δ18O) of snow precipitation at Vostok in 1970. They obtained linear relation between monthly values of δ and surface air temperature with the slope of 0.84 ‰ °C-1 that corresponds to the slope of 6.7 ‰ °C-1 for δD. This value agrees well with the theoretical slope from simple isotope model. In papers of Dansgaard et al. (1977) and Kolokolov et al. (1993) the isotope profiles are shown from two 1.2-m deep snow pits dug in 1975 and 1981. Mean δ18O values of snow deposits are, correspondingly, -57.9 and -56.3 ‰. No comparison of isotope data with temperature records was made.
In the other parts of Antarctica and in Greenland the comparison of δ and T is limited either by too short series of temperature observations or by too large distances between sites of isotope study and sites of temperature measurements. Nevertheless, in all cases the slope of temporal relationship between changes of δ and TS was found to be considerably (up to 40–50 %) less than corresponding present-day geographical slope (see review in [Jouzel et al., 1997]). The observed difference can be real or might be related to comparatively low correlation coefficients between the series either due to « stratigraphic » noise in the isotope composition records or due to long distance between sites.
Table of contents :
I. METEOROLOGICAL REGIME AND ISOTOPE COMPOSITION OF PRECIPITATION: REVIEW OF LITERATURE
I.1. Meteorological regime and precipitation formation in central Antarctica
I.1.1. Atmospheric circulation in the southern hemisphere
I.1.2. Surface temperature inversion at Vostok and wind regime
I.1.3. Precipitation and water vapor in central Antarctica
I.2. Isotope composition of precipitation and its relation to the conditions of formation: Theoretical considerations and empirical data
I.2.1. Theoretical basis of the relationship between isotope composition of precipitation and air temperature: Simple isotope models and GCMs
I.2.2. Empirical estimations of relationship between isotope composition of precipitation and temperature
Direct comparison of isotope composition and air temperature
Borehole thermometry
Use of melt layers
Correlation with snow accumulation rate
Data on gas inclusion
Isotope composition of trapped air
I.3. Factors influencing the relationship between snow isotope composition and surface air temperature
I.3.1. Moisture source conditions
I.3.2. Seasonality of precipitation
I.3.3. Microphysical conditions of precipitation formation
I.3.4. Difference between condensation and surface air temperature
I.3.5. Glaciological factors
I.3.6. Post-depositional processes
I.4. Conclusion of Chapter I
II. METHODS AND EXPERIMENTAL DATA
II.1. Experimental data
II.1.1. Meteorological data
II.1.2. Balloon-sounding data
II.1.3. Snow accumulation rate
II.1.4. Isotope composition of snow
II.2. Field works
II.2.1. Stratigraphic studies in pits
II.2.2. Snow sampling in pits
II.2.3. Sampling of precipitating and blowing snow
II.2.4. Construction of new snow accumulation-stake network
II.2.5. Snow surface leveling
II.3. Laboratory measurements
II.3.1. Isotope measurements
II.3.2. Measurements of beta-radioactivity
II.3.3. Measurements of liquid conductivity
II.4. Conclusion of Chapter II
III. METEOROLOGICAL CONDITIONS OF SNOW FORMATION
III.1. Contribution of different precipitation types in total precipitation amount
III.2. Temperature of condensation
III.3. Conclusion of Chapter III
IV. SPATIAL VARIABILITY OF SNOW ISOTOPE COMPOSITION: PLAYGROUND OF WIND
IV.1. Mega-dunes and micro-relief
IV.2. « Meso-dunes » signature in spatial and temporal series of snow build-up
IV.3. Relief-related oscillations in temporal isotope series Post-depositional changes of snow δD content in the past
IV.4. Conclusion of Chapter IV
V. MODERN TEMPORAL VARIABILITY OF SNOW ISOTOPE COMPOSITION
V.1. Seasonal variability of isotope composition of precipitation
V.2. Temporal variability of isotope composition and snow accumulation rate in the vicinity of Vostok Station over the last 50 years
V.3. The deuterium content – temperature slopes
V.4. Short-term variations of isotope composition in deep ice cores from Vostok
V.5. Conclusion of Chapter V
VI. CHANGES IN TEMPERATURE AND SNOW ACCUMULATION RATE AT VOSTOK STATION OVER THE PAST 200 YEARS
VI.1. Series of isotope composition and snow accumulation rate from deep pits
VI.2. 50-year cycle in changes of accumulation and isotope composition:
A teleconnection between central Antarctica and tropical Pacific?
VI.3. Secular trends of accumulation and isotopes at Vostok: Climate or mega-dunes?
VI.4. 200-year accumulation and isotope tendencies at other East Antarctic sites
VI.5. Conclusion of Chapter V
CONCLUSION AND PERSPECTIVES
REFERENCES
