The Annual Behavior of the Semidiurnal and Diurnal Pressure Variations in East Antarctica

Transcription

1 NOVEMBER 2002 PETENKO AND ARGENTINI 1093 The Annual Behavior of the Semidiurnal and Diurnal Pressure Variations in East Antarctica IGOR V. PETENKO A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia STEFANIA ARGENTINI Istituto di Scienze dell Atmosfera e del Clima, CNR, Rome, Italy (Manuscript received 15 April 2001, in final form 29 April 2002) ABSTRACT A 15-yr ( ) 3-hourly record of surface pressure from the Dome Concordia automatic weather station (74.30 S, E; 3280 m MSL), located in East Antarctica, was analyzed to study the annual behavior of the semidiurnal and diurnal variation associated with atmospheric tides excited by heating due to insolation absorption by ozone and water vapor. The mean daily behavior of the pressure variation shows maxima around 0900 and 2100 LT. This variation is more intense during the austral winter. The time series of a Total Ozone Mapping Spectrometer (TOMS) were analyzed to study the correlation between the local pressure tidal oscillations and the annual behavior of total ozone over the globe. A clear correlation between the intensity of the pressure semidiurnal variation and global total ozone in both the annual and long-term trends was found. The mean annual behavior of the semidiurnal tides and global total ozone is very similar, with two maxima and one deep minimum. Maxima in semidiurnal tides occur in May and September October; in ozone, they are observed in April May and September. A deep minimum in semidiurnal tides occurs in December February, and a minimum of ozone is observed in December. 1. Introduction A reasonably complete understanding of the dynamics of thermally forced atmospheric tides has been already achieved through the research of Siebert (1961), Bulter and Small (1963), Lindzen (1967, 1968), and Chapman and Lindzen (1970) and subsequent work. The daily behavior of the pressure p is determined primarily by the semidiurnal S 2 (p) and diurnal S 1 (p) harmonics. In general, two maxima in the mean pressure daily variation around 1000 and 2200 LT are observed (Chapman and Lindzen 1970). The largest magnitude of the daily variation (a few hectopascals) is observed in the Tropics and decreases with increasing latitude. At low latitudes, the amplitude of S 2 (p) is generally 2 3 times as large as S 1 (p). It is well established that the most important source of excitation for the thermal tides is heating due to the absorption of solar radiation by ozone in the stratosphere mesosphere and water vapor in the troposphere (Siebert 1961; Butler and Small 1963). The largest contribution to the semidiurnal oscillation is due to stratospheric ozone heating. If the current view of Corresponding author address: Stefania Argentini, Istituto di Scienze dell Atmosfera e del Clima, CNR, Area di Ricerca di Roma Tor Vergata, Via Fosso del Cavaliere 100, Rome 00133, Italy. s.argentini@isac.cnr.it the dynamics of thermal tides is correct, then it appears inevitable that S 1 (p) and S 2 (p) will follow the ozone variation. The study of the correlation between the pressure tidal variation and ozone is of interest for two main reasons. First, it might provide additional proof of the validity of current theoretical models of atmospheric tides. Second, it might offer the possibility of using surface pressure observations for monitoring the ozone variability. To the authors knowledge, Hamilton (1983) was the first to suggest such an idea. He analyzed a 21- yr record of surface pressure in the Tropics (Batavia), to detect the quasi-biennial oscillation and other longperiod variations in the solar semidiurnal barometric oscillation that were associated with stratospheric ozone variations. The presence of a peak between 24 and 30 months in S 2 (p) was shown. No direct comparison between the time behavior of ozone and S 2 (p) to demonstrate their synchronicity was given in that paper. The seasonal variability of S 2 (p) was shown by Chapman (1951), who analyzed the data of four stations in midlatitudes. Spar (1952) reported the monthly amplitude of the semidiurnal oscillations averaged over several tens of U.S. stations in the latitude belt from 25 to 45 N to show a seasonal cycle with prominent annual and semiannual components. Teitelbaum and Cot (1979) suggested that latitudinal asymmetries in the ozone con American Meteorological Society

2 1094 JOURNAL OF APPLIED METEOROLOGY VOLUME 41 centration in regions of significant heating can be responsible for the seasonal variation of semidiurnal tides at an approximate altitude of km. Later, Walterscheid and DeVore (1981) mentioned that the phases of the semidiurnal tides may depend on the relative contributions of sources that may be variable from month to month or year to year, such as hemispherically asymmetric ozone heating and cumulonimbus activity. Hamilton (1981a,b), following an idea by Lindzen (1978), showed that the latent heat forcing mechanism could be used to explain the observed seasonal cycle of the atmospheric tides in the subtropical and midlatitude regions. The seasonal variability of atmospheric tides observed on Mars, excited by heating due to solar absorption by dust and aerosols, was extensively discussed by Leovy and Zurek (1979), Zurek (1980), Leovy (1981), Zurek and Leovy (1981), and Wilson and Hamilton (1996). The observations of Viking Landers indicate two pronounced maxima in S 2 (p) during the Martian autumn and winter and a deep minimum during spring and early summer. Some aspects of the pressure behavior over a 1-day cycle are still not clear because the distribution of the pressure variation over the globe is complicated and variable, and it is sensitive to the topography and stratification of the atmosphere at higher altitudes. The variation in S 1 (p), in contrast to S 2 (p), is observed to have very large geographical variations (Hamilton 1981a). An important point is that S 2 (p) can reflect the local response to remote forcing, whereas the diurnal oscillation S 1 (p) is much more locally forced, and topography can lead to local variations in the amplitude and phase of the diurnal tide (Frei and Davies 1993). According to the classical theory, the atmospheric tides can be assumed to be negligible in Antarctica. However, as emphasized by Whiteman and Bian (1996), this theory loses its accuracy as the poles are approached and in proximity to mountain barriers. Both of these factors are important when we consider Antarctica. However, it must be said that there are very few polar observations to identify the possible tidal oscillations that may exist in this region. An abnormally strong diurnal pressure oscillation was observed on the Reeves Glacier (East Antarctica) by Viola et al. (1999). To the authors knowledge, Argentini et al. (2000a,b) and Petenko and Argentini (2001) were the first to report on a pronounced semidiurnal peak in the pressure spectra in East Antarctica. Some results confirming the presence of the tidal pressure variations in Antarctica and the importance of taking into account the daily variation of pressure on wind regime were given by Argentini et al. (2000b) and Petenko and Argentini (2001). In this paper, some features of the annual behavior of the semidiurnal and diurnal variations of pressure observed in East Antarctica from 1980 to 1995 are presented. Some possible explanations are given through the analysis of the ozone variability. FIG. 1. Map of Antarctica showing the locations of Dome C, D-80, and DdU. Contours are in meters. 2. Area and data description A dataset from the Dome Concordia Antarctic automatic weather station (AWS) (74.30 S, E; 3280 m MSL), recorded during , was taken to study the long-term behavior of the pressure tidal variation. In addition, the data from AWS D-80 (70.02 S, E; 2500 m MSL) and the meteorological station of Dumont d Urville, Petrels Island, Antarctica (66.42 S, E; 43 m MSL), obtained in 1994, were analyzed. These stations are located along the line connecting the plateau station of Dome Concordia (hereinafter Dome C) and the coastal station of Dumont d Urville (hereinafter DdU) (see Fig. 1). A detailed description of the orography and climate of this region was given by Gosink (1982), Périard and Pettré (1993), and Parish et al. (1993). The spectral characteristics of the meteorological parameters observed in this sector in 1994 are described by Argentini et al. (2000a, 2001). The pressure data refer to heights of 3 and 1.2 m above the surface at the AWSs and DdU, respectively. The 3- hourly 10-min-averaged data were taken from the automatic weather station Web site ( wisc.edu/aws/). For Dome C, the data are available for the period from 1980 to 1995, with the exception of 1983, when the AWS was not operating. Also, the data for the month of January from 1980 to 1982 and 1984 are missing. To study the ozone behavior, the Total Ozone Mapping Spectrometer (TOMS) time series from the Nimbus-7 satellite were taken for the period (obtained online at The global average of TOMS satellite ozone data is not strictly global, because TOMS does not measure ozone all the way to the poles. Here, the global average is defined as the average from 60 N to60 S. This is the largest latitude range for which TOMS has year-round coverage and comprises 87% of the surface of the globe. For our purposes, this average can be considered to be representative of a true global average.

3 NOVEMBER 2002 PETENKO AND ARGENTINI 1095 FIG. 2. The mean daily behavior of the pressure variation p (p p 0 ) and its diurnal S 1 (p) and semidiurnal S 2 (p) components at Dome C averaged over , where p 0 is the daily mean of the surface pressure p. 3. Results Studies by Petenko and Argentini (2001) showed that the mean daily behavior of pressure observed at Dome C, D-80, and DdU in indicates two maxima around 0900 and 2100 LT. The observed phase provides the foundation for supposing that the observed pressure variation is really caused by thermal atmospheric tides, because they induce oscillations with similar phase characteristics (Chapman and Lindzen 1970). The mean daily behavior of pressure at Dome C averaged over the period from 1980 to 1995 is given in Fig. 2. The semidiurnal S 2 (p) and diurnal S 1 (p) components are given on the same plot. Their amplitudes are similar and are equal to about and hpa, respectively. Because the signal-to-noise ratio of the daily oscillation is very much reduced at higher latitudes, some remarks concerning the accuracy of the presented data should be made. As mentioned by Whiteman and Bian (1996), the averaging provides an effective filter that allows the harmonic components to pass but filters out all other oscillations, including synoptic ones. Simple averaging, which is used in constructing the composite day, provides a high-pass filter. However, this is a simple firstorder filter. In our processing scheme, to avoid a possible distortion of the regular daily pattern of pressure by strong synoptic changes due to the passage of transient disturbances (which we treat as noise) before constructing a composite day, the data are filtered using an eighthorder high-pass filter with a cutoff frequency of 0.7 day 1. This seems to be more effective in eliminating low-frequency spectral components. To preserve the phase of the signal, a two-way filtering procedure (Hamming 1977) is used that implies processing the data by a digital filter in both a forward and backward direction. Such a procedure eliminates any phase shift between input and filtered signals for all frequencies. In addition, FIG. 3. Yearly average pressure power spectra at Dome C, D-80, and DdU observed in 1994; 95% confidence intervals are about 15% 20% of a spectral density magnitude. to eliminate the influence of sharp synoptic changes on the regular daily behavior, the days on which such synoptic changes occurred are left out of the analysis. Every point in the composite day is estimated over N 4700 points, with a standard deviation p 0.3 hpa. From this, the accuracy of the data presented in Fig. 2 can be roughly estimated as p / N hpa. Given that the mean surface pressure at Dome C is about 645 hpa, by converting the amplitudes to the corresponding values at sea level, we can compare them with values of S 1 (p) and S 2 (p) measured earlier at midlatitudes. In this case, for Dome C, the normalized amplitudes are about hpa, that is, about one order less than those at low latitudes (Chapman and Lindzen 1970). Another difference is that the 15-yr-average amplitudes of S 1 (p) and S 2 (p) are almost equal while at lower latitudes the S 2 (p) harmonic prevails. An observational analysis by Haurwitz and Cowley (1973) provides equations that fit for the global distribution of the mean of S 1 (p) and S 2 (p). The values of S 1 (p) and S 2 (p), estimated using their approximate formulas, are and hpa, respectively. These values are respectively 5.3 and 2.6 times as low as our results. The phase of S 2 (p) is consistent with that observed at lower latitudes. The first maximum in S 2 (p) occurs at about 0825 LT (time t 2 in Fig. 9), close to earlier observations at other sites (Chapman and Lindzen 1970). This statistically reliable result confirms the preliminary conclusion that we really are observing tidal pressure oscillations. The existence of the semidiurnal and diurnal variations of surface pressure is supported by spectral analysis results. Typical pressure power spectra observed in Antarctica are given for Dome C, D-80, and DdU in Fig. 3. These spectra are estimated for the entire year of 1994.

4 1096 JOURNAL OF APPLIED METEOROLOGY VOLUME 41 The 95% confidence intervals at a range of 1/2 1 day are about 15% 20% of the spectral density magnitude. A common feature in all spectra is a pronounced semidiurnal peak, whereas the diurnal component has no peaks, although it presents a larger intensity. As mentioned by Petenko and Argentini (2001), such behavior differs from that observed at midlatitudes (see, e.g., Gossard 1960), at which pressure spectra indicate two pronounced maxima, at 1/2 and 1 day, with the same spectral density magnitudes. The shape of the presented yearly averaged spectra is representative of all years throughout the analyzed period, although some year-to-year variation in the magnitude of spectral density is observed. Argentini et al. (2000b) showed that the pressure spectra vary during the year. To study the annual behavior of pressure spectral properties in more detail, we have considered the spectra estimated over time intervals of about 1 month. In Fig. 4, the pattern of the annual behavior of monthly estimated spectra at the period range between 1/4 and 1 day is shown as contour plots. Figures 4a c show the pressure spectra in 1994 at DdU, D-80, and Dome C, respectively. Figure 4d refers to Dome C in In these plots, the logarithm of the spectral density G(T) divided by the value of T 2 (T is period in days) is given versus yearday (x axis) and period T (y axis). Such normalization is done to compare spectral components with large magnitude differences. The resulting graphs have a similar behavior. The main common feature is that the intensity of the 1/2- and 1- day components is larger during austral winter and smaller during summer. Figure 5 shows the long-term variability of semidiurnal and diurnal spectral components at Dome C. The annual average values of G(1/2) and G(1) are given for the period. This plot demonstrates that a substantial decrease in G(1/2) occurred after The time variability of the semidiurnal and diurnal components is shown in Fig. 6, in which the magnitudes of G(1/2) and G(1), estimated over short intervals of about 1 month, are presented for the same years as in Fig. 5. These plots evidence a regular periodicity in the annual behavior of G(1/2) and G(1) over all the years included the study. Because ozone absorption is considered to be one of the main sources in driving thermal atmospheric tides, we compared the time variation of the intensity of the pressure spectral components with ozone observations. The monthly total ozone concentration averaged between 60 N and 60 S was derived from the TOMS time series. These data are also plotted in Fig. 6. The time behavior of the ozone concentration and the intensity of semidiurnal and diurnal oscillation look somewhat similar, the correlation coefficients being 0.62 for G(1/2) and 0.59 for G(1). An additional confirmation of the similarity in the variability of ozone and barometric tides is provided by the cospectral analysis. Coherence functions and phase spectra of the ozone concentration with G(1/2) and G(1) calculated for the period are given in Fig. 7. The coherence functions indicate a pronounced 1-yr peak as well as smaller 1/2- and 1/3-yr peaks for both G(1/2) and G(1). The phase spectra show that the variation of the considered parameters at the 1-yr period is in phase. The variation at 1/2 yr has a phase shift of about 1 month for G(1/2) and about 2 months for G(1/3). Because the annual behavior of ozone and tidal oscillations shows some similarity, their mean annual trend averaged over the whole analyzed interval is considered. The mean annual behavior of the global total ozone averaged over the period is given in Fig. 8; the magnitudes of G(1/2) and G(1) from 1980 to 1995 are given in Fig. 9. The 95% confidence intervals for the data presented in Fig. 9 are about and for semidiurnal and diurnal components, respectively. From Figs. 8 and 9, it can be seen that both the global total ozone and the spectral density G(1/2) have a seasonal cycle with prominent annual and semiannual components, with two maxima and one deep minimum. Maxima in G(1/2) occur in May and September October; in ozone, they are observed in April May and September. A deep minimum in G(1/2) occurs in December February, and a minimum of ozone is observed in December. Again, a delay of about 1 month in G(1/2), with respect to the ozone variation, is shown by cospectral analysis. The behavior of the semidiurnal oscillation is partly consistent with the result reported by Spar (1952). The main difference is that the primary minimum at Dome C occurs in December February but at northern midlatitudes this minimum appears in June. The clear similarity in the mean annual behavior of G(1/2) and the ozone content confirms that the semidiurnal tides really are the product of a global phenomenon governed by ozone. Although, on average, the mean annual behavior of the semidiurnal tides is similar to the global ozone variation, some differences are observed for individual years. This suggests that some additional factors influence the atmospheric tides. Teitelbaum and Cot (1979) suggested that latitudinal asymmetries of ozone concentration in regions of significant heating might account for the observed interequinoctial variations. Later, Walterscheid and DeVore (1981) mentioned that the phases of the semidiurnal tides may depend on the relative contributions of sources that may be variable month to month or year to year, such as hemispherically asymmetric ozone heating and cumulonimbus activity. The 15-yr-averaged mean annual trend of G(1) indicates only the annual component with a wide maximum between July and October, although its time behavior in Fig. 6 indicates a semiannual variation in some years. This result could be due to the larger diurnal tide forcing by the local features in the ozone and water vapor behavior. 4. Summary and conclusions This paper describes the results of an observational study on the annual behavior of the daily variation of

5 NOVEMBER 2002 PETENKO AND ARGENTINI 1097 FIG. 4. The annual behavior of pressure spectra in 1994 for (a) DdU, (b) D-80, and (c) Dome C, and in 1995 for (d) Dome C. The value of the spectral density G(T) divided by T 2 is given to compensate for the fall of spectra with frequency.

6 1098 JOURNAL OF APPLIED METEOROLOGY VOLUME 41 FIG. 5. Time variation of yearly spectral density G(1/2) and G(1) at Dome C from 1980 to pressure in East Antarctica that is associated with atmospheric tides. The analysis of a 15-yr ( ) record from the AWS Dome C was made to study the annual behavior of the semidiurnal and diurnal variation attributed to atmospheric thermal tides. The spectral analysis of 3-hourly pressure records from the Dome C and D-80 AWSs and the Dumont d Urville meteorological station in 1994 shows the presence of semidiurnal and diurnal variations and a similarity in their annual behavior. Yearly averaged spectra indicate a pronounced 1/2-day peak, which is not founded in the 1- day range. Nevertheless, the 1-day spectral density magnitude is of an order about one-half as large as that of FIG. 7. Coherence functions and phase spectra of G(1/2) and G(1) and the total ozone concentration averaged between 60 N and 60 S calculated for the same data as in Fig. 6. The accuracy of the coherence function estimate is about 25%. 1/2-day period. Such behavior is different from that observed at midlatitudes. The mean amplitudes of S 1 (p) and S 2 (p) at Dome C, averaged over this period, are close and equal to 0.04 hpa, being about one order less than those at lower latitudes. However, these values are larger than those obtained using a smoothed mathematical fit to the latitude distribution of S 1 (p) and S 2 (p) proposed by Haurwitz and Cowley (1973), by about 5.3 and 2.6 times, respectively. The reasons for these differences should be investigated in further studies. The annual behavior of the pressure daily variation was compared in detail with observations of total ozone between 60 N and 60 S derived from TOMS data. The mean annual trends of the intensity of the semidiurnal FIG. 6. The (top) global Nimbus-7 total ozone time series and time variation of the monthly spectral density (middle) G(1/2) and (bottom) G(1) for Dome C from 1980 to FIG. 8. The mean annual behavior of the global total ozone concentration averaged between 60 N and 60 S from 1979 to 1992.

7 NOVEMBER 2002 PETENKO AND ARGENTINI 1099 Atmospheric Physics of Moscow and Rome. The authors also acknowledge Progetto Strategico Artide of CNR, Piano Nazionale Ricerche in Antartide (PNRA), Italy, and the French Italian Dome Concordia Program for supporting part of this research. We are especially pleased to acknowledge the Automatic Weather Station Project run by Dr. Charles R. Steams of the University of Wisconsin Madison, which is funded by the National Science Foundation of the United States of America. We also thank Dr. F. Bignami and John Purtell for the help given in revising the text and thank reviewers for their helpful comments. REFERENCES FIG. 9. The 15-yr-average mean annual behavior of the spectral density G(1/2) and G(1) for Dome C from 1980 to The 95% confidence intervals are about and for semidiurnal and diurnal components, respectively. variation and global total ozone indicate a similar behavior, with two maxima and one deep minimum with about a 1-month delay of G(1/2) with respect to ozone. This result is in close agreement with well-accepted studies that found that the semidiurnal tides are the product of a global phenomenon determined by the global ozone concentration. The mean intensity of the diurnal oscillations indicates one maximum between July and October and one minimum in February, but for some individual years two maxima are distinguishable. The results of the performed analysis support the validity of the thermally driven tide theory advanced by Lindzen (1967) and the idea of Hamilton (1983) that observations of the pressure daily behavior can be used to monitor the global ozone variability. Of course, because of the possible ambiguity in interpreting observed trends in the daily pressure variation, this approach can be used only to corroborate other, more direct, measurements. Although the mean annual behavior of the intensity of the semidiurnal pressure oscillation and global total ozone showed excellent similarity, we have to say that year by year this coincidence may not be so good. This is because some additional factors, still not clear, can influence the annual behavior of the atmospheric tides. As an example of sources influencing thermal tides, we can mention the variability in water vapor and total ozone content, a rearrangement of the mean ozone profile without any change in the total column, as well as the seasonal latitudinal asymmetries in the ozone concentration in regions of significant heating. These factors should be considered and taken into account for a more comprehensive study of the seasonal variability of atmospheric tides. Acknowledgments. 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