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Gravity wave activity observed during a major Southern Hemisphere stratospheric warming by CHAMP/GPS measurements
M. Venkat Ratnam1, T. Tsuda1 , Y. Aoyama1 and Ch. Jacobi2
1Research Institute for Sustainable Humanosphere(RISH),Kyoto University, Uji 611 0011, JAPAN
2Institute for Meteorology, University of Leipzig, Leipzig – 04103, Germany.
Email: [email protected] ; [email protected] ; [email protected] ; [email protected]
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Figure 1. Time-height sections of zonal mean temperature gradient (from yearly mean) between 600S and the South Pole from 1 July to 31 December 2002.
Using temperature profiles measured by CHAMP/GPS occultations we analyzed the gravity wave activity during the major sudden stratospheric warming (SSW) occurred in the Southern Hemisphere (SH) in late winter/spring of 2002, which for the first time split the polar vortex into two parts. The observed temperatures show a rapid poleward moving increase with a maximum of about 25-30 K with respect to the undisturbed winter conditions, and a reversal of the latitudinal gradients at 30 km and below within 15 days (September 15-30). During this time gravity wave energy (potential energy, Ep) became 3 times higher than usual. This enhancement mostly occurred near the edge and outside the cold polar vortex, but not inside the vortex. We discuss the observed event with potential interaction of gravity waves, planetary waves and the mean circulation during the stratospheric warming.
Understanding stratospheric dynamics, particularly for the knowledge of the future evolution of the ozone hole is very important. The winter high-latitude stratosphere, particularly in the Southern Hemisphere (SH) consists of a cold and strong polar vortex. This vortex can be disturbed by sudden stratospheric warmings (SSW). Strong warmings that lead to a reversal of the zonal mean circulation at 10 hPa or lower are called major SSW. Major SSW are a common feature in the Northern Hemisphere (NH) but not in the SH. However, in September-October 2002 the Antarctic polar vortex split into two parts due to a major warming which was not hitherto observed since observations are available (from 1950) for the SH. This occurred about one month earlier than any final warming observed so far [Baldwin et al., 2003 and the references there in], resulting in unusual transport of ozone and high ozone column densities [Allen et al., 2003].
Even though the first reporting on an SSW has been long ago [Scherhag, 1952], still the detailed structure of warming evolution, wave-wave and wave-mean flow interaction, and interannual variability are under discussion. Case studies reveal that SSW is generally preceded by the large burst of upward wave flux into the stratosphere [Andrews et al., 1987]. It is thought that SSW is a transient phenomenon in the dynamics that is initiated by the propagation of planetary wave disturbances from the troposphere into the middle atmosphere and their interaction with the mean stratospheric flow.
Numerical weather prediction (NWP) models have been successfully used to predict wind and geo-potential height fields during warming events [Simmons and Hollingsworth, 2002] in medium-range 10-day forecasts, which shows the underlying processes are well captured by NWP models. However, previous studies on the details of SSW were based on somewhat limited observations (particularly in the SH), modeling or a combination of observations and models. To contribute to an improvement of the data situation, now for the first time an SH major SSW was measured using GPS radio occultations (RO), which reveals the potential application of GPS technique in studying the middle atmospheric dynamics especially at places where other ground-based observations are sparse.
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Figure 3. Polar stereographic charts showing southern hemispheric distribution of (a) radio occultations occurred, (b) temperature, (c) potential energy, and (d) total ozone observed during on selected days at 25-30 km (except ozone) during evolution of stratospheric warming in September 2002.
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The gravity waves generated in the troposphere during a variety of meteorological and orographic situations can have zonal asymmetries resulting from the distribution of orographic features and the flows in the troposphere. However in the SH the gravity wave process limits the generation due to orography unlike in the NH. The upward propagating gravity waves are filtered by the stratospheric circulation, which may be zonally asymmetric due to planetary waves. When these upward propagating gravity waves dissipate, the wave drag forcing will have planetary scale zonal asymmetries. Nevertheless, we need to estimate quantitatively the energy and momentum flux divergence of gravity waves whether they are significant enough to modify the background dynamical conditions, for example, polar vortex. Another reason for the observed enhanced wave activity may be the generation of GW in the shear zones near the vortex. However, to draw conclusions about the interaction of gravity waves with planetary waves and the mean flow the analysis of all waves is necessary. A study on observed planetary wave activity using combined measurements of CHAMP/GPS, SAC-C and JMA (Japanese meteorological Agency) is in progress.
Figure 4. Histogram shows the observed temperature and potential energy inside, near and outside the edge of the cold polar vortex during 15-30 September 2002. The numbers on the bars shows the total number of occultations used for respective day of the study.
Acknowledgements: We wish to thank GFZ Potsdam for providing CHAMP/GPS data through the ISDC data center. One of the authors (MVR) wishes to thank Kyoto University for providing fellowship to carryout this work. This study is partially supported by MOE's (Ministry of the Environment) Global Environmental Research Fund of Japan.
Introduction
Abstract
Data base For the present study we use level 3 version 004 data of CHAMP/GPS from July 2002 to December 2003, produced by GFZ Potsdam, in order to describe atmospheric conditions in early and mid winter that may have contributed to the observed major event.
References: Allen, D. R., R. M. Bevilacqua, G. E. Nedoluha, C. E. Randall, and G. L. Manney, Unusual stratospheric transport and mixing during the 2002 Antarctic winter, Geophys. Res. Lett., 30(12), 1599, doi:10.1029/2003GL017117, 2003.Andrews, D. G., J. R. Holton, and C.B. Leovy, Middle Atmosphere Dynamics. London, UK, Academic Press, 489, 1987. Baldwin, M., T. Hirooka, A. Oneill, S. Yoden, A. J. Charlton, Y. Hio, W. A. Lahoz and A. Mori, Major Stratospheric Warming in the Southern Hemisphere in 2002: Dynamical Aspects of the Ozone Hole Split, SPARC Newsletter, 20, 24-26, 2003.Scherhag, R., Die explosionsartige Stratosphärenerwärmung des Spätwinters 1951-1952, Ber. Deut. Wetterdienstes, 6, 51-53, 1952.Simmons, A. J., and Hollingsworth, A, Some aspects of the improvement in skill of numerical weather prediction, Quart. J. Roy. Meteorol. Soc., 128, 647-677, 2002.
Conclusions
The time-height section of the warming evolution in the latitude range from 600S to South Pole from 1 July to 31 December 2002 is shown in Figure 1. A substantial increase of the temperatures by about 25-30 K throughout the stratosphere by the end September is observed. This rapid increase has never been observed during this season in the SH. On 26 September the WMO criterion of a major warming is fulfilled with a reversal of the upper stratosphere temperature gradient at 30 km reaching down to 20 km. After a few days this temperature increase with respect to the time establishes once more for duration of about 3 weeks, finally leading to an early final warming at the end of October.
The latitude-height sections of potential energy observed in southern spring of 2001, 2002 and 2003 at all the heights under consideration are shown in Figure 2. The data are averages over two month at the respective height range.
Below 20 km near the equatorial latitudes there exists large potential energy during all the years, which decreases in between 20 and 25 km altitude region. For more details of the latitudinal variation of potential energy and the causative mechanisms the reader is referred to Tsuda et al., [2000] and Ratnam et al., [2004]. In the present case a temperature increase up to 30 K is observed during the SSW from 600S to the South Pole at 25-30 km altitudes (from Figure 1). Corresponding potential energy also increased by almost a factor of 3 during the SSW. From the gravity wave activity estimated using CHAMP/GPS observations for three years, it is found that the average potential energy during September-October is about 3 J/kg at 30 km altitude [Ratnam et al., 2004]. This also matches well that observed with GPS/MET satellite reported by Tsuda et al. [2000]. At high southern latitudes, during 2002 a clear enhancement of potential energy is evident showing the effect of the major SSW.
Figure 2. Inter-annual variations of potential energy observed in spring of 2001, 2002 and 2003.
During the middle of September, a warm pulse is found southwest of Australia, strengthening and moving pole-ward. During September 20-24, the polar vortex began to elongate and split into two centers, with the overall features continuing for another two days (not shown here). On 27 September the evolution of the major warming has completed, culminating in the breakdown of the polar vortex with associated warm air over the South Pole having developed within only 15 days. These values represent a temperature increase of 25-30K relative to the temperatures on 20 September. At the end of September, the vortex started to re-establish for about three weeks. The GOME total ozone maps show high ozone content in September 2002. The stream-function is nearly pole-centered on 15 September and changes its location by 20 September (Figure 3 fourth column). It is pushed off the pole and distorted into a dumbbell shape by 24 September. The separated ozone parts moved towards higher latitudes. This continued until 28 September and the total ozone hole split into two; one quickly disappeared, while the other returned to the pole by the end of September. An interesting feature noticed is that the wave energy is found to be high near the edge of the cold polar vortex during the warming and almost negligible inside the cold vortex. One more interesting feature is that four days before the major warming occurred the potential energy outside the vortex is found to be very high when compared to the energy observed during warming. Even though large burst of Ep (3 times) is observed 10 days before the warming, one more burst (2 times) is evident four days before warming occurred (more clear from Figure 4). On 24 September, Ep is decreased but still shows
higher values near the edges of the cold vortex. This feature continued almost throughout the warming until 28 September, when again Ep starts to increase. The observed temperature and corresponding Ep inside, near and outside the edge of the cold
polar vortex during 15-30 September 2002 is more clearly shown in the bar diagram of Figure 4. The various regions have been selected based upon the shape of the region of low ozone, which is represented in the fourth column of the Figure 3.
Results
SPARC 3rd General Assembly, Victoria, British Columbia, Canada, August 1-6, 2004
Note: Details of these results can be had from Enhancement of Gravity wave activity observed during a major Southern Hemisphere stratospheric warming by CHAMP/GPS measurements by M. Venkat Ratnam, T. Tsuda, Ch. Jacobi, and Y. Aoyama, , GRL, In press
