Project 1.3 Changes in MLTI dynamics and composition

From CAWSES

• Project leaders: G. Beig (India), C. Jacobi (Germany)
• Project members: M.Bittner (DE), K. Höppner (DE), J.Scheer (AR), S.Gurubaran (IN), D.Murphy (AU) J. Lastovicka (CZ), D.Pancheva (BG), P. Mukhtarov (BG), E. Merzlyakov (RU)

Introduction

Only, recently, a consistent picture of middle and upper atmosphere trends has been established (Lastovicka, 2009). However, the MLTI region responds to changes in solar/extraterrestrial forcing, changes of atmospheric composition, and to changes of the lower atmosphere dynamics. To estimate the relative roles of each of these forcing mechanisms, detailed knowledge on MLTI changes and trends not only in temperature, but also in dynamics and composition is required. The focus of the project is thus to analyze and quantify changes in wind and composition, and investigate the sources of MLTI changes in connection with forcing from above and especially below.

This raises the following questions: How do changes in tropospheric wave generation and their propagation through a changing atmosphere affect the dynamics of the MLT? Are there related trends observed in the MLTI?

Studies of Dynamics and Composition

Dynamics

Studies of the long-term variability of the mesosphere and lower thermosphere dynamics have been carried out for more than 40 years now, essentially by radar measurements, but also by OH emission (Höppner and Bittner, 2007). Already Greisiger et al. (1987) concluded that there is a possible quasi-decadal internal atmospheric variability of the prevailing circulation instead of a direct solar control. Additionally wind oscillations with periods from 2 to 5 years have frequently been reported (for instance, Namboothiri et al., 1994; Portnyagin, 1986). The year-to-year wind variability in the MLT region is observed on the background of secular changes in the circulation (for instance, Bremer, 1997; Jacobi et al., 2005; Merzlyakov et al., 2009). These studies investigated different time intervals of wind measurements, and they sometimes revealed opposite trends in the prevailing wind and in tidal parameters.

A mechanism linking the summer polar MLTI to the winter lower atmosphere has been proposed (Koernich and Becker 2010 ) to explain observations and modeling studies (e.g. Karlsson et al. 2009). This mechanism relates a modulation of the gravity-wave flux in the winter hemisphere to changes in the strength of the meridional circulation responsible for forcing the summer mesosphere away from radiative equilibrium. Further comparisons with observations will test this mechanism and provide insight into the effect changes in the lower atmosphere will have on the MLTI.

Composition

Long-term trends of lower ionospheric parameters like foE, are owing to the decrease of ionospheric layers through middle atmosphere cooling, changes in transport of ionized species, and changes of ionization through temperature dependence of reaction rates. Trend analyses are essentially based on ionosonde data. More detailed knowledge on ion composition trends is required.

Methods and Analysis

Fig. 1: Winter zonal prevailing winds over Collm (52N, 15E), and results of a piecewise linear trend analysis indicating possible structural changes of MLTI trends.

It should be noted that usually a simple linear regression model has been used for trend assessments. However, recent analysis have indicated that long-term trends show structural changes - often called breakpoints -, which partly have been analyzed using piecewise linear fitting (Merzlyakov et al., 2009; Jacobi et al., 2009). The results reveal changes of trends (see an example in Figure 1) which, through their coincidence with breakpoints in lower atmospheric dynamics or composition, frequently can be attributed to tropospheric or stratospheric changes (e.g., Offermann et al., 2006). The change in mesopause region seasonal variations observed since 2001 by Reisin and Scheer ( 2009) may be a related phenomenon. In particular, MLTI trend change breakpoints have been identified in connection with tropospheric temperature changes in the late 1970s, changes of oscillation patterns like NAO in the 1990s, global ozone tendencies. Use of advanced techniques for trend estimation thus allows drawing more conclusions on the origin of MLTI trends and changes, and on coupling processes throughout the atmosphere.

Long-term trend analyses of various parameters in the past have sometimes revealed different or even contradicting results for geographically relatively close sites and similar longitudes. This indicates the presence of stationary planetary waves and their changes. Satellite data analyses will provide more insight into planetary waves in the MLTI (Pancheva et al., 2010), and, together with local time series, explain differences in long-term time series analysis results.

Questions and Tasks

Questions

1. Analysis of variability of MLTI parameters at time scales of years to decades. Is it possible to derive a consistent picture of MLTI trends and changes of different parameters?
2. Which is the possible coupling of MLTI changes, breakpoints in trends, or shifts with lower atmosphere changes?
3. Are there regional features connected with planetary waves, which may lead to different trends and signatures of lower atmospheric dynamics in different MLTI time series. Satellite data analysis will be helpful in this context.
4. Is there an influence of regional gravity wave signatures on MLTI trends?
5. Results from existing networks like the NDMC should be used.

Activities

TREND2010 workshop

Several papers related to the project have been presented at the "6th IAGA/ICMA/CAWSES workshop on Long-Term Changes and Trends in the Atmosphere" [1], which was held at National Center for Atmospheric Research (NCAR) Center Green Conference Center, Boulder, Colorado, USA, June 15-18, 2010.

NDMC

NDMC: currently 49 NDMC airglow stations.

Recently, the Network for the Detection of Mesopause Change (NDMC,http://wdc.dlr.de/ndmc) has been established as a global program with the mission to promote international cooperation among research groups investigating the mesopause region (80-100 km) with the goal of early identification of changing climate signals. This program involves the coordinated study of atmospheric variability at all time scales, the exchange of existing know-how, and the coordinated development of improved observation, analysis techniques and modeling. The initial emphasis is on mesopause region airglow techniques utilizing the existing ground-based and satellite measurement capabilities. Participation or association of researchers using other techniques in the same altitude region will be actively developed. NDMC is concerned with coupling processes and will interface with related activities throughout the atmosphere. It is affiliated with the Global Atmosphere Watch program of the World Meteorological Organization and with the Network for the Detection of Atmospheric Composition Change.

Projects

Fig. 2: Obninsk (55°N, 37°E) and Collm (52°N, 15°E) winter MLT zonal prevailing winds as well as their differences (blue line). The zonal wind differences at this relatively short distance is correlated with stationary planetary waves (SPW) in the stratosphere (pink line).

Non-zonal stuctures seen in trends and interannual variability

Recent analyses have shown that long-term trends and interanual variability of MLTI parameter measurements distributed in longitude may be different, and tendencies are partly contradicting. This is the case also at relatively short zonal distances of 1000-2000 km and indicates quasi-stationary structures. Part of these non-zonal structures are connected with stationary planetary waves (SPW), as shown in Fig.1. The project aims at determining the spatial scales of these structures, and their origin. Work within the project includes:

1. Analysis of trends and variability from MLT radar winds at different stations, and correlation with stratospheric SPW,
2. Use of NDMC to analyse MLT temperature structures,
3. Analysis of in situ SPW from satellite measurements,
4. Gravity wave analysis,
5. Numerical modelling of wave propagation and effects on the MLTI,
6. Investigation of lower ionospheric non-zonal structures that may affect MLT dynamics.

References

Greisiger,K.M.,Schminder,R.,Kürschner,D.,1987.Long-period variations of wind parameters in the mesopause region and the solar cycle dependence. Journal of Atmospheric and Terrestrial Physics 49, 281–285.

Höppner, K., Bittner, M., 2007: Evidence for solar signals in the mesopause temperature variability?, Journal of Atmospheric and Solar-Terrestrial Physics 69, 431-448.

Jacobi, Ch., Portnyagin, Yu.I., Merzlyakov, E.G., Solovjova, T.V., Makarov, N.A., Kürschner, D., 2005. A long-term comparison of mesopause region wind measurements over Eastern and Central Europe. Journal of Atmospheric and Solar-Terrestrial Physics 67, 229–240.

Jacobi, Ch., P. Hoffmann, R.Q. Liu, P. Križan, J. Laštovička, E.G. Merzlyakov, T.V. Solovjova, and Yu.I. Portnyagin, 2009: Midlatitude mesopause region winds and waves and comparison with stratospheric variability. Journal of Atmospheric and Solar-Terrestrial Physics 71, 1540-1546.

Lastovicka, J., 2009: Global pattern of trends in the upper atmosphere and ionosphere: Recent progress, Journal of Atmospheric and Solar-Terrestrial Physics, 71, 14-15.

Karlsson, B, McLandress, M. and Shepherd, T. G. , 2009: Inter-hemispheric mesospheric coupling in a comprehensive middle atmosphere model, J. Atmos, Sol-Terr Phys., 71, 518-530.

Koernich, H. and Becker, E., 2010: A simple model for the interhemispheric coupling of the middle atmosphere circulation, Adv. Spac. Res. , 45, 5, 661-668.

Merzlyakov, E.G., Ch. Jacobi, Yu.I. Portnyagin, and T.V. Solovjova, 2009: Structural changes in trend parameters of the MLT winds based on wind measurements at Obninsk (55°N, 37°E) and Collm (52°N, 15°E). Journal of Atmospheric and Solar-Terrestrial Physics 71, 1547-1557.

Namboothiri, S.P., Meek, C.E., Manson, A.H., 1994. Variations of mean winds and solar tides in the mesosphere and lower thermosphere over time scales ranging from 6 months to 11 yr: Saskatoon, 52°N, 107°W. Journal of Atmospheric and Terrestrial Physics 56, 1313–1325.

Offermann, D., Jarisch, M., Donner, M., Steinbrecht, W., Semenov, A.I., 2006. OH temperature re-analysis forced by recent variance increases. Journal of Atmospheric and Solar-Terrestrial Physics, 68, 1924–1933.

Pancheva, D., Mukhtarov, P. , Andonov, B., Forbes, J.M., 2010: Global distribution and climatological features of the 5-6-day planetary waves seen in the SABER/TIMED temperatures (2002-2007), Journal of Atmospheric and Solar-Terrestrial Physics, 72, Pages 26-37.

Portnyagin, Yu.I., 1986. The climatic wind regime in the lower thermosphere from meteor radar observations. Journal of Atmospheric and Terrestrial Physics 48, 1099–1109.

Reisin, E.R., Scheer, J, 2009: Evidence of change after 2001 in the seasonal behaviour of the mesopause region from airglow data at El Leoncito, Advances in Space Research, 44, 401-412.