MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

1. MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

2. Spectra show that waves & tides of large amplitude dominate the MLT region A typical power spectrum of horizontal winds at a height of ~ 90 km. In this case the data are recorded by a meteor radar over Esrange (68oN). The spectrum is calculated using data for Jan-Dec 2000. (Younger et al., 2002). 1. Tides Well-defined oscillations occurring at harmonics of a solar day – 24, 12 and 8 hrs (others are very weak). Solar forced. 2. Gravity waves A continuous spectrum with periods from ~ 5 mins to 12+ hours 3. Planetary waves Particular frequencies, occurring in the period range ~ 2 – 16 days. Stationary planetary waves possible. All are “natural resonances of the atmosphere”

3. 300 Solar tidal forcing Altitude km Tides are thermally driven Absorption of solar radiation throughout the atmosphere, Absorption of UV radiation by stratospheric ozone and of infrared by water vapour in the troposphere. Plus Absorption of shortwave radiation by oxygen molecules and atoms in thermosphere Plus Interaction between tidal modes 200 O 100 O2 50 O3 H2O Convective 0 400 600 800 200 Temp K

4. Amplitude Growth with Increasing Height A wave of amplitude V ms-1 has energy per unit volume, E, Joules per m3 where: E = ½V2 ( = atmospheric density) If the wave is not dissipating, then E is a conserved quantity. Now,  decreases exponentially with height – a factor of ~ 300,000 from the ground to ~ 90 km. As the wave ascends, if energy is to be conserved, the amplitude, V, must rise to balance the decrease in density, . HEIGHT Wave source N Mitchell Sources inc. vigorous convection, flow over mountains, ageostrophic adjustment etc.

5. Breaking Waves Transfer Energy & Momentum to the Background Flow Wave amplitudes thus grow until a “breaking level” is reached. • Wave energy is no longer conserved. • Wave energy  turbulent energy • Momentum carried by the wave is deposited into the mean flow and imposes a force on the flow of the background atmosphere – “wave drag”. • Momentum deposited by waves provides up to ~ 70% of the momentum of the flow in the MLT. • The MLT has a wave-driven large-scale circulation. Breaking level HEIGHT Wave source N Mitchell

6. Dynamical instability J Plane

7. Wave Instabilities Constrain Wave Growth OH airglow images 16:19 – 17:25 UT, at a height of ~ 87 km, over Japan, 23/12/95. The images are spaced by ~ 3 minutes. The centre of each image is the zenith. The horizontal wavelength of the original waves is ~ 27 km and the period was deduced to be ~ 6 minutes Yamada et al., GRL, 2001

8. Tidal/Planetary-Wave Non-Linear Coupling - Theory Planetary Wave frequency, ω2 wavenumber, m2 Tide frequency, ω1 wavenumber, m1 Non-linear interaction A family of secondary waves, including two waves: “sum wave”: frequency (ω1 + ω2), wavenumber (m1 + m2) “difference wave”: frequency (ω1 - ω2), wavenumber (m1 - m2) Sum and difference waves can beat with the tide, causing a modulation of the tide’s amplitude at the frequency of the planetary wave How much does this process contribute to the observed variability of tides?

9. Diurnal tide over BrazilZonal and meridional winds at Sao Joao do Cariri 7°S, 36° W Diurnal tide

10. Semi-diurnal tide with planetary wave modulation Horizontal winds calculated from meteor drifts ZONAL WINDS OVER ESRANGE (68oN, 21oE) , AUGUST 5-20, 1999 Planetary wave modulation N. J. Mitchell

11. Tidal trends at 130 km from magnetometer data At mid-latitudes a 20% reduction in the amplitude of the tidal signature at ~ 130 km altitude since the middle of the 20th century May be linked to ozone depletion worldwide Ozone and water vapour heating are possible sources 60°N 20% 52°N 22°N M Jarvis

12. Modelling of Sq tidal signatures based on Ross and Walterscheid, GRL, 1991 > 12% Lower thermospheric tide Upward-propagating tide 7% 18% Upper stratospheric ozone 40 km 2000 1900 1950 Calculations suggest a decrease in the tidal signatures seen in geomagnetic Sq variation of >12%

13. Diurnal tide - zonal wind Lines model, 90 (solid) & 95 km (dash) Symbols, data (MF & meteor radar, 90km) Tidal Amplitude m/s Latitude Pancheva et al

14. Semidiurnal tide - zonal wind Lines model, 90 (solid) & 95 km (dash) Symbols, data (MF & meteor radar, 90km) Tidal Amplitude m/s Latitude Pancheva et al

15. Semi-diurnal tide Lerwick, (60ºN, 1ºW) Diurnal tide 16 day wave 5 day wave Wavelet analysis of magnetometer data. Peaks at tidal and planetary wave periods. Blue dotted line (winter) many planetary waves Red dotted line (summer) few planetary waves

16. Solar Max – Solar Min: Planetary Waves Du Du Du Changes in the reflection from planetary waves from the lower thermosphere Neil Arnold

17. Sources of gravity waves

18. Gravity wave momentum flux Observation at 25 km Model Scale! Ern et al. JGR 2004

19. Red arrow - direction of gravity waveYellow dot – all sky imager location Infra-red satellite image • Ray tracing shows deep convective plumes likely to be the source of gravity waves in the OH layer • Mostly direct propagation but ducted and reflected waves possible Vadas et al. Ann. Geophys. 2009