MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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 (68 o N). The spectrum is calculated using data for Jan-Dec (Younger et al., 2002). 3. Planetary waves Particular frequencies, occurring in the period range ~ 2 – 16 days. Stationary planetary waves possible. All are “natural resonances of the atmosphere” 2. Gravity waves A continuous spectrum with periods from ~ 5 mins to 12+ hours 1. Tides Well-defined oscillations occurring at harmonics of a solar day – 24, 12 and 8 hrs (others are very weak). Solar forced.

Altitude km Temp K O O2O2 O3O3 H2OH2O Convective Solar tidal forcing 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

Amplitude Growth with Increasing Height N Mitchell HEIGHT Wave source A wave of amplitude V ms -1 has energy per unit volume, E, Joules per m 3 where: E = ½  V 2 (  = 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, . Sources inc. vigorous convection, flow over mountains, ageostrophic adjustment etc.

Breaking Waves Transfer Energy & Momentum to the Background Flow N Mitchell HEIGHT Wave source Wave amplitudes thus grow until a “breaking level” is reached. Breaking level 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.

Dynamical instability J Plane

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

Tide frequency, ω 1 wavenumber, m 1 Non-linear interaction A family of secondary waves, including two waves: “ sum wave”: frequency (ω 1 + ω 2 ), wavenumber (m 1 + m 2 ) “difference wave”: frequency (ω 1 - ω 2 ), wavenumber (m 1 - m 2 ) 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? Planetary Wave frequency, ω 2 wavenumber, m 2 Tidal/Planetary-Wave Non-Linear Coupling - Theory

Diurnal tide over Brazil Zonal and meridional winds at Sao Joao do Cariri 7°S, 36° W Diurnal tide

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

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

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

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

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

Lerwick, (60ºN, 1ºW) 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 Semi-diurnal tide Diurnal tide 16 day wave 5 day wave

uu uu uu Solar Max – Solar Min: Planetary Waves Neil Arnold Changes in the reflection from planetary waves from the lower thermosphere

Sources of gravity waves

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

Red arrow - direction of gravity wave Yellow dot – all sky imager location Infra-red satellite image Vadas et al. Ann. Geophys 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