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MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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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 (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”

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

Amplitude Growth with Increasing Height

A wave of amplitude V ms-1 has energy per unit volume, E, Joules per m3 where:

E = ½V2

( = 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.

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

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

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?

Diurnal tide

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

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

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%

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

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

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

Solar Max – Solar Min: Planetary Waves

Du

Du

Du

Changes in the reflection from planetary waves from the lower thermosphere

Neil Arnold

Sources of gravity waves

Gravity wave momentum flux

Observation

at 25 km

Model

Scale!

Ern et al. JGR 2004

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