- Atmospheric tide
Atmospheric tides are global-scale periodic oscillations of the
atmosphere. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:
day/ nightcycle in the insolationof the atmosphere
gravitational fieldpull of the Moon
Non-linearinteractions between tides and planetary waves.
The largest-amplitude atmospheric tides are mostly generated in the
troposphereand stratospherewhen the atmosphere is periodically heated as water vapourand ozoneabsorb solar radiationduring the day. The tides generated are then able to propagate away from these source regions and ascend into the mesosphereand thermosphere. Atmospheric tides can be measured as regular fluctuations in wind, temperature, densityand pressure. Although atmospheric tides share much in common with ocean tides they have two key distinguishing features:
# Atmospheric tides are primarily excited by the
Sun's heating of the atmosphere whereas ocean tides are primarily excited by the Moon's gravitational pull. This means that most atmospheric tides have periods of oscillationrelated to the 24-hour length of the solar daywhereas ocean tides have longer periods of oscillation related to the lunar day(time between successive lunar transits) of about 24 hours 51 minutes.
# Atmospheric tides propagate in an atmosphere where density varies significantly with
height. A consequence of this is that their amplitudes naturally increase exponentiallyas the tide ascends into progressively more rarefied regions of the atmosphere (for an explanation of this phenomenon, see below). In contrast, the density of the oceans varies only slightly with depthand so there the tides do not necessarily vary in amplitude with depth.
At ground level, atmospheric tides can be detected as regular but small oscillations in surface pressure with periods of 24 and 12 hours. However, at greater heights the amplitudes of the tides can become very large. In the
mesosphere(heights of ~ 50 - 100 km) atmospheric tides can reach amplitudes of more than 50 m/s and are often the most significant part of the motion of the atmosphere.
The reason for this dramatic growth in amplitude from tiny fluctuations near the ground to oscillations that dominate the motion of the mesosphere lies in the fact that the density of the atmosphere decreases with increasing height. As tides or waves propagate upwards, they move into regions of lower and lower density. If the tide or wave is not dissipating, then its
kinetic energydensity must be conserved. Since the density is decreasing, the amplitude of the tide or wave increases correspondingly so that energy is conserved. The amplitude of a wave at a height of z can thus be described by the equation:
where is the initial amplitude of the wave, is height and is the
scale heightof the atmosphere.
Following this growth with height atmospheric tides have much larger amplitudes in the middle and upper atmosphere than they do at ground level.
olar Atmospheric Tides
The largest amplitude atmospheric tides are generated by the periodic heating of the atmosphere by the Sun - the atmosphere is heated during the day and not heated at night. This regular diurnal (daily) cycle in heating generates tides that have periods related to the solar day. It might initially be expected that this diurnal heating would give rise to tides with a period of 24 hours, corresponding to the heating's periodicity. However, observations reveal that large amplitude tides are generated with periods of 24 and 12 hours. Tides have also been observed with periods of 8 and 6 hours, although these latter tides generally have smaller amplitudes. This set of periods occurs because the solar heating of the atmosphere occurs in an approximate
square waveprofile and so is rich in harmonics. When this pattern is decomposed into separate frequency components using a fourier transform, as well as the mean and daily (24-hr) variation, significant oscillations with periods of 12, 8 and 6 hrs are produced. Tides generated by the gravitational effect of the sun are very much smaller than those generated by solar heating. Solar tides will refer to only thermal solar tides from this point.
Solar energy is absorbed throughout the atmosphere some of the most significant in this context are
water vaporat (~0 - 15 km) in the troposphere, ozoneat (~30 to 60 km) in the stratosphereand molecular oxygen and molecular nitrogen at (~120 to 170 km) in the thermosphere. Variations in the global distribution and density of these species results in changes in the amplitude of the solar tides. The tides are also affected by the environment through which they travel.
Solar tides can be separated into two components: migrating and non-migrating.
Migrating Solar Tides
Migrating tides are sun synchronous - from the point of view of a stationary observer on the ground they propagate westwards with the apparent motion of the sun. As the migrating tides stay fixed relative to the sun a pattern of excitation is formed that is also fixed relative to the Sun. Changes in the tide observed from a stationary viewpoint on the
Earth's surfaceare caused by the rotation of the Earth with respect to this fixed pattern. Seasonal variations of the tides also occur as the Earth tilts relative to the Sun and so relative to the pattern of excitation. [ [http://www.hao.ucar.edu/modeling/gswm/gswm.html Global Scale Wave Model] UCAR]
The migrating solar tides have been extensively studied both through observations and mechanistic models. [ [http://www.hao.ucar.edu/modeling/gswm/refs.html GSWM References ] ]
Non-Migrating Solar Tides
Non-migrating tides can be thought of as global-scale waves with the same periods as the migrating tides however, non-migrating tides do not follow the apparent motion of the sun. Either they do not propagate horizontally, they propagate eastwards or they propagate westwards at a different speed to the sun. These non-migrating tides may be generated by differences in
topographywith longitude, land-sea contrast and surface interactions.
The primary source for the 24-hr tide is in the lower atmosphere where surface effects are important. This is reflected in a relatively large non-migrating components seen in longitudinal differences in tidal amplitudes. Largest amplitudes have been observed over
South America, Africaand Australia. [ Hagan, M.E., J.M. Forbes and A. Richmond, 2003: Atmospheric Tides, Encyclopedia of Atmospheric Sciences] In contrast the 12-hr tide is thought to be primarily generated higher in the atmosphere and so has a relatively small contribution from non-migrating components.
Lunar Atmospheric Tides
Atmospheric tides are also produced through the gravitational effects of the Moon. "Lunar (gravitational) tides" are much weaker than "solar (thermal) tides" and are generated by the motion of the Earth's oceans (caused by the Moon) and to a lesser extent the effect of the Moon's gravitational attraction on the atmosphere.
Dampingof the tides occurs primarily in the lower thermosphere region, and may be caused by turbulencefrom breaking gravity waves. A similar phenomena to ocean waves breaking on a beach, the energydissipates into the background atmosphere. Molecular diffusionalso becomes increasingly important at higher levels in the lower thermosphere as the mean free pathincreases in the rarefied atmosphere.Fact|date=March 2008
Effects of Atmospheric Tide
The tides form an important mechanism for transporting energy input into the lower atmosphere from the upper atmosphere, while dominating the dynamics of the mesosphere and lower thermosphere. Therefore, understanding the atmospheric tides is essential in understanding the atmosphere as a whole. Modeling and observations of atmospheric tides are needed in order to monitor and predict changes in the Earth's atmosphere.
Notes and references
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