Atmospheric Tide - General Characteristics

General Characteristics

The largest-amplitude atmospheric tides are mostly generated in the troposphere and stratosphere when the atmosphere is periodically heated, as water vapour and ozone absorb solar radiation during the day. These tides propagate away from the source regions and ascend into the mesosphere and thermosphere. Atmospheric tides can be measured as regular fluctuations in wind, temperature, density and pressure. Although atmospheric tides share much in common with ocean tides they have two key distinguishing features:

1. Atmospheric tides are primarily excited by the Sun's heating of the atmosphere whereas ocean tides are excited by the Moon's gravitational pull and to a lesser extent by the Sun's gravity. This means that most atmospheric tides have periods of oscillation related to the 24-hour length of the solar day whereas ocean tides have periods of oscillation related both to the solar day as well as to the longer lunar day (time between successive lunar transits) of about 24 hours 51 minutes.
2. Atmospheric tides propagate in an atmosphere where density varies significantly with height. A consequence of this is that their amplitudes naturally increase exponentially as 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 depth and 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 energy density 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 height of 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.