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Anatmospheric wave is a periodic disturbance in the fields ofatmospheric variables (likesurface pressure orgeopotential height,temperature, orwind velocity) which may either propagate (traveling wave) or be stationary (standing wave). Atmospheric waves range inspatial andtemporal scale from large-scale planetary waves (Rossby waves) to minutesound waves. Atmospheric waves with periods which areharmonics of 1solar day (e.g. 24 hours, 12 hours, 8 hours... etc.) are known asatmospheric tides.
The mechanism for the forcing of the wave, for example, the generation of the initial or prolonged disturbance in the atmospheric variables, can vary. Generally, waves are either excited byheating ordynamic effects, for example the obstruction of the flow bymountain ranges like theRocky Mountains in theU.S. or theAlps inEurope. Heating effects can be small-scale (like the generation ofgravity waves byconvection) or large-scale (the formation ofRossby waves by the temperature contrasts between continents and oceans in theNorthern Hemisphere winter).
Atmospheric waves transportmomentum, which is fed back into the background flow as the wavedissipates. This wave forcing of the flow is particularly important in thestratosphere, where this momentum deposition by planetary scaleRossby waves gives rise tosudden stratospheric warmings and the deposition bygravity waves gives rise to thequasi-biennial oscillation.
In the mathematical description of atmospheric waves,spherical harmonics are used. When considering a section of a wave along alatitude circle, this is equivalent to asinusoidal shape. Spherical harmonics, representing individual Rossby-Haurwitz planetary wave modes, can have any orientation with respect to the axis of rotation of the planet.[1] Remarkably - while the very existence of these planetary wave modesrequires the rotation of the planet around its polar axis - the phase velocity of the individual wave modes doesnot depend on the relative orientation of the spherically harmonic wave mode with respect to the axis of the planet. This can be shown to be a consequence of the underlying (approximate) spherical symmetry of the planet, even though this symmetry is broken by the planet's rotation.[2]
Because the propagation of the wave is fundamentally caused by an imbalance of theforces acting on the air (which is often thought of in terms ofair parcels when considering wave motion), the types of waves and their propagation characteristics vary latitudinally, principally because theCoriolis effect on horizontal flow is maximal at thepoles and zero at theequator.
There are four different types of waves:
These arelongitudinal or compression waves. The sound wave propagates in the atmosphere though a series of compressions and expansions parallel to the direction of propagation.
At the equator, mixed Rossby-gravity andKelvin waves can also be observed.
