Atmospheric Waves Questions and Answers

Q: Which asymmetry between hemispheres does this explain?

With major mountain barriers and strong land-sea contrasts, the northern hemisphere supports large stationary waves, while the southern hemisphere flow is far closer to zonally symmetric. This is why blocking is common in the north and rarer in the south, and why the southern polar vortex is stronger and more persistent. The southern jet is in fact the stronger and steadier of the two. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Which criterion governs vertical propagation of planetary waves?

Charney and Drazin showed that stationary planetary waves propagate vertically only through westerlies weaker than a critical speed that decreases sharply as the wavelength shortens. This explains why only the very longest waves reach the stratosphere and why summer easterlies block them entirely. The other criteria concern instability, turbulence and wave-mean flow interaction respectively. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which season permits planetary waves to reach the stratosphere?

Only in winter are the stratospheric winds westerly and weak enough for the longest planetary waves to propagate upward, which is why sudden stratospheric warmings occur only then. Summer easterlies block propagation completely, making the summer stratosphere quiescent. The equinoctial transitions are brief and the winds are changing rapidly through them.

Q: Which dramatic event results from planetary wave breaking in the stratosphere?

Breaking planetary waves decelerate and sometimes reverse the winter polar vortex, driving descent over the pole and temperature rises of tens of kelvin within a few days. Major warmings occur roughly every other winter in the northern hemisphere. The ozone hole is a chemical phenomenon, and the quasi-biennial and semi-annual oscillations are driven by equatorial waves of much shorter wavelength. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Which characteristic of the quasi-biennial oscillation is most distinctive?

Each wind regime appears near 10 hectopascals and propagates downward to about 70 hectopascals over roughly a year, which is the signature feature of the oscillation. The descent occurs because each wave type is absorbed at the level where the shear allows, progressively lowering the absorption level. The period averages 28 months rather than exactly 24, which is why it is called quasi-biennial. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which property does an equatorial Kelvin wave have?

The Kelvin wave has zero meridional velocity everywhere, with the pressure gradient in that direction balanced entirely by the Coriolis force acting on the zonal wind. This constraint traps it about the equator with a Gaussian decay and forces it to propagate eastward only. The amplitude therefore decreases rather than increases away from the equator.

Q: Which direction does an equatorial Kelvin wave travel?

The trapping condition requires the wave to decay away from the equator, and only the eastward propagating solution does so, the westward one growing without bound. Kelvin waves are therefore strictly eastward at the equator. Along a coastal boundary the analogous wave travels with the coast on its right in the northern hemisphere, which is the same trapping argument applied to a different geometry. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Q: Which speed does an atmospheric Kelvin wave travel at?

Atmospheric Kelvin waves in the first baroclinic mode travel at roughly 20 to 60 metres per second, crossing the tropical Pacific in a few days. Oceanic Kelvin waves are far slower at about 3 metres per second, taking two months for the same journey, which is why the two are so often contrasted in discussions of El Nino.

21.

Which asymmetry between hemispheres does this explain?

A.The northern flow is less zonally symmetric
B.The southern flow shows much larger amplitude stationary waves than the northern flow does at all seasons
C.The two are identical
D.The northern hemisphere has a much weaker jet stream than the southern hemisphere does throughout the year

Answer: A

With major mountain barriers and strong land-sea contrasts, the northern hemisphere supports large stationary waves, while the southern hemisphere flow is far closer to zonally symmetric. This is why blocking is common in the north and rarer in the south, and why the southern polar vortex is stronger and more persistent. The southern jet is in fact the stronger and steadier of the two. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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22.

Which criterion governs vertical propagation of planetary waves?

A.The Eliassen-Palm criterion, which determines whether a wave will accelerate or decelerate the mean flow
B.The Charney-Drazin criterion
C.The Charney-Stern criterion, which gives a necessary condition for a zonal flow to be baroclinically unstable
D.The Richardson criterion

Answer: B

Charney and Drazin showed that stationary planetary waves propagate vertically only through westerlies weaker than a critical speed that decreases sharply as the wavelength shortens. This explains why only the very longest waves reach the stratosphere and why summer easterlies block them entirely. The other criteria concern instability, turbulence and wave-mean flow interaction respectively. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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23.

Which season permits planetary waves to reach the stratosphere?

A.Both equally
B.Spring and autumn only, when the stratospheric winds are weakest and the critical speed is most easily met
C.Winter
D.Summer, when the stratospheric winds are easterly and offer no resistance to upward propagating waves

Answer: C

Only in winter are the stratospheric winds westerly and weak enough for the longest planetary waves to propagate upward, which is why sudden stratospheric warmings occur only then. Summer easterlies block propagation completely, making the summer stratosphere quiescent. The equinoctial transitions are brief and the winds are changing rapidly through them.

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24.

Which dramatic event results from planetary wave breaking in the stratosphere?

A.The formation of the ozone hole, which results from chemical destruction on polar stratospheric cloud surfaces
B.The quasi-biennial oscillation
C.The semi-annual oscillation of the equatorial winds near the stratopause at about fifty kilometres altitude
D.A sudden stratospheric warming

Answer: D

Breaking planetary waves decelerate and sometimes reverse the winter polar vortex, driving descent over the pole and temperature rises of tens of kelvin within a few days. Major warmings occur roughly every other winter in the northern hemisphere. The ozone hole is a chemical phenomenon, and the quasi-biennial and semi-annual oscillations are driven by equatorial waves of much shorter wavelength. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

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25.

Which wave drives the quasi-biennial oscillation?

A.Equatorial Kelvin and mixed Rossby-gravity waves
B.Stationary planetary waves forced by the major mountain ranges of the northern hemisphere continents
C.Mountain waves
D.Inertia-gravity waves generated by the imbalance of the subtropical jet stream in the winter hemisphere

Answer: A

Eastward propagating Kelvin waves and westward propagating mixed Rossby-gravity waves deposit momentum of opposite sign at different levels, producing the alternating easterly and westerly regimes that descend through the equatorial stratosphere. The period averages about 28 months. Planetary and mountain waves belong to the extratropics and cannot reach the equatorial stratosphere in the same way. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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26.

Which characteristic of the quasi-biennial oscillation is most distinctive?

A.The oscillation occurs simultaneously at every level so that the whole stratosphere reverses together
B.The wind regimes descend with time
C.The wind regimes ascend with time, following the upward propagation of the waves that generate them
D.The period is exactly two years

Answer: B

Each wind regime appears near 10 hectopascals and propagates downward to about 70 hectopascals over roughly a year, which is the signature feature of the oscillation. The descent occurs because each wave type is absorbed at the level where the shear allows, progressively lowering the absorption level. The period averages 28 months rather than exactly 24, which is why it is called quasi-biennial. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

27.

Which property does an equatorial Kelvin wave have?

A.Westward propagation
B.An amplitude that increases steadily with distance from the equator on both sides of the waveguide
C.No meridional velocity component
D.A meridional velocity that reaches its maximum exactly on the equator itself where the wave is trapped

Answer: C

The Kelvin wave has zero meridional velocity everywhere, with the pressure gradient in that direction balanced entirely by the Coriolis force acting on the zonal wind. This constraint traps it about the equator with a Gaussian decay and forces it to propagate eastward only. The amplitude therefore decreases rather than increases away from the equator.

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28.

Which direction does an equatorial Kelvin wave travel?

A.Westward only, which is the direction taken by the Rossby gyres in the same equatorial waveguide
B.Either direction
C.Poleward, carrying energy away from the equator towards the subtropics of both hemispheres at once
D.Eastward only

Answer: D

The trapping condition requires the wave to decay away from the equator, and only the eastward propagating solution does so, the westward one growing without bound. Kelvin waves are therefore strictly eastward at the equator. Along a coastal boundary the analogous wave travels with the coast on its right in the northern hemisphere, which is the same trapping argument applied to a different geometry. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Discuss in Forum

29.

Which speed does an atmospheric Kelvin wave travel at?

A.Tens of metres per second
B.About 340 metres per second, which is the speed at which sound propagates through the lower atmosphere
C.A few metres per second
D.About 3 metres per second, which is comparable with the speed of the equivalent wave within the ocean

Answer: A

Atmospheric Kelvin waves in the first baroclinic mode travel at roughly 20 to 60 metres per second, crossing the tropical Pacific in a few days. Oceanic Kelvin waves are far slower at about 3 metres per second, taking two months for the same journey, which is why the two are so often contrasted in discussions of El Nino.

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30.

Which pattern describes the atmospheric response to equatorial heating?

A.A meridional circulation confined entirely to the longitude at which the heating is being applied there
B.A Kelvin wave to the east and Rossby gyres to the west
C.A symmetric pattern of outflow in every direction, since the equatorial atmosphere has no preferred direction
D.Rossby gyres to the east

Answer: B

Gill's solution shows the Kelvin response extending eastward and a pair of off-equatorial cyclonic Rossby gyres to the west, an asymmetry that follows directly from the propagation directions of the two wave types. This pattern is recognisable in observed circulations associated with tropical convection and with El Nino heating anomalies, and it is one of the most used results in tropical meteorology. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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