Atmospheric Waves Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.