Atmospheric Waves Questions and Answers

Q: Which region of the winter stratosphere is called the surf zone?

The mid-latitude surf zone is where planetary waves break irreversibly, mixing potential vorticity and forming a broad region of weak gradients between the tropics and the vortex edge. The vortex edge itself acts as a transport barrier. The name captures the analogy with ocean waves breaking on a shelving beach, and the sharp gradients at its edges are a striking feature of potential vorticity maps.

Q: Which consequence does that isolation have for polar ozone?

Because the vortex is isolated, air depleted of ozone by heterogeneous chemistry on polar stratospheric clouds is not replaced, and the depletion accumulates through the winter and spring. This is why the ozone hole is a southern hemisphere phenomenon, the southern vortex being colder and more isolated than the northern one. Descent does occur but the air remains extremely cold.

Q: Which hemisphere shows the stronger and more isolated polar vortex?

The southern vortex is colder, stronger and far less disturbed by planetary waves, because the southern hemisphere has weaker stationary wave forcing from topography and land-sea contrast. That is precisely why the ozone hole forms over Antarctica and not over the Arctic. The vortex is a winter feature, forming as the pole enters polar night.

Q: Which event can disrupt the vortex in mid-winter?

Breaking planetary waves can decelerate and reverse the vortex within a few days, raising polar stratospheric temperatures by tens of kelvin and sometimes splitting the vortex in two. Such major warmings occur in roughly half of northern winters and very rarely in the south. The spring reversal is the ordinary seasonal transition, and volcanic and tropospheric influences act far more slowly.

101.

Which quantity does the tape recorder signal measure directly?

A.The strength of the polar vortex
B.The frequency at which planetary waves are breaking in the surf zone of the winter stratosphere above
C.The tropical ascent rate in the lower stratosphere
D.The rate at which methane is being oxidised into water vapour within the middle stratosphere above it

Answer: C

By following the upward progress of a given annual maximum or minimum through successive years of observation, the mean vertical velocity of the tropical branch of the Brewer-Dobson circulation can be read off directly. Values around 0.3 millimetres per second are found. It is one of the few direct measurements of a large scale vertical velocity anywhere in the atmosphere.

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102.

Which region of the winter stratosphere is called the surf zone?

A.The tropical region in which air rises slowly across the tropopause into the stratosphere above it
B.The polar vortex interior
C.The layer immediately above the tropopause where the static stability increases sharply with height
D.The mid-latitude region where planetary waves break

Answer: D

The mid-latitude surf zone is where planetary waves break irreversibly, mixing potential vorticity and forming a broad region of weak gradients between the tropics and the vortex edge. The vortex edge itself acts as a transport barrier. The name captures the analogy with ocean waves breaking on a shelving beach, and the sharp gradients at its edges are a striking feature of potential vorticity maps.

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103.

Which property does the polar vortex edge have?

A.It acts as a barrier to horizontal transport
B.It permits free exchange of air between the vortex interior and the surrounding mid-latitude air mass
C.It moves with the mean flow
D.It coincides exactly with the position of the polar night jet at every level within the stratosphere

Answer: A

Strong potential vorticity gradients at the vortex edge inhibit mixing, so air inside remains isolated for months, which is what allows the chemical processing responsible for the ozone hole to proceed undiluted. The barrier is not perfect but is remarkably effective. The jet is associated with the edge but the two are not exactly coincident at every level.

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104.

Which consequence does that isolation have for polar ozone?

A.The vortex air warms steadily through the winter as it is compressed by descent within the circulation
B.Chemically processed air is not replenished from outside
C.Ozone rich air from the tropics is continually mixed into the vortex, offsetting any chemical loss there
D.There is no consequence

Answer: B

Because the vortex is isolated, air depleted of ozone by heterogeneous chemistry on polar stratospheric clouds is not replaced, and the depletion accumulates through the winter and spring. This is why the ozone hole is a southern hemisphere phenomenon, the southern vortex being colder and more isolated than the northern one. Descent does occur but the air remains extremely cold.

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105.

Which hemisphere shows the stronger and more isolated polar vortex?

A.Both equally
B.Neither, since the vortex forms only in the hemisphere experiencing summer at the time of the observation
C.The southern hemisphere
D.The northern hemisphere, where the greater land-sea contrast produces a more vigorous circulation aloft

Answer: C

The southern vortex is colder, stronger and far less disturbed by planetary waves, because the southern hemisphere has weaker stationary wave forcing from topography and land-sea contrast. That is precisely why the ozone hole forms over Antarctica and not over the Arctic. The vortex is a winter feature, forming as the pole enters polar night.

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106.

Which event can disrupt the vortex in mid-winter?

A.The seasonal reversal of the winds in spring as sunlight returns to the polar region after the winter
B.A volcanic eruption
C.The passage of a deep tropospheric cyclone directly beneath the centre of the polar region concerned
D.A sudden stratospheric warming

Answer: D

Breaking planetary waves can decelerate and reverse the vortex within a few days, raising polar stratospheric temperatures by tens of kelvin and sometimes splitting the vortex in two. Such major warmings occur in roughly half of northern winters and very rarely in the south. The spring reversal is the ordinary seasonal transition, and volcanic and tropospheric influences act far more slowly.

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107.

Which tropospheric consequence can follow a sudden stratospheric warming?

A.A shift of the storm track towards lower latitudes
B.An immediate warming of the surface across the whole of the polar region beneath the disturbed vortex
C.No consequence at all
D.A strengthening of the westerly flow across the Atlantic that persists for several weeks afterwards

Answer: A

The weakened stratospheric vortex is often followed within a few weeks by a negative phase of the Northern Annular Mode, with the storm track shifted equatorward and cold outbreaks over northern Europe and eastern North America. This downward influence is one route by which stratospheric prediction contributes to extended range surface forecasting, and the surface effect is cooling rather than warming.

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108.

Which practical value does stratospheric prediction have?

A.It replaces tropospheric forecasting entirely during the winter months in the northern hemisphere region
B.It extends useful surface forecast skill to several weeks
C.It allows the exact date of a sudden warming to be predicted many months in advance of its occurrence
D.It has no practical value

Answer: B

Because the stratospheric influence on the surface persists for several weeks after a major warming, forecasting the stratospheric state adds skill in the two to six week range where tropospheric memory has been lost. This is now exploited operationally at several centres. The onset of a warming is itself predictable only a week or two ahead, so long lead prediction of the event is not available.

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