Sudden stratospheric warming
Encyclopedia
A sudden stratospheric warming (SSW) is an event where the polar vortex
of westerly (eastward) winds in the winter hemisphere abruptly (i.e. over the course of a few days) slows down or even reverses direction, accompanied by a rise of stratospheric temperature by several tens of kelvin
s. This is considered to be the most dramatic meteorological event in the stratosphere
.
(~40 km). It was his persistence which led him to witness the first ever observed stratospheric warming on the 27th of January 1952. After his discovery, Scherhag created a team of meteorologists specifically to study the stratosphere at his university - the Free University of Berlin. This group continued to map the northern-hemisphere stratospheric temperature and geopotential height
for many years using radiosonde
s and rocketsonde
s. In 1979 then the satellite
era began, meteorological measurements became far more frequent. Although satellite
s were primarily used for the troposphere
they also recorded data for the stratosphere. Today both satellite
s and stratospheric radiosonde
s are used to take measurements of the stratosphere
.
Sometimes a fourth category, the Canadian warming, is included because of its unique and distinguishing structure and evolution.
) reverse, i.e. become easterly (westwards). A complete disruption of the polar vortex
is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.
According to the World Meteorological Organization
's Commission for Atmospheric Sciences (Mclnturff, 1978): a stratospheric warming can be said to be major if 10 mb or below the latitudinal mean temperature increases poleward from 60 degree latitude and an associated circulation reversal is observed (that is, the prevailing mean westerly winds poleward of 60 latitude are succeeded by mean easterlies in the same area).
Mclnturff states: a stratospheric warming is called minor if a significant temperature increase is observed (that is, at least 25 degrees in a period of week or less) at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly (eastwards) to easterly (westwards) is less extensive.
means that during winter the mean flow is westerly (eastward) and during summer it is easterly (westward). A final warming occurs on this transition, so that the polar vortex
winds change direction for the warming, however do not change back until the following winter. This is because the stratosphere
has entered the summer easterly (westward) phase. It is final because another warming cannot occur over the summer, so it is the final warming of the current winter.
and land-sea temperature contrasts are responsible for the generation of long (wavenumber
1 or 2) Rossby waves in the troposphere
. These waves travel upward to the stratosphere
and are dissipated
there, producing the warming by decelerating the mean flow. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed. This event to date is not fully understood.
At an initial time a blocking-type circulation pattern establishes in the troposphere. This blocking pattern causes planetary zonal wave number 1 and/or 2 to grow to unusually large amplitudes. The growing wave propagates into the stratosphere and decelerates the mean zonal winds. Thus the polar night jet weakens and simultaneously becomes distorted by the growing planetary waves. Because the wave amplitude increases with decreasing density this easterly acceleration process is not effective at fairly high levels. If the waves are sufficiently strong the mean zonal flow may decelerate sufficiently so that the winter westerlies turn easterly. At this point planetary waves may no longer penetrate into the stratosphere (due to the Charney-Drazen criteria
). Hence further upward transfer of energy is completely blocked and a very rapid easterly deceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere.
There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation
: If the QBO is in its easterly (westward) phase, the atmospheric waveguide
is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex
, intensifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).
Polar vortex
A polar vortex is a persistent, large-scale cyclone located near one or both of a planet's geographical poles. On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere...
of westerly (eastward) winds in the winter hemisphere abruptly (i.e. over the course of a few days) slows down or even reverses direction, accompanied by a rise of stratospheric temperature by several tens of kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s. This is considered to be the most dramatic meteorological event in the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
.
History
The first continuous measurements of the stratosphere were taken by Richard Scherhag in 1951. He used radiosondes to take reliable temperature readings in the upper stratosphereStratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
(~40 km). It was his persistence which led him to witness the first ever observed stratospheric warming on the 27th of January 1952. After his discovery, Scherhag created a team of meteorologists specifically to study the stratosphere at his university - the Free University of Berlin. This group continued to map the northern-hemisphere stratospheric temperature and geopotential height
Geopotential height
Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height"...
for many years using radiosonde
Radiosonde
A radiosonde is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required...
s and rocketsonde
Rocketsonde
A rocketsonde, a kind of sounding rocket, is a system for atmospheric observations that consists of a rocket that launches instruments that can make weather observations up to an altitude of 75,000 m. Common meteorological rockets are the Loki and Super Loki.A typical rocket is 3.7 m tall and is...
s. In 1979 then the satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
era began, meteorological measurements became far more frequent. Although satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s were primarily used for the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
they also recorded data for the stratosphere. Today both satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s and stratospheric radiosonde
Radiosonde
A radiosonde is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required...
s are used to take measurements of the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
.
Classification and description
Typically stratospheric meteorologists classify the vortex breakdown into three categories: major minor and final.Sometimes a fourth category, the Canadian warming, is included because of its unique and distinguishing structure and evolution.
Major
These occur when the westerly winds at 60N and 10hPa (geopotential heightGeopotential height
Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height"...
) reverse, i.e. become easterly (westwards). A complete disruption of the polar vortex
Polar vortex
A polar vortex is a persistent, large-scale cyclone located near one or both of a planet's geographical poles. On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere...
is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.
According to the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
's Commission for Atmospheric Sciences (Mclnturff, 1978): a stratospheric warming can be said to be major if 10 mb or below the latitudinal mean temperature increases poleward from 60 degree latitude and an associated circulation reversal is observed (that is, the prevailing mean westerly winds poleward of 60 latitude are succeeded by mean easterlies in the same area).
Minor
Minor warmings are similar to major warmings however they are less dramatic, the westerly (eastward) winds are slowed, however do not reverse. Therefore a breakdown of the vortex is never observed.Mclnturff states: a stratospheric warming is called minor if a significant temperature increase is observed (that is, at least 25 degrees in a period of week or less) at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly (eastwards) to easterly (westwards) is less extensive.
Final
The radiative cycle in the stratosphereStratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
means that during winter the mean flow is westerly (eastward) and during summer it is easterly (westward). A final warming occurs on this transition, so that the polar vortex
Polar vortex
A polar vortex is a persistent, large-scale cyclone located near one or both of a planet's geographical poles. On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere...
winds change direction for the warming, however do not change back until the following winter. This is because the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
has entered the summer easterly (westward) phase. It is final because another warming cannot occur over the summer, so it is the final warming of the current winter.
Canadian
Canadian warmings occur in early winter in the stratosphere of the Northern Hemisphere, typically from mid November to early December. They have no counter part in the southern hemisphere.Dynamics
In a usual northern-hemisphere winter, several minor warming events occur, with a major event occurring roughly every two years. One reason for major stratospheric warmings to occur in the Northern hemisphere is because orographyOrography
Orography is the study of the formation and relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain...
and land-sea temperature contrasts are responsible for the generation of long (wavenumber
Wavenumber
In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...
1 or 2) Rossby waves in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
. These waves travel upward to the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
and are dissipated
Dissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
there, producing the warming by decelerating the mean flow. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed. This event to date is not fully understood.
At an initial time a blocking-type circulation pattern establishes in the troposphere. This blocking pattern causes planetary zonal wave number 1 and/or 2 to grow to unusually large amplitudes. The growing wave propagates into the stratosphere and decelerates the mean zonal winds. Thus the polar night jet weakens and simultaneously becomes distorted by the growing planetary waves. Because the wave amplitude increases with decreasing density this easterly acceleration process is not effective at fairly high levels. If the waves are sufficiently strong the mean zonal flow may decelerate sufficiently so that the winter westerlies turn easterly. At this point planetary waves may no longer penetrate into the stratosphere (due to the Charney-Drazen criteria
). Hence further upward transfer of energy is completely blocked and a very rapid easterly deceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere.There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation
Quasi-biennial oscillation
The quasi-biennial oscillation is a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at...
: If the QBO is in its easterly (westward) phase, the atmospheric waveguide
Atmospheric waveguide
An atmospheric waveguide is an atmospheric flow feature that improves the propagation of certain atmospheric waves.The effect arises because wave parameters such as group velocity or vertical wavenumber depend on mean flow direction and strength...
is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex
Polar vortex
A polar vortex is a persistent, large-scale cyclone located near one or both of a planet's geographical poles. On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere...
, intensifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).