Observation of Upper Tropospheric Jetstreams
Short History
Zonal winds in midlatitudes have a tendency to increase vertically in the troposphere,
forming high speed westerlies that peak near the tropopause at around the 200mb level.
This tendency had long been termed as the Jetstream (originally 'Strahlströme', Seilkopf
1939).
Observations of swiftly moving winds, increasing with altitude were reported since the
early 1920's using kites and pilot balloons. It was not until WWII, however, that reports of
high velocity winds made by military aircrafts ignited a large scale study of the
phenomenon.
Perhaps the most remarkable early observation was the one made by Wasaburo Ooishi,
head of Japan’s first upper-air observatory at Tateno (W. Ooishi, 1926). Ooishi (who by the
way, published the article in Esperanto) made the observation seen in figure 1 using a
balloon followed by a single theodolite.
FIG. 1: Seasonal mean
wind speed profiles from
pilot balloon observations at
Tateno,
Japan,
for
the
period 1924–26 (speed is
given in m/s and height in
km). The labeling is in
Esperanto, where Vintro,
Printempo,
Autuno,
Somero,
indicate
and
winter,
spring, summer, and fall,
respectively.
Although his publication did not receive much attention in Europe, Ooishi’s
observations were not ignored in Japan. His observations were instrumental to the Japanese
military Balloon bomb program whereby 9000 balloon bombs were launched at the last
stages of the war, 3% of which had actually reached North America.
Jetstreams may be defined as "a strong narrow current, concentrated along a quasihorizontal axis in the upper troposphere or in the stratosphere, located above regions of
strong temperature contrast." Bergren, Gibbs and Newton (1958) add the following criteria:
"Normally a jetstream is thousands of kilometers in length, hundreds of kilometers in width
and some kilometers in depth. The vertical shear of wind is of the order of 5-10m/s per km
and the lateral shear is of the order of 5m/s per 100km. An arbitrary lower limit of 30m/s is
assigned to the speed of the wind along the axis of a jet stream."
There are essentially five major definable jetstreams in earth's atmosphere. The highest
is the westerly stratospheric polar night jetstream which forms during the polar winter. The
upper troposphere hosts the westerly polar jetstream and subtropical jetstream as well as
the easterly equatorial jet which forms during the northern hemisphere summer. Another
group of jets is the low level jets such as the low level night jets which typically form at
night above temperature inversions and the African easterly jetstream, located below the
equatorial jetstram.
Data Sources
Since the early 90's, the meteorological global community has experienced the advent
of Reanalyzed Data. Advanced assimilation methods are used to reprocess all past
environmental observations, combining them with numerically calculated short forecasts in
order to produce an improved estimate of the state of the environment in an easily
accessible data base. Since the process of assimilating real observed data with artificially
generated forecasts is denoted as 'Analysis', the new method is known as 'Reanalysis'.
Three major global atmospheric reanalysis projects have been already carried out:
1)
The NCEP/NCAR 50-year Reanalysis 1, covering from 1948 to the present,
followed by the NCEP/DoE 20-year Reanalysis 2.
2)
The ECMWF 15-year Reanalysis, covering 1979-1993.
3)
The NASA/DAO 17-year Reanalysis, covering 1980-1996
The most significant contribution of reanalyzed data is the possibility of conducting
statistical research of climate properties in a matter of a few clicks. One must keep in mind,
however, that the global 4 dimensional set provided by reanalysis is not real. It is merely
the best available estimate. For example, about 80% of radiosonde stations are located in
the northern hemisphere. The quality of observation therefore, varies globally. In particular,
over areas such as the southern oceans the data is composed of almost purely simulated
fields (I. James, 1994).
Observation
An annual average of the zonal wind speed as seen in figures 1 and 2 shows a distinct
maximum of speed at about the 200mb level ranging from 40m/s up to 80m/s located
slightly above the 30º longitude line in the northern hemisphere and a milder though more
uniform maximum ranging from 30m/s to 40m/s located at around the 45ºS longitude line
in the southern hemisphere. This persistent trend is termed the Subtropical Jet Stream
(STJ), typically located above the subtropical high pressure ridge (Reiter and Whitney,
1964).
Figure 1: NCEP/NCAR reanalysis of the annual and zonal mean of the zonal wind in the years 1968-1966.
Figure 2: NCEP/NCAR reanalysis of the annually averaged zonal wind at the 200mb level (2000-2005).
One of the most pronounced properties of the jetstreams is seasonal variability, as can be
seen in figure 3. During winter, as the meridional temperature gradient increases due to the
drastic fall in temperature at the poles, so does the amplitude of the subtropical jetstream.
In the northern hemisphere, during December-January we see one dominant subtropical jet
whereas during July-August the subtropical jet weakens to speeds averaging 20m/s and an
easterly equatorial jet forms, reaching winds as high as 15m/s. The change in the amplitude
of the subtropical jetstream coincides with a poleward migration of the jet maximum to the
north during the summer and equatorward migration of the jet maximum during winter.
The Southern hemisphere offers a similar yet less pronounced picture.
Figure 3: NCEP/-NCAR reanalysis of the 200mb level monthly averaged NH zonal
wind (m/s) during 2001. We see that the intensity of the NH STJ peaks during winter
and its core migrates northward to around 45N where its intesity reaches its minimum
value in summer. The migration of the STJ to the north and the decrease in amplitude
in summer is coincident with the formation of an equatorial easterly jet at around 5N.
A weaker, yet observable, maximum in the zonal wind is present some years along the
polar front near the tropopause at an altitude of about 300mb - termed the Polar Front Jet
(PJ), seen on figure 4. The polar jet and the subtropical jet normally meander in broad
loops sweeping north and south, splitting into two or three branches that occasionally
converge (Ahrens, 2005).
As seen on figure 4, Jet streams are not continuous but can be as much as 3000 – 5000
km long, 100 – 300 km wide and span 2 – 3km of altitude, depending on the definition of
the velocity cutoff. The Subtropical jet is not symmetrical about its core (defined by the
region within the jet where the zonal velocity is maximal). About 60% of the width tends to
be on the equatorial side of the core. The wind speeds usually decrease by 1 – 2 m/s per
100m vertically reaching half the maximal value about 5km below the level of maximum
wind. Horizontally, the wind speed decreases by about 5m/s per 100 km reaching half the
value about 200-300km from the core, so that jets are fairly thin and narrow, with
horizontal dimensions exceeding the vertical dimensions by about two orders of magnitude
(Raethjen, 1960 ; Reiter, 1960).
Figure
4:
San
Francisco
State
University's California Regional Weather
Server
May/8/05.
Jetstream
The
reanalysis
for
discontinuous
and
meandering nature of the Jetstream is
clearly evident as well as the converging
of the branches of the Polar and
Subtropical Jets. On a global scale, the
core of the jetstream has a wavelike
eastward propagating phase. This is Why
Jetstreams are often described using
Long Rossby Wave Theory.
Typology
The upper troposphere hosts 3 identifiable jets. The subtropical jet, which is a consequence
of the conservation of angular momentum by air rising in the equator and traveling
poleward as part of the tropical circulation termed the Hadley cell. The Midlatitude and
Polar jets are 'eddy-driven jets', meaning that they are a result of deviations from the mean
flow which produce strong pressure and temperature gradients. The polar and midlatitude
jets emerge at such unstable regions and are sometimes referred to as turbulent jets.
The Polar Jet Stream
The northern polar jet stream (also called the polar jet, or the mid-latitude jet stream, or
just the jet stream) is the one having the most influence on weather across much of the
United States and Canada, and thus it gets the most attention on local and national
weathercasts. The polar jet stream can usually be found somewhere in the latitude belt from
40-60 degrees at an altitude from 7600 metres to 10,600 m (25,000-35,000 ft).
The highest wind velocity is found in the jet stream core where speeds can be as high as
460 km/h (290 mph) in the winter. The jet stream core region averages 160 km/h (100
mph) in winter and 80 km/h (50 mph) in summer. Those segments within the jet stream
where winds attain their highest speeds are commonly known as jet streaks.
The polar jet stream position marks the location of the strongest temperature contrasts
between polar and subtropical latitudes on the Earth surface. Therefore, the strongest polar
jet stream velocities usually occur during the winter months. During the summer months,
when the equator-pole surface temperature differences are less dramatic, the jet winds blow
slower and are usually found at higher latitudes.
Jet Stream Formation
The polar jet stream is formed in the region of greatest contrast between polar and
subtropical air known as the polar front where the cold, dry polar air meets the warm,
moist air from the subtropical regions. From a climatological viewpoint, the position of the
polar front forms a more or less even band around the globe, slipping north and south with
the seasonal changes.
In the vicinity of the polar front, the air pressure drops more rapidly with increasing
altitude in the denser cold air than in the less dense warm air. This temperature effect on air
density results in air pressure at any given altitude being higher on the warm (equatorward)
side of the polar front than on the cold (poleward) side. When cold air and warm air masses
sit side by side, the higher the altitude, the greater the pressure difference between the cold
and warm air. Thus, across the polar front, the horizontal pressure differential (or gradient)
causes air to flow from the warm side of the front towards the cold side.
Once the air begins to flow, it is deflected by the Earth's rotation (called the Coriolis effect)
and prevented from flowing directly from high to low pressure. Air flowing from higher
pressure towards lower pressure is deflected to the right in the Northern Hemisphere (or to
the left in the Southern Hemisphere). The resulting flow produces a westerly wind current
generally flowing towards the east, parallel to and above the polar front. Very strong
temperature and pressure gradients in the polar frontal zone can intensify these wind speeds
to over 94 km/h, thus forming a jet stream.
Unlike the climatological position of the polar front, its day-to-day position and shape can
vary in latitude and develop north-south loops in the frontal boundary The streambed of the
jet stream can vary widely with these frontal position changes and kinks much like a
riverbed develops serpentine bends and meanders. When the flow is mostly west to east,
the jet stream is said to be in zonal flow. But the jet can take wild north-south swings as it
flows, a condition termed meridional flow.
The polar front and its associated jet stream have a major influence on the weather
conditions surrounding it. Many storms form along the polar front in the vicinity of the jet
stream's maximum winds. (For more on this see The Highs and Lows of Weather: Part 2 - The Low) Because of the association of the jet stream with the polar front, media weather
maps often include the jet stream's position as a rough indicator of continental divisions
between warm and cold air masses. Here is why.
From a climatological viewpoint, the position of the polar front forms a more-or-less even
band around the globe at mid-latitudes in both hemispheres. The average polar-front
position slips north and south with the seasons in response to the annual hemispheric
heating cycle. Like my waistline's annual cycle, the polar frontal belt shrinks poleward in
the summer months and then expands toward the subtropics during the winter.
From a meteorological viewpoint, however, the polar front is not an evenly encircling
latitude belt but an undulating zone around the globe that is ever-changing as masses of
warm and cold air push away from their regions of origin. Of particular interest to
meteorologists and weather forecasters is the ever-changing pattern of long-waves that
form around the polar front. These long-waves, very visible on polar projection maps,
undulate around the hemisphere with three to six wave cycles. (Long waves are also known
as Rossby waves in honour of Swedish meteorologist C.G. Rossby who gave us many early
insights into the impacts of upper atmosphere features on weather.)
At times, that global belt fits tight, having three or four, small-amplitude undulations (little
north-south latitude variation) around the hemisphere. At other times, it has as many as six
large-amplitude loops (great north-south variation). How those wave loops sit over the
hemisphere, or portion thereof, determines what temperature regimes are experienced on
the surface below.
When a long section of the polar front, as seen on continental weather maps, is smooth like
the surface of a calm sea, the upper-level winds, including associated jet streams, run
generally parallel to the latitude lines, a condition called zonal flow. Under zonal-flow,
north-south undulations of the frontal boundary are small, and the surface temperatures
across the continent, as seen in the isotherm (lines of equal temperature) pattern on the
weather map, layer in zonal (east-west) bands with warm air to the south and cold air to the
north.
But like occupants of a small boat riding that calm surface, we may be lulled during
persistent zonal flow by a stretch of nearly indistinguishable day-to-day temperature
changes, often remaining near the climatological mean for that time of year. Despite the
unfounded desires of those living in the mid-latitudes to believe there is a truly "normal"
state of weather from which departures are abnormal, such stable zonal patterns are but a
temporary state. We live not in a meteorological Camelot, where it never rains til after
sundown — or the Tropics where it always rains in late afternoon — but in a region subject
to very changeable-weather regimes.
Thus, when a zonal flow pattern cuts off the exchange of heat between the polar region and
the tropics, great thermal contrasts develop across the polar front. And this becomes the
zonal pattern's downfall. Eventually, some chink, some small perturbation, develops in the
zonal pattern, and a short wave, with initially small north-south extent, arises upon the
polar front. If the short wave amplifies (grows in size), it can distort the flow into a new
pattern that crosses the latitude lines, a pattern termed meridional. In meridional flow, cold
air rushes southward while warm air streams northward.
As the polar-front meridional pattern rises into Rossby waves of deep north-south extent, it
severely rocks our "temperature boat." When the wave, and associated jet stream, plunges
southward, we find ourselves in a deep upper-level trough with temperatures quite cold
below it. When we rise high on the upper-level ridge, or crest of the passing Rossby wave,
the surface experiences warm, even hot, temperatures.
Often, the intense temperature changes brought by the meridional flow are expressed by
frontal patterns associated with a surface low-pressure system: warm air moving northward
behind the warm front; cold air descending southward behind the cold front. Not only are
cyclonic storms spawned in the vicinity of the peak jet stream winds (called jet streaks),
but they are often pushed rapidly eastward by the jet stream above.
So when you look at a weather map with the polar jet stream position drawn on it, you can
generally tell what type of weather you will be having. If north of the jet, it should be
relatively cold, if south, warmer conditions should prevail. And, if the jet stream is flowing
overhead, look for stormy weather to dominate.