by John Mason, Skeptical Science, May 22, 2013
Barely
a week goes by these days in the Northern Hemisphere without the jet
stream being mentioned in the news, but rarely do such news items
explain in detail what it is and why it is important. As a severe
weather photographer this past 10+ years, an activity which requires
successful DIY forecasting, I've had to develop an appreciation into
what makes it tick. This post, then, is a start-from-scratch primer
based on that knowledge plus some valuable assistance from academia into
where the current research is heading. Because of its length and
breadth of coverage, I've broken it up into bookmarked sections for easy
reference: to come back here click on 'back to contents' in each
instance.
Earth's Troposphere - an introduction
We live at the bottom of a soup of gases, constantly moving in all directions -- our atmosphere. Virtually all of our tangible weather goes on in its lowest major division, the Troposphere.
This division varies in average thickness from about 9,000 m over the
poles to 17,000 m over the tropics -- in other words, it's thinnest in cold
areas and thickest in hot areas, because hot air is more expansive than
cold air. Likewise it fluctuates in thickness on a seasonal basis
according to whether it's warmer or colder. Above it lies the Stratosphere, while below it lies the surface of the Earth.
The junction with the stratosphere is known as the tropopause, and as the diagram below shows, it is a major temperature inversion: although it gets colder with height in the troposphere, at the tropopause
it suddenly warms. The inversion is so strong that convective air
currents, which involve parcels of warm air rising buoyantly through
cooler surroundings, fail to penetrate it. That is why the flat,
anvil-shaped tops of convective cumulonimbus (thunderstorm) clouds
spread out laterally beneath the tropopause, as though it were some ceiling in the atmosphere.
Above: section through the lower 100 km of Earth's atmosphere.
The thick black zigzagging line plots typical changes in temperature
from the surface upwards; height above surface is the left-hand scale and
typical pressure with that height is the right-hand scale.
The troposphere, which this post concerns, can be divided into two subsections: an upper layer, known as the Free Atmosphere,
and a lower layer, known as the Planetary Boundary Layer. The Boundary
Layer usually runs up from the surface to about 1,000 m above it
(sometimes a bit more, sometimes a bit less), but basically it's a
relatively thin layer in which the air movements and temperatures are
influenced not only by major weather patterns but also by localized
effects relating to the interaction of the air with the planet's
surface. Such effects include frictional drag as winds cross land areas,
eddies, veering and lifting due to hills and headlands, and convection
initiated directly by heat radiation from sun-warmed ground. Low-level
air currents, such as the cool sea breezes that push inland from coasts
on warm summer days, likewise aid and abet convection
and thereby thunderstorm formation as they undercut and lift warmed
air masses along zones of convergence -- where different air currents come
together. These factors are all low-level forcing mechanisms that set
air currents in motion or perturb existing currents.
Above
the Boundary Layer, winds are directed by two factors: the gradients
that exist between centres of high and low pressure (anticyclones and
cyclones respectively) -- air will always flow from a high-pressure zone
to a low-pressure zone -- and the modifying factor known as the Coriolis
Effect, which is the force exerted by the Earth's rotation. In the
Northern Hemisphere, it causes air masses to be deflected to the right of
their trajectory, and this effect is strongest at the poles and weakest
at the Equator. In the Northern Hemisphere, the effect is to make the
winds around a high-pressure centre circulate in a clockwise manner and
those around a low-pressure centre circulate in an anti-clockwise manner:
on a larger scale, the Coriolis Effect helps to maintain the prevailing
west-to-east airflow.
Although the weather charts seen on
TV forecasts show only what is happening close to the surface, the
forecasts themselves are made with much reference to goings-on in the
upper troposphere.
In upper-air meteorology, pressure patterns are as important as they
are down here at the surface. Atmospheric pressure is simply an
expression of the force applied by a column of air upon a fixed point of
known area and is measured in pascals (Pa). Meteorologists use the
hectopascal (hPa) because the numbers are the same whether expressed in hectopascals or the older unit, millibars.
The
greater the altitude, the lower the atmospheric pressure because
there's less air above. In meteorology, above-surface observations are
made remotely with satellites and directly by weather balloons carrying
measuring instruments. The results of the balloon ascents, called
soundings, are plotted on charts at different pressure levels, some
typical examples of which are as follows:
Pressure at any given height can
change quite drastically as weather systems move through, just as it
does at the surface. Taking the UK as an example, as an Atlantic
low-pressure system moves through and is then replaced by a large
high-pressure area, the pressure over a few days at sea level can rise
from 970 hPa to 1,030 hPa.
The same applies aloft, but unlike surface charts, where the data are
plotted in terms of pressure, the upper-air data are plotted in terms of
geopotential. Geopotential is the height above sea level where the
pressure is, say, 850, 500 or 300 hPa, and is measured in Geopotential Metres (gpm or gpdm).
Other properties of the upper air, such as temperature, are important, too. For example, storm formation in an unstable lower troposphere
is markedly encouraged if cold, dry air is present aloft, which makes
the rising warm, moist air much more buoyant, increasing the instability.
Storm forecasters will look at soundings for indications that cold,
upper air is either already present or is upwind and can be expected to
be transported into the forecast area. The process by which air (with
its intrinsic physical properties such as temperature or moisture
content) is transported horizontally is known as advection, an important term that will appear elsewhere in this post.
Weather systems aloft - the Polar Front and the jet stream
The interaction of warm tropical
and mid-latitude air and cold polar air is what drives much of the
Northern Hemisphere's weather all year round. For a variety of reasons,
the change in temperature with latitude is not gradual and even but is
instead rather sudden across the boundary between mid-latitude and polar
air. This boundary, between the two contrasting air masses, is known as
the Polar Front. It is the collision zone where Atlantic depressions
develop, and their track is largely directed by its position. The steep
pressure gradients that occur aloft in association with this major,
active, air mass boundary result in a narrow band of very strong
high-altitude winds, sometimes exceeding 200 miles per hour, occurring
just below the tropopause.
Such bands occur in both hemispheres and are known as jet streams. The
one in the Northern Hemisphere, associated with the Polar Front, is
often referred to as the Polar jet stream. The greater the temperature
contrast across the front, the stronger the Polar jet stream: for this
reason it is typically strongest in the winter months, when the contrast
between the frigid, sunless Arctic and the mid-latitudes should normally
be at its greatest.
Above: section through the atmosphere
of the Northern Hemisphere. Air rises at the Intertropical Convergence
Zone and circulates northwards via the Hadley and Ferrel Cells
(sometimes separated by a relatively weak Subtropical jet stream) before
meeting cold Polar air at the Polar Front, where the Polar jet stream
is located. Graphic: NOAA.
Waves on the jet stream - upper ridges and troughs
The Polar jet stream is readily
picked out on upper-air wind charts, as in the example below. This is a
Global Forecasting System (GFS) forecast-model chart for wind speeds and
direction of flow at the 300-hPa pressure level; in other words, at an altitude a little higher than the summit of Everest and not far beneath the tropopause.
Highest winds are red, weakest blue. The most obvious thing that
immediately catches the attention is that the jet stream doesn't always
run in a straight, west-east line, even though that's the prevailing
wind direction in the Northern Hemisphere.
Instead, it curves north and south
in a series of wavelike lobes, any one of which can half-cover the
Atlantic. These large features, which are high-pressure ridges and
low-pressure troughs, are known as Longwaves or Rossby Waves, of which
there are several present at any given time along the Polar Front. A key
ingredient in their formation is perturbation of the upper troposphere
as the air travels over high mountain ranges, such as the Rockies. Warm
air pushing northwards delineates the high-pressure ridges. Cold air
flooding southwards forms the low-pressure troughs. The two components
to jet stream flow -- west-east and north-south -- are referred to as
zonal and meridional flows respectively. The straighter a west-east line
the jet stream takes, the more zonal it is said to be. The greater the
north-south meandering movement, the more meridional it is said to be.
In
addition to the Longwaves, there are similar, but much smaller ridges
and troughs, known as Shortwaves. The chart above also shows how,
locally, the jet stream can split in two around a so-called cut-off
upper high or low, reuniting again downstream. Longwaves, shortwaves and
cut-off highs and lows all have a strong bearing on the weather to be
expected at ground level.
Several factors are important with
regard to the Polar jet stream and its effect on weather. Again taking
the UK as an example, the position of the Polar jet stream is of
paramount importance. If it sits well to the north of the UK, residents
can expect mild and breezy weather, and occasional settled spells. The
Atlantic storms are passing by to the north, so they only clip
north-western areas. However, if the Polar jet stream runs straight
across the UK, then the depressions will run straight over the country,
with wet, stormy weather likely. If it sits to the south, depressions
take a much more southerly course, bringing storms to Continental
Europe, and, in winter, the risk of heavy snow for the southern UK, as
the prevailing winds associated with low-pressure systems that are
tracking to the south of the UK will be from the east, thereby pulling
in colder continental air.
Above: typical zonal (red) and
meridional (orange) jet stream paths superimposed on part of the
Northern Hemisphere. Extreme meridionality can bring very cold air
flooding a long way south from the Arctic, while warm air is able in a
different sector to force its way into the far north. The most extreme
version of this I have seen was on the morning of November 28th, 2010: at
06:00, parts of Powys (Mid Wales) were down to -18 C, whilst at the same
time Kangerlussuaq, within the Arctic Circle in Western Greenland, was
at +9 C -- or 27 degrees warmer!! Graphic: author
In highly
zonal conditions, weather systems move along rather quickly, giving
rise to changeable weather. However, in highly meridional conditions,
the Longwaves can slow down in their eastwards progression to the point
of stalling, to form what are known as blocks. When a block forms,
whatever weather type an area is experiencing will tend to persist.
During some winters, for example, a blocking ridge forms in the
mid-Atlantic, with high pressure extending from the Azores all the way
up towards Greenland. Provided the block is far enough west, it can
induce a cold northerly-to-easterly airflow over NW Europe, a synoptic
pattern that brings cold weather and, in recent winters, heavy
snowfalls.
To complete this section, here are a
couple of Flash animations of different jet-stream patterns by Skeptical
Science team-member 'jg' that illustrate how the waves progress
eastwards. First, zonal, with the longwaves moving through briskly:
Next: meridional: the longwaves are progressing eastwards much more slowly in general. In a blocked scenario, imagine the 'pause' button has been pressed and the whole lot has stopped for a while:
Now, let's move onto some of the
important weather-forcing mechanisms that are associated with the jet
stream and its wave patterns.
Positive vorticity - a driver of severe weather - and the jet stream
Another important factor associated with any jet stream is vorticity advection.
The jet flowing around a lobe of cold polar air (an upper Longwave or
Shortwave trough), orientated north-south, first runs S, then SE, then
E, then NE, then N, i.e., its motion is anti-clockwise, or cyclonic.
Watch a floating twig in a slow-moving river. As it turns a bend, it will
slowly spin. It's spinning because the water upon which it floats is
spinning -- it has vorticity. You can't necessarily see the water doing
this, but the floating twig gives the game away! Vorticity is a measure
of the amount of rotation (i.e., the intensity of the "spin") at a given
point in a fluid or gas. And, in the air rounding an upper trough,
anti-clockwise vorticity is induced. This is known as Cyclonic Vorticity
(or frequently as Positive Vorticity).
Above: how the eastwards
progression of upper ridges and troughs affects vorticity which in turn
affects lift in air masses. Areas of positive vorticity advection (PVA) occur ahead of approaching troughs, aiding severe weather development, whereas areas of negative vorticity advection (NVA) cause air to sink, inhibiting developments. Graphic: jg.
Positive vorticity in the upper troposphere encourages air at lower levels to ascend en masse. Rising air encourages deepening of low-pressure systems, assists convective storm development and so can lead
to severe weather such as heavy precipitation and flooding. As an upper
trough moves in, air with positive vorticity is advected ahead of its
axis in the process known as positive vorticity advection,
usually abbreviated to PVA. Thus, to identify areas of PVA when
forecasting, look on the upper-air charts for approaching upper Longwave
or Shortwave troughs: PVA will be at its most intense just ahead of the
trough and that is where the mass ascent of air will most likely occur.
The reverse, anticyclonic or negative vorticity advection
(NVA) will occur between the back of the trough and crest of an upper
ridge, due to the same process but with a clockwise (anti-cyclonic)
spinning motion induced into the air as it runs around the crest of the
ridge. In such areas, air is descending en masse instead of ascending. Descent is very adept at killing off convection and cyclonic storm development. Thus as the upper trough passes, severe weather becomes increasingly unlikely to occur.
Wind shear - a driver of severe weather - and the jet stream
Wind shear, involving changes in
wind speed and/or direction with height, is an important factor in
severe weather forecasting. Shear in which wind-speed increases occur
with height (speed shear) is common, as you will notice when climbing a
mountain: a breeze at the bottom can be a near gale at summit level. But
in the upper troposphere, the proximity of the Polar jet stream can lead
to incredibly strong winds. Speed shear is important in convective
storm forecasting as it literally whisks away the "exhaust" of a storm,
thus helping to prolong it: the storm's up-draught and precipitation core
(down-draught) are kept apart, instead of the down-draught choking the
up-draught. It's a bit like an open fire drawing well. The strongest
speed shear occurs when the jet is racing overhead. In this environment,
cumulonimbus anvils may stretch for many miles downstream due to the
icy cirrus of the anvil being dragged downwind. When there's hardly any
speed shear, the storm tops have a much more symmetrical shape to them.
Directional shear basically means
that winds are blowing in different directions at different heights
from the surface. Drawing from my experience in weather photography, I
know that a warm early summer's day where the synoptic pressure pattern
gives a light northerly airflow at say 850 hPa,
coupled with some instability, is a consistently productive set-up for
thunderstorms and funnel clouds. Why? Well, I live ten miles due east of
the Welsh coast, surrounded by hill country. As warm sunlight heats the
lower troposphere over the hills, air will begin to rise by convection:
at the same time, a sea breeze will set in, flowing west to east inland
from the coast. These two air currents will meet -- or converge -- along a
linear front somewhere over the hills. Because the sea breeze is
relatively cool, along the front it undercuts and lifts the warm air,
strongly aiding convective storm initiation. In addition, the developing
storms are moving north-south along their steering flow, but the air
flowing into the western side of their up-draughts -- the sea breeze -- is
coming in at right angles to that. That's a lot of low-level,
rotation-inducing, directional shear, more than sufficient for
funnel cloud development, something I have witnessed along sea-breeze
fronts on a number of occasions.
In situations where major instability
(and therefore the potential for severe storms) is present, directional
shear can be of critical importance in the formation of tornadic
supercells, in which the up-draught is rotating strongly from near
ground level, all the way up to the top of the storm cloud. These tend to
be the most violent members of the thunderstorm family because of the
persistence and strength of their up-draughts.
Above: speed shear revealed by a
convective shower cloud. High-speed upper winds are dragging the upper
parts of the cloud well over to the right. Below: speed and directional shear
revealed by a small supercell thunderstorm: the up-draught is tilted
R-wards so that the rain is falling well over to the R, several miles
down wind from the up-draught base. The seat of the up-draught is indicated
by the dramatically lowered rotating wall cloud reaching halfway down
to the sea from the overall cloud base. This storm persisted for over 90
minutes as it tracked across over 100 km of the seas and mountains of
Wales. Photos: author.
Jet streak development along the jet stream - a driver of severe weather
Within the overall circumglobal,
ribbon-like, wind field of the Polar jet stream, there occur local
sections with much stronger winds than elsewhere. These are called
jet streaks. They form in response to localised but major
temperature gradients, and they move around the lobes, following the
troughs and ridges, and affect these in their passing, strengthening
them as they move in, and weakening them as they move out. They also
influence the weather below, even if moving in a fairly straight line
when there are few long-wave ridges/troughs about.
Graphic: model output plot Wetterzentrale; annotation: author
Fast jet streaks with winds as high as 200 knots pull in air upstream (to their west) at what is called an Entrance Region and throw it out down stream (to their east) at what is called an Exit Region.
These are further subdivided, as in the diagram above, into Left (to
the north) and Right (to the south). Because the behaviour of air
currents is determined by the interaction of the Coriolis effect and the
pressure gradient, the Right-Entrance and Left-Exit regions
of jet streaks are areas where winds aloft diverge, allowing air below
to rise. This in turn further encourages storm development. In Right-Exit and Left-Entrance regions, the opposite occurs, with upper-level winds converging leading to air sinking
and inhibiting storm formation. The reason, in terms of storm
development, it is divergence as opposed to convergence that is
important at height (the opposite being the case at low levels) is
that converging air at height cannot go upwards because of the
effective ceiling provided by the tropopause. There is only one vertical direction in which the air can freely go -- downwards.
What this means on the ground is
that if your area is near to a developing low-pressure system or a
convectively unstable air mass, and an upper trough is approaching with a
jet streak heading towards the base of the trough with its Left-Exit region
heading straight for where you are, you have the ingredients for
explosive severe weather development. The low can deepen intensively to
bring a storm system with tightly packed surface isobars giving severe
gales and flooding rains. Alternatively, convection may lead to the development of severe thunderstorms because that critical combination of mass ascent and high shear is in place.
Northern Hemisphere atmospheric circulation patterns: the Arctic and North Atlantic Oscillations
Atmospheric pressure patterns in
the Northern Hemisphere feature several semi-permanent features and
patterns. By semi-permanent I mean that areas of high and low pressure
are normally to be found in certain places or that pressure patterns
tend to switch from one type to another and then back. The low pressure
of the Intertropical Convergence Zone is a good example of a
semi-permanent feature: it is normally close to the Equator, but it is
not always in the same place -- it can shift a little north or south in
its position. A good example of a switching pressure pattern occurs in
the Arctic and is known as the Arctic Oscillation (AO). When atmospheric pressure over the Arctic is low and pressure over the mid-latitudes is high, the AO
is said to be in its positive phase, which supports a tight and
fast-moving zonal, west-to-east airflow -- the Polar Vortex -- as the
diagram below shows:
Graphic: author
The next diagram is an example of what
happens when the Arctic Oscillation is in its negative phase, with high
pressure over the Arctic:
Graphic: author
The flow becomes more meridional, with
big meanders occurring in the long-wave ridges and troughs that then
tend to move eastwards much more slowly. Rossby Wave theory predicts
this, but there is a simple analogy: think of a river's flow weakening as
it leaves the mountains and enters the lowlands, where it becomes
sluggish and meanders develop and propagate seawards along the flood
plain over many decades. A negative Arctic Oscillation pattern with
these high-amplitude longwaves has the effect of permitting warm air to
penetrate much further north (in the ridges) and cold air to plunge much
further south (in the troughs), something that is obviously of
relevance in the resultant weather conditions.
The North Atlantic Oscillation
is a numerical index that describes the average difference in surface
air pressure between Iceland and coastal southern Europe (the data sources
used are Reykjavík in the north and either the Azores, Portugal or
Gibraltar in the south). Although daily data are available, the NAO is typically expressed in monthly or seasonal terms.
Here's the NAO in its positive phase:
Graphic: author
With a positive NAO,
the Atlantic pressure pattern essentially features a dipole, with low
pressure over Iceland (the Icelandic Low) and high pressure off the
Iberian coast (the Azores High). These are both good examples of
semi-permanent features -- if they were not so commonplace, they would not
have been so named. South of the Icelandic Low, the southwesterlies blow
mild air and moisture towards NW Europe, whilst SW
of Iberia, on the southern flank of the Azores High, we find the
northeasterly Trade Winds (so important to merchant shipping back in the
days of sailing).
Now let's see a slightly negative NAO:
Graphic: author
The low- and high-pressure centres are still there but are both much weaker, leading
to a strongly reduced pressure gradient between the two and a slacker
airflow. With the southwesterlies much suppressed, colder winter weather
can develop more easily over NW Europe. But what happens if the NAO
is strongly negative, as it was during the cold spell of March 2013
when it dipped at one point to a phenomenal value of -5 (typical values
are between +2 and -2)?
Graphic: author
The normal pressure pattern is
reversed: pressure over Greenland and Iceland is high, whilst the
mid-Atlantic is dominated by low pressure. In winter, this has the
effect of vigorously pulling in moisture from the Atlantic but also cold
air from either northern or eastern sources, a mixture which can lead
to severe weather developing: the pressure pattern in the diagram is
similar to those of both January 9th, 1982, and March 22nd, 2013 -- dates
that have gone down in UK weather history for the unusually severe
blizzards that occurred. The March 2013 blizzards were disastrous: it
was very late in the winter to have such cold over here, and the losses
to farmers of livestock have been significant, with drifting snow having
buried sheep, cattle and ponies to a depth of five metres or more in
places.
Above: the late March 2013 blizzards struck parts of the UK with a fury not seen in decades. A strongly negative NAO/AO
with blocking patterns in the jet stream can bring a complete spectrum
of weather extremes and this is just one of them. This was on March
29th, a week after the storm occurred. Photo: author.
Another pressure pattern that has been
recognised in recent years and which has been linked to the rapid warming of
the Arctic is the Arctic Dipole:
Graphic: author
In the Dipole pattern, high pressure
sits over the Canadian side of the Arctic and low pressure sits over the
opposite, Siberian, side. This setup has some similarity to a negative
Arctic Oscillation phase in that the strong west-east zonal flow is not
supported but, more importantly, two things are facilitated: cold air is
churned out on the North Atlantic side of the system and may flood
southwards for great distances, but conversely warm air is pulled into
the Arctic on the Pacific side. The Dipole pattern is thus a major
heat exchanger between the Arctic and the mid-latitudes.
The Arctic and North Atlantic Oscillations tend to behave in step with one another, as the following superimposed plots show:
In the plots, the thin lines are the NAO (with a black trend line denoting the moving average) and the bars the AO.
It is apparent that there are periods dominated by either positive or
negative values in both indices: the 1990s were strongly positive,
whereas the late 2000s, which have featured several very cold winters,
have seen many and often strongly negative excursions.
Climate change and the future: how will the jet steam and pressure-patterns respond?
Wave theory tells us that the
west-east progression of the Rossby waves is influenced by their size:
larger waves move more slowly. Negative NAO/AO
setups promote such meridionality and, according to recent research,
that meridionality seems to be on the increase. A possible cause of this
effect is the warming of the Arctic, which has become so profound (twice
that of the rest of the world) that it has been given a term: Arctic
Amplification. Arctic Amplification manifests itself not only in the
temperature record but also in physical features like the strong and in
2012 record-shattering seasonal melting of Arctic sea ice, a process which itself leads to more accumulation of heat energy as the ice-free sea water absorbs incoming solar radiation that would have otherwise been mostly reflected back out into space.
Further heat, independent of sea ice or snow cover, is transported into the Arctic by the increased global water-vapour content of the atmosphere, a factor that has three effects. Firstly, water vapour is of course a potent greenhouse gas:
secondly, as moist air cools as it comes into the Arctic the water
vapour condenses, releasing latent heat; and thirdly, condensation forms
clouds, increasingly regarded as heat-trapping agents. Such warming is
particularly important in the sunless winter months and at higher
atmospheric levels: at 500hPa and above it is the major component of
Arctic Amplification, compared to the loss of albedo due to melting sea ice
and snow close to the surface. Arctic Amplification is a relatively new
phenomenon which has emerged as a signal in recent years: how it will
interact with variations in existing circulation patterns like the NAO/AO, ENSO (the El Nino-La Nina oscillation) and the PDO
(Pacific Decadal Oscillation) remains to be fully understood. However,
in a system full of variables, it generally holds that if major
variables undergo major changes there will be knock-on effects elsewhere in the
system.
Above: a very simplified diagram of
how things were prior to Arctic Amplification, with a steep temperature
gradient between the warm Equator and the cold Arctic. below: the
situation now -- while the low and mid-latitudes have warmed a bit, the
Arctic has warmed a lot. As a consequence, the temperature gradient
between the two has a gentler slope. Graphic: author
As the simple diagram above shows,
one consequence of Arctic Amplification is to reduce the
temperature gradient between the Arctic and the warmer latitudes. Given
that the strength of the jet stream is influenced by the magnitude of
the temperature gradient, it follows that warming of the Arctic could lead
to a weakening of the jet stream and a greater tendency to meander as
it slows down [Readers, note that the jet stream's west-to-east progression slows down, not its wind speeds, which may even increase]. As this meandering develops, troughs may be expected to
extend further southwards and ridges to push further northwards.
However, recent research suggests a greater northwards component to this
behaviour (the ridges are pushing further northwards than the troughs
are nosing southwards), meaning that in overall terms the Polar jet
stream has moved northwards. The wavier state of the jet stream also
causes more mixing of warm and cold air in the Northern Hemisphere. More
importantly, situations where the eastwards progression of these upper
waves becomes sluggish or stalls lead
to prolonged weather conditions of one type or another. Unseasonably
cold, wet, hot or dry conditions that last for weeks at a time can be
just as destructive as storms: their effects on biodiversity and
agriculture can be disastrous, leading variously to reduced crop yields, crop failure, biodiversity loss and wildfires, to name but a few effects.
Recent research into the Polar jet
stream has been focused on the 500-hPa height/windfield, because for a
number of reasons it is easier to work with. This lies below the height
of the strongest jet stream winds, but a look at the charts below,
300-hPa windfields above and 500-hPa windfields beneath, shows that the
tightest gradients and strongest winds are colocated.
Above: 300-hPa windfields for April
14th, 2013, 0600z. Below: plot for the same date and time at the 500-hPa
level. The tightest gradients and strongest winds occur in the same
places, meaning the 500-hPa pattern can be used to make deductions about
the 300-hPa pattern. Model output plot - Wetterzentrale
The research has indeed found a correlation between 500-hPa-height autumnal wind speeds and Arctic sea ice annual minima -- both have gone down, as the following graph shows:
Above: how the drop in
high-altitude winds in autumn over the past 30 years (solid line) has
closely tracked the decline in Arctic sea ice (dashed line).
Graphic: Jennifer Francis, based on data from the National Center for
Environmental Prediction, National Center for Atmospheric Research, and
National Snow and Ice Data Center.
That's for autumn, and in recent
years blocked patterns have often persisted into the winter, but what
about the rest of the year? The tendency for the jet stream to slow down
and meander more seems to have become a summer feature, too, well
before the annual sea ice minimum. However, there is another important regional
and seasonal variable: lying snow, both in the Arctic and sub-Arctic.
This snow is melting progressively earlier over time: the sooner it
melts, the sooner the soil beneath is warmed by the spring sunshine.
There has been approximately 2 C of late spring-early summer warming over
high-latitude land areas since the mid-1980s, heat which is
contributing to the Arctic Amplification effect during the summer
months. Again, the probability is that Arctic Amplification can slow the
jet stream and amplify its waves into slow-moving blocking patterns,
bringing prolonged weather of one kind or another to various parts of
the Northern Hemisphere.
In researching this post I had a
useful discussion with Dr Jennifer Francis of the Institute of Marine
and Coastal Sciences at Rutgers University, New Brunswick. Jennifer has
published extensively on Arctic climate change
and in recent years has been studying changes to the jet stream. I
finished my Q&A session with a look at the future. What, I wanted to
know, was the outlook? Would any pattern of change to the jet stream be
linear in fashion? Jennifer replied:
"Hard to say if it's linear or
otherwise -- not enough years of data yet, and it's not clear if models
are able to capture the behavior realistically. Some recent papers
suggest they don't simulate blocking patterns well, for example, which
are key for extreme weather. We have looked at a 4xCO2 run of the NCAR GCM, however, which suggests that (like the real atmosphere) the 500-hPa
zonal winds will weaken substantially in all seasons (not just fall,
which is the strongest signal in the real world), and also that the flow
will become more meridional, that is, the ratio of north-south winds
relative to the total flow will increase. I think the tendencies we're
seeing in the real world will continue to increase. As we lose all the
summer ice, the response in the fall may plateau somewhat (although
Arctic Amplification will continue via the other factors), but as ice in
the other seasons declines, we should see the response become stronger
all year long."
That modelling jet stream behaviour is difficult should come as no surprise: we are entering Terra Incognita here, with Arctic sea ice melting far more rapidly than most previous predictions have suggested. It makes sense to suggest that -- if sea ice melt is a prime driver here -- that once all the variability in the system is 'used up' (i.e., when we see a seasonally sea-ice-free Arctic), then we should see a plateau effect in autumn/fall, but
this is but one part of Arctic Amplification and the way the other
variables such as poleward water vapour transport behave is just as
important.
Conclusion
The Arctic has warmed about twice as much as the rest of the world, and the responses to the warming by some variables such as sea ice have greatly exceeded expectations. Evidence is mounting to indicate that the response of the jet stream to this new thermal regime
has been to tend to slow down [its west-to-east progression is slowing] and meander more, with a greater tendency
to develop blocking patterns. In the UK, the run of wet, dull summers
and the run of prolonged cold outbreaks in recent winters show what can
occur when the jet steam behaves in a meridional and sluggish fashion.
At the moment it's more active: on the morning that this was written,
April 14th, 2013, a 130-knot jet streak was racing NE over the northwestern UK
on the eastern limb of a deep upper trough: it was mild and wet with a
southwesterly gale blowing but with alternating bouts of sunny and
cloudy, wet weather forecast for the week ahead. Changeable weather is
the norm for NW Europe: prolonged periods of any weather type are
historically atypical and may be noteworthy when they occur. Clearly,
we need to get a good handle on what is going on here and how future
responses may play out in our weather patterns: already it seems to be
the case that we are going to have to develop greater adaptability to a
greater range of prolonged weather extremes. How that plays out in terms
of agriculture and economics remains to be seen, but there should be no
room for complacency.
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