Rosby waves and the jet stream?

Hi Jeff,

A little about the Polar Jet Stream (PJS) and Atmospheric Rossby Waves (ARW’s) first then I’ll come to your specific questions. The full process is complex and a lot of detail is missing from my answer, hope it suffices…

ARW’s occur when a westward flowing air-mass encounters a barrier such as a mountain range. The air is forced upward and over the mountains, the amount of vertical compression increases with altitude and the movement of air accelerates as it passes through the vertical constriction. Once clear of the summits the air parcel can vertically expand again and it decelerates.

At the same time, the air-mass will also try and find it’s way around the mountains. With respect to the PJS, the rotation of the Earth deflects the air-mass northward (south if in the southern hemisphere, a consequence of the Coriolis Force.)

Once around the obstacle, the air that was pushed to the north tracks south and converges with the air that is flowing down the leeward slopes.

This process generates the ARW’s, the dynamics of which will be determined by velocity, mass, volume, pressure, humidity etc. In general, the higher these metrics the larger the waves will be. Wavelengths will typically be in the order of a few hundred kilometres but can be as much as 1,000km.

If an ARW has a high zonal index, that is to say, it occupies a narrow latitude band and follows a relatively straight track, then it can encounter the opposing easterly flow of the PJS and more or less pass through it. If an ARW has a low zonal index it meanders within a large latitude and altitude band and can have a larger cross-sectional area than the PJS. In extreme cases a meander forms a complete loop and can detach from the rest of the current, something known as a drop.

Alternatively, the two currents could pass close by each other – one moving east and the other moving west, this generates turbulent air vortices between the currents. Similarly, they could strike each other at a glancing angle causing a deflection in the track of both currents (the level of deflection being determined by the characteristics of each current).

In terms of weather variation, the greatest influence occurs when a PJS meets an opposing ARW head-on, this can cause both systems to stall leading to a blocking event.

Perhaps the most significant such event occurred in 2010 when the PJS ground to a halt after colliding with an ARW. In the lead up to this collision an almost complete loop within the PJS had formed trapping within it a mass of unusually warm air.

When the PJS stalled, this hot air sat above Russia and led to a devastating heatwave. Without the ARW, Russia would have had a couple of hot days whilst the warm weather passed by, instead the stalled air-mass lingered for weeks, growing in intensity with each passing day.

It was this same blocking event that prevented the migration of the PJS across the Himalayan Plateau, this caused the seasonal Monsoon cell to stall over Pakistan and instead of delivering the Monsoon rains over a wide area, most of the rain fell in the same place leading to extensive flooding for 40 million people.

The mechanics of the Polar Vortex are slightly different in that it requires the addition of an errant mass of cold air. This can be provided when the typically stable Arctic weather systems are thrown into reverse.

The established pattern over the Arctic is for cyclonic conditions to exist over Siberia and anticyclonic conditions over Canada, this sets in motion an anticlockwise movement of air. In recent years this pattern has become increasingly disrupted in what is known as an Arctic Dipole Anomaly (ADA). When an ADA is established there’s a reversal of pressure systems, places that are normally milder receive the cold air whilst the normally cold places can warm significantly, the anomaly can be as much as 30°C.

If a stray mass of unusually cold air gets swept up by the PJS then it will transport the cold air along its track. Or, as happened in the Polar vortex encountered last winter in the US, the cold air becomes trapped within a loop of the PJS. If the PJS is then stalled by an ARW you get the extreme cold persisting for several days or weeks.

CAN ANYONE EXPLAIN WHAT THE INTERACTION BETWEEN ROSBY WAVES AND THE JET STREAM CAUSES?

The Polar Jet Stream acts like a weather delivery system, it pulls in air-masses and the weather associated with them from the east. In areas of high continentality this means dry and warm air, where continentality is low the result is cooler, moister air.

The PJS, like all jet streams, acts as a barrier preventing the north-south and south-north passage of air, generally this means cold and wet weather trapped to the north and warmer, drier and calmer weather to the south. All the while, there is the west to east movement of weather systems.

When a westerly flowing ARW opposes the PJS there can be stalling or deflection, as mentioned above. There can also be a slowing, divergence, partial splitting or looping. This disruption drags weather systems into areas that would otherwise have been unaffected whilst at the same time allowing for greater north↔south transit.

The PJS meanders north and south in accordance with the seasons, when it becomes disrupted and takes a more northerly or southerly course, the result is unseasonal weather. For example, the PJS failed to migrate northwards during the summer of 2012 across western Europe. Instead of delivering wet weather to the north of Europe the result was months of unusual rainfall leading to record breaking flooding. This was a rapid transition, the last week of March saw a heatwave but the first week of April saw the start of six months of rain and a dramatic end to one of the longest droughts on record.

Because the track of the PJS is mainly confined to the mid northerly latitudes it’s here that the greatest weather disruption occurs. This disruption can take on pretty much any form depending just where the PJS has migrated to and how fast it is moving.

WHY DOES THIS OCCUR?

As the world warms up and weather patterns generally become disrupted, there is a redistribution of cyclonic and anti-cyclonic systems. This leads to changes in pressure gradients which in turn affect the movement and strength of wind patterns. With a low P Grad the winds are weaker but with a higher P Grad the conditions are set for the formation of stronger ARW’s.

WHAT IMPACT HAS A WARMING ENVIRONMENT HAD THUS FAR

ARW’s and ADA’s are two of the factors that influence the PJS. Other factors also affect the jet streams including sea surface temperatures and enhanced temperature gradients (particularly between the Arctic and the mid northerly latitudes where the T Grad has increased significantly).

The PJS is the most significantly affected of the five jet streams due to it’s latitude and proximity to the Arctic. In recent years it has become increasingly erratic and this manifests itself as periods of unusual weather. The stalling or blocking events also mean that when a weather system becomes established it’s likely to remain in-situ for a longer period of time

The ADA is an entirely new phenomenon, it had long been predicted as a consequence of global warming and first manifested itself in 2000. Since then there have been about 12 ADA’s of varying intensity and they’re becoming more common. These events have been the primary contributors to the severe winter conditions that have affected parts of North America and Eurasia in recent years.

Disruption of the PJS, caused more by ARW’s than ADA’s, has prevented the dissipation and transit of warm air masses and this has enhanced heatwave conditions. The deadliest heatwaves on record have occurred comparatively recently with the two deadliest both occurring in Europe in 2003 and 2010 (approx 70,000 and 50,000 deaths respectively).

The generally warmer atmosphere will add to the likelihood of heatwaves and heatwave conditions occurring, but it’s the blocking of the PJS and stalling of hot air-masses in one place that really causes the problems.

When hot air becomes stalled it warms the ground, trees, lakes, roads etc. These normally act as heat sinks and remove excess heat from the air above them. If hot air passes overhead then enough heat is removed to moderate the temperatures. When the air-mass stalls the ground etc gets warmer and warmer and the amount of heat absorbed drops significantly. Whilst global warming may cause a 1°C rise in temps, the stalling of the PJS can locally cause in excess of 10°C of warming.

AND WHAT CAN WE EXPECT IN THE FUTURE?

More of the same unfortunately. As temps continue to rise there will be greater T Grads and P Grads, these will enhance the conditions needed for the formation of ARW’s and ADA’s, which in turn will continue to impact on the movement of the PJS. We should be prepared for more periods of unusual weather and be aware that when they do occur they’re likely to linger for some time.

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EDIT: TO MIKE

Both Wallen’s comments and mine are accurate, they’re two separate pieces of a much larger and more complex picture. If I were to go into more detail then I’d introduce the role of temperate zone circulatory systems and other factors, but this is beyond the scope of my answer (I did state at the start that a lot of detail was missing, for just this reason).

There’s still a lot we don’t know about the jet streams and how they’re affected by AGW. The formation or Rossby Waves is just one of many factors that influence them.

EDIT: TO PEGMINER

Thanks for the correction; I’d confused the low and high zonal indices (now corrected).

I cant see how it is temperature, as global temperatures have been fairly stable.

I think it is effected by solar changes as we know (or rather NASA knows) that the height of the mesosphere changes with solar cycles (in 2009 in shrunk dramatically) whether it is solar wind or AP magnectic index I couldn't say.

What effect well more unpredictable weather, some very cold winters and some very mild winters, floods in some places droughts in others depending on where the blocking occurs, we might know about hadley cells ferrel cells polar cells, jet streams and rosby waves, but I am not sure we know what could influence them, hell it could even be the moon you know it has an 18.6 yr cycle

Sorry Jeff I just watched video, I have some issue with it, the arctic warming causing weakness of the jet stream, it could be the other way around as the stream meanders farther south it is allowing the cold air from the arctic to escape, it is also noticeable that the Arctic is warmer in the winter but has been colder in the summer.

The Rossby Wave pattern sets up the general environment in which storm systems move. If they are not very loopy, so that the motion through the pattern is mostly west to east, the, the pattern is called "zonal". That's because things tend to primarily stay in the same zone of latitude. I actually think this is called a "high zonal index" pattern--Trevor might have mixed this up. The standard zonal index is calculated by the pressure difference between 35N and 55N, the larger the pressure difference, the faster the mean westerly winds and the larger the zonal index. When the zonal index is large, storm systems will move quickly from west to east, so nothing sticks around too long.

Contrast this with the situation when the zonal index is smaller. Then the Rossby wave pattern is more like a slow moving river with large meanders. In this type of situation, you can have a pattern of large troughs (bends concave away from the equator) and ridges (bends concave away from the pole). From the mid-line of the troughs eastward to the mid-line of ridges you generally expect stormy weather, and the complementary area will have fair weather. The actual storms will be associated with patterns modulated on top of this ("short waves").

Generally, waves in the atmospheric circulation have a propagation speed that is dependent on their wavelength, with shorter waves moving faster. Rossby waves have very long wavelengths, and their speed is the difference between the zonal mean speed and their intrinsic NEGATIVE speed (without the zonal wind they would propagate east to west). For particular wavelengths and mean zonal wind speeds, they can become quasi-stationary when they're in the low zonal index (meridional) pattern, which is why you can get persistent patterns that may last for several weeks. They will finally move when a "kicker" comes along (a short wave that shortens the wavelength of the entire pattern and changes the wave speed to a higher value).

Warming in the arctic relative to the mid-latitudes will decrease the meridional pressure gradient, lower the zonal index and make the pattern subject to more meanders, which are the things that can bring very cold air equatorward.

You can find some information on Rossby wave speed and zonal index in the Glossary of Meteorology:

http://glossary.ametsoc.org/wiki/Rossby_wave

http://glossary.ametsoc.org/wiki/Zonal_index

EDIT for Mike: I think what he is saying is probably correct, but taken out of context in a popular magazine article it is a bit confusing. The video and related paper are based on reanalysis, by the way, I tool that was not available until the 1990s.

An asteroid, a planetoid, an optical illusion, The Sentinile that puts you through a worm hole to a distant land, which means Arthur Clarke and Stanley Kubrick got it wrong! AND, Clarke wanted it to be Saturn! Isn't that a hoot! He might have got it right, except for the shape!

@Trevor, Jeff

What you are saying contradicts previous climate research. For example, in a Science News article from 1975 (back when they were discussing global cooling), C.C. Wallen at the WMO states:

"During warm periods a "zonal circulation" predominates, in which the prevailing westerly winds of the temperate zones are swept over long distances by a few powerful high and low pressure centers. The result is a more evenly distributed pattern of weather, varying relatively little from month to month or season to season.

"During cooler climatic periods, how-ever, the high-altitude winds are broken up into irregular cells by weaker and more plentiful pressure centers, causing formation of a "meridional circulation" pattern. These small, weak cells may stagnate over vast areas for many months, bringing unseasonably cold weather on one side and unseasonably warm weather on the other." https://www.sciencenews.org/sites/default/files/89...

Jeff, The Encyclopedia of World Climatology has a pretty detailed section on the jet stream here: http://books.google.ca/books?id=-mwbAsxpRr0C&pg=PA...

Wouldn't that book make a lovely gift for climate change enthusiasts?

Edit: Wow, that book is pricey!! http://www.amazon.ca/Encyclopedia-World-Climatolog...

Why not. Global warming causes cooling. It already cause more hurricanes, less hurricanes, tsunamis, earthquakes, cold winters, warm winters, flooding, and droughts in the same location, male pattern baldness, the the end of red-heads, and every other stupid prediction imaginable.

Just predict everything and claim victory when you see anything. And if we were not seeing the polar vortex and saw less cold coming from the North Pole, they would have simply pulled OM's article and claimed victory once again.

Palm reading has more credibility.

Nice answer zippi. What we don't know far out weighs what we know in this regard. As pointed out by Ottawa, the same arguments were provided 40 years ago for exactly the opposite effect. It is an explanation that evolved out of convenience and need to explain failed predictions and models. I know it feels better to have those explanations but instead, a little skepticism might be more healthy.

You would read it in a way that concurs with your religious "Climate Science".

When you can explain to people how much air space difference (in volume) there is between the "surface (ground) to 1 mile" vs. "14 mile to 15 mile", then maybe you will get a clue as to "how and why" as it pertains to the "importance" of the "Jet-Stream" and also how this effects temperatures.

(4piR2 might help, but I severely doubt it.)

Is the Earth shaped like a potato?

510,000,000 km2 is the approximate surface area (includes sea surface area). What is it if the surface was actually 14 miles higher?

Does the "Jet Stream" dip in altitude also along with varying in latitudes and longitudes? What "effect" does that have on surface temperatures?

Is this "undulating signal" part of Earth "correcting itself"?

Who is in control here? Climate science? ... or Earth itself?

Love your arrogant inferences!

I remember seeing a picture of a man with his own head stuck up in his own buttocks. Now I'm thinking it was a picture depicting a future presentation of how arrogant "Climate Science" really is.

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Jeff M - In direct answer to your comment - Do you have an answer as to why the "anomalies" undulate so much? ... and do you have an answer as to why there is such a "variance" in global average temperatures from month to month when averaging raw data?

I do know that "natural climate variability" still has a strong role on how the climate reacts to anything. I can safely say that 97% of climate scientists agree with that!!!

You're a "climate clown" IMHO!!! You wouldn't be asking, if you knew anything about what is making the Jet Stream undulate as it is currently being shown to you. The undulations don't show the 3D view.

How many dimensions of temperature are there in the atmosphere again? "Infinity" maybe?

I like how Trevor backs up my answer with this statement at the end of his post : " ... There’s still a lot we don’t know about the jet streams and how they’re affected by AGW. The formation or Rossby Waves is just one of many factors that influence them. ... "

The temperature variables in the atmosphere are far too numerous to come to any conclusions, but climate clowns still try and explain things as if they know.

Thanks Jeff for the question and the link. Thank you Trevor for informative and understandable answer.