Block That Jet Stream

This dynamic and wildly variable part of the weather/climate system helps us visualize and understand extreme and persistent weather events

Aug 08, 2022

Extreme heat waves in Europe, devastating floods in Australia, heat, drought, and fires in the Western U.S. - the stories are unrelenting and can seem like a concatenation of random events.

But they aren't. These tragic stories fit into a pattern of a changing climate system in ways that can be understood, and they occur within the constraints of the long-term changes in climate driven primarily by increasing greenhouse gas concentrations in the atmosphere.

We currently benefit from the inertia in the climate system provided by the ability of the oceans to absorb both gases and heat, and by the major ice sheets in Greenland and Antarctica to resist rapid melting. The flip side of this inertia is that there is now momentum in these changes in oceans and ice that will be very hard to deflect, but at least we can now see where we are headed and understand why.

By contrast, the dynamics of weather within these long-term, large-scale controls are chaotic and impossible to predict more than a few days out. And embedded in this system are seasonal to yearly oscillations like El Niño that can make long-term trends harder to detect.

I find that some of the most intriguing weather stories occur where these long-term constraints impact short-term changes in weather. We'll look at two examples below that relate to one of the most dynamic parts of the weather/climate system for the northern hemisphere: the polar jet stream.

Let's start with the textbook climatology of the jet streams.

We did not really know about the jet streams before the age of high elevation aviation. Benjamin Franklin could make a rough chart of the Gulf Stream flowing up the east coast of North American in the 1770s because, as sailing ships had been crossing the Atlantic for a couple of centuries at that point, many, many observations of ocean temperatures and currents had been made. In contrast, it was not until the 1930s, and really until WWII, that high-elevation aviation revealed the nature and extent of the jet streams.

And that is because jets are fast moving "rivers" of air at high elevation. The figure above presents the standard climatology of global atmospheric circulation with three adjacent "cells" (Hadley, Ferrel and Polar) that together redress the uneven loading of solar energy across the globe (highest at the equator) by transferring heat towards the poles. The turbulence created at the interfaces of these cells generates the high-elevation jets which are sent racing west to east by the Earth's rotation (the Coriolis effect). We will concentrate here on the polar jet in the northern hemisphere.

These static images bely the wildly dynamic nature of the polar jet, which can be linear or "wavy" and can change location and shape on an hourly to daily basis - or not. In general, a linear or straight polar jet (as in the image below on the left) indicates less interaction between the warmer Ferrel cell and the colder Polar cell, and so less turbulence, less uplift, less weather.

Steep dips and curves in the jet as in the image on the right (called Rossby Waves) signal stronger interactions between the air masses and a greater chance for storms. Rossby Waves tend to move west to east, driving storm systems in that direction as well, although individual low pressure systems (storms) tend to travel along an existing front between cells, often tracing the path of the polar jet.

If you like to look at the incredible graphics on either commercial or government weather sites (as I do daily) you will see that radar images of current rainfall across North America often coincide with the current location and strength of the polar jet. It's not that the jet stream has "caused" the storm, but the jet reflects the dynamics of the warring air masses that drive storms.

This relatively simple picture can be complicated when major low pressure storms, or major high pressure calms, get "cut-off" from the usual west to east flow of the jet stream. Without the guiding energy of the jet, either can remain in place for days to weeks, leading to persistent weather where they occur. Cutoff lows can cause days of stormy weather leading perhaps to flooding or major snow accumulation. Cutoff highs can mean stable and calm weather, but can also mean extreme heat or persistent drought.

As these cutoff systems are not driven by the usual flow reflected in Rossby Waves and the polar jet, it is very hard to know how long they will persist, making prediction of the end of these long-lasting weather events difficult. Another term related to these persistent systems is "blocking" - a tendency for relatively immobile systems to hinder or stop the normal west to east movement of Rossby Waves and associated weather.

A first example of climate change and the jet stream is from a recent study that links these to the persistent summer heat waves that have been increasingly common in Europe. The authors of that study conclude that an increase in warming of arctic land masses due to general arctic warming and earlier loss of snow cover creates energy gradients that tend to cause a cutoff high pressure system to persist over Europe, leading to what they term a "double jet." Instead of a single flow across central Europe in summer, the jet tends to split into two streams navigating around a persistent high pressure area between them.

In the absence of recurring changes in weather that an active jet stream would create, hot and stagnant air tends to build up under this dome of high pressure. An absence of storms can also lead to drought, which can further accentuate the heat by cutting off evaporation and also transpiration (the evaporation of water from the leaves of plants) that tend to cool the air. All of this leads to a season-long increase in average temperatures and more persistent extremes. So changes in climate (warming arctic, earlier snow melt) appear to lead to the double jet condition and the weather phenomenon of recurring heat waves.

Let's look at an even longer pattern of change in the jet stream, and another case of extreme weather.

The winters of 2013-2015 were record-breakers in the northeastern U.S. and eastern Canada. February 2015 averaged 5 degrees Celsius (9 degrees Fahrenheit) or more below average in this region, while the rest of the world was, as usual, well above historical averages. I was in Montreal on January 7, 2014 when the temperature was colder than either the north or south pole, and was compared to summer temperatures on Mars!

You may remember hearing in those years about the polar vortex. A quick search for polar vortex cartoons will turn up some humorous versions of the story, a sure indicator that it was a news-worthy event.

So what happened? Once again, it's far northern warming and a major shift in the polar jet.

The temperature gradient between the Polar and Ferrel cells in the northern hemisphere is greater in winter, reflecting the greater difference in solar energy received due to the tilt of the north pole away from the sun. This greater temperature gradient energizes the polar jet which will run faster than in the summer (sometimes up to 200 miles per hour). That super-fast circular flow traps the super-cold air over the pole, keeping it from leaking to the south.

This tight circulation pattern is the real polar vortex and it is a good thing if you want to avoid the kind of extreme cold that hit the area in dark blue in the figure.

Those frigid winters actually resulted from a breakdown in the polar vortex.

Sudden Stratospheric Warming describes the occasional, very rapid increases in temperature in the stratosphere, or upper atmosphere, over the far north. Increases of as much as 25 degrees Celsius (45 degrees Fahrenheit) can occur over just a few days. This warming reduces the temperature gradient south to north, weakening the polar jet, and leading to the escape of frigid arctic air to the south.

In 2013-2015, the breakdown in the polar vortex was paired with what was called the Greenland Block - essentially a blocking pattern that trapped the circulation of cold arctic air in place over the northeast.

Record cold driven by global warming can be a tough sell, and it only makes sense in terms of the global average temperatures in the figure above. This was a local effect; it just happened to occur in an area with lots of people and lots of news outlets! The rest of the Earth remained warmer than average.

Do we understand the causes of these patterns well enough to predict them, and how will they be affected by climate change?

There is a U.S. Weather Service site on blocking that gives predictions for several days into the future, but unlike most of their pages, there is no range of model predictions included, and no indication of how accurate previous forecasts were. They do discuss interactions with El Niño and plot the frequency of blocking patterns by longitude. The authors of the study mentioned above on heat waves in Europe note that blocking patterns are very hard to predict.

The Weather Service does have a page with predictions for the next 14 days for the Arctic Oscillation, which it also identifies as the polar vortex. The index for this oscillation indicates how tightly the polar jet or polar vortex spins, and so how likely it is that arctic air will escape to the south.

As with many other important weather indicators, the Weather Service presents both the range of possible values for the Arctic oscillation in the future as predicted by different models, and graphics that summarize the accuracy of previous predictions for conditions 7, 10 and 14 days into the future. And as with forecasts for temperature, accuracy drops off quickly over this 14 day period, so long-term prediction is not likely. (I have said this several times before: kudos to the U.S. Weather Service for their clear presentations of the accuracy of previous predictions!)

I can't find any official sites that predict future patterns for the jet stream. Weather maps predicting temperature and precipitation should relate to the position of the jet stream, but I find no official pages that explicitly predict this river of air. Interestingly, there are at least two non-government sites that provide detailed, mapped predictions for the next 6 to 14 days, and one cites a U.S. Weather Service model as its source.

So we apparently can't predict the occurrence of blocking events very well, and official predictions of changes in the jet stream are hard to find. General warming in the Arctic appears to drive shifts in the jet stream that lead to both persistent summer heat in Europe and the occasional breakdown of the polar vortex in winter. We might expect both to continue to defy long-term prediction, but perhaps both will become more common or more extreme as the Earth warms and as warming occurs more rapidly in the Arctic than elsewhere.

A basic familiarity with atmospheric cells, jet streams, blocking patterns and indexes like the Arctic Oscillation can help explain what is happening now, even if predictions are not all that accurate very far into the future. I take some comfort in knowing that there is knowledge out there about this complex energy machine we call weather, and that the stories we hear about extreme events (always couched as "the worst ever…") are not just random but result from observable and understandable patterns. Our ability to watch the dynamics of these weather systems through the graphics on commercial and government websites is truly astounding.

But finally, it is important to note again that all of this chaotic weather is firmly constrained within the long-term, measurable, irresistible momentum of climate change we have set in motion through our emissions of greenhouse gases and other changes to the Earth system. We know where we are headed, but we can't predict very well what will happen along the way.

Sources

A very compelling figure on the direct impact of cumulative carbon dioxide emissions on past and projected global temperature is here:

https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf

figure SPM.10 - p.28

A news story on the European heat waves is here:

https://www.nytimes.com/2022/07/19/climate/europe-heat-wave-science.html

Information on jet streams is here:

https://scijinks.gov/jet-stream/

https://www.climate.gov/news-features/blogs/enso/what-jet-stream

The two figures are derived from:

https://en.wikipedia.org/wiki/Jet_stream

and attributed originally to NOAA.