From NOAA (Tim Woollings):
This is a guest blog by Tim Woollings , a professor of climate science in the Department of Physics at the University of Oxford in the UK. Tim recently won the Louis J. Battan Award from the American Meteorological Society for his book “ Jet Stream : A Journey Through our Changing Climate.”
The term jet stream is used increasingly in both weather forecasts and news reports of extreme events, from cold spells and flooding to heatwaves and droughts. But what is the jet stream, and why do we care about it so much?
The jet stream is a fast, narrow current of air flowing from west to east that encircles the globe (not to be confused with the Gulf Stream which is instead an ocean current of drifting seawater). It was first documented by Wasaburo Oishi, whose regular weather balloon launches from Japan in the 1920s invariably ended up flying out over the Pacific Ocean at a considerable rate (footnote #1).
As revealed in Oishi’s measurements, the jet is strongest in the upper troposphere, around 6-8 miles up. This is roughly the level at which airplanes fly, and indeed they often pay careful attention to the jet stream: fuel is saved by staying in the jet if you’re heading east, and by avoiding it if you’re going west. While we describe the jet as narrow, this is somewhat subjective—it’s typically a couple hundred miles across, which gives a wide enough corridor for flight planning.
If the jet stream is so high up, why do we care about it?
It turns out that the jet, on a number of counts, directly impacts surface weather patterns.
Firstly , the jet acts to steer mid-latitude weather systems, so it can control which regions are storm-bound and, in the extreme, which can become dangerously dry.
Secondly , while the fastest winds of the jet core are far above the surface, weaker winds often extend all the way down to the surface. Many of us who live in the midlatitudes are acclimatized to prevailing westerly winds (i.e. winds that blow from west to east), and these winds are the very underbelly of the jet stream dragging along Earth’s surface.
Thirdly , the jet stream also acts as a sharp boundary between contrasting air masses, with relatively warm, tropical air to the south and much colder air on the northern, poleward side. The location of the jet, and hence also the boundary, has a strong influence on temperatures down at ground level.
Why do jet streams exist?
Because of two crucial ingredients: rotation and heating.
The planet is unevenly heated due to the disproportionate concentration of solar heating in the tropics and lack of heating in the poles. This forces the atmosphere into motion, as some bits of air become warmer and more buoyant than others. But the planet also rotates, and this imposes constraints on the resulting fluid flow, particularly in the north-south direction (footnote #2). These two ingredients mean that the resulting flow stretches out in the west-east direction and jet streams are formed.
Striking lab experiments to show that these two ingredients result in jet streams were first performed by Dave Fultz , who had witnessed the power of the jet stream first-hand as a meteorologist at the US air base on Guam during World War II. Returning to Chicago after the war, he set out to reproduce such a flow by building a spherical water tank with a heating element underneath. Then, to simulate the Earth’s rotation, he spun the tank and saw the flow break down into chaotic waves and, perhaps more excitingly, several west-to-east jet streams.
Not just one jet stream
So far we have somewhat loosely referred to the jet stream, while in fact there are several in both Northern and Southern hemispheres. One type, termed a subtropical jet, is predominantly a high-altitude feature invariably found at the edge of the tropical Hadley Cells . In contrast, eddy-driven jets (footnote #3) are deeper, and it is these specifically, which reach all the way down to the surface. They are often called sub-polar jets, even though they can be found over a wide range of latitudes. The two jets are sometimes separated in latitude (often this is the case over the Atlantic Ocean), but sometimes merged together (as is more common over the Pacific Ocean) forming a broader region of midlatitude west-to-east winds.
In much the same way that the ocean surface is never completely flat everywhere, the jet streams are generally always in motion, sometimes shifting north or south across huge regions, sometimes meandering like a lazy river. Hence their importance for forecasting; the jets can change from one week or month to the next and regional patterns of heat and cold, wet and dry will shift along with them (footnote #4).
For much of the midlatitudes, the local jet stream is typically the single most important factor in regional weather variations on these timescales, so that if you only had one piece of information about the atmosphere, you would want to know about the jet. Commonly used circulation indices such as the North Atlantic Oscillation (NAO) or Arctic Oscillation (AO) are dominated by jet variations as a result. A positive NAO index, for example, is a sign that the eddy-driven jet over the Atlantic is stronger than usual and shifted to the north.
Given this is the ENSO Blog…
Climate forecasters are largely concerned with trying to predict how the jets may behave over the coming months or seasons. While still very challenging, this task is made possible because the jets don’t just vary chaotically as the butterflies flap (although they certainly do lots of that). Particularly on seasonal timescales the jets can be nudged, or influenced, by other factors in the climate system, and ENSO is the prime example of this.
An El Niño event can affect the jets in a couple of ways: firstly, by warming the tropics and amplifying that crucial temperature contrast between latitudes, and secondly by triggering regional patterns associated with Rossby waves . The atmospheric circulation over the North Pacific this winter provides a good example of this latter effect.
In an average year, the jet extends across the Pacific towards North America, although it’s strongest over Japan. The jet first became widely known when American forces were caught off-guard during World War II, with their high-altitude bombers often stranded over Japan by raging headwinds. Meanwhile, the Japanese Fu-Go campaign used the jet to carry balloon-bombs downwind across the Pacific to America, albeit with little success because most of the bombs dropped too early, landing in the ocean.
But December 2021 was no average year; instead the classic La Niña signal, often referred to as the negative phase of the Pacific-North America pattern , was evident. Instead of the jet stream flowing due east from Japan and over the North Pacific Ocean, it snaked northward to Alaska before returning southward over the contiguous United States. This is indicative of a wave of influence originating in the tropical Pacific and curving across the North Pacific Ocean to North America.
So the art of subseasonal-to-seasonal forecasting for the midlatitudes is largely concerned with predicting the state of the jet streams. Cases such as this with a regional Pacific response to ENSO are often well predicted, and December 2021 was no exception. Other features such as the Atlantic jet are harder to predict, yet modern models often have useful skill here too (footnote #5). And hardest of all to predict is the longer-term trend: as the climate warms further, how will the all-important jet streams be affected? Clever ideas on this abound in the literature, but overall our confidence in what will happen unfortunately remains low.
Lead Editor: Michelle L’Heureux (NOAA CPC)
(1) In an attempt to reach the widest possible audience, Oishi published his results in Esperanto—an experimental “international” language invented in the late 1800s—and as a result the jet stream remained little known outside of Japan.
(2) These constraints are essentially due to conservation of angular momentum, as an air parcel moving north or south is changing its distance from Earth’s axis.
(3) ‘Eddy’ is used to describe a swirl in the atmosphere, just like a swirl of water in a river. Weather systems such as cyclones and anticyclones are examples of atmospheric eddies, so a hidden complexity here is that these weather systems actually help to ‘drive’ the eddy-driven jets.
(4) This reveals a complex two-way coupling between the jets and the temperatures. While the jets are formed in response to the same heating which gives us warm tropical and cold polar regions, the dynamics of the jet cause it to move around and the sharp boundary between the air masses is constrained to follow suit.
(5) It has been shown the jet over the Atlantic can be partially predicted for the coming season (i.e. a forecast made in November for December-January), and even beyond. However, it has less accuracy than ENSO, which can be used many months in advance (e.g. when predicting the Pacific jet).