Posted by Greg Laden on June 4, 2013
Global Warming is the increase in the Earth’s temperature owing to the greenhouse effects of the release of CO2 and other gasses into the atmosphere, mainly by humans burning fossil fuel, but also by the release of Methane from oil wells and melting of Arctic permafrost, natural gas from leaky pipes, and so on. This increase in temperature occurs in both the atmosphere and the oceans, as well as the land surface itself. During some periods of time most of the increase seems to happen in the atmosphere, while during other times it seems to occur more in the oceans. (As an aside: when you use passive geothermal technology to heat and cool your home, the heat in the ground around your house is actually from the sun warming the Earth’s surface.)
“Weather” as we generally think of it consists partly of storms, perturbations in the atmosphere, and we would expect more of at least some kinds of storms, or more severe ones, if the atmosphere has more energy, which it does because of global warming. But “weather” is also temperature, and we recognize that severe heat waves and cold waves, long periods of heavy flooding rains, and droughts are very important, and it is hard to miss the fact that these phenomena have been occurring with increasing frequency in recent years.
We know that global warming changes the way air currents in the atmosphere work, and we know that atmospheric air currents can determine both the distribution and severity of storms and the occurrence of long periods of extreme heat or cold and wet or dry. One of the ways this seems to happen is what is known as “high amplitude waves” in the jet stream. One of the Northern Hemisphere Jet Streams, which emerges as the boundary between temperate air masses and polar air masses, is a fast moving high altitude stream of air. There is a large difference in temperature of the Troposphere north and south of any Jet Stream, and it can be thought of as the boundary between cooler and warmer conditions. Often, the northern Jet Stream encircles the planet as a more or less circular stream of fast moving air, moving in a straight line around the globe. However, under certain conditions the Jet Stream can be wavy, curving north then south then north and so on around the planet. These waves can themselves be either stationary (not moving around the planet) or they can move from west to east. A “high amplitude” Jet Stream is a wavy jet stream, and the waves can be very dramatic. When the jet stream is wavy and the waves themselves are relatively stationary, the curves are said to be “blocking” … meaning that they are keeping masses of either cold (to the north) or warm (to the south) air in place. Also, the turning points of the waves set up large rotating systems of circulation that can control the formation of storms.
So, a major heat wave in a given region can be caused by the northern Jet Stream being both wavy (high amplitude) with a big wave curving north across the region, bringing very warm air with it, at the same time the Jet Stream’s waves are relatively stationary, causing that lobe of southerly warm air to stay in place for many days. Conversely, a lobe of cool air from the north can be spread across a region and kept in place for a while.
Here is a cross section of the Jet Streams in the Northern Hemisphere showing their relationship with major circulating air masses:
Cross section of the atmosphere of the Northern Hemisphere. The Jet Streams form at the highly energetic boundary between major circulating cells, near the top of the Troposphere.
The Jet Streams moving around the planet. Not indicated is the Intertropical Convergence Zone (ITCA) around the equator which is both not a Jet Stream and the Mother of All Jet Streams. This post mainly concerns the “Polar Jet.” Note that the wind in the Jet Streams moves from west to east, and the Jet Streams can be either pretty straight or pretty curvy. Curvy = “high amplitude.” This figure and the one above are from NOAA.
The Jet Stream in a particularly wavy state.
Research published in the Proceedings of the National Academies of Science last February, in a paper titled “Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes,” links global warming to the setup of high amplitude waves in the Jet Stream, as well as relatively stationary, blocking, waves that cause extreme warm or cold conditions to persist for weeks rather than just a few days. According to lead author Vladimir Petoukhov, “An important part of the global air motion in the mid-latitudes of the Earth normally takes the form of waves wandering around the planet, oscillating between the tropical and the Arctic regions. So when they swing up, these waves suck warm air from the tropics to Europe, Russia, or the US, and when they swing down, they do the same thing with cold air from the Arctic…What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays.”
So how does global warming cause the northern Jet Stream to become wavy, with those waves being relatively stationary? It’s complicated. One way to think about it is to observe waves elsewhere in day to day life. On the highway, if there is enough traffic, waves of cars form, as clusters of several cars moving together with relatively few cars to be found in the gaps between these clusters. Change the number of cars, or the speed limit, or other factors, and you may see the size and distribution of these clusters (waves) of cars change as well. If you run the water from your sink faucet at just the right rate, you can see waves moving up and down on the stream of water. If you adjust the flow of water the size and behavior of these “standing waves” changes. In a baseball or football field, when people do “the wave” their hand motions collectively form a wave of silliness that moves around the park, and the width and speed of that wave is a function of how quickly individuals react to their fellow sports fan’s waving activity. Waves form in a medium (of cars, water molecules, people, etc.)
following a number of physical principles that determine the size, shape, speed, and stability of the waves.
The authors of this paper use math that is far beyond the scope of a mere blog post to link together all the relevant atmospheric factors and the shape of the northern Jet Stream. They found that when the effects of Global Warming are added in, the Jet Stream becomes less linear, and the deep meanders (sometimes called Rossby waves) that are set up tend to occur with a certain frequency (6, 7, or 8 major waves encircling the planet) and that these waves tend to not move for many days once they get going. They tested their mathematical model using actual weather data over a period of 32 years and found a good fit between atmospheric conditions, predicted wave patterns, and actual observed wave patterns.
The northern Jet Stream originates as a function of the gradient of heat from the Equatorial regions to the Polar regions. If air temperature was very high at the equator and very low at the poles, the Jet Stream would look one way. If air temperatures were (and this is impossible) the same at the Equator and the poles, there would probably be no Jet Stream at all. At various different plausible gradients of temperature from Equator to the poles, various different possible configurations of Jet Streams emerge.
One of the major effects of global warming has been the warming of the Arctic. This happens for at least two reasons. First, the atmosphere and oceans are simply warmer, so everything gets warmer. In addition, these warmer conditions cause the melting of Arctic ice to be much more extreme each summer, so that there is more exposed water in the Arctic Ocean, for a longer period of time. This means that less sunlight is reflected directly back into space (because there is less shiny ice) and the surface of the ice-free northern sea absorbs sunlight and converts it into heat. For these reasons, the Arctic region is warming at a higher rate than other regions farther to the south in the Northern Hemisphere. This, in turn, makes for a reduced gradient in the atmospheric temperature from tropical to temperate to polar regions.
Changing the gradient of the atmospheric temperature in a north-south axis is like adjusting the rate of water flowing from your faucet, or changing the number of cars on the highway, or replacing all the usual sports fans at the stadium with stoned people with arthritis. The nature of the waves changes.
In the case of the atmosphere of Earth’s Northern Hemisphere, global warming has changed the dynamic of the northern Jet Stream, and this has resulted in changes in weather extremes. This would apply to heat waves, cold snaps, and the distribution of precipitation. The phenomenon that is increasingly being called “Weather Whiplash” … more extremes in all directions, heat vs cold and wet vs. dry, is largely caused by this effect, it would seem.
This study is somewhat limited because it covers only a 32 year period, but the findings of the study are in accord with expectations based on what we know about how the Earth’s climate system works, and the modeling matches empirical reality quite well.
Petoukhov, V., Rahmstorf, S., Petri, S., & Schellnhuber, H. (2013). Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes Proceedings of the National Academy of Sciences, 110 (14), 5336-5341 DOI: 10.1073/pnas.1222000110
