We have some new research in the form of a Science article called “Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification” by Michael Mann, Stefan Rahmstorf, Kai Kornhuber, Byron Steinman, Sonya Miller, Stefan Petri, and Dim Coumou. (Vol 4(10))
I have been discussing on this blog for a few years not the problem of quasi-resonant amplification (QRA) of the jet stream. Let me quickly review what that is, then tell you about the new research.
The Earth is encircled by giant twisting donuts of air. The two main donuts lie side by side along the equator. Air warmed at the point where the sun is strongest (a climatological equator that moves north and south with the seasons) rises. It traverses, at altitude, either north or south, towards the polls, then drops and then circles back towards the equator. This drives wetness at the equator as moist air hits cold air aloft and thunderstorms are everywhere.
These primary giant twisting donuts, called Hadley Cells, set up a second set of twisting donuts to the north and south. These donuts, called mid-latitude cells, tend to cause a dry zone to form. Look at a map of the planet, and you can trace the dry zone across the northern hemisphere from the deserts of Central Asia, to the deserts of the US Southwest. In the south, the deserts of Namibia, Botswana and South Africa line up with dry regions of South America and, pretty much all of Australia.
There is a third cell, the Polar cell, north and south of the mid latitude cells.
These cells, as they move around the spinning earth, are the trade winds. Near junctures of the cells, at latitude, air molecules face an interesting mathematical problem. Air pressure, temperature, cell-driven winds, and all the various factors set up a situation where those air molecules sitting between the upper parts of the cells are supposed to be somewhere where they are not, pretty much all the time. In order to solve that problem, the air has to move very rapidly in one direction. This is a bit like nature abhorring a vacuum, large scale. That rapidly moving river of air is the jet stream.
A combination of trade wind effects and the jet stream tends to move storm systems around the planet in the mid latitudes. Under pre-climate change conditions, a low pressure system might ride along just south of the Jet Stream, moving across the planet at a few tens of km an hour, bringing rain followed by fair weather. But if the jet stream either slows or changes direction somewhat, that conveyor belt effect can get kinked up, and the low pressure system can sit in a giant meteorological kink, causing a large region to experience wet conditions for days or weeks at a time. Meanwhile, on the other side of the jet stream, in the counter-kink that a curved jet stream might cause, you can get a stalled high pressure system bringing dry conditions for longer than normal, causing what meteorologist Paul Douglas calls a “flash drought.”
Go back to the beginning a second. This entire process is controlled by the global process of heat accumulated in abundance at the equator moving to the north and south poles. But in recent years, the arctic has warmed considerably. Lack of snow cover in northern Canada and Siberia, loss of sea ice, and, probably, darkening of glacial ice in Greenland, combine to cause the Arctic to warm to a much greater degree than the rest of the planet.
This is a little like putting your refrigerator too close to the wall and building a cabinet around it without proper ventilation. The heat pump that runs your refrigerator will stop working. The behavior of the giant twisting donuts and the jet streams changes.
What occurs is this: The jet stream gets wavy, and that waviness can form a standing wave, like a swirl you see in a running brook that sits in one place because of an underwater obstruction like a rock or log. The wave, in a sense, resonates with the circumference of the earth, so you get a regular number of waves around the planet, and they tend to move only very slowly, or not at all, for months at a time.
There are two phenomena that have caused the plethora of wild and wicked weather we have been experiencing across the globe for the last five or six years. One is the increase in strength and possibly frequency of various storm systems as a nearly direct effect of warming. The other is this QRA system causing major weather patterns to pan out abnormally.
These two problems can interrelate, by the way, but that is a subject of a different essay, perhaps.
The result of quasi-resonant waves? The California drought, massive multi day rainfall events in Calgary, Boulder, Minnesota, China, Japan, Mediterranean Europe, and on and on and on.
Two questions arise from the research showing this effect. One: is it real, is there really a QRA effect? Two: will this persist, get worse, or get better, over time?
The answer to the first question has been getting more and more solid with the publication of research paper after research paper. There isn’t any longer a doubt, in my view, that this phenomenon is for real and seroius. The second question is harder. The paper that came out today on this topic says that the degree of extra warming in the Arctic is probably the biggest factor affecting the future of QRA effects. The research also suggest that it could get worse and it could persist. But there still is some uncertainty.
Real Climate has a detailed article on the QRA phenomenon, and concludes, in part:
We find that the incidence of QRA events would likely continue to increase at the same rate it has in recent decades if we continue to simply add carbon dioxide to the atmosphere. But there’s a catch: The future emissions scenarios used in making future climate projections must also account for factors other than greenhouse gases. Historically, for example, the use of old coal technology that predates the clean air acts produced sulphur dioxide gas which escapes into the atmosphere where it reacts with other atmospheric constituents to form what are known as aerosols.
These aerosols caused acid rain and other environmental problems in the U.S. before factories in the 1970s were required to install “scrubbers” to remove the sulphur dioxide before it leaves factory smokestacks. These aerosols also reflect incoming sunlight and so have a cooling effect on the surface in the industrial middle-latitudes where they are produced. Some countries, like China, are still engaged in the older, dirtier-form of coal burning. If we continue with business-as-usual burning of fossil fuels, but countries like China transition to more modern “cleaner” coal burning to avoid air pollution problems, we are likely to see a substantial drop in aerosols over the next half century. Such an assumption is made in the Intergovernmental Panel on Climate Change (IPCC)’s “RCP 8.5” scenario—basically, a “business as usual” future emissions scenario which results in more than a tripling of carbon dioxide concentrations relative to pre-industrial levels (280 parts per million) and roughly 4-5C (7-9F) of planetary warming by the end of the century.
As a result, the projected disappearance of cooling aerosols in the decades ahead produces an especially large amount of warming in middle-latitudes in summer (when there is the most incoming sunlight to begin with, and, thus, the most sunlight to reflect back to space). Averaged across the various IPCC climate models there is even more warming in mid-latitudes than in the Arctic—in other words, the opposite of Arctic Amplification i.e. Arctic De-amplification (see Figure below). Later in the century after the aerosols disappear greenhouse warming once again dominates and we again see an increase in QRA events.
Author Michael Mann notes, “Most stationary jet stream disturbances will dissipate over time. However, under certain circumstances the wave disturbance is effectively constrained by an atmospheric wave guide, something similar to the way a coaxial cable guides a television signal. Disturbances then cannot easily dissipate and very large amplitude swings in the jet stream north and south can remain in place as it rounds the globe.”
From the abstract of the original paper:
Persistent episodes of extreme weather in the Northern Hemisphere summer have been associated with high-amplitude quasi-stationary atmospheric Rossby waves, with zonal wave numbers 6 to 8 resulting from the phenomenon of quasi-resonant amplification (QRA). A fingerprint for the occurrence of QRA can be defined in terms of the zonally averaged surface temperature field. Examining state-of-the-art [Coupled Model Intercomparison Project Phase 5 (CMIP5)] climate model projections, we find that QRA events are likely to increase by ~50% this century under business-as-usual carbon emissions, but there is considerable variation among climate models. Some predict a near tripling of QRA events by the end of the century, while others predict a potential decrease. Models with amplified Arctic warming yield the most pronounced increase in QRA events. The projections are strongly dependent on assumptions regarding the nature of changes in radiative forcing associated with anthropogenic aerosols over the next century. One implication of our findings is that a reduction in midlatitude aerosol loading could actually lead to Arctic de-amplification this century, ameliorating potential increases in persistent extreme weather events.
Here is Michael Mann discussing the research: