|A shot-down Japanese fire balloon|
reinflated by the US
File uploaded by Bkwillwm to
Wikipedia, public domain
In a new study by Dim Coumou and a team from Potsdam Institute for Climate Impact Research published August 11 in the Proceedings of the U.S. National Academy of Sciences (ref. below), Coumou points out that the large number of very high-impact extreme weather events over the past decades has seemed out of proportion to the rate of warming of the atmosphere caused by increased CO2. The authors rely on, and quote, an earlier paper in PNAS by Petoukhov, et al. (of the same institute) reporting the same thing: the frequency of these extreme events over the past decade is such that it is unlikely to be just a "stochastic mechanism of extremes."
And, here's where the Japanese discovery of the jet stream becomes relevant, because it's in the jet stream that the changing flow patterns are driving the weather extremes.
|From the cited PNAS article. Shows the increase in so-called "boreal summer weather extremes."|
Now back to the research of Coumou and his team: According to the theory advanced in the article (based on analysis of meteorological conditions from 1979 to 2012), there are resonances in the atmosphere that trap the Rossby waves into certain configurations for long periods of time. Thus, a heat wave that would not be dangerous if it were a few days long, becomes extreme when its duration increases. (The paper is limited to analysis of the Northern Hemisphere.)
The speed at which a wave travels along the jet stream (the "phase speed") is, in one approximation, directly proportional to the mean zonal wind speed. To first order, synoptic waves with a wave number (k) equal to 6-8 travel at this speed. The zonal mean wind speed changes with season, being less in the summer. Because the zonal mean wind speed is lower in the summertime, the phase speed is also lower because of this direct proportionality. In fact, in the "boreal summer"--July, August--the phase speed can be close to zero (the waves are quasi-stationary, especially for wave number 6) or even negative (that is, the waves would travel to the west instead of the east). This weakening of the zonal wind speed and, hence, the wave speed is one mechanism explored. Free-traveling waves are simply slowed down or stopped. If the waves are stationary, then the troughs and ridges of the Rossby waves are stationary, setting in the northerly or southerly flow of air for long periods.
The second mechanism is the amplification of quasi-stationary waves by resonance between free and forced waves in the midlatitudes. Looking at the Petoukhov et al paper, the quasi resonance hypothesis is as follows. (1) Generally, the large-scale atmospheric circulation at mid latitudes is characterized by traveling Rossby waves with zonal wave numbers (k) equal to or greater than 6 propagating in the longitudinal direction at a phase speed of c~6-12 m/s as discussed in the paragraph above. (2) The circulation is also characterized by quasi stationary planetary-scale Rossby waves with c~0, frequency w~0, and various zonal wave numbers m that develop in response to orographic obstacles or weather sources and sinks, that is, to "conditions in the atmosphere that differ from place to place on the earth. Their hypothesis is that during the extreme summer events, persistent wave structures with high amplitudes evolved and made an unusually large contributions that the usually weak midlatitude response to the thermal and orographic sinks was strongly magnified at wave numbers 6,7 and 8.
They assert that the apparent cluster of resonance events observed in their data set (see their figure reproduced above) is due to an increased wave 7 and 8 resonances, and that furthermore, these resonances high in the atmosphere are coupled to persistent weather patterns at the surface, and thus the extreme weather events. The changes observed are statistically significant at the 95% confidence level.
The theory and data (from 1979-2012) suggest that because of warming in the Arctic, temperature differences between the Arctic and tropics are decreasing. Temperature differences drive the atmospheric circulation patterns, and changes in these differences (the temperature "gradients") are causing the atmospheric circulation patterns to change. Although much more detailed work and analyses needs to be done, their tentative projection (Figure 7) of conditions a century away shows t"similarities with the recently observed anomalies). According to RCP8.5 climate model (one in which we don't curb our CI2 emissions very much), the July-August thermal gradients will increase northward of 50N and decrease southward of 50N, leading to strengthening of the sub polar jet and weakening of the subtropical jet.
**I have used the report in ScienceDaily.com for parts of this post: http://www.sciencedaily.com/releases/2014/08/140811170106.htm
The abstract for the Coumou PNAS article is here and the full text is here.
The PNAS Petoukhov et al. article referenced is here.