The human release of greenhouse gasses has ultimately caused changes in weather patterns so that major storm systems in the Northern Hemisphere get wetter and move along more slowly, causing significant rainfall events to occur at a much higher rate than previously. This has become a nearly ongoing phenomenon, with major floods in Canada, Colorado, Texas, Western Europe, Texas again, various places in Azia, more in Europe, Texas again, and so on.

The short version of the story: The jet stream is often fairly linear, traveling around the planet at a high speed, but it can also get all wavy and those waves can become “quasi resonant” meaning that they sit in the same place for a long period of time. Also, they go slower and thus move weather patterns along more slowly. This can cause the aforementioned major rainfall events, as well as persistent droughts. And we’ve had plenty of both of those.

I have written quite a bit about this, but especially this item (but see also this). And now we have more research confirming the findings.

The same (or overlapping) team of researchers that did this earlier work has a new paper out in PNAS. Here’s the summary material from the paper:

Significance:

Weather extremes are becoming more frequent and severe in many regions of the world. The physical mechanisms have not been fully identified yet, but there is growing evidence that there are connections to planetary wave dynamics. Our study shows that, in boreal spring-to-autumn 2012 and 2013, a majority of the weather extremes in the Northern Hemisphere midlatitudes were accompanied by highly magnified planetary waves with zonal wave numbers m = 6, 7, and 8. A substantial part of those waves was probably forced by subseasonal variability in the extratropical midtroposphere circulation via the mechanism of quasiresonant amplification (QRA). The results presented here support the overall hypothesis that QRA is an important mechanism driving many of the recent exceptional extreme weather events.

Abstract
In boreal spring-to-autumn (May-to-September) 2012 and 2013, the Northern Hemisphere (NH) has experienced a large number of severe midlatitude regional weather extremes. Here we show that a considerable part of these extremes were accompanied by highly magnified quasistationary midlatitude planetary waves with zonal wave numbers m = 6, 7, and 8. We further show that resonance conditions for these planetary waves were, in many cases, present before the onset of high-amplitude wave events, with a lead time up to 2 wk, suggesting that quasiresonant amplification (QRA) of these waves had occurred. Our results support earlier findings of an important role of the QRA mechanism in amplifying planetary waves, favoring recent NH weather extremes.

The paper is: Role of quasiresonant planetary wave dynamics in recent boreal spring-to-autumn extreme events, by Vladimir Petoukhov, Stefan Petri, Stefan Rahmstorf, Dim Coumou, Kai Kornhuber, and Hans Joachim Schellnhuber.