Saturday, June 16, 2012

A Message from the Unknown

In a comment on the post below, a bunny who wishes to remain Unknown points to a Jennifer Francis's work on how global climate has been changed by climate change and specifically ice melt in the Arctic

which is summarized as 
If the theory is correct, that Arctic amplification reduced ice pack alters the jet stream patterns, leading to prolonged weather patterns, then we have entered a new meteorological system. Then shifted probability curves (based on the old meteorology) don't really make sense. The new meteorology we have created could have a very different probability curve for prolonged heat waves (as well as droughts, cold spells, extreme and prolonged flooding events, major snowstorms) in the NH than the prior meteorology.
but in passing makes an important point that Eli has repeated several times, most often over at Open Mind.  One can too easily fall into the trap of only looking at the statistical properties of some occurrence, it is even more important to understand the physical origins, as a matter of fact the later in IEHO is more important, sometimes you have to wait a bit on the statistics, but the physics (and chemistry) is basic.
Another approach to studying these events tries to identify whether the meteorology of the earth has changed. Prolonged weather patterns produces a heat wave, instead of a hot week. And prolonged weather patterns depend on the positioning of the jet stream. The direct cause of all three events in your post was a stalled jet stream (a blocking pattern in the jet stream). 


Steve Bloom said...

Here's a public-access copy of Francis & Vavrus (2012), plus see this Yale Environment 360 article by Francis explaining the results.

There's also a fresh review paper on the general subject, although it puts the major emphasis on the expectation of winter polar outbreaks.

Ron Broberg said...

Thanks for pulling this up out of the comments and for anon's comment.

This intersection of climate and weather is a very interesting arena because people don't live in 30 year averages - and as far as I know, this kind of detail is not caught in the current generation of AOGCMs.

Yet, it seems like this is precisely the kind of thing that can drive western US snowpack and eastern US snowstorms. And I wonder if the increased amplitude and slower phase of the Rossby waves makes a play in the recent heat wave in Russia and extended rain in Pakistan.

In a previous post you pointed out that many people's opinion of climate science are driven by their personal experience of the degree of match between local weather and what they believe to be GCM predictions. When weather seems warmer their belief goes up. When weather seems colder and/or snowier, it goes down.

The whole 'weighted dice' narrative is an attempt to get beyond that, to help align climate predictions to experienced weather. But it is a strained relationship since, AFAIK, GCMs are not producing the natural range of variability that can be considered weather-like. And, AFAIK, aren't showing anything like this arctic impact on the Rossby waves.

Very interesting line of research. Thanks for the post.

Anonymous said...

Cohen and co-authors had this in GRL recently (draft pdf here):

Asymmetric seasonal temperature trends

Current consensus on global climate change predicts warming trends driven by anthropogenic forcing, with maximum temperature changes projected in the Northern Hemisphere (NH) high latitudes during winter. Yet, global temperature trends show little warming over the most recent decade or so. For longer time periods appropriate to the assessment of trends, however,
global temperatures have experienced significant warming for all seasons except winter, when cooling trends exist instead across large stretches of eastern North America and northern Eurasia. Hence, the most recent lapse in global warming is a seasonal phenomenon, prevalent only in boreal winter. Additionally, we show that the largest regional contributor to global temperature trends over the past two decades is land surface temperature in the NH extratropics. Therefore, proposed mechanisms explaining the fluctuations in global annual temperature trends 13 should address this apparent seasonal asymmetry.

Their Fig. 3 illustrates the point very clearly. Interesting stuff. I did *not* know that DJF temperatures in the NH extratropics were doing this.


Aaron said...

F&V12 looks back at recent changes without considering what happens as these processes continue, until the Arctic becomes a net source of water vapor (heat).

Statistics of past weather/climate will not give a clue as to future weather/climate as the very thermodynamics of the weather machine change.