The relationship between climate change and weather is becoming increasingly understood. One of the crucial links in that relationship is related to that river of air circling the northern hemisphere known as the jet stream. Most of us have been wondering what has caused the weird weather patterns we have noted in recent years throughout the USA. New understanding of the relationship between climate change and the jet stream does much to answer such questions and is very well explained in a new U-tube video available at http://www.youtube.com/watch?v=u7EHvfaY8Zs
In order to help understand the contents of this video, let me first list its major points.
As the Earth has warmed up, temperatures in the Arctic regions have increased more than those nearer the Equator. Thus the temperature difference between the Arctic and Equatorial regions has decreased.
The jet stream is powered by that temperature differential between the Arctic and Equatorial regions. As that difference decreases, the jet stream changes – as described in the video. One of those changes is that it then wanders farther north and farther south as it circles the Earth. Another is that the intensity of the jet streams weakens and the specific location of this river of air changes more slowly. Thus, existing weather patterns a given location tend to stay longer.
Thus, if a specific region of the USA or Canada finds itself south of the jet stream at a given point in time, that region will be warmer than usual. Conversely, regions that find themselves north of the jet stream will be more strongly coupled to air masses to the north and, therefore, will experience lower temperatures. Thus, as the jet steam wanders farther north or farther south than it usually does, the weather in those regions below will become increasingly unusual.
From one year to the next, the jet stream can change its location so that very different types of seasons can be experienced one year to the next or even one month to the next.
All of this is superbly explained with pictures in the video, so please do have a look.
ericgrimsrud 
It has been a while!
When global temperatures rise, high latitudes warm much faster than the equator:
http://diggingintheclay.wordpress.com/2010/12/30/dorothy-behind-the-curtain-part-2/
Since 1850 the IPCC informs us that the global average temperature increased by ~0.8 Kelvin whereas Greenland warmed by ~2.3 Kelvin. This seems to confirm that temperature changes are magnified at high latitudes. If this is true it follows that during warming perioods the temperature gradient from low to high latitudes will diminish.
Regardless of what James Hansen says, most scientists agree that “Extreme Weather” is driven by tempertaure gradients so it follows that if our climate warms extreme weather will become less common.
While the video linked above is “Touchy Feely” it seems to be in agreement.
[Response by EPG: Gallopingcaml, Touchy Feely ? The folks in this video are reporting direct observations of our weather. No theory, just observations. The jet stream can be measured and weather patters are easily observed. Also you are confused on another poiht – the decreasing temperature difference between the Arctic and lower latitudes weakens the jet stream but does not diminish the severity of weather events. More extreme weather is caused by the presence of the increased levels of water vapor in our now warmer air masses. ]
By: gallopingcamel on June 11, 2013
at 11:02 pm
What a wonderful explanation! Thank you for that (Yale forum) site, Eric. Is there any predictability season to season–Is the movement and the undulation of the jet stream pushed solely by the temperature gradients at different levels of the atmosphere. Dr. Francis explanation is wonderful!
[ Priss, I do not believe that the movement of the jet stream with time is well understood yet. Also I don’t believe we can predict the magnitude of the N / S undulations yet. Glad you liked the video. I believe it explains a lot about recent weather patterns – albeit much more to learn. ]
By: Priss on July 16, 2013
at 12:01 pm