Northward Jetstream Migration

[Mike Peregrine]

http://www.msnbc.msn.com/id/12974363/

Study finds jetstreams are moving closer to the poles, allowing wider areas to become warmed, and also affecting location and motion of storms. Could this explain the generally northward shifts in severe weather over the last few years?

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:huh:

 

[Jeff Snyder]

It's kind of a chicken v. egg idea... If the mean jetstream position is moving northward (not saying it is, but IF it is), then the expected risks for severe storms would move northward as well. Now, this isn't the issue in terms of cause. Warmer temps in the US would lead to higher heights aloft, which may lead to less mean height gradient aloft across the US, effectively shifting the 'jetstream' northward. So, it's not necessarily that jetstreams are moving closer to the poles that allows for more warming, since it may just as well be that more warming is resulting in the jet stream moving closer to the poles. There's a lot of interaction and feedback involved, and an easy cause-and-effect may not be attainable in this case.

I think most have noticed a general northward shift in areas that see several supercell or tornado days. I'd be interested to see Brooks' Severe Thunderstorm Climatology for the 1995-2005 period. We can't really conclude that there are long-term changes going on when only looking at severe weather occurrence in the past 5 years (a similar argument used by those who attribute the rise in hurricane activity and intensity the past few years to the multi-decadal oscillation that's been known for decades). I odn't know a whole lot more about this subject, so I'll yield to those who may be more knowledgeable in this regard.

 

[Rich Thompson]

It's kind of a chicken v. egg idea... If the mean jetstream position is moving northward (not saying it is, but IF it is), then the expected risks for severe storms would move northward as well. Now, this isn't the issue in terms of cause. Warmer temps in the US would lead to higher heights aloft, which may lead to less mean height gradient aloft across the US, effectively shifting the 'jetstream' northward. So, it's not necessarily that jetstreams are moving closer to the poles that allows for more warming, since it may just as well be that more warming is resulting in the jet stream moving closer to the poles. There's a lot of interaction and feedback involved, and an easy cause-and-effect may not be attainable in this case.

I think most have noticed a general northward shift in areas that see several supercell or tornado days. I'd be interested to see Brooks' Severe Thunderstorm Climatology for the 1995-2005 period. We can't really conclude that there are long-term changes going on when only looking at severe weather occurrence in the past 5 years (a similar argument used by those who attribute the rise in hurricane activity and intensity the past few years to the multi-decadal oscillation that's been known for decades). I odn't know a whole lot more about this subject, so I'll yield to those who may be more knowledgeable in this regard.
[/b]

Jeff's correct in that the "jet stream" is simply a reflection of the mean temperature gradient, and being deeper in the "warm" air suggests you are farther removed from the gradient zone. We won't know if the past few years represent a longer term shift for at least another 10 years. We can only hope that such a change is not occurring, or there will be huge consequences for native vegetation, agriculture, commerce, energy consumption, etc., regardless of the reasons for the change.

I lived and attempted to chase through the late 80s, so these past two years on the Plains aren't the worst case scenario for chasing. However, the warm temperatures and drought are quite bad. It sure would be nice to revert to the early 90s spring patterns.

Rich T.

 

[Dan Dawson]

I would argue that ultimately, in the long-term, tropospheric motions are forced by surface processes. When you get right down to it, the reason we have weather at all is ultimately due to differential heating and evaporation of water from the earth's surface, and these effects propagate upwards into the atmosphere to ultimately give us the weather we see. In the short term (i.e. on the order of days), however, we tend to focus on what the mid and upper levels are doing to determine what is going to happen at the surface. With this in mind, I would say that the mean positions of jet streams (which, as already stated by Jeff and Rich, are upper-level reflections of large-scale atmospheric temperature gradients, as described by the thermal wind relation) would respond to changes in the patterns of low-level temperature gradients, and not the other way around. So, if we are heating up the interior of N. America more often and to a greater degree, that would tend to shift the mean position of the jet stream further north, because that would be where the temperature gradients would set up. Keep in mind that this is over long time scales. In the short term, the position of jet streams and the troughs and ridges that set up within them can just as legitimately be said to be "causing" the movement of surface fronts and cyclones as the other way around.

 

[Cstok]

Another article on the subject:

http://www.breitbart.com/news/2006/05/25/D8HR4K6G0.html