Projected Changes in Storm Tracks

We had quite a flood event in Austin and counties immediately north and south last night, but nothing of the sort in counties east and west. (A classic Mesoscale Convective Complex, methinks. It was interesting to watch it pop up out of  a background of small thunderstorms and take over the air circulation of Texas for a half a day.) This reminds me that as you move away from the poles (as I certainly have done!) intuitions about major weather events can be wrong – to my Montrealer brain, big precipitation is typically part of very large scale events.

Nevertheless, those large scale events are important. What’s more, they are big enough that climate models can get a handle of them. And a recent study out of PIK by Lehmann et al of the big GCM outputs has shown a fairly complicated set of trends.

The upshot is this: the northern hemisphere and the southern hemisphere will have different fates.

We already knew that storm tracks would move poleward.

The changes in the model results are far more spectacular in the southern hemisphere than the northern. In the southern hemisphere, there is a dramatic increase in their winter (June-August) storm activity, while summer activity remains about the same. However, this is accompanied by southward migration of the storm tracks, so of populated places only southern Australia and New Zealand really feel the brunt.

In the Northern hemisphere summer, which affects more people, large scale storms noticeably decrease as well as moving north. Cool air has more trouble moving south in summer, presumably because there is so much less of it. This ties in to the idea of continued drying of the subtropics and adjacent regions. In the Northern hemisphere winter, large scale storms on the whole become weaker. But storm tracks move northward into populated areas, so the recently perceived trend of an increase in snowy winters in the eastern US and Europe may actually be a harbinger of the future. (See Fig 1.)

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Unfortunately the color scheme here is confusing. Roughly speaking the areas in red will see more disturbed weather (wetter) and the areas in blue less disturbed weather (drier). I think the projection for snowpack in California and the southwest is especially dire from this picture.

Again, I emphasize that not all precipitation comes from large scale events, and that the closer you get to the equator, the less they dominate. But this paper claims that these shifts are robust in the models.



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