Monday, November 05, 2018


Picture a river flowing down from the mountains to the plains. In the high country, where the river's descent is steep and hedged in by canyon walls, it's likely to follow a relatively straight path, basically tracing the shortest route down. Once it gets to the plains, however, there's no big difference between one path and another, and the river will begin to meander.

 Meandering river
Credit: Alana Whitman

Something similar is true for the jet streams, vast rivers of air that circle the globe from west to east, sometimes reaching speeds of 250 miles per hour. Like rivers, jet streams typically meander north and south. The size of those meanders and the speed with which they gradually drift eastward have major impacts on the weather. The UK, for example, was deluged by record floods in 2007 and again in 2012 when the jet stream looped far to the south and then stalled, unleashing extended periods of heavy rains.

Meandering polar jet stream

That, in turn, gets us to climate change and quasi-resonant amplification (QRA). 

One of the most dramatic impacts of global warming and climate change is that the far north and south are warming much faster than the mid-latitudes and the tropics, a phenomenon known as polar amplification. What this means is that the temperature difference between the poles and the tropics-- which is what creates and drives the jet streams--is reduced. And like a river crossing a plain, this reduced gradient lets the jet stream meander farther north and south, and also tends to make the individual loops, known as Rossby waves, drift more slowly from west to east.

At the extreme, the meanders in the jet stream can form a stable sequence of exaggerated loops around the planet that can stay in place for weeks or months. This is known as quasi-resonant amplification  (QRA) and its results can be devastating. Warm, sunny days can extend into deadly heat waves, droughts and wildfires; while cool, rainy weather can linger, drenching an unlucky region with unprecedented rains and floods.

Michael Mann, a distinguished professor of atmospheric science and Pennsylvania State University, says that as of 2018 this phenomenon has now passed from the theoretical to the all-too-real and threatening. "It played out in real time on our television screens and newspaper headlines in the form of an unprecedented hemisphere-wide pattern of extreme floods, droughts, heat waves and wildfires," he says.

Unfortunately, Mann adds, climate models indicate that such QRA-caused weather extremes will grow more frequent and more severe unless we get a handle on global warming. We can hope that humanity (and the politicians who hold the reins) will get their act together. In the meantime, however, we can all expect more QRAs and the devastating weather extremes they bring coming to a region near us.


You can read an earlier post on this subject here.

For more detail on arctic warming and its impacts on our weather click here.

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