Here’s a simple explanation, from The Wildlife News, of the jet stream and how it can cause some of the extremes of weather we’ve been seeing over the United States in recent years.
The post also describes why the climate has changed and won’t change back. That’s because once the Arctic begins to melt, the ice, which is white and reflects heat, is replaced by water, which is dark and absorbs heat. The strong winds that make up the jet stream are the result of temperature difference between the cold arctic and the warmer air to the south. So as the Arctic warms, the jet stream begins to slow down.
The polar jet stream … circles the northern part of the globe from west to east. It is the product of the rotation of the Earth and the temperature difference between the frozen Arctic and … [warmer] lands to its south. Temperature differences cause wind.
A powerful polar jet stream forms a vortex and keeps the frigid Arctic air locked up near the Arctic Circle, though it moves southward in the winter. This jet stream is never a perfect circle around the top of the globe. It has dips (“troughs”) and ridges. When a trough comes over your area, e.g., state or region, we expect the storm track will move over you. That means wet stormy weather, especially in the winter. A jet stream ridge allows high pressure to dominate to its south. In the spring, winter and fall this means little precipitation.
In the summer, sub-tropical air comes up from the south and creates thunderstorms around the edges of the clockwise rotating high pressure area. These thunderstorms on the margin of the high pressure are called “the ring of fire.”
A regular, faster-moving jet stream and a weaker, slow-moving jet with troughs and ridges. (From The Weather Doctor)
As the jet stream slows down, its path begins to meander and the troughs and ridges become more pronounced. One part dives to the south, leaving another part to shrink further to the north. If you live where it’s diving to the south, you’re going to get colder, wetter weather – which is exactly what happened in western half of the country last March. And if you live below where it’s shrinking back to the north, you’re going to get warmer weather – which is exactly what happened in New England in March, when temperatures soared into the 90s.
One of the reasons there was so much wet weather in the western part of the country is that part of the weakened jet stream broke off, leaving a region of low pressure just circling around, with nothing to push it along, and dumping rain for days on end.
A weak polar jet makes whatever weather we are having more persistent. Rain storms move more slowly and drop more water. Droughts form under very stable, unchanging ridges. They last much longer. One result is gigantic wildfire seasons. Mid-latitude temperatures show greater extremity over a short period of time.
People think climate change means warmer temperatures. It is not so simple. Yes, the average temperature will probably rise, but more importantly the variations of the temperature around the mean temperature have grown in size.
Here’s the jet stream with some troughs and ridges from July of this year:
Once the water in the Arctic has started warming, it’s going to keep warming. And water all over the planet is warming, too. The warming is going to continue; it’s going to escalate; and this is going to continue for a long time to come.
Once ice has melted, very much heat must be shed before it freezes again. The Arctic Ocean melted much more quickly than predicted and all the efforts to curb greenhouse gases will not refreeze it, except over a period of hundreds of years. We are once again talking climate change, but now the climate has changed.
Our recognition comes far too late.