Viv - I'm a jet stream freak; have been studying it for decades, literally. Part of being an ecologist, for me; should any ecologist attempt to understand weather and climate? Sure. And we've known for a long time that fluctuations in the jet streams drive a whole lot of weather.
My favorite comprehensible model is here. The "streams" have not ever been what we think of looking at a river; they come, go, pause, start. But. The "norm"; possibly now gone forever, is that both hemispheres have (had) two jet streams; the polar jet, and the subtropical jet. In both hemispheres, the streams blow from west to east.
That normal pattern is now hard to even see in that model. Two streams? Where? Looks like a mess, right? You did used to be able to see the "2 streams" clearly and consistently, fluctuations notwithstanding.
Up until 5 or 6 years ago, it was almost unheard of for - the polar jet, and the subtropical jet to MEET. They pretty much stayed in their own backyards. But a few years back it started becoming more and more common for the northern hemisphere streams to shift so far they would actually bump into each other. Then - they started not only meeting - but - for long distances and long periods of time; they would MERGE - leaving great chunks of the northern hemisphere with only ONE jet stream.
That does crazy things to weather, and was a primary driver in the years long California drought (now over). Meterologists have been privately goggling at each other and muttering "Merge?? They merged?? WTF!!??"
Viv - here's the specific thing- until just the past 6 months or so; the southern hemisphere jet streams were still behaving "normally" - 2 jets, one polar, one subtropical. But. If you look at the model; they are now starting to form huge loops - and meet - and merge. You are not likely to hear that from any official meteorology expert any time soon; the phenomenon is too new (though it's been consistent for 6 months now) - and they have reputations to protect. But. I've been watching; closely. The consistency, and the parallel with the process that has gone on for years now in the northerns hemisphere are significant.
Yes, it's going to mess up the weather, and normal seasonal expectations, all over the planet. And for the most part it is outside all the training and expectations of the best meteorologists- just very hard to guess what comes next. Best bets, based on the past years in the northern hemisphere; droughts, floods, bigger and more frequent storms.
One of the things we do know about jet streams is - we don't know ANYTHING about jet streams. We know this from? Space exploration. Below is the polar jet stream on - Saturn; viewed from directly about Saturn's north pole. As reported by Voyager in 1981-82; and Cassini in 2006-2009. Courtesy of NASA.
It forms a hexagon. ???!!! And - in all the time we've known about it, and watched it - it has been STABLE. It doesn't change. No loops up, or down. When the first images from Cassini came in, the exo-meteorologists were stunned - no one had expected that weird hexagon to persist over 25 years.
What if- Earth's jet streams suddenly hit whatever conditions are required for our jet streams to drop into this kind of stable configuration? Could that happen?
We don't have a clue. The one thing we know for sure- weather and climate would be hugely, drastically, affected. And would stay in whatever pattern showed up.
Seriously. Buckle up. If you haven't already started.