Hmm... what's up with the jet stream?

It snowed on our winter festival this year.

It never snows on the winter festival. Ever since I've moved here, it's been the big joke in town: at the end of January, right at the time of the winter festival, the snow will all melt. And when La Nina kicked into gear this fall, season passes at the ski area seemed like a really bad idea.

We were wrong. We've had a good three feet of snow at the foot of the mountains, and up high, on the Continental Divide, there's a lot more. Good news for skiers and for places that rely on water from the Colorado and Rio Grande. Not such good news for geology field trips (though my classes, for some reason, are not complaining).

So what's going on?

We're on the south side of a mountain range that has several 14,000 foot peaks, and that averages 10,000 feet in elevation. That means that we get rather strange orographic effects: in order to get big dumps, we need our air to come from the southwest, from San Diego or Baja, rather than directly from the west. And this year, when we've had the big storms, the jet stream has done this:



(Source: San Francisco State University/California Regional Weather Server.)

When it gets to the Pacific, the jet stream makes this big loop north, sometimes all the way to the Bering Sea, and then goes almost straight south again, down to the Mexican border, and then shoots back north again... right over us. Over the course of a few days, the entire pattern shifts eastward, so later in the same week, there's a big loop from Alberta down to the Gulf of Mexico and back up the East Coast to the Canadian Maritimes.

Now, I've been hearing about the jet stream for years, and even though I briefly touch on it in my intro earth science course, I don't fully understand it. The jet stream is a region of strong winds high in the troposphere (the bottom layer of the atmosphere), and is caused by temperature differences between the tropics and the poles. The temperature differences create pressure differences (because warm air is less dense than cold air, and less dense air rises), and the addition of the Coriolis effect means that the winds blow mostly in big circles around the globe. "Mostly," that is, because there are also waves in the jet stream (called "Rossby waves"), and their movement around the globe creates troughs and ridges that, in turn, create low and high pressure systems.

But why are the Rossby waves so big this winter? And, if this is a typical La Nina pattern, why didn't it happen last time there was a La Nina event? (La Nina events have colder than normal water in the eastern Pacific, and warmer than normal water in the western Pacific, and strong east-to-west trade winds. And they're traditionally associated with dry winters in the Southwest. I've always explained that pattern by invoking the lower evaporation of colder water over the Pacific, but perhaps I've understood it wrong all along.) And why do the big Rossby waves seem to develop over the Pacific - the winds through Asia are fast and strong and mostly west-to-east, but when the big storms develop here, the winds start looping way north and south across the Pacific.

How do ocean temperatures affect the development of these big waves in the jet stream? Does it have anything to do with ocean surface temperatures in the Bering Sea? How far south is the sea ice this winter, for that matter? Is this some of the stuff that's so hard to model in global climate models?

And is it going to dump snow on me again on Monday? I've run out of indoor things for the intro class to do.

And they won't be covering weather until the end of the semester.

(For the Accretionary Wedge, February 2008: "Things that make you go hmmmmmmm.")