Alaska is likely to stay warm this month, while much of the Lower 48 experiences a cold snap. It’s a flip-flop of the expected weather pattern that’s not uncommon, especially in winter.
Brian Brettschneider, with our Ask a Climatologist segment, says the culprit is a feature of the jet stream called a Rossby wave.
Brettschneider says the jet stream moves around the planet from west to east, but it doesn’t necessarily take the direct route.
Brian: It waves up and it waves down, kind of like a river meandering. We call these waves Rossby waves and there are generally about five of these waves traveling around the globe. But where ever it goes up in one place, it has to come back down somewhere else. It’s kind of like squeezing a balloon. When you squeeze a balloon in the middle, there’s a reaction, it bulges out on other sides of the balloon. So where these waves move northward, like what’s happening now near Alaska, it’s dragging lots warm air from the subtropics. So we feel that as warm. But if you do the math, the average distance between these peaks and troughs is about two to four thousand miles. If you move east two to four thousand miles, you’re talking about the central part of North America. So the jet stream is now racing south over there and it’s pulling in cold air from the north.
Annie: Is this more likely to happen in the winter?
Brian: We definitely see this in much larger magnitude in the winter. In winter, we have much bigger temperature gradients between lower and higher latitudes and that creates a stronger jet stream and it’s able to move north and south a much greater area and then either draw in warm air from much farther south or cold air from much farther north.
Annie: And what about climate change? Is climate change making this phenomenon more noticeable or worse?
Brian: We need to be careful. Whenever we have a warm spell, it can be convenient to say, oh this is global warming. Or when we have a cold spell, we can say what global warming? So you have to remember that global warming, climate change, is like this background noise, so our warm spells are a little bit warmer and our cold spells are also a little bit warmer. As the earth warms, the temperature gradient decreases a little bit, so it gets warmer at higher latitudes, much warmer, and it gets somewhat warmer at lower latitudes. So the temperature gradient is a little bit less, the jet stream is a little bit weaker, and is susceptible, perhaps, to greater waviness. It can be a little bit paradoxical, that in the lower 48 they can have more frequent cold outbreaks, but perhaps not as intense as before or not as long lived as before. We do still get warm and cold, it’s just a little bit less intense than it would be in a pre-warming environment.