Jesse Zhang – TRANSCRIPT
I visited the East Coast this past winter, and I was greeted by a site similar to this. We can all relate to the severe snowstorms that affected the US in January and February.
Coming back, my flight was delayed a whole day, but I mean, when it’s going to snow it’s going to snow – fine. What stood out was this: in places like Philadelphia, New Jersey, and New York City a potentially historic snowstorm was predicted. But just a few days later, after policy makers had already acted accordingly, only a few inches fell. The bottom line is that even our best weather forecast models have a lot of room for improvement. In particular, I’m sure most of you if not all of you have heard the term “polar vortex” all over the news.
Polar vortex, polar vortex, polar vortex. What exactly is the polar vortex, and why do we even care about it? The polar vortex is a large-scale cyclone that forms over the poles of the Earth during winter time. As winter approaches, the air above the pole becomes a lot colder than the air around it. This creates something called a pressure gradient, which causes air to start flowing inward towards the pole. This, combined with the Coriolis effect, causes the air to turn and start rotating eventually forming a cyclone.
The interesting part about this is that the wind speed at the edge of the vortex can reach up to 80 meters per second. That’s 180 miles per hour, about the speed of a small plane. This means that all the cold air during winter time is trapped inside and warmer air can’t get in, so as winter passes, the air inside becomes increasingly cold. With the coming of spring, the polar vortex will eventually collapse and when it does, all of this cold air that was trapped inside comes rushing down to lower latitudes like North America. And that’s why we get these severe winter patterns.
A particularly relevant aspect of the polar vortex is called polar vortex weakening or PVW. Polar vortex weakening is an atmospheric anomaly that occurs before the the spinal warming and before the final collapse during the months of December, January, or February, and it’s when the polar vortex weakens suddenly and dramatically. Unfortunately, the ultimate cause of the polar vortex is still a mystery. No one knows what triggers it.
I was curious: if we know so much about the polar vortex, why are the weather measurements so off? I’ve always been interested in math and science so I decided I wanted to investigate this issue of the polar vortex weakening, and conduct some of my own research.
At first, I was very reluctant because the polar vortex is obviously very well studied, and in the past, scientists have come up with some theories regarding why it happens, but they’ve never been able to find strong enough empirical evidence. Experts have never been able to find strong correlations in observed data. And I was just a high school student finishing up his last two years. Luckily, I had the opportunity to work as a research intern at the University of Colorado at Boulder. With my new environment and my new resources, I dove into my projects.
First off, what exactly do I mean when I say that polar vortex weakens? To give you a better idea of what’s actually happening, this plot shows the year 1985. In fact, it shows how two things change over time. One, the blue curve is the wind speed at the edge of the vortex, and the black curve is the temperature in the center of the vortex. So you can see on the day of the polar vortex weakening, which is indicated by the vertical red bar, the blue curve deeps suddenly, and at exact same time there’s an upward spike in the black curve. This is because, as the blue curve, which is the wind speed, slows down on the edge, some warm air is allowed to get in, and this, in turn, warms up the center of the vortex.
So this is a very, very prevalent trend right here, as you can see, there’s the two peaks that happen simultaneously. And this phenomenon is known as the polar vortex weakening. In a lot of atmospheric science the focus in on the Sun, because it is such a big part of everything that goes on on Earth. But in the course of my projects, I realized that the stratosphere, which is the layer of the atmosphere where the polar vortex is, the stratosphere and up only absorb 1% of total solar irradiance. In other words, there must be some significant energy source coming up from down below.
This led me to think about oceans. Another main detail that really reinforces this idea is that, of course, in the South Pole there is also a polar vortex, but it does not display any of these weakening effects, and it does not have any PVW at all. Obviously, the biggest difference between the North Pole and the South Pole is that there is no ocean in the South Pole. So this really narrowed down my thought process to two main things: one, the Atlantic Ocean, and the other one, the Pacific Ocean. In the Atlantic Ocean, there is a major warm water current called the North Atlantic Current that flows up into the Arctic Circle from between Greenland and Scandinavia. It’s part of the Gulf Stream.
And the Pacific Ocean, there is also opening into the Arctic Circle between Alaska and Russia. I’m sure many of you are familiar with it, it’s called the Bering Strait, but it’s much more narrow, and there is really no significant current flowing up through there. So, in the end, my idea was that North Atlantic Current carries warm energy, heat energy into the Arctic Circle. This energy eventually propagates up, and that’s what weakens the vortex.
So, this was the hypothesis that really drove my project. The hard part about all this is quantifying something as large as the North Atlantic Current, and correlating it with both the timing and the strength of the polar vortex weakening. I was sifting through a lot of really raw satellite data that just were a bunch of numbers, incomprehensible and hard to read, and trying to look for patterns, trying to find evidence for my hypothesis. After several months and a lot of trouble there, I was trying different procedures, trying different variables to represent the heat energy, trying different regions of the oceans to indicate the current, I was able to get a 99.95% level of correlation between the weakening and the current.