Move Over Polar Vortex: A Tale of Two Extreme Storms

ImaGeo iconImaGeoBy Tom YulsmanFeb 15, 2014 2:40 AM


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Nature has really been dishing out the misery to millions of people on opposite sides of the Atlantic in the past few days. It might be tempting to conclude that the cold and snow that has reached from Mississippi to Maine this week (as seen in the animation of satellite images above), and the extreme storms that have caused devastating flooding in the United Kingdom, each are separate examples of the kind of weather mayhem that just happens to occur from time to time. But this winter, these events have not been a time-to-time phenomenon. The cold and snow on one side, and intense wind and rain storms on the other, have come one after the other, starting in late December. So how unusual is that, and what's going on? If a new scientific analysis is correct, the repeated bouts of extreme weather on either side of the Atlantic are indeed unusual — and both are manifestations of a chain of climatic "teleconnections" that reach half way around the globe and all the way to the tropics. According to the new analysis, produced this week by the U.K.'s Met Office, they are tied together by a distorted and, in some places, unusually fast jet stream, abnormally warm sea surface temperatures in the West Pacific, and by a particularly intense polar vortex — not a weak one, as has been widely reported before. And the analysis does not stop there. It also raises the possibility that the extreme weather may have gotten a boost from us, through human-caused climate change. The report itself was cautious in its conclusions about climate change links, pointing out possible connections and saying that more research is necessary. But the head of the Met Office, Julia Slingo, went a little further. Here's what she told reporters at a press briefing on Feb. 10 about what's been happening (as quoted in the British newspaper the Independent):

“Is it consistent with what we might expect from climate change? Of course, as yet there can be no definitive answer on the particular events that we have seen this winter, but if we look at the broader base of evidence then we see things that support the premise that climate change has been making a contribution."

Her comments are in contrast to what five prominent climate scientists have to say in a paper published today in the journal Science. They conclude that the recent discourse focusing on the possible connections between winter weather and climate change distracts from the bigger issue: that is, regardless of extreme winter weather, climate change is undoubtedly real, and that harsh winters are not what we really need to be concerned about going forward as the climate continues to change. To this I might add a more down to Earth reaction: When you've just developed a sore throat, and then you sneeze, you don't need to spend that much time wondering whether that particular sneeze is the result of the cold coming on, or perhaps a momentary allergic reaction to the cat that has just walked in the room. You're getting a cold, and you know that means you're going to be sneezing a whole lot more in coming days. It is undeniably interesting to know whether these particular storms are connected to climate change. And it is without question important to pursue the question scientifically so that we might get a better idea of the types of things that could be headed our way — for planning on things like re-construction of sea walls in the U.K. But we don't need to answer the question to know that the climate system has the equivalent of a cold, and that if we don't take care, it could develop into pneumonia. I'll delve more deeply into these issues in a bit. But first, some details on what's been happening, and how they may actually be linked together. On Wednesday, the latest wintry mix of cold, ice and snow in the U.S. left Atlanta coated in ice. Throughout the Southeast, more than 600,000 people lost power. And then the misery spread northeast, leaving a region from Mississippi to Maine afflicted by cold, ice and snow. Again.

On Wednesday, NASA's Aqua satellite captured this dramatic image of the powerful cyclonic storm pushing across Ireland and into England, Wales and Scotland. (Image: NASA) Meanwhile, across the Atlantic, the United Kingdom was hammered Wednesday by a powerful storm that brought 100 mile per hour gusts in places, torrential rain, and further devastating flooding. To make things even worse, a new deluge of a month's worth of rain is forecast for some areas through the weekend — on top of record and near-record rains that have already fallen in January and February. Scotland, for example, had its wettest December and southern England its wettest January since record keeping began in 1910. Moreover, this may have been one of the most exceptional periods of winter rainfall in England and Wales in 248 years or more. The duration of this stormy weather "and the clustering of deep depressions has been a notable feature of this winter," according to the Met Office report. Moreover, that astonishing storminess "was part of a hemispheric pattern of severe weather. More specifically, it has happened concurrently with the outbreaks of bitter, arctic cold across Canada and large parts of the United States, which have affected more than 200 million people. The Met Office report documents in detail how the events on either side of the Atlantic have been connected — through strange twists and turns of the jet stream, and conditions half a world away in the tropics, where sea surface temperatures have been running anomalously warm.

Source: U.K. Met Office You can see the contortions of the jet stream during January in this image from the Met Office report. The arrows show the direction of the winds in the upper troposphere, and the colors show their speeds in meters per second. The red streak in the western Pacific is part of the Asian-Pacific jet stream. Follow it across the ocean and you'll note a sharp upward buckling toward Alaska. This is a significant departure from the normal pattern. It contributed to exceptional warmth in Alaska in January. Meanwhile, the downward buckling over the U.S. midsection allowed bitter cold to spill south. Now, keep following the jet. It turns to the northeast, tracking up the U.S. East Coast, and then heads out over the North Atlantic. Here, it is known as the North Atlantic jet. As it turns out, cold air from arctic outbreaks in the United States was able to enter this part of the jet stream — with dramatic effects. The temperature differential this introduced caused the jet to strengthen by as much as 30 percent, according to the Met Office report. And this, in turn, enhanced the formation of cyclonic storms, the very ones that have been pummeling the U.K. But what's the connection to the western Pacific Ocean? Check out this graphic:

Source: U.K. Met Office Both show how the rate of precipitation varied from the long-term average during December (on the left) and January (on the right). Take a look at the western Pacific above Australia. Those purple areas indicate above normal rainfall rates. That's the area of the Pacific that has also been experiencing higher than average sea surface temperatures, and not just this winter but for the better part of a decade. There is a connection between warmer surface waters and enhanced precipitation: The warmth leads to greater evaporation and, as a result, more water vapor in the atmosphere. This provides more moisture for storms. Now, go back to the maps and follow those curving dashed lines in each one. They trace areas of disturbances associated with precipitation across the Pacific, the Gulf of Mexico, up the Eastern Seaboard of the United States, out across the North Atlantic, and all the way to... The United Kingdom. The connections aren't as simple and straightforward as these graphics and my shortened explanation might imply. Suffice it to say that the rainfall patterns seen in December and January in the western Pacific and eastern Indian Ocean are known to enhance the movement of disturbances in the Pacific. And in the eastern part of the ocean, they became entrained in the deep downward buckle of the jet stream this winter, which helped propel them up the east coast of the U.S. and then out across the Atlantic. According to the Met Office report, once there the disturbances got a boost from the faster jet stream, and also from another source: a stronger than normal polar vortex — not a weaker one, as has been reported recently. This, in turn, helped whip up the succession of storms that have been battering the United Kingdom, the report says. What's the explanation for the discrepancy between what the Met Office is saying about the polar vortex and what other scientists have previously said? I'm not sure. I'm trying get some insight into that from Julia Slingo, head of the Met Office. When I learn something, I'll update this post. What about possible connections to climate change, particularly the storminess in the U.K.? The report cautiously points out that attributing changes in storminess and rainfall intensity to human-caused climate change will require development of climate models with the ability to simulate weather at the resolution of individual storms. But it also notes that models like this are just coming into use, so more definitive science on climate change connections to events such as these may be on the way soon. With that caveat, the report notes a number of observations related to climate — the "broader base of evidence" that Slingo referred to in her press briefing — including these:

  • The persistence of the recent storminess is unusual, "and although clustering of storms is quite common, the continued run of deep depressions, through December, January and on into February, is not."

  • Although the number of strong cyclonic storms in winter in the North Atlantic has not increased since 1871, the mean intensity of very strong cyclones has risen by quite a lot.

  • There is evidence that rainfall in the United Kingdom has been increasing in intensity. And globally, there's "robust evidence" for an increase in the frequency and intensity of extreme rainfall events.

  • North Atlantic storms that take a relatively southerly track — typical of those that have battered the U.K. this winter — have been getting more intense.

And then there is the question of whether the strange contortions of the jet stream are related to a changing climate. From the report:

The persistence of the weather patterns affecting both the UK and also the US, where abnormally cold conditions have continued to affect the eastern and southern states through January, has raised questions about whether the jet stream is making greater excursions, north and south, and whether these waves in the jet stream are becoming more locked in one position. This is a critical question because it raises the possibility that disruption of our usual weather patterns may be how climate change may manifest itself.

What to make of all this? Slingo told reporters that what we've been seeing is "consistent" with our understanding of the physics of climate change. On its surface, that seems reasonable. But dig deeper and its significance begins to weaken a bit. As my colleague here at the University of Colorado, Roger Pielke, Jr., has tried to help me understand, if you think about it, just about everything is consistent with climate change. A day when it does not rain buckets is consistent with climate change. A week when the jet stream is not weirdly contorted and weather patterns appear more "normal" is also consistent with climate change. The sun shining, the clouds rolling in, the wind whipping the trees around my house right now — all of it is consistent with climate change. How's that? As Kevin Trenberth of the National Center for Atmospheric Research (and an author of the Science paper) has said on a number of occasions, all weather events are now taking place against the backdrop of a climate system that we've altered. So on some level, however small, climate change is involved in some way in all of them. But if all weather events have some sort of nudge from a changed climate system, then they're all "consistent" with climate change. So making the consistency argument for one particular set of events doesn't really say very much. What we really want to know is how particular aspects of a changing climate system are contributing to particular kinds of weather events. In their technical report, U.K. Met Office scientists did their best to help us with that. For example:

The climatological context discussed earlier was unusual, with the Atlantic jet stream being more intense and reaching further back into the tropical East Pacific than normal. Those factors in themselves would allow warmer and moister air to enter the storm systems. It is also the case that the sub-tropical Atlantic is now warmer than it was several decades ago and that too would act to enhance the moisture content of the storms.

My translation — with additional elaboration: We know that global warming can theoretically contribute to storminess by heating up the surface of the seas, which leads to increased evaporation and thus more water vapor available in the atmosphere to form storms. We also know that a warmer atmosphere can carry more moisture to begin with. Now, combine that with a jet stream set up in a pattern that would shunt warmer and moister air from the western Pacific into developing storm systems. Then guide those storms along an abnormal path that winds up targeting parts of the U.S. as well as the United Kingdom. That is what the Met Office is saying we've seen this winter. So there are plausible ways that climate change could have played an important role in both storms specifically. But "could" doesn't mean "did." So can scientists say definitively whether climate change acted in that way? The answer is no. Perhaps more important, at least for me (because I don't need yet another example of the impact of climate change to convince me that action is prudent), these two storms should tell us something about how we interpret our experiences of a world that is changing. If the Met Office scientists are right, what we've been experiencing in the U.S. and U.K. are linked through a series of complex climatic teleconnections that span more more than half the globe. This reminds me that the weather I will experience when I step out the door in a little while is not some isolated event. Rather, it may well be the result of a chain of countless complex interactions spanning every latitude and every longitude, and across many different timescales ranging from seconds to decades. It's all connected, all part of a living, breathing planet. It also makes me see the wisdom of the "butterfly effect," at least metaphorically. That's the idea that in a non-linear system, the origins of hurricane in, say, the Atlantic Ocean, can actually be found in the flapping of a butterfly's wings somewhere on the other side of the globe. In this case, the butterfly is that patch of warm water in the Pacific. I also draw another lesson from the Met Office Report: that Earth scientist Wally Broecker of Columbia University's Earth Institute was very wise when he said, "The climate system is an angry beast and we're poking it with sticks." The thing about an angry beast is that it can surprise you. It might actually do something unexpected, something unbeastly. It might, for example, cower for a bit behind a tree. It might even run away. But then it may suddenly rise up with fury and swat you down. Metaphorically, again, I get a sense that this is what we're seeing with the climate system. That isn't an expert scientific assessment. It's coming from both my knowledge of climate science and from intuition. So take it for what its worth.

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