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Post by swamprat on Feb 28, 2018 21:21:05 GMT -6
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Post by swamprat on Mar 19, 2018 8:58:04 GMT -6
Warm Arctic episodes linked with increased frequency of extreme winter weather in the United StatesJudah Cohen, Karl Pfeiffer & Jennifer A. Francis Published online: 13 March 2018
Abstract Recent boreal winters have exhibited a large-scale seesaw temperature pattern characterized by an unusually warm Arctic and cold continents. Whether there is any physical link between Arctic variability and Northern Hemisphere (NH) extreme weather is an active area of research. Using a recently developed index of severe winter weather, we show that the occurrence of severe winter weather in the United States is significantly related to anomalies in pan-Arctic geopotential heights and temperatures. As the Arctic transitions from a relatively cold state to a warmer one, the frequency of severe winter weather in mid-latitudes increases through the transition. However, this relationship is strongest in the eastern US and mixed to even opposite along the western US. We also show that during mid-winter to late-winter of recent decades, when the Arctic warming trend is greatest and extends into the upper troposphere and lower stratosphere, severe winter weather—including both cold spells and heavy snows—became more frequent in the eastern United States.
Introduction Variability in the day-to-day weather is due to a combination of forced and natural variability. Forced variability results from boundary conditions, such as sea-surface temperatures, and natural or internal variability results from the chaotic nature of dynamical systems. While the tropics are usually considered the main driver of boundary-forced variability, recent studies have argued that the Arctic is playing an increasingly important role as a boundary-forcing agent owing to its accelerated warming relative to other regions of the globe.
Increasing greenhouse gases are contributing to a general warming of the atmosphere and oceans globally. Over recent decades, warming has dominated global temperature trends during three of the seasons. In winter, however, cooling trends have been observed across Eurasia and the eastern US along with rapid warming in the Arctic. This seesaw winter temperature pattern is known as the “warm-Arctic/cold-continents pattern”. A vigorous debate in the climate community is whether and/or how much the Arctic can influence mid-latitude weather and, in particular, whether a warmer Arctic increases the likelihood of severe cold spells in the mid-latitude continents.
Anthropogenic global warming is widely expected to increase certain types of weather extremes, including more intense and frequent heat waves and droughts as well as heavy precipitation events. Surprisingly, however, over the past two to three decades, the increase in extreme weather has included more (not fewer) severe cold-air outbreaks and heavy snowfalls observed both in North America and Eurasia.
Previous studies have shown qualitatively that anomalously high geopotential heights across the Arctic are linked with extreme weather events across the mid-latitudes in winter and even into spring. However, those studies were limited to just a few months of one particular year. Here we present a more extensive, quantitative analysis of the link between Arctic variability and severe winter weather across the mid-latitudes. In this study we find a robust relationship between Arctic temperatures and severe winter weather in the United States. When the Arctic is warm both cold temperatures and heavy snowfall are more frequent compared to when the Arctic is cold. We also found that during the period of accelerated warming when the Arctic warming reaches into the upper troposphere and lower stratosphere during mid-winter to late-winter severe winter weather has been increasing.
Results
Metrics analyzed We employ three metrics to diagnose the relationship between Arctic temperatures and severe winter weather. The first two are called the polar cap geopotential height anomaly (PCH) index and the polar cap air temperature anomaly (PCT) index. The PCH and PCT indices measure the area-averaged geopotential height and temperature anomalies poleward of 65° N and from 1000 to 10 hPa. Both PCH (units in meters) and temperature (units in °C) are normalized by their standard deviation. The PCH values incorporate air temperature and surface pressure, thus combining both thermodynamic and dynamic influences. PCT reflects only thermodynamic effects.
The third metric is the Accumulated Winter Season Severity Index (AWSSI). We analyzed changes in daily and cumulative AWSSI in relation to changes in PCH/PCT at diverse geographic locations in order to explore the relationship between Arctic variability and severe winter weather (Methods section). The AWSSI diagnoses severe weather owing to extreme snowfall and temperatures at individual stations across the US. It is reported as an accumulated value throughout the winter season, which informs comparisons of weather severity between years. Daily accumulated changes in AWSSI provide insight into episodic severe winter weather. For our study, the AWSSI is advantageous because it integrates both intensity and duration of temperature, snowfall, and snowcover into one index to measure weather severity across seasons and stations. However, the thresholds used to create the index are somewhat subjective. The AWSSI index is incremented based on thresholds of maximum and minimum temperature, snowfall and snow depth. Because the AWSSI index is not increased unless temperatures fall below freezing and snowfall or snowcover exists, the index better represents winter weather variability in cities that experience colder temperatures and/or heavy snowfall, such as those in the mid-west relative to those in the southern US or the west coast.
Arctic variability and mid-latitude weather The daily change in seasonal AWSSI (or the daily accumulation for that day) is composited for all standardized PCHs at 12 representative cities across the US (see Fig. 1 for the geographic distribution of the chosen stations) by computing the mean change in AWSSI associated with daily PCH values at each isobaric level during winter (DJF) from 1950 to 2016 (Fig. 2). A strong relationship between a warmer Arctic and increased frequency of severe winter weather is apparent for all stations east of the Rockies, with the strongest association in the eastern third of the US, where we find a statistically significant (p < 0.01) and nearly linear relationship between Arctic height changes throughout the troposphere and AWSSI. When Arctic heights are at their lowest (PCH < ~ −1), severe winter weather is unlikely. For larger values of PCH (PCH > +1), the likelihood of severe weather increases, with correlations peaking when the PCH is greater than +1.5. This relationship is fairly consistent throughout the troposphere over the full range of Arctic height anomalies. The correlation generally holds in the stratosphere (below the 30 hPa surface) as well. In the Rockies and along the west coast, however, the relationship is weak, and some stations even exhibit the opposite relationship, i.e., a relatively warm Arctic favors milder winter weather. This result is consistent with the predominance of an anomalous western ridge during the recent period of pronounced Arctic warming.
In deciding whether you believe it, it’s important to realize that measurements are in a different category in science from analysis. The March 13 study – authored by Judah Cohen of MIT, Karl Pfeiffer of Atmospheric & Environmental Research, Inc. and Jennifer Francis of Rutgers – used the tools of science to analyze existing data. These authors described their study as an: "… extensive, quantitative analysis of the link between Arctic variability and severe winter weather across the mid-latitudes. In this study we find a robust relationship between Arctic temperatures and severe winter weather in the United States. When the Arctic is warm, both cold temperatures and heavy snowfall are more frequent compared to when the Arctic is cold."
See Next Post for Page 2
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Post by swamprat on Mar 19, 2018 9:01:53 GMT -6
Page 2
Jennifer Francis commented in a statement from Rutgers on March 13: "Basically, this confirms the story I’ve been telling for a couple of years now. Warm temperatures in the Arctic cause the jet stream to take these wild swings, and when it swings farther south, that causes cold air to reach farther south."
These researchers found that when Arctic warming occurred near Earth’s surface, the correlation with severe winter weather was weak. But when the warming over the Arctic extended high into the atmosphere, into the stratosphere, disruptions of the stratospheric polar vortex were likely.
Global warming skeptics pounced on this study. For example, Steven Milloy – who is, among other things, the publisher of the website JunkScience.com, a former columnist for Fox News and a co-creator and manager of the Free Enterprise Action Fund – posted the following tweet, which lifts language directly from Cohen et al.’s Nature paper:
He’s pointing to the part of Cohen, Pfeiffer and Francis’ own study where they themselves point out some of this study’s unknowns and challenges, and, by extension, some of the unknowns and challenges inherent in modern climate science. Do these acknowledged unknowns and challenges undercut this study – or almost all climate studies – as Milloy suggests?
Let’s look at the answer in a wider context. Does asking scientific questions in any field suggest that area of science is not worth pursuing? Of course not.
If it did, science as a whole would have come to a dead standstill long ago, and our lives would be far less easy and comfortable than they are today. Think of electricity. Do you suppose Thomas Edison had questions, as he figured it out? Do you think he might have had challenges?
The fact is, scientists are supposed to question themselves and each other. They’re supposed to work through challenges. It’s what they’re trained to do. It’s how science gets done. It might be helpful here to mention that all science is a process, as all scientists and many non-scientists know. Scientists question, and try to answer their own questions or learn how other scientists have answered them, and this constant questioning-and-answering pushes their investigations of nature forward … or, I should say, our investigations of nature, since science is a cultural activity, paid for in large part by our tax dollars.
Do global warming skeptics like Steven Milloy understand that questioning is part of the process of science? I have no idea. It’s possible he doesn’t; he’s trained as a lawyer, not a scientist.
Should we believe that Arctic warming is correlated with colder winters as this study suggests? Belief or disbelief doesn’t enter into it, for scientists, and it shouldn’t for you either. The results are just out there, for you and me to read about, be informed about and think about, and for future scientific studies to either confirm or refute.
This study is one small clue in the investigation of climate change, which has already been going on for decades. Will this one little clue be swept away by better ones? Maybe. Time will tell.
Until then, claims that the study has been undercut because scientists are questioning themselves and each other … well, those claims just show some writers’ igannance of – unawareness of, unconsciousness of, unfamiliarity with, inexperience with, lack of information about – the way science works. It may be willful igannance, or not.
By the way, someone is bound to ask in the comments who supported the study by Cohen, Pfeiffer and Francis. That is a valid and excellent question. For virtually any published science study, you can find a section at the very bottom called Acknowledgements. These authors’ acknowledgements are as follows: "We are grateful to Barbara Mayes-Boustead and Steve Hallberg for generously sharing with us the AWSSI [Accumulated Winter Season Severity Index] data. J.C. is supported by the National Science Foundation grants AGS-1303647 and PLR-1504361. J.F. is supported by NASA grant NNX14AH896 and NSF/ARCSS grant 1304097."
In his criticism of the study, Steven Milloy didn’t have as established a mechanism for mentioning who currently funds him, but he’s well-known for having been a paid advocate for Philip Morris, ExxonMobil and other corporations. You can read more about who funds Milloy here: www.sourcewatch.org/index.php/Steven_J._Milloy#Funding
Bottom line: A warm Arctic means colder, snowier winters in the northeastern U.S., according to a new study. You don’t have to believe it; just think about it.
You can look at more visual aids and see the data links in the actual article here: www.nature.com/articles/s41467-018-02992-9#Fig1
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