Researchers may have found the missing link that explains why a warming world is likely to result in more extreme weather.
More research is needed, but the explanation is stunningly logical. It starts with this simple fact: The Arctic is warming faster than the central part of the Northern Hemisphere. As it warms, there's less of a temperature difference between the Arctic and areas further south.
That's no small matter.
This temperature difference is critical to weather because it creates the jet stream.
Simply put, the jet stream is the boundary between cold Arctic air and warm tropical air. The sharper the difference in temperature, the stronger the jet stream. The stronger the jet stream, the more power it has to move in a rapid, straight line. The weaker it gets, the loopier it gets.
Below is an e-mail interview with an atmospheric scientist who is conducting the cutting- edge research that may explain why a warming world is leading to more extreme weather.
This e-mail with Jennifer Francis of Rutgers University grew out of an earlier interview that I did with her in March.
The underlying phenomenon of rapid warming in the Arctic is known as Arctic amplification.
The research focuses on weather in the mid-latitudes. In North America, that's the central part of the continent.
Here's the interview:
Q. Do you see the same factors at play with the jet stream now, as you did in March 2013, and thus would you draw a connection between the changes in the Arctic and the behavior in the jet stream now? (Currently the jet stream has a bulge that is contributing to heat and fires in the Southwest and a dip bringing coolness, floods to the Midwest and upper Midwest.)
A. In both cases the jet stream was in a very amplified pattern, i.e., there were large north-south swings in its path around the Northern Hemisphere. The causes for this are many and difficult to separate, but I believe there is a link to Arctic amplification (AA: the enhanced warming in Arctic relative to mid-latitudes).
The factors contributing to AA, however, are different in March than they are in May-July.
In March we believe the major factors are extra water vapor in the atmosphere compared to a few decades ago (water vapor is a greenhouse gas, and it also releases heat when it condenses into clouds, which also trap heat below them like a greenhouse gas) and reduced sea ice.
In May-July, an important factor may be the rapid decline of snow cover on high-latitude land areas. The rate of snow loss in May and June exceeds that of sea ice loss in summer! Earlier snow loss means that the soil underneath will be exposed to the strong spring sunshine earlier, which will allow it to dry out and warm earlier, effectively contributing to the bubble of warm air over high-latitude land areas. We think this helps the ridges extend northward, increasing the chances of heat waves and droughts over the continents.
Q. A parenthetical, locally driven question: Do you have an opinion about whether the heavy rains in May 2011 in the upper Missouri River basin (Montana) were another manifestation of this type of jet stream/Arctic influence? Those rains led to historic flooding along the Missouri River valley.
A. It looks like the flooding that year was due to a combination of two main effects: melt of a very heavy snow cover upstream and very heavy rains later in the month. It does appear that a persistent trough was located just west of Missouri, and it's very possible that its persistence was linked in part to AA that year. Hard to say for sure, though.
Q. Bottom line: Can you assess whether a “new normal” has arrived? This may be premature to ask, but have we shifted toward a jet stream that meanders more and travels less often in a straight line?
A. We are definitely in a new normal because of a variety of climate-change-related factors: warmer atmosphere (more energy for storms to work with), more water vapor (more energy and more moisture for storms to work with), higher sea levels, warmer oceans (more energy for tropical systems and contributes to higher evaporation rates), and AA.
While we still can't say for sure that AA is the main factor or even an important factor driving the increasingly amplified jet stream in recent years, evidence is piling up that suggests that it is. Given the Arctic's key role in the Earth's energy balance, it's inconceivable that it's NOT having an impact, but the details are a topic of active research.
Q. Absent the effects of shrinking Arctic sea ice, is there another explanation for what's causing the jet stream to take on a different personality?
A. As I mentioned above, both increased water vapor and spring snow loss contribute to the AA we observe in seasons other than fall/early winter, when sea ice loss has its largest effect. The differential heating rates of continents versus oceans may also be disrupting the normal jet stream patterns, and there are other large-scale oscillations in the atmosphere/ocean system that undoubtedly modulate the effects of AA, such as ENSO, the PDO (Pacific Decadal Oscillation), etc.
Untangling all these influences is another hot research topic.
Q. Anything else?
A. Related to the previous question, I think it's interesting that ENSO (El Niño and La Niña) has been in a nearly neutral phase for much of the past year, so it could be that its lack of influence on large-scale weather patterns is allowing us to see a stronger expression of AA. When either a La Niña or El Niño does come back, the locations of ridges and troughs (bulges and dips in the jet stream) may shift dramatically...I think AA will always have the effect of causing those features to be larger than in previous decades.
Mother Nature certainly is giving us a lot to talk about these days!