On Monday, I wrote about a study that pushed back against a theory that a warming Arctic and melting ice are leading to a more volatile jet stream and an increase in extreme weather in the U.S. and elsewhere.
Jennifer Francis, an atmospheric scientist at Rutgers University and leading proponent of the theory, has sent me some comments vigorously defending it.
Her comments are technical, but I’ll try to extract the key points (I reproduce the comments, in their entirety, at bottom of this post).
Francis begins by questioning the motivation of Elizabeth Barnes, the author of the study which claims the link between Arctic warming and extreme weather is “an artifact of the methodology” and “unsupported by the observations.”
She calls Barnes’ approach “less than objective” and “a direct attempt to disprove work” she authored.
Francis’ take-home point is that Barnes’ work actually backs up her theory, rather than refuting it.
“While her overarching conclusion is that the connections between [Arctic amplification] and mid-latitude extreme weather are unfounded, I see a great deal of support for our results in her new work,” Francis writes.
The data are a bit noisy and subject to differences in interpretation, but in Barnes’ work Francis sees evidence for an increase in the jet stream’s waviness and a slow down in its speed. It’s these changes in jet stream characteristics that Francis believes are making the weather more extreme in the mid-latitudes.
Francis concedes the connections between mid-latitude weather patterns and Arctic warming are “clearly not simple” and that “we still have much to learn.”
She closes by taking a shot at Barnes: “I welcome and appreciate Dr. Barnes’ contribution to the community’s efforts to understand the effects of [Arctic amplification] on large-scale circulation changes, but perhaps a more balanced approach to interpreting the results could be applied going forward.”
My take: I see both sides here. Francis’ work is intriguing and the manic weather we’ve experienced in recent years certainly seems consistent with the idea the jet stream is behaving erratically. On the other hand, the basis for the work is just 10 years or so of data, placing it firmly in the speculative category. Barnes’ work shows the signal of Francis’ theory is ambiguous over longer time scales. It has yet to pass the test of time.
Here are Jennifer Francis’ comments in full…
Comments on Barnes 2013: Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes. GRL. by J.A Francis
I am pleased that Dr. Barnes, a respected and talented atmospheric dynamicist, has taken an interest in the topic of linkages between the rapidly changing Arctic and the large-scale circulation. The emerging influence of Arctic amplification (AA) on mid-latitude weather patterns is complex, and her expertise will help resolve some fundamental dynamical questions that are relevant to understanding mechanisms driving these linkages as the Arctic continues to warm faster than elsewhere.
What perplexes me, however, is that her intent in interpreting the new results in Barnes (2013) seems less than objective and is a direct attempt to disprove the work presented in Francis and Vavrus (2012; hereafter FV12). A very different interpretation of the results could be made. While her overarching conclusion is that the connections between AA and mid-latitude extreme weather are unfounded, I see a great deal of support for our results in her new work. For example:
Figure 2 presents time series of wave amplitudes (or extents) measured using two methods: one similar to ours and an alternative based on seasonal latitude differences. In all cases the trends are positive, suggesting an increase in amplitude during fall and summer, albeit only some of the trends are statistically significant. Because AA has emerged from the noise of natural variability only in the last 15 year or so, it is not surprising that its influence would not drive 30-year trends in a statistically significant way. Note that her new method does exhibit significant trends. This supports FV12.
My interpretation of the results in her Figure 3 is that in the ranges of 500 hPa heights that typically occur in mid-latitudes during summer (5.6 to 5.8 km) and autumn (5.5 to 5.7 km), the wave amplitudes are increasing from the early to the later part of the record. This, once again, supports FV12. She claims that because warming is shifting a particular height contour northward, it is incorrect to conclude that wave amplitudes are increasing. In fact, it is this northward shift – in particular the larger shift in high latitudes where warming is greatest – that we hypothesized would be a factor causing the waves to elongate.
Figure 4 presents measures of wave phase speed. While FV12 did not present wave speeds, we speculated that larger amplitude waves should have slower wave speeds. Her measure of phase speed for waves at 500 hPa slows with time, supporting our speculation. She then measures speeds at the 250 hPa level and finds no change in speed. This much higher level is near the tropopause, often above the jet stream, and can be affected by dynamics of the stratosphere. The stratosphere is cooling with increasing greenhouse gases, leading to very different dynamical changes. Why did she choose to analyze this level? My only guess is to deliberately cast doubt on FV12.
The mechanisms linking Arctic amplification with large-scale circulation patterns are clearly not simple and we still have much to learn. These new results provide additional insight into those linkages, but it appears that the interpretation of these results in Barnes (2013) was conducted with a particular intent. I welcome and appreciate Dr. Barnes’ contribution to the community’s efforts to understand the effects of AA on large-scale circulation changes, but perhaps a more balanced approach to interpreting the results could be applied going forward.