Een video van Simon Clark
The answer may surprise you!
Recently the media has been abuzz with talk of the 'polar vortex' which they describe as being an outbreak of cold air over the United States and Canada. In this video I break down what the polar vortex is and isn't, talk about why it's suddenly so extremely cold, and ask whether this weather was caused by climate change.
I'd like to stress that this video is specifically looking at a) what the polar vortex exactly is, and b) whether the vortex is experiencing sudden warmings more frequently under climate change. There are however, as I hint in the video, multiple factors forcing all events in the atmosphere, including the current cold snap. The jet stream is ultimately the main mechanism (as forced by the sudden warming) and the effect of climate change on the jet stream itself is still debated. See for example this thread from Dr Amy Butler: https://twitter.com/DrAHButler/status.... The conclusion that I draw on this video is perhaps phrased too generally - in my opinion the SSW, which was a significant factor in the current cold snap, was not made more likely by climate change. There of course remains the possibility that other forcing factors, such as the long-term average strength of the jet stream, *were* altered by climate change to make the cold snap more likely. Again this is just my personal opinion, and I encourage informed debate in the comments!
- (1) Wind on 10hPa surface. Created using the excellent http://earth.nullschool.net
- (2) I will concede that there is a certain degree of ambiguity in the literature about the term 'polar vortex', with some referring to the low pressure system in the stratosphere using the term. Most however use the term to refer to the circulation itself, sometimes also called the polar night jet. It is however NOT a surface or even tropospheric phenomenon.
- (3) See Butler et al, 2017 https://www.earth-syst-sci-data.net/9/63/2017/essd-9-63-2017.pdf
- (4) My PhD thesis: https://www.simonoxfphys.com/blog/2018/10/15/my-phd-thesis
- (5) Baldwin (my PhD supervisor) and Dunkerton, 2001: http://science.sciencemag.org/content/294/5542/581.full
- (6) See, e.g., https://en.wikipedia.org/wiki/Thermal_wind
- (7) Temperature anomalies, 1880-2015: https://www.youtube.com/watch?v=gGOzHVUQCw0
- (8) Ayarzagüena et al, 2018: ttps://www.atmos-chem-phys.net/18/11277/2018/acp-18-11277-2018.pdf
- (9) As I say in my interjection, there is evidence to suggest that the jet stream is meandering more due to a warmer Arctic, though that's not the focus of this video. Remember that for simplicity I'm only focusing on the influence of the SSW! See e.g. Francis and Vavrus, 2015. https://iopscience.iop.org/article/10.1088/1748-9326/10/1014005.pdf
- (10) Graphs from http://www.theclimateconsensus.com/content/satellite-data-show-a-cooling-trend-in-the-upper-atmosphere-so-much-for-global-warming-right
- (11) Note that I'm not saying stratospheric cooling is why tropospheric warming is not affecting the vortex! I'm simply using this as an example of how two contributing factors to the dynamics can be forced in opposite directions by climate change.