The extraordinary series of storms that hit the UK in January and February 2014 was due to the position of the Northern Polar Jet Stream. It was stuck in a position too far south. The image above is for 20 Feb 2014, when things have eased up a bit. Over the past weeks there has been a dark streak of wind sitting in mid-Atlantic.
If you click on the link under the image, it takes you to the excellent netweather.tv site, where you can look at a lovely JS forecast every day.
The question is, why is the Jet Stream (JS) behaving like this?
As ever in the case of climate, there is more than one factor at work. Climate sceptics stumble every time on this point. They think that climate scientists think only in terms of greenhouse gases. This is emphatically not the case. They try to gather in as many factors that they know about. Then they try to quantify the effects in computer models, and improve the models until they approach ever closer to reproducing the observed reality.
Jet Stream work is exciting, in that it is beginning to cross the bridge between climate and weather. It is new work, and therefore the uncertainties are high.
There are two main lines of enquiry: the impact of Arctic Warming on the JS, and the effect of the weakening sun.
In 2012, Jennifer Francis and Stephen Vavrus published Evidence linking Arctic amplification to extreme weather in mid-latitudes. They suggested that the fact that the Arctic was warming faster than the rest of the atmosphere weakens the temperature gradient between mid latitude air and Arctic air. This causes the Jet Stream to become more wavy, pushing great loops further south, which also tend to get stuck in one position. This is a good explanations for many extreme weather events not just in 2014, but over the past few years, not least the dull wet summers we had in the UK 2011/2012.
Elisabeth Barnes (EB) tested the hypothesis and claimed it was wrong, and that Jennifer Francis (JF) had misinterpreted some aspects of the data.
JF came back at EB, and claims that Barnes’ work actually backs up her theory, rather than refuting it. Her reasoning is on this page, and it seems convincing. EB was being overly critical.
Screen and Simmonds also failed to give JFs work a full confirmation. They concluded
" possible connections between Arctic amplification and planetary waves, and implications of these, are sensitive to how waves are conceptualized. The contrasting meridional (N-S) and zonal (E-W) amplitude trends have different and complex possible implications for mid-latitude weather, and we encourage further work".
Weakening Solar Activity
Another possible factor is that the Sun is entering a weaker, less active phase.
Here is a long term look back at sunspot numbers:
Here is a plot of Solar activity since 1885 (red), with temperature (blue) just to show how poor the relation is between solar activity and global temperature.
However, Mike Lockwood of Reading, a solar scientist, finds that solar activity is falling very rapidly. Below is an image created on the excellent www.WoodforTrees.org site. It is from 1975 onwards, and shows sunspots in green, Total Solar Irradiance (TSI) in red (they are both indices of solar activity) and global temperature in blue. Clearly there is a pretty significant falling off in solar activity. This falling off probably accounts for part of the hiatus in global warming that we are experiencing. Deniers are predicting that it will usher in a new Ice Age, but the greenhouse forcing is stronger than the reduction in solar forcing, so it is unlikely that we will see Ice Fairs on the Thames again.
This is not a case of either-or, of either a cool sun and Arctic warming. Both hypotheses are under examination, and of course, in science, hypotheses are never "Proved", only "not-yet-disproved".
Global weather is certainly weird, that is a fact. There must be a cause or causes for this fact. Whatever the final outcome, global warming is a problem, one that is bound to affect our weather in the end. Sea level is rising, rainfall is intensifying, that is undeniable. If it is concluded that warming is a significant factor in Jet Stream aberrations, surely even mainstream politicians will see that it is time for us to stop messing about with atmospheric chemistry, and make the transition to a carbon free world economy.