Temperatures in the UK fell rapidly again on Thursday. I watched the thermometer drop from about 7°C mid-morning to -1°C by 4pm and another 5-6 inches of global warming accumulated by late evening. More snow also fell on Friday. Blame a negative North Atlantic Oscillation. In fact it is rather disturbing to find that the NAO index has been negative since December 2009. So we had a cool summer and now another chilly winter. Not since 1968/69 (Figure 1) have we had this happen for such a long duration.
I can’t better the recent discussion provided by Joe D’Aleo over at Icecap and WUWT about the Arctic Oscillation. The shift between strong polar highs and lows affects the behaviour and position of the Polar Jetstream and the position and strength of the Atlantic pressure systems also (Figure 2). Although this is a variable system (weather) that changes on a period of days/weeks/months; at the moment the negative phase dominates and has done for quite some time. It would also be true to say that the effects of a negative NAO are felt more acutely in the UK in winter than in summer.
The NAO is described as having a multidecadal component in winter (Figure 3) such that the negative phase was dominant in the 1950s through to the late 1970s and the positive phase has been dominant for the last 30 years. So of course the question is – are we now experiencing returning to the dominance of the negative phase and all that may bring?
One of the tenets of global warming is that the GHG-induced warming is likely as a cause of the recent prolonged dominance of the positive NAO.
Over the last 3 decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values. Much remains to be learned about the mechanisms that produce such low frequency changes in the North Atlantic climate, but it seems increasingly likely that human activities are playing a significant role. Visbeck et al., (2001) 
More than that, much of the research suggests that the positive phase will also dominate in the future. For example Spangehl et al (2010)  reports on the use of simulations using a stratosphere-extended general circulation model (bold mine):
“One simulation is driven by changes in total solar irradiance due to solar activity as well as volcanic eruptions and changes in greenhouse gas (GHG) concentrations. A second simulation additionally includes changes in short-wave heating due to prescribed photochemical changes in ozone. […] Both simulations reveal a shift of the North Atlantic Oscillation toward a more positive phase from the Maunder Minimum to present day, mainly attributed to anthropogenic increase in concentration of well-mixed GHG. Increase in GHG is related to a more disturbed stratospheric polar vortex resulting in an only moderate strengthening of tropospheric westerlies over Europe compared with the tropospheric version of the model.”
The effects of a positive NAO on the Arctic sea ice (Figure 4) are interesting. We hear again and again that the climate has reached a tipping point, that in summers the Arctic will be ice-free etc etc. and again this insistence that the positive phase NAO will continue to be dominant seems to be a major factor in this projection.
Climate Change natural causes and anthropogenic influences, from which Figure 4 is taken, explains the implications of a positive NAO, including changes in Arctic sea ice cover. The left-hand panel of Figure 4 looks to me remarkably like the sort of distribution of sea ice we have been seeing in the past couple of summers – and note that this figure was published in March 2000. If we are entering a period when a negative NAO is dominant, this should help the sea ice recovery by both reducing the ingress of warmer Atlantic water and the conditions that promote transport of ice out of the Fram Strait.
With the current prolonged cold in the (sub-Arctic) Northern Hemisphere, is it frequently pointed out that the temperature in the Arctic is ‘above normal’ compared to the recent long term average (1971-2009). Yet this would have been ‘normal’ in the 50s, 60s and 70s and still the Arctic ice not only survived but increased.
All the over reliance on models and overestimation of the effect of CO2 within models can’t be good for us 😉 We don’t know what drives the AO and NAO between negative and positive phases, but I doubt that CO2 has anything much to do with it.
Now about that title…
Now is the winter of our discontent
Made glorious summer by this sun of York;
And all the clouds that lour’d upon our house
In the deep bosom of the ocean buried.
Now are our brows bound with victorious wreaths;
Our bruised arms hung up for monuments;
Our stern alarums changed to merry meetings,
Our dreadful marches to delightful measures.
(Richard III, William Shakespeare)
This describes the ascent to the throne of Edward IV (“Blazing Sun“, son of York). The meaning is that a time of unhappiness is past and better times are ahead. [This was the start of the period of English history known as the “Wars of the Roses” between the houses of York and Lancaster, each eyeing the throne.]
So, a bit obscure, but the title is an allegorical aspiration hoping that better times are ahead here too, not just a dominant negative NAO and cooling climate, but also a cooling of the climate debate, more dialogue and increased understanding.
 Martin H. Visbeck, James W. Hurrell, Lorenzo Polvani, and Heidi M. Cullen (2001) The North Atlantic Oscillation: Past, present, and future. PNAS vol. 98 no. 23:12876–12877
 Spangehl, T., U. Cubasch, C. C. Raible, S. Schimanke, J. Körper, and D. Hofer (2010), Transient climate simulations from the Maunder Minimum to present day: Role of the stratosphere, J. Geophys. Res., 115, doi:10.1029/2009JD012358.