Outlook For The Upcoming UK Winter

By: Christopher O’Reilly

In this post I discuss the outlook for the 2022/23 winter from a UK perspective: what do the forecasts predict and what physical drivers might influence the upcoming winter?

 An important winter

 The price of utilities has risen dramatically over the last year for people, businesses and organisations in the UK. As we move towards winter there is great concern about the effect of these price rises on people’s lives. In the UK, winter temperatures have a strong impact on the demand for gas and electricity. For example, a winter with a 1 degree temperature anomaly results in roughly a daily average gas demand anomaly of 100 GWh over a winter season. In monetary terms, based on the UK October gas price cap (i.e. 10.3p/kWh), this equates to about £1 billion for each 1 degree UK temperature anomaly (though likely much higher due to the higher unit costs for businesses/organisations – not to mention the governments costs to underwrite the price cap). The numbers are pretty big, and the stakes are pretty high.

What do the forecast models predict?

So can we predict what is in store for the UK this winter? Seasonal forecasts out to six months in the future are performed operationally by weather centres across the world. The European Commission’s Copernicus Climate Change Service (or, more snappily, “C3S”) coordinates these long-range forecasts from 7 international centres (including the UK Met Office). When forecasting many months ahead we cannot predict the weather on a particular day, however, forecasts demonstrate some skill in determining average conditions on monthly timescales.

Ideally, we would examine the 2m temperature from the forecasts but these do not demonstrate clear skill over the UK. However, there is skill in the sea-level pressure over the North Atlantic and this can be utilised to provide predictions of the UK temperature (as demonstrated in several previous studies).

Figure 1: (Top) Forecasts of sea-level pressure (SLP) anomaly for the early winter (ND) and late winter (JF) from the C3S multi-model forecasts, initialised at the start of September. (Bottom) Observational SLP anomalies for the early winter (ND) and late winter (JF) during La Nina winters with respect to other years (1954-2022).

For the upcoming winter it is useful to first consider predictions of the large-scale atmospheric circulation because the winter temperatures in the UK are largely determined by wind anomalies (and the associated advection) in the surrounding Euro-Atlantic sector. The multi-model forecasts of the sea-level pressure anomalies for the 2022/23 winter are plotted in Figure 1.

The sea-level pressure anomalies over the North Atlantic exhibit notable changes in characteristics between early winter and late winter. In the early winter period (November-December or “ND”) there are high pressure anomalies across most of the midlatitude North Atlantic, extending into Europe. In the late winter period (January-February or “JF”) there are low pressure anomalies over Iceland and high pressure anomalies further south, more closely resembling the positive phase of the North Atlantic Oscillation or “NAO”, which typically causes warmer winters in the UK. But what is driving these signals?

La Nina conditions in the Tropical Pacific

As we approach this winter, forecasts are confident that we will have La Nina conditions, associated with cooler sea surface temperatures in the eastern/central Tropical Pacific. The observed impact of La Nina on the large-scale atmospheric circulation in the Euro-Atlantic sector shows a clear difference in the early winter compared with late winter. Composite anomalies during observed La Nina years are shown in Figure 1.

The resemblance of this observational composite plot to the predictions from the seasonal forecasts is clear. In the early winter the ridging over the North Atlantic is followed by the emergence of positive NAO conditions in late winter. The La Nina conditions are clearly, and perhaps inevitably, driving the circulation anomalies in the seasonal forecasts and the comparison with observations suggests that this is a sensible forecast.

A possible role for the Quasi-Biennial Oscillation (QBO)? 

Another driver that can confidently be predicted (mostly) several months in advance and can influence the extratropical large-scale circulation is the Quasi-Biennial Oscillation (QBO). The QBO refers to the equatorial winds in the stratosphere that oscillate between eastward and westward phases, which have been shown to influence the large-scale tropospheric circulation in the Euro-Atlantic region. The QBO is currently in a “deep” westerly phase, with strong westerly winds that span the depth of the equatorial stratosphere. Winters with westerly QBO conditions in observations demonstrate a clear signal in early and late winter, both of which project onto the positive phase of the NAO.

A number of studies have shown that seasonal forecasting models capture the correct sign of the relationship between the QBO and the NAO but that it is substantially weaker than in observations. We might therefore reasonably expect/anticipate/suppose that this effect is not adequately represented in the forecasts for this winter.

What does this mean for UK temperatures?

The La Nina and deep QBO-W conditions tend to favour milder winters for the UK, however, there remains significant variability. For example, the record cold period during early winter in 2010/11 occurred during La Nina and deep QBO-W conditions (and was possibly linked to North Atlantic SST anomalies). Nonetheless, the drivers analysed here tend to favour circulation anomalies in both the early and late winter that favour milder UK conditions, and support the signals seen in the seasonal forecast models.

So we can be cautiously optimistic…?

Milder conditions would certainly be welcome this winter in the UK so it’s positive that the forecasts and drivers seem to point in this direction. However, there is of course still a clear possibility for cold conditions. One possible cause would be a sudden stratospheric warming event, in which the stratospheric polar vortex breaks down, favouring the development of negative NAO conditions and associated cold conditions in the UK. An example of this was the “Beast from the East” event in February 2018. Weather geeks get very excited about sudden warmings – and understandably so – but we might hope to forego such excitement this winter. The C3S seasonal models show no clear signal on the probability of a sudden stratospheric warming event occurring at present.

So milder conditions might be on the cards for the UK this winter, which would be good news. But warmer winters also tend to be wetter here in the UK, so at least we’ll still have that to moan about.

A version of this blog is also available with additional figures, references and footnotes here.

This entry was posted in Atmospheric circulation, Climate, Climate modelling, ENSO, North Atlantic, Oceans, Seasonal forecasting, Stratosphere, Teleconnections. Bookmark the permalink.

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