Winter half way point

Almost halfway through the winter, so how do we stand and what does the second half hold in prospect ?

As per original expectation, the winter across much of North-west Europe has been dominated by unsettled cyclonic conditions with a pronounced westerly flow in place. January, the month where wintry potential was least expected, looks like being an exceptionally mild month.

Figure 1: observed sea level pressure anomalies first half winter 2019/20

From the range of potential drivers (or more pertinently the lack of them), this winter seems to be manifesting the effects of a weak or very weak strength El Nino event under conditions where there is little other coherent driving, possibly some influence from North Pacific sea surface temperatures and a more regionalised impact associated with the strong Indian Ocean Dipole.

Crucially, there appears to be little evidence of any influence from the low level of solar activity whilst the Quasi Biennial Oscillation (QBO) is notably in a transitional and non-influential state from west to east phase. The narrative of a hemispheric pattern devoid of any strong, coherent drivers able to stimulate large scale variances across the mid and high latitudes continues.

In the last thirty years, probably only two similar winters to this narrative have been observed, the last being 2006/07. The combined composite for these two years shows excellent symmetry with that observed thus far.

Figure 2: comparison of sea level pressure anomalies for two similar years December – January and those for current winter

The Arctic Oscillation (AO) was negative during the middle part of December but since then has become strongly positive as the tropospheric polar vortex has intensified. January is likely to feature a high end positive Arctic Oscillation value.

Figure 3: Arctic Oscillation behaviour since September

The last time such a high AO index for January was recorded was, interestingly, in January 2007. This underlines how the polar vortex will manifest itself in the absence of destructive interference from favourable tropical forcing and an east phase of the QBO during the winter with a highly volatile index behaviour.

It surprises me that fewer people make use of the Southern Hemisphere as a predictive tool, as it can offer great insight into how the known unknowns and unknown unknowns might play out. With the current drivers in a similar state, the Southern version of the Arctic Oscillation went from +1.47 in the first winter month to -1.08 in the last winter month. The Southern Hemisphere winter time polar vortex therefore started very strong and weakened considerably by the final third.

Figure 4: Southern Hemisphere Polar Vortex behaviour during the last winter

Whilst there will be some differences in timing of the peak positive polar vortex, the broad gist suggests that there will be a weakening trend in the strength of the polar vortex, and with it, an increasing trend for more traditional wintry weather across North-west Europe. During 2007, the three AO index values recorded from December through February were +2.28, +2.03 and -1.3.

Changes in the pattern of tropical convection (intense thunderstorm activity) can have a predictive value of over a month. In the last week, a coherent tropical wave has emerged in the Western Pacific and is forecast to move steadily east over the next few weeks. This will echo what happened in January 2007 where the tropical signal largely remained across the central part of the Tropical Pacific.

Figure 5: Observed pattern of tropical convection during January 2007
Figure 6: Forecast tropical convection into January

The long lead expectation has always been for February to hold the greatest potential for wintry outbreaks in North-west Europe and half time analysis continues to suggest this.

With 2007 seemingly offering very good similarity on a number of fronts, it is worthwhile looking at the temperature anomalies for that month.

Figure 7: temperature anomalies for January 2007 (left) and February 2007 (right)

Compare the temperature map for January with that of February 2007. Whilst western Europe remained milder than average, Eastern Europe and Scandinavia cooled significantly. This pattern is likely to repeat again although it’s worth stating that the above average temperature plot for Western Europe and particularly the UK for February masked some strong intramonthly variations and the pattern was much more wintry compared to the previous January – it was the lack of any pre-established cold air across continental Europe that was largely driving this milder outcome in February for Western Europe.

So the expectation for the final half of the winter would be for a continuation of the milder conditions across NW Europe during January with a shift to colder weather outbreaks in Eastern Europe and Scandinavia during February, with colder episodes extending westwards across the UK at times. With the Arctic Oscillation remaining strongly positive into January, drier conditions are likely to characterise the remainder of January but a shift back to more unsettled conditions is expected during February. For the first time this winter, snowfall for much of Central and North-western Europe becomes a much more realistic proposition during the final month.

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