A “winter heatwave” in a warming world

The final week of February 2019 has been characterised by anomalously warm, record-setting conditions over NW Europe. The United Kingdom broke its all-time maximum record temperature for February on several occasions and at several stations – the previous record of 19.7C from 1998 was obliterated, replaced with a new record of 21.2C (a huge difference of 1.5C, which were it to be replicated in August would see the UK experience 40C). For the first time, the UK experienced 20C during a winter month, and this moved the date of the first recorded 20C forward from March 2nd to February 26th. This was by all counts a “winter heatwave”, in magnitude and duration, and widely produced temperatures which wouldn’t be out of place in summer.

At the University of Reading, we also saw a new all-time (since 1908) record maximum for February – the previous record of 17.4C (which was first tied on Feb 25th!) was replaced with 19.5C on Feb 26th.

A classic “heatwave” sunset on February 27 from Whitley Wood Road, Reading, after temperatures reached 17.9C. A touch down on the previous day’s 19.5C, but still 0.4C above the old record!

Why was it so warm?

This is a difficult question to answer, but there’s several components which seem to have been required in order to get the atmospheric configuration such that high temperatures were possible over the UK. Here I present a few that I’ve noticed, but there’s likely other finer components, too (these are not necessarily in any meaningful order):

Rossby wave train: evident in Figure 1, there is a pattern in the 200 hPa height anomalies suggesting a Rossby wave train propagating out of East Asia and the Pacific has been evident for the last week. This provides the enhanced meridional flow associated with blocking weather regimes. Figure 2 also shows anomalously weak 250 hPa zonal flow in the mid-latitudes, suggesting reduced propagation speeds of weather systems allowing for (and associated with) extended blocking regimes.

200z_reanal — **Figure 1:** 7-day (20-26 Feb) mean 200 hPa height anomalies from NCEP/NCAR Reanalysis. Apparent Rossby wave trains are shown with superimposed black arrows.

**Figure 2:** 250 hPa zonal-mean zonal wind anomalies from NCEP/NCAR Reanalysis for 20-26 Feb 2019. Note the anomalously weak zonal winds in the N Hemisphere mid-latitudes.

Extreme eastern USA jet streak & cyclogenesis: the record-setting jet stream winds seen on Tuesday 19th preceded the development of the blocking ridge. This may be associated through the downstream impacts of such extreme winds (Figure 3) – decelerating an unusually strong jet requires a very active jet exit region, leading to strong (anti)cyclogenesis. A series of deep cyclones (Figure 4) developed in the jet exit region, and when combined with other factors aiding their meridional track, the cyclones likely acted to build the downstream ridge, with positive feedbacks, helping to amplify the pattern. HYSPLIT trajectories also suggest some of the air over the UK originated within the extreme jet streak prior to undergoing strong descent, which may have been aided by its unusually strong nature driving unusually strong descent.

**Figure 3:** 250 hPa winds on Feb 19th showing a possible downstream impact of ridge amplification over NW Europe.

**Figure 4:** MODIS view of a 938 hPa cyclone in the central North Atlantic on Feb 20, 2019.

Strong adiabatic descent: HYSPLIT back-trajectories shown in Figure 5 reveal the airmass over the UK originated near the tropopause a few days prior, before descending through the depth of the troposphere. This not only adiabatically warms the air (on top of its warm source region), but also dries out the entire column, allowing for strong insolation needed for the sensible heating to generate strong surface warming.

183836_trj001 — **Figure 5:** Ensemble of 84 hour backwards trajectories for air at 1500 m AMSL over London at 12Z Feb 25th based on GFS 0.5 degree data.

Anomaly persistence: once established, the block lasted for several days. This allowed for further descent of air which also underwent diabatic warming thanks to the intense radiation under cloudless skies – recirculating around the anticyclone (a similar pattern existed during the summer 2018 European heatwave).

These are the weather components which contributed. They describe the prior and contemporary state of the atmosphere. To relate this to the climate, I’ll draw an analogy. You exeperience a car crash. Why? What I have presented so far would be equivalent to saying “You ran a stop sign”. Now we naturally ask, “what about climate change?”. In my analogy, this is asking, “Were you intoxicated?”. Being intoxicated doesn’t mean you will run a stop sign, and you certainly can do so without being drunk, but it will increase your risk of doing so.

There is no doubt that the configuration of the atmosphere during the last week has been extreme, and primed for producing these warm temperatures. However, in a stationary climate we do not expect to break records with the frequency that we are doing, especially given a lengthening record (e.g. Kendon 2014). Now that we have warmed the mean temperatures, an extreme dynamical perturbation to the mean state (e.g. a monster blocking ridge) will produce an even more extreme temperatures than we would have seen beforehand.

This mechanism is supported by looking more closely at the University of Reading’s weather data record (Figure 6 & Table 1). Similar events, even with similar sunshine, have historically produced cooler temperatures. The recent frequency of extremely warm February temperatures is also evident, and you can also see recent cases of very warm temperatures with much less sunshine than older cases that matched the temperatures but only with strong solar forcing – suggesting, as I mentioned earlier, that it doesn’t take as much of a ‘push’ to equal temperatures which were once close to a “theoretical maximum”, such that now we can obliterate those records with sufficiently unusual large-scale anomalies.

sun_tmax_feb — **Figure 6:** Data from Reading University Atmospheric Observatory 1957-2019 showing daily maximum temperatures above the monthly 95th percentile and associated sunshine hours. Red indicates February 2019, grey indicates pre-2000, black post-2000. The 2019 record is shown with a red star. Tmax exceeding 16C is selected for further analysis in Table 1.

tmax — **Table 1:** Data corresponding to the points within the box in Figure 6 plus the 2019 record value.

But sunny, warm weather in February is nice!

Indeed it is – it was my birthday on February 24th, and I never expected to be celebrating it sitting outside! This event didn’t have the same severe impacts as a summer heatwave, but to me it almost felt more disturbing – the knowledge of what this might mean should a similar extreme be generated in the summer months, and that climate change was “eating away” at winter’s very existence. Unusual late winter/spring temperatures mainly impact the natural world which is highly sensitive to temperature and sunshine at this time of year (e.g. Figure 7), and this is why we should care – this could have many wide-ranging impacts on the ecology which supports our existence.