Category Archives: Weather Events

The link between climate change and Britain’s winter storms

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This article originally appeared in the February 16th 2020 edition of The Sunday Times, and was co-written with Shingi Mararike.

Every winter Britain gets hit by a series of storms. Ciara and Dennis are just the latest — but with two key differences.

The first is their strength. Our storms get their energy from temperature gradients in the atmosphere over north America. Recently that gradient has been much greater than normal.

The second is good PR. In 2015 the Met Office decided to start naming storms, which gave them a much higher media profile.

In 1993 Britain had the powerful Braer storm. In 2013-14 we faced about 12 of these weather systems. But in other years there are hardly any, which is why this year might be feeling so extraordinary to people coping with flooding, high winds and lots of rain.

A key question: why does the number vary so much?

Part of the answer lies in the jet stream, the powerful westerly wind blowing about six miles above us which, driven by that steep temperature gradient, has accelerated and got bigger. That energy feeds into our storms.

On their own, Ciara and Dennis are not symptomatic of climate change or a global weather crisis. What climate change does is to alter the likelihood of such events. Computer models of the impact of climate change predict an increase in winter rainfall for the UK, along with warmer atmospheric temperatures and changes to the tracks followed by storms across the north Atlantic. This year may not be a sign of things to come, but we will probably see more severe winter flooding in future.

January 2020 was the warmest or second warmest on record in every global temperature dataset. It was rivalled only by 2016, when there was a strong El Niño event in the Pacific that temporarily raised global temperatures. Given that there is no El Niño this year, these record global temperatures — up to 1.5C above pre-industrial levels — are a cause for concern.

It emphasises the rapid warming of the planet. These record temperatures are consistent with recent events such as the Australian wildfires, the rising temperatures in the Antarctic and the unprecedented lack of ice and snow in parts of Europe.

What does this mean for Britain’s weather? So far the world has seen warming of about 1C. That is going to continue and the best guess is that the world could be — in a worst-case scenario — 4C-6C warmer by 2100.

That may not sound much – but multiplied by the area of the planet it means that the atmosphere will hold an enormous amount more energy.

That energy will not only be felt as heat. It will also power our weather like never before. That means more and bigger storms, stronger winds and changes in the temperature of the oceans, which will make the sea levels rise. If our weather is exciting now, it may soon be overwhelming.

An earlier version of the article implied a greater likelihood of 4-6C of warming. This is at the very top-end of climate projections, following the extreme RCP8.5 scenario, and is not the most likely outcome now – but still possible.

A “winter heatwave” in a warming world

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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.

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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.
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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.

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

 

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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.
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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.

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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.

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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.

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Figure 7: Blackthorn blossom, complete with Honey Bee (if you look closely), on Feb 23rd in Reading. This blossom is more likely in March and April.

NCEP/NCAR anomaly plots credit https://www.esrl.noaa.gov/psd/map/.

The January 2009 SSW

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This blog was originally published on 24 January 2019, and updated on 24 January 2020.

January 24th, 2009. This was the ‘central date’ (defined as the day on which the 10 hPa 60N zonal-mean zonal wind reverses from westerly to easterly) of a remarkable, record-breaking major Sudden Stratospheric Warming event, and there are several reasons why this event is worth a revisit.

The Jan 2009 event set a large number of significant records in the stratosphere – and it still holds almost all of these to this day, despite strong competition from warming events in 2016 and 2018.

The aspect of the event that I always recall is the monumental (you might say…stratospheric) deceleration of the 10 hPa 60N zonal wind, which you can see in Figure 1. In early January 2009, the stratospheric polar vortex was date-record strong, with westerly zonal-mean winds ~70 m/s. Only a few days later, they were all-time record-weak, with zonal-mean easterlies of ~30 m/s. Only the final stratospheric warming (FSW) event of March 2016 comes close to rivalling 2009’s easterlies – the event is in the clear in terms of SSWs that are ‘major mid-winter warmings’. The mean deceleration rate between the peak (Jan 8th) and trough (Jan 28th) was an astonishing 10 m/s/day!

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Figure 1: 10 hPa 60N zonal-mean zonal winds for 2008-09 from MERRA-2 (via NASA Ozonewatch).

Associated with this rapid deceleration of the vortex was a rapid warming. This is the most wonderful example of why we call them “sudden warmings”. On January 12th, the mean 60-90N (polar-cap) 10 hPa temperature was 202K. On January 23rd, it was 253K – a rise of 51K in 11 days! The peak of 253K remains a satellite-era record for this region of the stratosphere, as you can see in Figure 2.

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Figure 2: 10 hPa 60-90N average temperatures for 2008-09, according to MERRA-2 data.

Of course, to produce such a huge warming, you need a massive heat flux (which also indicates huge amounts of wave activity propagating into the stratosphere, through the Eliassen-Palm relation). The 45-75N heat flux at 10 hPa was another metric which hit outrageously high values that have never been matched. The peak of 564 K m/s on Jan 19th is 6.5 times larger than the daily-mean climatology (and 2.5 times larger than the 90th percentile!), which is shown in Figure 3. 

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Figure 3: 45-75N 10 hPa eddy heat flux ([v*T*]) for 2008-09, according to MERRA-2 data.

This all came together to produce a textbook wave-2 vortex split. Figure 4 shows this fantastic evolution (thanks to Patrick Martineau for the excellent graphics) – you can really see the cold, strong vortex that existed beforehand, and the strong heat flux from the Atlantic sector. I think you could call this “catastrophic vortex failure”. It’s also worth comparing the location of the daughter vortices in this event versus what’s been happening so far in 2019.

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Figure 4: Animation of 10 hPa geopotential height (left) and temperature (right) during the Jan 2009 SSW. Animation created by Patrick Martinaeau (http://p-martineau.com/ssw-animations/).

This SSW was also a Polar-night Jet Oscillation (PJO) event (Hitchcock et al. 2013), so had the associated impressive appearance on time-pressure plots of polar-cap geopotential height (Figure 5) – with a long-lasting signal in the lower-stratosphere. According to Karpechko et al. (2017)’s Table 1, 100% of the days 8-52 after the central date had a negative 150 hPa NAM (one of only 5 times that has occurred since 1979). I always think Jan 2009 looks like a side-on view of a foot stamping down on the troposphere! You can also see that the event affected the troposphere until late March – over 2 months after the initial warming and a great example of lower-stratospheric persistence.

The downward penetration of the 2009 SSW was tremendous – it remains one of the few SSWs associated with reversing the 60°N zonal-mean zonal wind all the way down to 100 hPa, primarily because of how effectively the vortex was ripped in two throughout depth of the stratosphere (check out the barotropic wave-2 structure in Figure 6).

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Figure 5: 65-90N standardized geopotential height anomalies for JFM 2009 (credit: NOAA CPC).

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Figure 6: Departures from zonal-mean geopotential height averaged across 45-75°N for Jan 24, 2009, from ERA-5 reanalysis.

This moves the discussion nicely onto the downward propagation of the event. It wasn’t as strong as some SSWs – only 69% of the days 8-52 after the event had a negative 1000 hPa NAM index, and therefore ranks nearer the bottom end of Karpechko et al.’s “downward-propagating” SSWs (dSSW) (and that’s not surprising looking at Figure 5). However, it was nevertheless a dSSW, and had various impacts across the N Hemisphere.

Figure 7 shows the British Isles were influenced by easterlies 7-13 days after the SSW – which was the only case of mean easterlies in the 45 days following the event. Using this metric… the 2009 SSW isn’t special at all, although does pass my semi-arbitrary threshold of 5 consecutive easterly days for a true “outbreak”. However, I still do find it incredible that such a huge, hemispheric phenomenon as a major SSW, involving massive planetary waves and propagation from 30-50 km above our heads, can have a detectable response in such a small area as the British Isles. That’s one of those “isn’t the atmosphere amazing?!” moments.

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Figure 7: Average 850 hPa zonal winds across the British Isles in the 45 days following the January 2009 SSW, according to JRA-55 reanalysis.

This easterly spell brought with it colder-than-normal temperatures and snow. The Met Office’s monthly summary for Feb 2009 notes that “it was very cold during the first part of the month with snowfalls in many areas. This was the most widespread snowfall as a whole since February 1991”. Figure 8 shows the Met Office surface analysis for 18Z Feb 1st, with a negative NAO pattern and an easterly flow over NW Europe evident.

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Figure 8: 18Z Met Office surface pressure and frontal analysis for Feb 1st 2009.

Finally, the January 2009 SSW will always be special to me on a personal level, as it was the first time I had heard of sudden stratospheric warming and its influence on the tropospheric weather patterns. A schematic posted by the Met Office (Figure 9) in a press release (announcing an increased likelihood of cold weather in the next few weeks), showed wind reversals propagating down from the stratosphere to the troposphere and eventually the surface. This fascinated me, and it started a journey which had led me to where I am now.

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Figure 9: Met Office diagram showing the downward propagation of zonal wind reversals associated with a major SSW.

 

References

February 2009 – Met Office: https://www.metoffice.gov.uk/climate/uk/summaries/2009/february

Hitchcock, P., T. G. Shepherd, and G. L. Manney, 2013: Statistical Characterization of Arctic Polar-Night Jet Oscillation Events. J. Climate., 26, 2096-2116, https://doi.org/10.1175/JCLI-D-12-00202.1.

Karpechko, A. Y., P. Hitchcock, D. H. W. Peters, and A. Schneidereit, 2017: Predictability of downward propagation of major sudden stratospheric warmings. Quart. J. Roy. Meteor. Soc., 143, 1459-1470, https://doi.org/10.1002/qj.3017.

NASA MERRA-2 Annual Meteorological Statistics: https://acd-ext.gsfc.nasa.gov/Data_services/met/ann_data.html

NOAA CPC Stratosphere-Troposphere Monitoring: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/

Polar vortex animation during Stratospheric Sudden Warming [Patrick Martineau]: http://p-martineau.com/ssw-animations/

Wetter3 UKMO surface chart archive: http://www1.wetter3.de/Archiv/archiv_ukmet.html

Heatwave Summers: There’s more than 1976 & 1995

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2018 has been a remarkable summer. On the back of the warmest May on record (since 1910) for the UK, we saw the 3rd warmest June (featuring the 2nd warmest daytime maxima) which was also the 5th sunniest and 9th driest (3rd driest for England). The first half of this summer has been the driest on record for the UK. Temperatures have remained consistently very warm, with localised regions seeing prolonged and sometimes record-breaking dry spells. Were it not for a wet spring, we might have more concerns than we already do about water supplies (with only United Utilities so far issuing a hosepipe ban). 2018 has yet to see a very hot spell, though that may change in the coming few weeks – climatologically the warmest time of the year (for example, the UK’s record temperature of 38.5°C was set on August 10th 2003).

But will this heatwave be remembered?

I pose this question because the manner in which this summer has been reported would seem to suggest we’ve only ever had one heatwave in the UK: 1976. At a push, maybe 1995 too. But the truth is, of course, far from that.

Even just last year featured a memorable heatwave. June 2017 saw 5 consecutive days of temperatures exceeding 30°C somewhere in the UK, with a peak of 34.5°C on June 21 marking the highest temperature recorded in June since…yes, you guessed it…1976.

Until this year, the driest first half of a summer was 2013, which also featured a 19-day streak of temperatures exceeding 28°C somewhere in the country during July (which was the 3rd warmest and 3rd sunniest). Yet, aside from the astute meteorological observer, no-one I speak to seems to remember it happening – something I find astonishing because of the contrast after the 2007-2012 spell of very wet summers!

Other remarkable summers have occurred in recent times. July 2006 is the warmest month on record for the UK, and set the warmest July maximum temperature record (later beaten in 2015). 2003 saw a severe heatwave across Europe which resulted in setting the UK’s all-time maximum temperature record of 38.5°C on August 10th, a month which went on to become the UK’s 5th warmest August. August 1997 was the 2nd warmest on record for the UK, only slightly behind 1995. And before 2006, July 1983 was the warmest month on record for the UK.

In terms of mean temperatures for the UK, 1976 is only 3rd (tied with 2003) with 2006 taking pole position. However, 1976 and 1995 are the top 2 in terms of maximum temperatures, followed by 2006 and 2003. Rainfall wise, the driest summers are 1995 and 1976, followed by 1983.

I can’t deny that the string of hot temperatures, and the truly “flaming” June of 1976, were incredible. The water shortages caused by the preceding hot summer of 1975 (even more forgotten, with a hotter August than 1976!) and dry 1975-6 winter, were historic. But other historic heatwaves have happened since.

So, will people look back and remember the dry and hot summer of 2018? Only time will tell, but the evidence of “forgotten” recent heatwaves seems to suggest it won’t get the recognition it deserves.

Perhaps it’s a generational thing.

Perhaps people don’t spend as much time outside anymore – as someone who’s outside a lot (aside from being a meteorologist) I have always noticed the warmer spells even in a poor summer.

Perhaps, because the summers preceding 1976 were so much poorer (the 1960s lacked anything that could be called a ‘heatwave’ summer), we’ve just become accustomed to warmer summers and there’s less of a ‘wow’ factor when a heatwave does come along.

Despite all of that, I’d like to think people will remember this summer due to England’s performance in the World Cup and how well it timed with the peak (so far) of the heatwave. If only we’d won it, then it would really be a magnificent combination!

I’ll close with this thought: has anyone ever said “Summer weather was so much worse when I was a child!”.

Winter 2013-14: When the Rain was Tropical

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This morning, I watched as my aneroid barometer here in Harrogate, North Yorkshire slowly crawled upwards towards 1030 hPa. It’s a far cry from the forecast surface pressure in the next few weeks across the USA, but it’s also a far cry from what was happening on this day 4 years ago.

At least in Harrogate, 18 December 2013 marked the start of the infamously cyclonic winter of 2013-14. I took “daily” barometer observations throughout that winter. (Rather unscientifically, I don’t actually know what time each day I took these readings and it definitely wasn’t consistent – but it still serves as one record of the extraordinary winter.)

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The transition on the 18th from a strong anticyclonic regime to the cyclonic “hell” that followed is clear, and I’ve shown that with 2 different averages (dashed lines). The period during February also pushed my barometer to its limits…the scale runs to 965 hPa, which is the minimum value I’d noted down during the winter (Feb 8th).

I distinctly remember the onset of the winter reminding me of the wet summer of 2012, as both came on the back of a dry spell (though 2012 was much more significant in that regard) [and believe it or not, I stood beside Thruscross Reservoir in early December 2013 and remarked of its low water levels that “if we don’t have significant rain soon, we’ve got a problem”…we ended up with a very different problem!]. The swing from extreme to extreme is a signature of how both events were driven by stationary amplified patterns in the jet stream – both before and afterwards – something which has been a subject of recent research in the context of climate change.

Based on analysis from a Met Office report and subsequent journal articles, an enhancement of convection in the equatorial western Pacific (plus a few other things en-route) played a strong role in driving the wet winter of 2013-14 in the United Kingdom. Isn’t it just fascinating to think of torrential tropical downpours in Indonesia – an entirely different kind of rain – driving wet day after wet day in the UK? I think this serves as a reminder to always enjoy the weather – there’s always some fantastic dynamics behind it, even if it is just another rainy day.

I co-authored a summary report of the winter whilst studying Synoptic Meteorology Laboratory at the University of Oklahoma last year, which if you’re interested is available here: Winter 2013-14 Summary.