Category Archives: Weather Events

10 years later: the January 2009 SSW

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 10 years later.

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!


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.


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. 


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.


Figure 4: Animation of 10 hPa geopotential height (left) and temperature (right) during the Jan 2009 SSW. Animation created by Patrick Martinaeau (

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.


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

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. I’m somewhat limited in my analysis by the current US government shutdown, so I’ll focus on the British Isles.

Figure 6 (a snippet from some work I’ve been doing on SSWs and easterly outbreaks in the UK) 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.


Figure 6: 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 7 shows the Met Office surface analysis for 18Z Feb 1st, with a negative NAO pattern and an easterly flow over NW Europe evident.


Figure 7: 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 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, 10 years later, finds me doing a stratosphere-related PhD project.

met office ssw

Figure 8: Met Office diagram showing the downward propagation of zonal wind reversals associated with a major SSW.


February 2009 – Met Office:

Hitchcock, P., T. G. Shepherd, and G. L. Manney, 2013: Statistical Characterization of Arctic Polar-Night Jet Oscillation Events. J. Climate., 26, 2096-2116,

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,

NASA MERRA-2 Annual Meteorological Statistics:

NOAA CPC Stratosphere-Troposphere Monitoring:

Polar vortex animation during Stratospheric Sudden Warming [Patrick Martineau]:

Wetter3 UKMO surface chart archive:

Heatwave Summers: There’s more than 1976 & 1995

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

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.)


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.