Low Ozone Events between January 2000 and December 2009
A low ozone event is defined as a departure of more than two standard deviations below the long-term monthly mean value for the site and is based on provisional data which may be subject to change. Between July 2000 and December 2009, the following notable low ozone events were observed at Lerwick, Camborne and Reading:

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October 2009

This event on the 14th October (Figure A) was part of large scale European dynamical disturbance which affected much of the troposphere and lower stratosphere. It was characterised by an intrusion of low ozone air over Lerwick. Examination of the Northern Hemisphere thickness (Figure B) reveals that this event was part of a warm tongue of tropospheric air pushing in from the south over Lerwick.   The relatively warm tropospheric air lifted the tropopause height and also advected low ozone from the South over Lerwick. This pool of low ozone air was then advected westwards over Scandinavia and Russia.

Figure A Ozone anomaly maps during the peak of the ozone event on 14th October.

Figure B Atmospheric thickness (m) on the 14th October. Figure shows warm tongue reaching to Lerwick.

August 2009

The 13th August event was part of a long-term disturbance dating back to at least the 10th of August. It was caused by an intrusion of a tropospheric anti-cyclone which caused upper level divergence (ozone advected away locally out f the column) and increases in tropopause height (cooling). The event is relatively small scale but slow moving and quasi-stationary over Europe (still detectable on the 15th August).

Figure A Ozone anomaly map on the 12th August showing low event over Reading.

July 2009

The event was caused by exceptionally warm weather due to tropospheric anticyclone, and this advected warm air from the south and lifting the tropopause thus reducing the ozone column locally. The event was centred over UK and Scandinavia as demonstrated by the satellite maps (Figure A). Note there was a simultaneous low ozone anomaly near New Foundland.

Figure A Ozone anomaly maps during the peak of the ozone event on 3rd July.

Examination of the Northern Hemisphere geopotential maps reveals that this event was part of a relatively low wave number 4 disturbance (Figure B).

Figure B Geopotential height (m)  at 500 hPa for the 3rd July.

These hemispheric wide upper level pressure wave patterns tend to be slow moving and quasi-stationary which accounts for the relatively long duration (3 days) of this event. In final stages the low ozone anomaly moved into Central Europe (5th July). These types of low wave number disturbances are unusual at this time of year.

The UK data agree well with the pattern observed at other nearby monitoring locations, and that the low centre moved away to the north-east, clearing the Valentia monitoring station in southern Ireland first. Low ozone alerts were recorded on four consecutive days during the period, three from the Dobson instrument at Lerwick and a later one from the Brewer instrument at Reading.

Data from the OMI satellite showed a similar decline to the ground-based monitoring stations but were consistently lower than both the Dobson and the Brewer measurements, indicating a possible systematic bias in the satellite results (there are recent reports of some loose material on the satellite possibly interfering with measurements). Note that there are also differences between satellites as for example GOME appears to show lower values early on than the OMI.

November 2008
This mini ozone hole can be traced back to a meteorological disturbance causing an ozone low forming off the East Coast of Canada around the 11th November; it was then advected over the Atlantic and became quasi-stationary over the UK intensifying somewhat on the 14th and 15th. This mini-hole covered most of the UK mainland (see Figure A). It then weakened and drifted into mainland Europe.

Figure A Ozone anomaly for the 14th November which was the day of maximal extent.

February 2008
This event was intermittently recorded between the 15th and the 21st February covering much of the UK. Figure A shows the scale of the event on the 18th and 21st. During this the troposphere geopotential height anomaly shows a wave number one structure i.e. high in western hemisphere and low in the eastern hemisphere (Figure B). Anti-cyclonic conditions favour low ozone events because of tropopause lifting. In this case the UK was part of planetary pressure wave anomaly which may explain the longer than typical duration. The anti-cyclone also moved back and forwards which is characteristic of planetary event rather than a smaller anomaly which is mostly advected eastwards. It is likely that some of the ozone loss is due to chemistry. During this period threshold temperatures in the stratosphere to form polar stratospheric clouds were met over large areas of Northern Scandinavia. These clouds provide surfaces for chlorine activation and hence ozone destruction. Although the stratospheric flow was predominately from the north west (i.e. not directly from the region of the coldest temperatures), it is possible that some ozone depletion would have recirculated to enhance the dynamic loss. The dynamic loss mechanism is the dominant cause of this event.

Figure A Ozone anomaly composite map of satellite and ground based anomaly (%) for the 18th February 2008 which was part of an extended low ozone phase.

Figure B 200 hPa Geopotential Height anomaly for the 18th February 2008. the figure shows high a wave number one type structure with high anomalies in the western hemisphere and low anomalies on the eastern hemisphere.

November 2007
The events of the 27th and 30th November were part of a wave disturbance of a sequence of high and low events. It was not a severe mini-hole but recorded by both satellite and ground-based instruments. Figure A below shows the anomaly on the 27th and a mini-hole to the east of the US is clearly already visible. This wave disturbance was advected to the east so that a further mini-hole appeared on the 30th (Figure B). Reading reported daily mean ozone of 227 DU and 222 DU on the 27th and 30th . These events are confirmed by the Manchester readings of 220 DU and 188 DU respectively. The OMI satellite reports 232 and 213 DU respectively.  There is therefore overall all reasonable agreement with the satellite for this event.

The autumn has seen an active storm generation in the mid-Atlantic coupled with persistent anti-cyclonic conditions over the UK. These conditions are favourable for mini-holes over the UK. The frequency may be explicable by the natural low seasonal amount of ozone and an unexplained long-term decline in this season.

Figure A Ozone anomaly on the 27th November 2007.

Figure B Ozone anomaly on the 30th November 2007.

On the 7th of November there was minor event (less than 1% below the alert threshold at Reading). This anomaly was first generated off the coast of Greenland and then advected westwards over the entire UK. It has its tropospheric origin in a tropospheric anticyclone near Spain. This caused tropopause lifting and advection of poor ozone air. This relatively minor anomaly intensified over France.

October 2007
The event centred on the 21^st of October was particularly severe with a daily averaged column reported of 189 DU. This was the lowest recorded ozone column for over 40 years at Lerwick and is similar to Antarctic Ozone hole levels. However, the corresponding satellite OMI overpass observation for that day was 220 DU. This discrepancy of 16% is too large to be explained by instrument noise and more likely due to differences in the data reduction polynomials. There is however no doubt that there was a mini hole over Lerwick at this time.

This event commenced as a relatively small mini-hole near Iceland on the 17th October. This is a frequent zone of the genesis of mini-holes that arrive over Lerwick. This anomaly was then advected towards Lerwick and became stationary and intensified over Lerwick and Scandinavia until the 24th October. It did not cover much of the UK as can be seen by the map (Figure A) and this is confirmed by unremarkable ozone columns at Manchester and Reading.

Figure A Total Ozone Departures (%) for 21^st October 2007

The origin of this event was a persistent deep anti-cyclonic anomaly over Scandinavia that lifted the tropopause causing divergence of ozone in the column and advection of ozone poor air from the South-West. Tropopause and stratospheric temperatures over Scandinavia were 8-10K below their climatological mean, but not sufficiently low to generate polar stratospheric clouds. It is therefore unlikely that there is a significant chemical contribution to this event. This is the season of annual mid-latitude ozone minimum. Therefore any dynamic mini-hole is likely to cause an extreme ozone low.  The short-term daily ozone change in the order of 50 DU is typical for such an event, but when this happens during a seasonal low records can be broken.

However, of some concern is that this event is consistent with a long-term decline of 3% per year since 1979 during the autumn season. It may be more instructive to search for a chemical signal in the seasonal ozone minimum rather than for example the maximum. This is largely because of a bigger signal to noise ratio where ozone variability and absolute levels are smaller in autumn, so there is a better chance of detecting a given absolute amount of ozone depletion. However, this season is not affected by chlorine activation (polar stratospheric clouds) and may therefore have been neglected by past research. There is no explanation as to the cause of the long-term trend. One may hypothesise that the seasonal minimum is a good indicator of global mean ozone levels. The chemical contribution is not clear at the moment. Figure B shows the ERA-40 trends of Autumn 200 hPa geopotential height - it seems that there is a significant increase near Lerwick. This would be consistent with a negative ozone trend. These trends occur irregularly for other months and locations, so it may just be a co-incidence of a tropospheric trend with the seasonal ozone minimum.

Figure B ERA-40 trends of Autumn 200 hPa geopotential height

April 2007
Lerwick reported a low ozone event of 281 DU on the 10^th April 2007. OMI overpass data reported a value of 286 DU, which is within the expected rms range of 2-5% depending on the wavelength pair chosen. This event was part of an extensive dynamic disturbance that reached into Northern Scandinavia. (Figure A)

Figure A WMO Ozone anomaly plot combing ground based and satellite climatology.

The origin of this event was traced back to the anti-cyclone located to the west of Ireland at this time. This relatively deep feature caused tropospheric warm advection at its northern flank over Scotland. The tropopause was thus lifted and the pressure anomalously low (Figure B). In this case stratospheric ozone advection from the (relatively low ozone) South seems modest, so that the tropopause lift appears the dominant cause of this event.

Figure B Operational analysis (GDAS) of tropopause pressure (Pa)

November 2006
On the 3^rd November 2006, low ozone was measured at Lerwick. This was a dynamic mini hole that had originated to the East of Greenland and was then advected over Lerwick. However the discrepancy with the OMI is quite high at 14%.

October 2006
From the 25^th October 2006, low ozone was measured over the UK and Western Europe.  This was a persistent event lasting for about 5 days.  Low ozone events were reported at Lerwick on the 27^th and at Reading on the 29^th and 30^th October.  Figure A below shows the ozone anomaly on the 28^th, which was the day of the maximum area of ozone depletion for this event.  At this time there was simultaneous anomalous high ozone over the Arctic.  This pattern is known as a dynamical wave number one disturbance.  Stratospheric temperatures are also unusually low in over the UK and high over Alaska.  There was a high-pressure ridge and seasonally very high surface temperatures over the UK.  In the stratosphere, low-ozone air was advected from the South.

Figure A - Ozone Anomaly on the 28th October, which was the Day of the Maximum Area of Ozone Depletion for this Event.

Comparison with the satellite OMI observations showed more extreme depletion reported by the ground stations.  This is a consistent pattern of previous low ozone events.

June 2006 at Lerwick
This event was part of a 10-day low ozone anomaly that started over the Eastern North Atlantic. It was then blocked and quasi-stationary for nearly a week near Lerwick. This episode was associated with a tropospheric anticyclone advecting warm air from the south and lifting the tropopause thus reducing the ozone column locally.

Figure. A shows the anomaly and spatial extent of this event as seen by the satellite OMI.

Figure. A The anomaly of ozone expressed as a percentage of the long term mean for the 15th of June

March 2006 at Lerwick
Lerwick recorded low ozone (>2 s.d. below the Lerwick long term monthly mean) on the 18th and 19th March with values of 244 and 246 respectively. The interpolated lower limits are 267 and 268 respectively. This mini-hole originated west of Greenland and then intensified travelling eastwards. Two independent satellites confirm local dynamic ozone depletion.

January 2006 at Reading
The Reading instrument recorded low ozone on the 18^th January (192 DU) and 19^th January (177 DU). This appears to be an extremely severe dynamically driven mini hole event. The scale of the depletion was nearly 50% below mean and can be classified as a "hole". It was part of an upper level depression that originated over the North Atlantic on about the 16^th and then drifted over the UK and intensified, finally moving on into Central Europe. Tropospheric conditions were anomalously warm. However, temperatures at 100 mob were very close to PC thresholds of 195K.

October 2005 at Lerwick
The Lerwick observatory recorded low ozone during the period 14^th to 18^th October. The lowest daily value during the period was 210 DU recorded on the 15^th. The event was due to a current and persistent blocking high pressure ridge stretching into Scandinavia, which led to unusual tropopause warming and tropopause lifting causing local ozone deplacement.

26^th May 2005 at Reading
This event was centred over Brittany and in relatively small spatial extent. It coincided with some unusual warm surface temperatures over the UK (many places reported the warmest or 2^nd warmest day of the year up to then). Mid-troposphere warming was particularly pronounced with south-westerly bringing warm air into the South-East and Northern-France. This lifted the tropopause giving rise to the event. This anomaly then moved slowly eastwards before dissipating over Poland on the 31^st of May.

7^th March 2005 at Lerwick
Data from the Lerwick observatory indicates that low ozone was recorded on 7th March. The daily value day was 257 DU. This event is largely attributable to chemical depletion. The vortex of ozone depleted air was over Lerwick. The previous record chemical ozone loss was reported in 1995-1996, with a depletion of up to 65% at some heights from January to April.

31^st October 2004 at Lerwick
The October 31st value of 189 DU (TOMS reported 198 DU) is the lowest daily ozone value recorded at Lerwick at least since 1981. Inspection of meteorological analysis confirmed standard features of mini-hole dynamics i.e. tropopause lifting (high tropopause pressure) and strong advection of ozone poor air from the south near the tropopause. The figure below shows the TOMS map for October 31^st.

It should be noted that the Lerwick observation was based on only 4 zenith sky measurements. Nevertheless, the TOMS record confirms two severe ozone mini holes over the North Atlantic and a simultaneous hole over Finland. The double hole structure is somewhat unusual. On the 29th September Lerwick was at the edge of the mini hole, but on the 31st October it was closer to the centre of it. Ozone values over the rest of the UK were not exceptionally low on either day.

29^th September 2004 at Lerwick
Inspection of the TOMS map for the 29th September showed a relatively large area of depletion (mini hole) near the east coast of Greenland and Iceland. Lerwick is at the edge of this feature. The upper level wind pattern indicated strong southerly flow (associated with an upper level depression to the west of Greenland) bringing ozone poor area into the region.

27^th July 2004 at Lerwick and Reading
Lerwick reported a low ozone event on July 27th and this feature covered most of the UK. Reading also reported the lowest values of the year during this time, but there is insufficient climatology to quantify the size of the anomaly. Valencia in Ireland showed an anomaly of about -10%. This feature can be traced to strong cyclonic anomaly centred east of Greenland at this time, which by southerly warm advection lifted the tropopause and transported ozone poor air from over the UK.

16^th and 27^th September 2002 at Lerwick:
The figures below show the ozone anomaly map as reported by Northern Hemisphere ground based instruments. Anomalous low ozone values of up to 20% lower than the long-term mean were observed in Northern Europe on both days. However, the global structure of the anomaly is somewhat different. There is a clear planetary wave pattern visible on the 27th but only a dipole structure (high values over Central Europe) on the 16th. Comparison with TOMS on the relevant days shows some discrepancy with the ground based reporting 226 and 229 DU versus TOMS of 253 and 245 DU on the 16th and 27th respectively. As noted before TOMS appears to be reporting significantly higher values than the ground-based station. However, inspection of the TOMS map for the relevant days does show low ozone values over Northern Europe for both days. It is plausible that the viewing average of TOMS continues to under report the severity of local fluctuations.

        
Ozone Anomaly Maps for the Low Ozone Events Occurring on 16th September 2002 (left-hand panel) and 27th September 2002 (right-hand panel), as reported by Ground-based Instruments

The figures below show the tropopause pressure for the 16th and 27th September, as reported by the Global Data Assimilation System. Low tropopause pressures are evident over the same region as for the reported low ozone anomalies. These low values indicate low stratospheric mass and therefore low column ozone are very likely. There is a global wave-like structure with tropopause increases and decreases around middle latitudes. Particularly interesting to note is the low pressure anomaly over the East Pacific on the 27th September. This may suggest that the origin of the most recent event could be linked to the developing El Niño in the Pacific, and the northward propagation of Rossby waves caused by tropical heating anomalies. These waves can modify the jet stream. Figure 4.10 Maps of the Tropopause Pressure for the Low Ozone Events Occurring on 16th September 2002 (left-hand panel) and 27th September 2002 (right-hand panel). Concurrent with the low troposphere pressure anomaly is a surface blocking anti-cyclone giving a prolonged dry spell during September. As an anomalous high tropopause pressure in the south over the Atlantic (e.g. the Azores) was evident, it would be reasonable to conclude that the origin of both disturbances is in the tropics (low latitudes) and that the signature of this disturbance then spans the middle latitudes across the globe. It would appear that the position of this planetary wave will have a critical influence on whether the UK experiences a low ozone event or not. The wind fields at 100 mb for these days showed that there was a southerly flow over Lerwick on the 27th September, but northerly flow on the 16th September. The southerly flow will advect low ozone from lower latitudes and reduce the ozone column and thus further enhance the tropopause effect discussed above. However, the northerly flow would tend to have the opposite effect. It therefore seems that the dominant effect is the modification of the tropopause pressure and not advection. There is thus a reasonable dynamic explanation for both of these low ozone events.

(2) 17th June 2002 at Lerwick:
The left-hand panel below shows the ozone anomaly map as reported by Northern Hemisphere ground based instruments. Anomalous low ozone values of up to 20% lower than the long-term mean were observed in Northern Europe and appear centred over the North Sea. Lerwick reported a value of 296 DU, which was a high quality direct sun and blue zenith measurements. At local noon TOMS reported a value of 303 DU over Lerwick. As stated above, the ground-based value is lower than that reported by TOMS. Inspection of the TOMS global map on this day does not show any distinctive pattern. There are ongoing calibration problems with the TOMS instrument with may introduce errors of up to 5%. Inspection of the meteorological analysis reveals a likely dynamic origin of the reported low ozone event. The right-hand panel below shows the tropopause pressure for the 17th June, as reported by the Global Data Assimilation System. Low tropopause pressures are evident over the same region as for the reported low ozone anomalies. These low values indicate low stratospheric mass and therefore low column ozone are very likely. The wind field at 100 mb for this day provided evidence for a significant southerly flow into the North Sea. This will advect low ozone from low latitudes and reduce the ozone column and thus enhance the tropopause effect discussed above. In conclusion, there is a reasonable dynamic explanation of this event.

        
Maps of (a) the Ozone Anomaly for the Low Ozone Event Occurring on 17th June 2002 (left-hand panel) and (b) the Tropopause Pressure (right-hand panel).

        
Maps of the Tropopause Pressure for the Low Ozone Events Occurring on 16th September 2002 (left-hand panel) and 27th September 2002 (right-hand panel)

28^th May 2001 at Camborne:
The Camborne station reported an ozone column of 300 DU. Inspection of the TOMS data showed that this event was part of a planetary event with low ozone values also over the US, the Bering Strait and the Pacific. The synoptic situation thus favoured transport of low latitude air that is poor in ozone. Over Camborne itself TOMS reported a somewhat higher ozone amount of 311 DU.

24th September 2000 and 15th November 2000 at Lerwick:
Using stratospheric temperatures at 50 mb as a diagnostic for dynamic disturbances, these low ozone values could all be accounted for by intrusion of low-latitude ozone-poor air rather than as chemical depletion. It is however difficult to quantify the extent of chemical depletion without a comprehensive suite of chemical measurements. To first order, the dynamic variations appear to dominate the column ozone variations. The event on the 15th of November was the lowest ozone value reported in the last 20 years (194 DU). The 50 mb temperature plot for the day shows a classic wave number. Cold temperature will give rise to low ozone as air is either ascending and/or transported from low latitude. However, it must be borne in mind that there is a seasonal minimum at this time and that a natural fluctuation about a low mean can be expected to give new absolute records. Furthermore, there is a disagreement of the severity of these two events with TOMS reporting nearly 10% higher values (The TOMS satellite recorded a value of approximately 207 DU over the region on the 15th November, as shown in the figure right).

TOMS Map of the Low Ozone Event which Occurred on 15th November 2000.

In addition, there were two other low ozone events (on 20th December 2002 and 13th January 2003) which were not observed at either Lerwick or Camborne because the event either occurred during the period when the ozone measurements are out of range at the Lerwick station or the measurement was just within the site's own climatological two standard deviation anomaly band ( e.g. Camborne on 13th January 2003).