"Evolution of the Southern Annular Mode during the past millennium," by N. J. Abram et al., Nature Clim. Change (2014); doi:10.1038/nclimate2235

Nature Climate Change (11 May 2014); doi:10.1038/nclimate2235

Evolution of the Southern Annular Mode during the past millennium

• Nerilie J. Abram,
• Robert Mulvaney,
• Françoise Vimeux,
• Steven J. Phipps,
• John Turner and 
• Matthew H. England

Abstract

The Southern Annular Mode (SAM) is the primary pattern of climate variability in the Southern Hemisphere^1,2, influencing latitudinal rainfall distribution and temperatures from the subtropics to Antarctica. The positive summer trend in the SAM over recent decades is widely attributed to stratospheric ozone depletion²; however, the brevity of observational records from Antarctica¹—one of the core zones that defines SAM variability—limits our understanding of long-term SAM behaviour. Here we reconstruct annual mean changes in the SAM since AD 1000 using, for the first time, proxy records that encompass the full mid-latitude to polar domain across the Drake Passage sector. We find that the SAM has undergone a progressive shift towards its positive phase since the 15th century, causing cooling of the main Antarctic continent at the same time that the Antarctic Peninsula has warmed. The positive trend in the SAM since ~AD 1940 is reproduced by multimodel climate simulations forced with rising greenhouse gas levels and later ozone depletion, and the long-term average SAM index is now at its highest level for at least the past 1,000 years. Reconstructed SAM trends before the 20th century are more prominent than those in radiative-forcing climate experiments and may be associated with a teleconnected response to tropical Pacific climate. Our findings imply that predictions of further greenhouse-driven increases in the SAM over the coming century³ also need to account for the possibility of opposing effects from tropical Pacific climate changes.

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Link:  http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2235.html

NewScientist: Antarctic wind vortex is strongest for 1,000 years

The winds ripping around Antarctica. (Image: earth.nullschool.net)

by Michael Slezak, NewScientist, May 11, 2014 

Our greenhouse gas emissions are helping to spin up a giant vortex of winds around Antarctica.

Antarctica has been warming relatively slowly compared with the rest of the world. The explanation seems to be that the winds spinning clockwise around the continent have been getting stronger, preventing warm air from entering.

In a way, those winds have done us a favour by keeping warm air away from the South Pole. Otherwise it might be melting. But as this atmospheric maelstrom accelerates, it shrinks, leaving the most vulnerable parts of Antarctica out in the warm and dragging winter rain away from Western Australia.

In 2009, it seemed that the hole in the ozone layer above Antarctica was responsible for boosting the winds. Now Nerilie Abram from the Australian National University in Canberra and her colleagues have shown the ozone hole is only part of the story. Global warming is just as important.

Warming powers winds

The team reconstructed Antarctic temperatures over the past 1,000 years, using an ice core from James Ross Island near the Antarctic Peninsula. The temperatures correlated with how strong and tight the winds are, so they could construct a record of wind strength.

They found that the current strength of the winds is unprecedented over the past millennium. But the surge in strength started in the 1940s, decades before the ozone hole.

So Abram's team simulated the last millennium using 8 climate models, driven by actual greenhouse gas levels previously reconstructed from ice cores. All the models predicted that the winds would pick up by the 1940s, suggesting greenhouse gases were playing a role. That may be because the Northern Hemisphere is warming faster than the south – because it has more continents – creating a strong temperature gradient that boosts the winds.

Such historical data is vital, says Wenju Cai from the CSIRO, Australia's national research agency, in Melbourne. In as-yet-unpublished work, he estimates that ozone depletion has caused two-thirds of the impact on the Antarctic winds, with greenhouse gases responsible for the rest.

Futureshock

If greenhouse gases really are contributing to the winds, it changes our expectations for what will happen to the climate in Australia and Antarctica.

The ozone hole is expected to heal in the coming decades, and if it was the only factor controlling the winds they would weaken and expand. So Australia would get its rain back, while the western parts of Antarctica might get some more protection against warming.

However, Abram says rising global temperatures will counteract this weakening effect on the winds. That means Western Australia will stay dry and the western parts of Antarctica, stranded outside the winds, will keep melting.

Cai estimates that, on our current emissions pathway, the two factors will counteract each other until 2045, so the winds will stay constant. After that, without reducing our emissions, greenhouse gases will boost the winds further.

Journal reference: Nature Climate Change, DOI: 10.1038/NCLIMATE2235

http://www.newscientist.com/article/dn25547-antarctic-wind-vortex-is-strongest-for-1000-years.html#.U4I2Y_ldXX4

Ozone hole over Antarctica causing circumpolar winds to speed up and move polewards

by Lauren Morello, Climate Central, January 31, 2013

High above Antarctica, the atmosphere is slowly recovering from the decades-long barrage of manmade chemicals that ate a hole in the protective ozone layer.

But the legacy of that destruction lingers. Scientists have linked the ozone hole that forms each Antarctic spring high above Earth to changes in the fierce band of westerly winds that swirls around Antarctica. Those winds, closer to the continent's surface, have grown stronger and moved poleward over the past several decades.

This NASA animation shows variations in the ozone holes that developed over the South Pole each Antarctic spring from 1979 to 2013. Purple and blue areas have the least ozone, while yellows and reds have the most. Credit: NASA.

And now a new study suggests that the ozone hole has an even broader reach. It finds evidence those shifting winds are speeding circulation patterns in polar waters. That shift is important because it may already be weakening the Southern Ocean's ability to absorb carbon dioxide from the atmosphere and slow the march of manmade climate change.

Even a small change in the Southern Ocean carbon sink could have a noticeable impact, because the region's waters takes in about 40% of the total carbon absorbed by the world's seas.

"The models were indicating there could be some change in ocean circulation (caused by ozone depletion), but there was a lot of debate about whether what the models were saying was actually happening," said lead author Darryn Waugh, a climate scientist at Johns Hopkins University.

His research, published Thursday in the journal Science, bears those models out. It was published alongside a separate study, from researchers at Pennsylvania State University, that affirms the ozone hole has been the main driver of the changes in Antarctica's winds, dwarfing the role played by climate change.

Waugh and colleagues in the U.S. and Australia found that in the subtropical Southern Hemisphere, water 500 to 1,000 meters deep appear to be growing "younger." That's a sign that north-south circulation in the deep ocean has been speeding up, sending surface water from the ocean surface near the pole to those intermediate depths more quickly, he said.

At the same time, the currents closer to Antarctica's shores appear to be pushing more old, deep water up to the ocean surface.

Scientists worry that the increasing upwelling of that water, hundreds of years old and naturally rich in carbon dioxide, is reducing the amount of manmade carbon absorbed by sub-polar waters.

NOAA staff at the United States' South Pole research station prepare to release a balloon that will measure the strength of the ozone layer high above Antarctica.

Click to enlarge. Credit: NOAA.

 

"The amount of carbon that goes from the atmosphere into the ocean depends on the balance between the amount of carbon in the atmosphere and the amount of carbon in surface water," Waugh said.

If surface waters are already rich in carbon, "that would mean more of the carbon we're producing would stay in the atmosphere, and that would contribute more to climate change," he said.

Michael Meredith, a physical oceanographer with the British Antarctic Survey who was not involved in either study, said the new research drives home the importance of the Southern Ocean carbon sink. "It's doing us a very big favor, if you like, by taking carbon from the atmosphere and slowing the rate of atmospheric climate change," he said.

Meredith, who called the new study "a strong and important paper," said the question now is what will happen as the ozone layer slowly heals and human activities pump out increasing amounts of greenhouse gases.

With the 1987 Montreal Protocol, which bans the use of ozone-destroying chemicals, now in force, researchers expect the ozone layer to recover by mid-century.

"The future of the circulation in the Southern Ocean, and the impact that it has on global climate change now seems to be very strongly tied to what happens to the westerly winds in the future," Meredith said.

The second study, by Pennsylvania State University researchers, is a small step toward answering that question, said Julie Arblaster, a climate scientist at Australia's Bureau of Meteorology, who called the analysis' use of wind speed and direction observations "sophisticated." 

It's the first paper to use such observations — not model simulations — to determine what roles ozone depletion and climate change have played in shifting westerly winds poleward.

That could help researchers identify which climate models will do the best job projecting how the wind pattern will change as the ozone layer strengthens and climate change intensifies.

But first, they'll have to figure out just how both factors alter westerly winds — no small task.

"The next step is to understand the mechanism," Arblaster said. "Because if we can understand the mechanism, we can increase our confidence in projections for the future." 

Related Content
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Forget the Melting Arctic, Sea Ice in Antarctica Is Growing!
The Ozone Hole Opens Up Again. It's Still Not the Same as Climate Change
Report: Most Antarctic Peninsula Warming Human-Caused

Follow the author on Twitter @lmorello_dc or @ClimateCentral. We're also on Facebook and other social networks.

http://www.climatecentral.org/news/ozone-holes-shifting-winds-may-be-sapping-major-carbon-sink-15530

"UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor," by James G. Anderson*, David M. Wilmouth, Jessica B. Smith and David S. Sayres, Science, DOI: 10.1126/science.1222978

Science DOI: 10.1126/science.1222978

• REPORT

UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor

1. James G. Anderson*, 
2. David M. Wilmouth, 
3. Jessica B. Smith and 
4. David S. Sayres

1. ¹Department of Chemistry and Chemical Biology, Department of Earth and Planetary Sciences and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA.

1. ↵*Correspondence: anderson@huarp.harvard.edu.

ABSTRACT

The observed presence of water vapor convectively injected deep into the stratosphere over the United States fundamentally changes the catalytic chlorine/bromine free radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer. Were the intensity and frequency of convective injection to increase as a result of climate forcing by the continued addition of CO₂ and CH₄ to the atmosphere, increased risk of ozone loss and associated increases in UV dosage would follow.

http://www.sciencemag.org/content/early/2012/07/25/science.1222978

SELECTED TEXT FROM THE FULL PAPER:

Because the binary sulfate-water aerosols are ubiquitous in the lower stratosphere, if the necessary temperature and water conditions are met, then heterogeneous conversion of inorganic chlorine to free radical form can occur anywhere, not just in the polar regions. While the Arctic lower stratosphere is marginally colder than the mid-latitude lower stratosphere over the US in summer, what matters is the combination of water vapor concentration and temperature.

The in situ observations of H₂O obtained from both the high altitude NASA ER-2 and WB-57 aircraft extending over a number of recent missions are summarized in Fig. 1B. The data shown were retrieved during flights originally selected to observe the outflow from typical convective storms over the US in summer. What proved surprising is the remarkable altitude to which large concentrations of water vapor are observed to penetrate. The convective injection of water into the stratosphere was also observed with surprising frequency, occurring in approximately 50% of the summertime flights over the US. The convective origin of this water vapor is established by simultaneous in situ observations of H₂O and the HDO isotopologue (19, 20), the concentration of which differentiates between direct convective injection and other pathways linking the troposphere and stratosphere (19, 21–23). The observed presence of water vapor enhancements reaching and occasionally exceeding 12 ppmv at temperatures in the vicinity of 200 K in the altitude region between 15 and 20 km, as displayed in Fig. 1B, has significant consequences.

The initiation of fundamental changes in the photochemistry of the lower stratosphere in summer is captured in Fig. 1C, that superimposes on the threshold plot for chlorine activation over the range of 2–10 μm²/cm³ for reactive surface area, the observed in situ H₂O mixing ratios and temperatures at 90 ± 10 mb pressure. It is clear that, at observed water vapor concentrations and temperatures, the threshold for chlorine activation converting inorganic chlorine to free radical form is routinely crossed in the summertime. The result is that ClO can become a major component of the available inorganic chlorine budget within regions of high water vapor. Convective injection of water vapor to heights reported here can occur in storm systems that are ~50 km across, with smaller domains of high altitude injection embedded within them at their origin (24). The elevated concentrations of water can spread to 100 km or more in horizontal extent within a few days (19, 25), and remain at the elevated levels reported here over a period of days. This phenomenon has been analyzed by Newman et al. (26) using high altitude (70–100 mb) observations of rocket plume dispersion that defines the rate of horizontal spreading from a point source. Additionally, the circular flow pattern of air in the lower stratosphere over the US resulting from the North American summer monsoon provides the potential for repeated convective injection events into the summer lower stratosphere over the US.

***************

There are a number of important considerations associated with the issue of convective injection of water vapor inducing chlorine activation and catalytic removal of ozone over mid-latitudes of the NH in summer. First is the fact that a remarkably dry stratosphere characterizes the current climate state. However, the paleorecord holds evidence that the stratosphere, under conditions of high CO₂ concentrations, was characterized by significantly higher water vapor concentrations than is the case today (43, 44). If currently increasing concentrations of CO₂, CH₄ and other infrared active gasses force the stratospheric system to a state of increasing water vapor concentrations, the impact on ozone is of significant concern given the concentrations of chlorine and bromine in the stratosphere today.

Second, the loss of ice from the Arctic Ocean opens the possibility for significant increases in CO₂ and CH₄ release from melt zones in the Arctic. A release of just 0.5% per year of the carbon tied up in the soils of Siberia and Northern Alaska alone will double the carbon added to the atmosphere each year from the combustion of fossil fuels world-wide (45). This release of carbon from clathrates and permafrost will accelerate the forcing of the climate that is potentially linked to the intensity and frequency of convective injection of water into the stratosphere.

Third, engineering the climate by the addition of sulfates to increase reflective aerosol concentrations and thereby reduce climate forcing by reflecting sunlight back to space (46, 47) would significantly increase reactive surface area which would accelerate the processing of chlorine to free radical form (Fig. 1, A and C), thereby decreasing ozone concentrations. In the same vein, the convective injection of water vapor into the stratosphere increases the sensitivity of ozone loss to volcanic injection of sulfates into a stratosphere with current loading of chlorine and bromine. Evidence for this was presented for the eruption, in 1991, of Mt. Pinatubo by Salawitch et al. (35).

Fourth, from the perspective of human health, a primary concern is that decreasing ozone concentrations, particularly in summer over populated areas, results in increased UV dosage levels. Sustained increases in UV dosage levels are in turn associated with the increased incidence of skin cancer (48, 49), which is currently 1 million new cases a year in the US (49).

Lastly we emphasize that, because chlorine activation depends exponentially on water vapor and temperature, and in turn that the forcing of climate may well control the convective injection of water into the lower stratosphere, the idea that ozone “recovery” is in sight because we have controlled CFC and halon release is a potentially significant misjudgment.                

2011 Network for the Detection of Atmospheric Composition Change Symposium. Session 5 - Tropospheric Observations and Analyses: Is there a hole in the global OH shield over the tropical western Pacific warm pool? by Markus Rex et al.

Is There a Hole in the Global OH Shield Over the Tropical Western Pacific Warm Pool? 


Markus Rex (Alfred Wegener Institute for Polar and Marine Research), Theo Ridder (Institute of Environmental Physics, University of Bremen), Justus Notholt (Institute of Environmental Physics, University of Bremen), Franz Immler (GRUAN lead centre, German Meteorological Service), Kirstin Krüger, Viktoria Mohr and Susann Tegtmeier (IFM-GEOMAR)


Hundreds of organic species are emitted into the atmosphere mostly from biogenic processes. The rapid breakdown by reactions with OH radicals prevents most of them from reaching the stratosphere. Hence, the omnipresent layer of OH in the troposphere shields the stratosphere from these emissions and is particularly relevant for those species that do not photolyse efficiently. The dominant source of OH in clean tropical air are reactions involving ozone. Hence the OH concentration is closely coupled to ozone abundances. Biogenic halogenated species, biogenic species containing sulphur and perhaps anthropogenic SO2 emissions play an important role in the stratospheric composition and ozone chemistry. Changes in their abundance and tropospheric breakdown processes provide an ozone climate feedback mechanism. The dominant source of OH in clean tropical air is the photolysis of ozone at wavelengths shorter than about 340 nm producing O(1D) radicals followed by the reaction O(1D) + H2O -> OH + OH. This couples the OH concentration and hence the oxidizing capacity of tropospheric air closely to the concentration of ozone. The area of the Western Pacific warm pool is known to be key for troposphere to stratosphere exchange (e.g., Newell & Gould-Stewart, JAS, 1981; Fueglistaler et al., JGR, 2004). Vertical profiles of tropospheric ozone or OH from that part of the Pacific are not available so far. Measurements from the south east edge of the warm pool area during the Central Equatorial Pacific Experiment in 1993 (Kley et al., Science, 1996) and individual profiles from the station Samoa in the same geographical area (Solomon et al., GRL, 2005) showed extremely low ozone concentrations in the marine boundary layer and at tropopause level but still significant amounts of ozone in most of the free troposphere, where most of the oxidation of biogenic species occurs. We report ship based ozonesonde measurements from the center of the Western Pacific warm pool in October 2009. During a 2500-km portion of the ship track between 10S and 15N we found ozone concentrations below the detection limit of the sondes throughout the troposphere. Based on comprehensive CTM modelling we show that the observations suggest the existence of a pronounced minimum in the tropospheric OH column well correlated with the region where most of the vertical transport of air into the stratosphere occurs. The consequences for the tropospheric lifetimes of chemical species in that key geographical area and implications for the transport of emissions from that area into the stratosphere will be discussed. The results highlight the importance of setting up longer term observational  capabilities in the westernmost part of the tropical Pacific.


http://ndacc2011.univ-reunion.fr/fileadmin/documents/Abstracts_and_documents/abstracts/Abstracts_Session_5.pdf

NASA Leads Study of Unprecedented Arctic Ozone Loss

Left: Ozone in Earth's stratosphere at an altitude of approximately 12 miles (20 kilometers) in mid-March 2011, near the peak of the 2011 Arctic ozone loss. Red colors represent high levels of ozone, while purple and grey colors (over the north polar region) represent very small ozone amounts. Right: chlorine monoxide – the primary agent of chemical ozone destruction in the cold polar lower stratosphere – for the same day and altitude. Light blue and green colors represent small amounts of chlorine monoxide, while dark blue and black colors represent very large chlorine monoxide amounts. The white line marks the area within which the chemical ozone destruction took place. Image credit: NASA/JPL-Caltech
› Full image and caption

October 2, 2011

PASADENA, Calif. - A NASA-led study has documented an unprecedented depletion of Earth's protective ozone layer above the Arctic last winter and spring caused by an unusually prolonged period of extremely low temperatures in the stratosphere.

Video

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Unprecedented Arctic Ozone Loss in 2011. 

The study, published online Sunday, Oct. 2, in the journal Nature, finds the amount of ozone destroyed in the Arctic in 2011 was comparable to that seen in some years in the Antarctic, where an ozone "hole" has formed each spring since the mid-1980s. The stratospheric ozone layer, extending from about 10 to 20 miles (15 to 35 kilometers) above the surface, protects life on Earth from the sun's harmful ultraviolet rays.


The Antarctic ozone hole forms when extremely cold conditions, common in the winter Antarctic stratosphere, trigger reactions that convert atmospheric chlorine from human-produced chemicals into forms that destroy ozone. The same ozone-loss processes occur each winter in the Arctic. However, the generally warmer stratospheric conditions there limit the area affected and the time frame during which the chemical reactions occur, resulting in far less ozone loss in most years in the Arctic than in the Antarctic.


To investigate the 2011 Arctic ozone loss, scientists from 19 institutions in nine countries (United States, Germany, The Netherlands, Canada, Russia, Finland, Denmark, Japan and Spain) analyzed a comprehensive set of measurements. These included daily global observations of trace gases and clouds from NASA's Aura and CALIPSO spacecraft; ozone measured by instrumented balloons; meteorological data and atmospheric models. The scientists found that at some altitudes, the cold period in the Arctic lasted more than 30 days longer in 2011 than in any previously studied Arctic winter, leading to the unprecedented ozone loss. Further studies are needed to determine what factors caused the cold period to last so long.


"Day-to-day temperatures in the 2010-2011 Arctic winter did not reach lower values than in previous cold Arctic winters," said lead author Gloria Manney of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the New Mexico Institute of Mining and Technology in Socorro. "The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time. This implies that if winter Arctic stratospheric temperatures drop just slightly in the future, for example as a result of climate change, then severe Arctic ozone loss may occur more frequently."


The 2011 Arctic ozone loss occurred over an area considerably smaller than that of the Antarctic ozone holes. This is because the Arctic polar vortex, a persistent large-scale cyclone within which the ozone loss takes place, was about 40 percent smaller than a typical Antarctic vortex. While smaller and shorter-lived than its Antarctic counterpart, the Arctic polar vortex is more mobile, often moving over densely populated northern regions. Decreases in overhead ozone lead to increases in surface ultraviolet radiation, which are known to have adverse effects on humans and other life forms.


Although the total amount of Arctic ozone measured was much more than twice that typically seen in an Antarctic spring, the amount destroyed was comparable to that in some previous Antarctic ozone holes. This is because ozone levels at the beginning of Arctic winter are typically much greater than those at the beginning of Antarctic winter.


Manney said that without the 1989 Montreal Protocol, an international treaty limiting production of ozone-depleting substances, chlorine levels already would be so high that an Arctic ozone hole would form every spring. The long atmospheric lifetimes of ozone-depleting chemicals already in the atmosphere mean that Antarctic ozone holes, and the possibility of future severe Arctic ozone loss, will continue for decades.


"Our ability to quantify polar ozone loss and associated processes will be reduced in the future when NASA's Aura and CALIPSO spacecraft, whose trace gas and cloud measurements were central to this study, reach the end of their operational lifetimes," Manney said. "It is imperative that this capability be maintained if we are to reliably predict future ozone loss in a changing climate." 

http://www.jpl.nasa.gov/news/news.cfm?release=2011-308&cid=release_2011-308

Richard Black, BBC: Arctic ozone loss at record level

Arctic ozone loss at record level

by Richard Black, October 2, 2011Environment correspondent, BBC News, 

The Arctic ozone hole lay over over populated regions for parts of winter and spring

Continue reading the main story

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Ozone loss over the Arctic this year was so severe that for the first time it could be called an "ozone hole" like the Antarctic one, scientists report.

About 20 km (13 miles) above the ground, 80% of the ozone was lost, they say.

The cause was an unusually long spell of cold weather at altitude. In cold conditions, the chlorine chemicals that destroy ozone are at their most active.

It is currently impossible to predict if such losses will occur again, the team writes in the journal Nature.

Early data on the scale of Arctic ozone destruction were released in April, but the Nature paper is the first that has fully analysed the data.

"Winter in the Arctic stratosphere is highly variable -- some are warm, some are cold," said Michelle Santee from Nasa's Jet Propulsion Laboratory (JPL).

"But over the last few decades, the winters that are cold have been getting colder.

Continue reading the main story

“Start Quote

Why [all this] occurred will take years of detailed study”

Michelle SanteeJPL

"So given that trend and the high variability, we'd anticipate that we'll have other cold ones, and if that happens while chlorine levels are high, we'd anticipate that we'd have severe ozone loss."

Ozone-destroying chemicals originate in substances such as chlorofluorocarbons (CFCs) that came into use late last century in appliances including refrigerators and fire extinguishers.

Their destructive effects were first documented in the Antarctic, which now sees severe ozone depletion in each of its winters.

Their use was progressively restricted and then eliminated by the 1987 Montreal Protocol and its successors.

The ozone layer blocks ultraviolet-B rays from the Sun, which can cause skin cancer and other medical conditions.

Longer, not colder

Winter temperatures in the Arctic stratosphere do not generally fall as low as at the southern end of the world.

Ozone destruction takes place within polar stratospheric clouds, with chlorine the main culprit

No records for low temperature were set this year, but the air remained at its coldest for an unusually long period of time, and covered an unusually large area.

In addition, the polar vortex was stronger than usual. Here, winds circulate around the edge of the Arctic region, somewhat isolating it from the main world weather systems.

"Why [all this] occurred will take years of detailed study," said Dr Santee.

"It was continuously cold from December through April, and that has never happened before in the Arctic in the instrumental record."

The size and position of the ozone hole changed over time, as the vortex moved northwards or southwards over different regions.

Some monitoring stations in northern Europe and Russia recorded enhanced levels of ultraviolet-B penetration, though it is not clear that this posed any risk to human health.

While the Arctic was setting records, the Antarctic ozone hole is relatively stable from year to year.

This year has seen ozone-depleting conditions extending a little later into the southern hemisphere spring than usual - again, as a result of unusual weather conditions.

Chlorine compounds persist for decades in the upper atmosphere, meaning that it will probably be mid-century before the ozone layer is restored to its pre-industrial health.

http://www.bbc.co.uk/news/science-environment-15105747