Changing Antarctic Winds Could Accelerate Sea Level Rise

science20.com, July 7, 2014

Changes to Antarctic winds have been implicated in southern Australia's drying climate but a new estimate says they may also have a profound impact on warming ocean temperatures under the ice shelves along the coastline of West and East Antarctica.

Projected changes in the winds circling the Antarctic may accelerate global sea level rise significantly more than previously believed. Most sea level rise studies focused on the rate of ice shelf melting due to the general warming of the ocean over large areas. 

Using super computers at Australia's National Computational Infrastructure (NCI) Facility the researchers were able to examine the impacts of changing winds on currents down to 700 m around the coastline in greater detail than ever before. The authors believe previous global models did not adequately capture these currents and the structure of water temperatures at these depths. 

"When we included projected Antarctic wind shifts in a detailed global ocean model, we found water up to 4 °C warmer than current temperatures rose up to meet the base of the Antarctic ice shelves," said lead author Dr. Paul Spence from the ARC Centre of Excellence for Climate System Science (ARCCSS). "The sub-surface warming revealed in this research is on average twice as large as previously estimated with almost all of coastal Antarctica affected. This relatively warm water provides a huge reservoir of melt potential right near the grounding lines of ice shelves around Antarctica. It could lead to a massive increase in the rate of ice sheet melt, with direct consequences for global sea level rise." 

Unexpectedly, this more detailed approach suggests changes in Antarctic coastal winds due to climate change and their impact on coastal currents could be even more important on melting of the ice shelves than the broader warming of the ocean.

"When we first saw the results it was quite a shock. It was one of the few cases where I hoped the science was wrong," Spence said. "But the processes at play are quite simple, and well-resolved by the ocean model, so this has important implications for climate and sea-level projections. What is particularly concerning is how easy it is for climate change to increase the water temperatures beside Antarctic ice sheets."

The believe their new estimate may help to explain a number of sudden and unexplained increases in global sea levels that occurred in the geological past. And recent estimates suggest the West Antarctic Ice Sheet alone could contribute 3.3 metres to long-term global sea level rise.

According to co-author Dr. Nicolas Jourdain from ARCCSS, the mechanism that leads to rapid melting may be having an impact on the Western Antarctic right now. Jourdain said it may help explain why the melt rate of some of the glaciers in that region are accelerating more than expected. "Our research indicates that as global warming continues, parts of East Antarctica will also be affected by these wind-induced changes in ocean currents and temperatures. Dramatic rises in sea level are almost inevitable if we continue to emit greenhouse gases at the current rate." 

Source: University of New South Wales

http://www.science20.com/news_articles/changing_antarctic_winds_could_accelerate_sea_level_rise-139999

"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

Bloomberg: West Antarctic Ice Sheet Melt Past 'Point of No Return,' NASA Says

by Alex Morales, Bloomberg, May 12, 2014

Source: NASA/GSFC/SVS via Bloomberg

Although the Amundsen Sea region is only a fraction of the whole West Antarctic Ice Sheet, NASA estimates the region contains enough ice to raise global sea levels by 4 feet. Close

A glacial region of western Antarctica that’s already melting rapidly has passed “the point of no return,” according to the National Aeronautics and Space Administration.

“The collapse of this sector of West Antarctica appears to be unstoppable,” Eric Rignot, a glaciologist at NASA’s Jet Propulsion Laboratory and the University of California, Irvine said today in an e-mailed statement.

NASA estimates the glaciers, in the Amundsen Sea region, contain enough water to raise global sea levels by 4 feet (1.2 meters). United Nations researchers in September said sea levels have risen by 19 centimeters (7.5 inches) since the Industrial Revolution, and may rise an additional 26 centimeters to 98 centimeters by 2100.

“This sector will be a major contributor to sea level rise in the decades and centuries to come,” Rignot said in the statement. “A conservative estimate is it could take several centuries for all of the ice to flow into the sea.”

Rignot is lead author of a study that has been accepted for publication in the journal Geophysical Research Letters, NASA said. The team used radar observations from the two European Earth Remote Sensing satellites, ERS-1 and ERS-2, to track the movement of the “grounding lines,” the place where the floating portion of glacier meets land.

Floating Ice

They determined that the glaciers have become so thin that they are now floating in areas where they used to be grounded. As the grounding lines retreat inland, there’s more space below the ice for sea water, which accelerates melting. The researchers also found the masses of ice are flowing faster toward the sea, causing further thinning.

Scientists have homed in on the West Antarctic Ice Sheet because the level of melting detected there is greater than the far bigger and more stable Eastern portion of the continent.

Antarctica’s ice sheets hold enough water to raise sea levels by 187 feet, according to UN estimates, though that’s not likely for thousands of years.

http://www.bloomberg.com/news/2014-05-12/antarctic-glacier-melt-appears-unstoppable-nasa-says.html

Australia: Wilder winds, less rain, as Roaring Forties become Furious Fifties

by Peter Hannam, Environment Editor, The Sydney Morning Herald, May 11, 2014

Photo: Fairfax Graphics

The Roaring Forties, the Southern Ocean winds which once bore European sailors to Australia and the East Indies, are becoming more like the Furious Fifties as climate change triggers a shift in key weather patterns poleward, an Australian-led team of scientists has found.

Using data derived from Antarctic ice cores and other sources, the researchers found Southern Ocean winds are now stronger than at any time in the past 1,000 years.

Greenhouse gases are what are causing the winds to intensify now and that’s really moving the system beyond the natural range,” said Nerilie Abram of the Australian National University’s Research School of Earth Sciences and lead author of the research, published on Monday in Nature Climate Change.

In the past half century, the westerly winds have quickened 10-15% and moved 2-5 degrees closer to the South Pole – meaning fewer storms are reaching as far north as Australia.

“That isn’t good news for farmers in the southern parts of Australia who are reliant on the winter winds that come out of the Southern Ocean,” Dr Abram said. Winter rainfall has dropped 20% in southwest Western Australia since the 1960s, with cool-season rain tallies also lower in Australia’s southeast.

The stronger winds also help resolve a climate-change conundrum – why Antarctica is not warming as fast as other continents and the Arctic. “Over a large part of Antarctica we don’t get much warming at all,” Dr Abram said. [This may be changing - temperature anomalies over Antarctica have become very significant this year.]

The reason for the discrepancy is that cool air is being trapped over Antarctica, resulting in increased snowfall for some regions. However, areas exposed to stronger winds and warming seas, such as the Antarctic Peninsula, are heating up faster than anywhere else in the Southern Hemisphere.

“The West Antarctic Ice Sheet [adjacent  to the peninsula] is probably the bit of the Antarctic ice mass that we’ve been most concerned about for the longest time,” said Matthew England, from the University of NSW’s Climate Change Research Centre, and a co-author of the paper. If it all melted, that ice sheet could lift global sea levels by 4-5 metres, he said.

Professor England said the changes to atmospheric variability that see the band of westerly winds oscillate north or south – known in the Southern Hemisphere as the Southern Annular Mode – are driven roughly equally by the effect of rising greenhouse gases and the ozone hole.

The relative contribution, though, should alter as internationally agreed constraints on the use of chemicals that destroy the protective ozone layer take effect, potentially slowing the pick-up in wind speeds.

“Going forward, the greenhouse aspect will dominate as the ozone hole starts to repair and, of course, greenhouse gases are going terrifyingly upwards in their concentration,” he said.

Wenju Cai, an atmospheric scientist at the CSIRO who was not part of the research team, said the findings would assist the study of other key processes, such as whether the rate at which the Southern Ocean absorbs heat and carbon dioxide is changing.

The faster winds “may have a lot of influences that we do not know now,” Dr Cai said. “We may even solve some of the big issues that have been puzzling scientists for many, many years.”

http://www.smh.com.au/environment/weather/wilder-winds-less-rain-as-roaring-forties-become-furious-fifties-20140511-zr9b1.html

Readers, a must-read: The Antarctic Half of the Global Thermohaline Circulation Is Faltering

byFishOutofWaterFollowforOcean Advocates,  Daily Kos, April 10, 2013

The sudden cooling of Europe, triggered by collapse of the global thermohaline circulation in the north Atlantic and the slowing of the Gulf Stream has been popularized by the movies and the media. The southern half of the global thermohaline circulation is as important to global climate but has not been popularized. The global oceans' coldest water, Antarctic bottom water forms in several key spots around Antarctica. The water is so cold and dense that it spreads out along the bottom all of the major ocean basins except the north Atlantic and Arctic. Multiple recent reports provide strong evidence that the formation of Antarctic bottom water has slowed dramatically in response to massive subsurface melting of ice shelves and glaciers. The meltwater is freshening a layer of water found between depths of 50 and 150 meters. This lightened layer is impeding the formation of Antarctic bottom water, causing the Antarctic half of the global thermohaline circulation to falter.

Update from the comments

I have been asked what's going to happen in response to the faltering of the thermohaline circulation around Antarctica. This post is based on a synthesis of very recent research reports. The key report, that found the layer of fresh water between 50 and 150 meters deep, was just published. Deward Hastings explained, in a comment, how disruptive this lens of freshened water could be to the earth's climate system and our models of it:

it IS complicated, and confusing

That lens of (relatively) fresh water that is forming around Antarctica is challenging, and changing, almost everything in global circulation patterns.  It freezes sooner (and at a higher temperature).  That shields the water from the wind, and reduces wind-driven mixing.  It reduces, perhaps to the point of stopping altogether, the present global ocean circulation patterns.  That in turn will change global atmospheric weather.

Nobody knows exactly what comes next.  We've never seen it happen, and our models, not terribly accurate in describing the world we know, are completely untested in the coming world that we don't know.

Without a constant flow of cold water from the poles the Abyss will warm . . . and without cold slowly rising from the Abyss the mid-ocean and ocean surface will warm (already happening).  That will lead to more evaporation (driving a different haline circulation in the tropics) and stronger tropical winds driving different surface currents and greater mixing.

Pretty much everything changes as a result . . . pretty much everywhere.  After it's all over some places will have it better and some worse.  While it's changing everywhere will be worse, because there is no way to know what to expect (except that it won't be what you've prepared for).

The best guesses we can make now about the effects of this melt layer are based on paleoclimatology research. Possible effects, based on paleoclimatology studies, are presented in the last few paragraphs. The results of these new studies will be challenging climate modelers for many years.

Sea ice extent has been increasing around Antarctica. In September 2012, while Arctic sea ice was at record low levels, Antarctic sea ice extent hit a record high. Climate skeptics jumped on the Antarctic record as evidence of cooling, while sea ice researchers blamed it on the wind.

Since the start of the satellite record, total Antarctic sea ice has increased by about 1% per decade. Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent. Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.

NSIDC scientist Ted Scambos said, "Antarctica's changes—in winter, in the sea ice—are due more to wind than to warmth, because the warming does not take much of the sea ice area above the freezing point during winter. Instead, the winds that blow around the continent, the "westerlies," have gotten stronger in response to a stubbornly cold continent, and the warming ocean and land to the north."

Several recent reports, however, paint a more complex and disturbing picture where the intensifying winds are speeding up below surface currents bringing more above freezing water in contact with deep ice around Antarctica. Twenty of the ice shelves and many of the glaciers that feed them are melting from below.

Researchers used 4.5 million measurements made by a laser instrument mounted on NASA’s ICESat satellite to map the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt across the continent. Of the 54 ice shelves mapped, 20 are being melted by warm ocean currents, most of which are in West Antarctica.

Figure 2 | Antarctic ice-shelf ice-thickness change rate DT/Dt, 2003–2008.

Seaward of the ice shelves, estimated average sea-floor potential temperatures (in uC) from the World Ocean Circulation Experiment Southern Ocean Atlas (pink to blue) are overlaid on continental-shelf bathymetry (in metres)30 (greyscale, landward of the continental-shelf break, CSB) Grey circles show relative ice losses for ice-sheet drainage basins (outlined in grey) that lost mass between 1992 and 2006 (after ref. 2).

The melting from below is creating a layer of relatively fresh water 50-150 meters below the surface around Antarctica. This layer of light fresh water is floating above a  salty layer below. When ice forms at the surface in the Antarctic winter, it creates cold dense salty water that tends to sink to the bottom, forming bottom water. However, this layer of light melt water is tending to block the water in the top 50 meters from sinking. The area of Antarctic sea ice has expanded because the layer of cold water has stayed on top and expanded outwards instead of sinking. Melting from below has created 2 stratified cold layers in the top 150 meters.

Note the bright pink area in the top 25 meters between 65° and 70° S. This top layer is becoming more saline. Brine is rejected from ice when sea ice forms. It isn't sinking because it is ponding above a freshening layer located at depths between 50 and 150 meters.

The freshened water column around Antarctica has become more stable between depths of 100 and 150 meters. This increasing stability is impeding the formation of Antarctic bottom water. Water that does sink is freshened through incorporation of glacial melt water.

Figure 3.  Austral winter half-year (April–September) zonal mean trends (1985–2010) of observed salinity, vertical density gradient and potential temperature, in the Southern Ocean. a, Salinity. b, Vertical density gradient. c, Potential temperature. Contours indicate the 1985–2010 mean state (psu; kg m^-4, °C). Colouring (bright or faint) indicates whether the trend is significant (yes or no) at p<0:1 65="" 70="" a="" according="" analysis="" and="" based="" between="" brine="" due="" en3="" font="" forms.="" from="" ice="" in="" increase="" is="" likely="" met="" most="" near-surface="" observations.="" observations="" ocean="" office="" on="" rejection="" salinity="" sea="" situ="" sub-surface="" t-test.="" taken="" the="" to="" two-sided="" were="" when="" which="">

Analysis of potential temperatures, which are temperatures adjusted for the effects of increasing pressure with depth, shows the surface water in the top hundred meters is cooling over a vast area from 40°-80° S, while the water in that vast area below 150 meters is warming.

These results show a trend towards reversal of vertical motions around Antarctica. Intermediate water is welling up around Antarctic melting ice from below, creating a freshened layer. Strengthening winds are blowing the cold surface water away from Antarctica. Bottom water formation, caused by the sinking of cold salty water formed by brine rejection, is declining.

The results of this study are confirmed by a detailed study of anthropogenic tracers in the Weddell sea.   Chlorofluorocarbon (CFC) observations showed increasing average ages of the deep water in the sea from 1984–2010. The average age increased because because bottom water formation, and outflow from the Weddell sea, declined.

...we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the anthropogenic Carbon increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations.

The decline in Antarctic bottom water formation, combined with the southward expansion of warm subtropical water in the south Pacific and south Indian oceans has led to the rapid heating of intermediate and deep ocean water in the southern hemisphere.

Figure: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.

A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth, and Källén (2013).  There are several important conclusions which can be drawn from this paper.

• Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.

• As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.

As the earth has warmed in response to the effects of increasing levels of greenhouse gases the southern subtropical belt in the oceans and atmosphere has expanded, tightening the rings of winds and ocean currents around Antarctica. Enormous volumes of warm subtropical water have been added to the southern ocean at depths greater than 300 meters (greater than approximately 1000 feet).

Observed temperature trends in the Indian Ocean present complex patterns that cannot be explained by surface heating alone. The heat storage has apparently increased more in the southern part than in the northern part of the Indian Ocean (Levitus et al. 2005), although this result may be biased by the sparse data coverage, particularly in the south (Harrison & Carson 2007). The strongest warming is found near the subtropical front and extends as deep as 800 m; it is not directly linked to surface heating but rather due to a southward shift of the oceanic gyre circulation and associated thermal structure (Alory et al. 2007).

Another recent detailed study of the water properties of the southern ocean has independently determined that the southern branch of the global thermohaline circulation has slowed dramatically, contributing to a large uptake of heat by the deep southern ocean.

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the Meridional Overturning Circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW.

Rates of change in AABW-related circulation are estimated in most of the world’s deep
ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The
Southern Ocean is losing water below θ = 0 °C at a rate of -8.2 (±2.6) × 106 m3 s-1.

The budget calculations and global contraction pattern are consistent with a global scale slowdown of the bottom, southern limb of the MOC.

The slowdown of the southern branch of the thermohaline circulation and the cooling of the surface waters close to Antarctica are enhancing the thermal gradient from the tropics to the pole, speeding up the winds in the Southern Hemisphere. These increases in wind speeds are likely increasing the flow of water from the Pacific to the Atlantic ocean, enhancing the northward flow of water, salt and heat from the south to the north Atlantic. Moreover, the southward movement of the subtropical front allows more flow of the Agulhas current around the south African capes from the Indian ocean to the south Atlantic.

Thus, increased melting of Arctic sea ice may be related to declines in Antarctic bottom water formation. Likewise, the cool Pacific, warm Atlantic pattern causing increased U.S. droughts and storminess in the north Atlantic may be tied to these changes in ocean circulation patterns. Paleoclimate studies have consistently shown oscillations between Antarctic and north Atlantic bottom water formation and between relative coolness around Antarctica and north Atlantic warmth.

The Arctic melt down that is far exceeding model predictions is connected to the slow down in Antarctic bottom water formation. Climate modelers will be challenged to model the connections and the details. The cooling waters around Antarctica, while apparently good news, are not. The rapid melting of the Arctic will be enhanced.

http://www.dailykos.com/story/2013/04/10/1200602/-The-Antarctic-Half-of-the-Global-Thermohaline-Circulation-Is-Faltering

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
Winds Seen As Key Driver Of Antarctica's Growing Sea Ice
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

Seth Borenstein, AP: Antarctica's floating ice shelves are melting from beneath due to warmer ocean waters being pushed closer by changing winds

Study: Antarctic Ice Melting From Warm Water Below 


by Seth Borenstein, Associated Press, April 25, 2012


Antarctica's massive ice shelves are shrinking because they are being eaten away from below by warm water, a new study finds. That suggests that future sea levels could rise faster than many scientists have been predicting. 


The western chunk of Antarctica is losing 23 feet of its floating ice sheet each year. Until now, scientists weren't exactly sure how it was happening and whether or how man-made global warming might be a factor. The answer, according to a study published Wednesday in the journal Nature, is that climate change plays an indirect role — but one that has larger repercussions than if Antarctic ice were merely melting from warmer air. 


Hamish Pritchard, a glaciologist at the British Antarctic Survey, said research using an ice-gazing NASA satellite showed that warmer air alone couldn't explain what was happening to Antarctica. A more detailed examination found a chain of events that explained the shrinking ice shelves. Twenty ice shelves showed signs that they were melting from warm water below. Changes in wind currents pushed that relatively warmer water closer to and beneath the floating ice shelves. The wind change is likely caused by a combination of factors, including natural weather variation, the ozone hole and man-made greenhouse gases, Pritchard said in a phone interview. 


As the floating ice shelves melt and thin, that in turn triggers snow and ice on land glaciers to slide down to the floating shelves and eventually into the sea, causing sea level rise, Pritchard said. Thicker floating ice shelves usually keep much of the land snow and ice from shedding to sea, but that's not happening now. That whole process causes larger and faster sea level rise than simply warmer air melting snow on land-locked glaciers, Pritchard said. 


"It means the ice sheets are highly sensitive to relatively subtle changes in climate through the effects of the wind," he said. What's happening in Antarctica "may have already triggered a period of unstable glacier retreat," the study concludes. 


If the entire Western Antarctic Ice Sheet were to melt — something that would take many decades if not centuries — scientists have estimated it would lift global sea levels by about 16 feet. 


NASA chief scientist Waleed Abdalati, an expert in Earth's ice systems who wasn't involved in the research, said Pritchard's study "makes an important advance" and provides key information about how Antarctica will contribute to global sea level rise. 


Another outside expert, Ted Scambos of the National Snow and Ice Data Center, said the paper will change the way scientists think about melt in Antarctica. Seeing more warm water encircling the continent, he worries that with "a further push from the wind" newer areas could start shrinking.


http://abcnews.go.com/Technology/wireStory/study-antarctic-ice-melting-warm-water-16212616#

"Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf " by Stanley S. Jacobs, Adrian Jenkins, Claudia F. Giulivi & Pierre Dutrieux, Nature Geosci., doi: 10.1038/ngeo1188

Nature Geoscience,  doi: 10.1038/ngeo1188

Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf

• Stanley S. Jacobs,
• Adrian Jenkins,
• Claudia F. Giulivi and
•  Pierre Dutrieux

• Received December 1, 2010; accepted May 20, 2011; published online June 26, 2011.

Abstract

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In 1994, ocean measurements near Antarctica’s Pine Island Glacier showed that the ice shelf buttressing the glacier was melting rapidly¹. This melting was attributed to the presence of relatively warm, deep water on the Amundsen Sea continental shelf. Heat, salt and ice budgets along with ocean modelling provided steady-state calving and melting rates^2, 3. Subsequent satellite observations and modelling have indicated large system imbalances, including ice-shelf thinning and more intense melting, glacier acceleration and drainage basin drawdown^{4, 5, 6, 7, 8, 9, 10}. Here we combine our earlier data with measurements taken in 2009 to show that the temperature and volume of deep water in Pine Island Bay have increased. Ocean transport and tracer calculations near the ice shelf reveal a rise in meltwater production by about 50% since 1994. The faster melting seems to result mainly from stronger sub-ice-shelf circulation, as thinning ice has increased the gap above an underlying submarine bank on which the glacier was formerly grounded¹¹. We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 °C above freezing can now more readily access the grounding zone.

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Link to abstract:  http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1188.html