By Matt Owens November 20, 2013
Two circular bands of winds called the westerlies are being changed by human-caused global warming. The consequences from these changes could become quite large and come on suddenly - quite the surprise for anyone who still thinks climate change is a future "slow" problem.
In the words of Paul Mayewski, director of the University of Maine's Climate Change Institute, these and associated climate changes are “just not part of a natural cycle.” From his perspective, an abrupt climate change has also just taken place - in the Arctic. And more could be on the way at the other side of the world.
Mayewski's research has taken him to remote corners of the globe like the Himalayas and Antarctica. And his work has covered a number of interdisciplinary climate topics, not the least of which is abrupt climate change. I spoke to him last week about some of his latest published findings which show that the austral westerly winds have changed rapidly before, and that regional-scale climate has also rapidly changed as a result.¹ One of the most surprising aspects of his findings is that these significant changes have happened in as little as one year.
The westerlies are associated with the polar jet streams, but unlike the jets, the westerlies reach all the way to the surface of the ground. Westerly winds are found in both hemispheres, and both sets of winds have been changing, although somewhat differently in their respective hemispheres. Westerlies in the Southern Hemisphere (also known as austral westerlies) are usually the stronger of the two because very few land masses or mountains block their circular flow around the globe.
[See the westerlies in motion in the two animations at the end of the article.]
The westerly winds are the same winds that racing yachts still use in some parts of the oceans. Because they reach the ground level (or sea level), they have a significant impact on ocean surface currents as well as the patterns of ocean mixing. Mixing is the process where deep waters and surface waters of the ocean mix together. This mixing enables exchange of heat, nutrients, and greenhouse gases. The exchange of heat between the surface and deeper layers in turn has a strong impact on things like rainfall distribution and ice melt, especially around the edges of Antarctica. In sum, the westerlies are extremely important, especially in the ocean-dominated Southern Hemisphere.
On that austral side of the globe, it's a combination of ongoing human greenhouse gas pollution and previous decades' worth of ozone depletion from CFC pollution that are combining to cause the problem. Mayewski explained it like this: “Human source increases in greenhouse gases, such as CO2, has led to warming in the troposphere over much of the Southern Hemisphere. But human source destruction (through CFC's) of the ozone that traps heat over Antarctica in the lower stratosphere over Antarctica has led to a slight cooling over Antarctica.”
So in just the Antarctic area of the globe, we are seeing two opposite effects from human pollution: local Antarctic cooling and a global warming. These effects combine to produce “a steeper thermal gradient in heat from pole to mid-latitudes and resulting stronger zonal (westerly) winds around Antarctica. Strengthened zonal winds help to prevent southward penetration into Antarctica of warmer air. Therefore, despite the dramatic warming of the Antarctic Peninsula and coastal regions of Antarctica, the interior of the ice sheet is not yet feeling the impact of warming. It will once the ozone hole over Antarctica heals.”
However, on the Northern Hemisphere side of the planet where there isn't nearly as much of a hole in the ozone layer, the westerlies have been weakening.
This is similar to the observed recent pattern of weakening and meandering seen in the Arctic polar jet stream, a trend that Rutgers climate scientist Jennifer Francis has brought to the world's attention recently. She has said that the jet stream has been changing largely due to reductions in Arctic sea ice and thus a resulting lowering of the thermal gradient between the Arctic and mid-latitudes. She's also attributed an increased frequency of extreme weather events in the Northern Hemisphere to the increased meandering wave pattern of the Arctic jet.
When there's a weakening north-south thermal gradient, the winds (including both the jet stream and the westerlies) move much more like a lazy meandering river on a flat coastal plain instead of a rapidly-flowing and mostly-straight river of the type you'd expect to find in steep foothills.
Right: a NASA satellite view of a meandering river in the mostly-flat Amazon.
The atmospheric “river” of winds acts like a barrier between polar-sourced cold air and mid-latitude-sourced warm air. The more meandering the waveform, the further south cold arctic air can penetrate - and likewise, the further north warm air can penetrate. The speed at which the waveform itself progresses also changes with its amplitude. So when the meandering pattern grows more accentuated, then progression of the wave slows, and in turn this leads to persistent and unusual weather as air on each side of the line reaches further south or north than usual and also sticks around for longer than usual.
While Francis' explanation makes sense intuitively, National Center for Atmospheric Research (NCAR) climate scientists Kevin Trenbreth has said that Francis has overstated the importance of the Arctic sea ice as the root cause of the recent changes in zonal wind patterns. Trenbreth instead has said that he places more credit on the growing heat surplus of the tropical latitudes. He has pointed out that while the increasingly-open Arctic Ocean is pumping more heat into the atmosphere, the waters there are so cold and geographically small, that they are dwarfed by the massive tropical ocean areas which are also pumping increasing amounts of extra heat into the air.
Research published in 2006 confirmed that the Northern Hemisphere westerlies, while less impressive than the austral set of westerlies, have nevertheless been playing a significant role in the intensity of drought cycles experienced in North America.² And since that 2006 paper, there has been a significant change in the Arctic and an accompanying major reduction in the north-south thermal gradient there.
“We just experienced an abrupt climate change event in the Arctic,” Mayewski said. “Portions of the Arctic have warmed as much as 9°F in the last few years. This is possibly the most dramatic event of the last several thousand years and a harbinger of more abrupt events in onset of drought, storm frequency and heat waves to come.”
Regarding just the austral westerlies, Mayewski pointed out that recent research by others supports a connection between changes in the westerlies and the decades-long, ongoing Australian drought.³ And research he took part in (published in 2012) has also connected drought in southern Africa to the westerlies. That research predicts substantially more drought if the westerlies continue to move closer to the pole.⁴
The most recent work from Mayewski¹ used Antarctic ice cores and the preserved concentration of particles in the ice layers, including sea salt, land-sourced dust, and methylsulfonate, to produce a paleorecord of the austral westerlies. It shows that they can move both abruptly and gradually, and have been doing so at a remarkable pace in recent decades. According to this research, they are now “at their maximum southerly extent” of the past 100,000 years. In our conversation, he said that he expects them to keep pushing further south as global warming continues. This trend is also predicted by a number of global and regional climate models.
When abrupt changes in the westerlies happened in the past, Mayewski said that they tended to be preceded over a few decades by a handful of excursions towards the impending new climate state. And once the abrupt shift happened, there were a handful of similar excursions back towards the old climate state, also spread out over a number of years.
Mayewski's 2013 research paper¹ concludes: “our investigations point to the likelihood that the drying trend will continue in these regions [southern areas of the Africa, Australia, New Zealand and South America] and to the possibility that there will be in the future abrupt transitions in the continued poleward migration of the westerlies.” This poses numerous serious risks to people, including for example, significant changes in agricultural output, fisheries production, water supplies, and wildfire.
There's also the issue of sea level rise, since the changing westerlies will impact Antarctica's massive ice sheets. “Once warm air masses begin to penetrate inland more vigorously than today and as the Southern Ocean continues to warm, West Antarctica will be more likely to experience ice loss,” according to Mayewski.
Precipitation, especially rainfall, on the surface of the floating West Antarctic Ice Shelf could be quite problematic. If enough surface water were to accumulate at any one time, it would lead to hydraulic fracturing by that water - right through the shelf to the ocean below. Extensive hydraulic fracturing would break apart large sections of the massive floating ice shelf, and thus remove the buttressing effect which the intact shelf now provides. The land-based ice would then flow out into the ocean much faster, thus accelerating sea level rise.
Apparently, a lot more than we thought rests on the wind.
¹ Mayewski et al. 2013 - “West Antarctica’s Sensitivity to Natural and Human-forced Climate Change Over the Holocene.”
² Booth et al. 2006 - “A Reanalysis of the Relationship Between Strong Westerlies and Precipitation in the Great Plains and Midwest Regions of North America.”
³ For example, see Ommen et al. 2010 - “Snowfall increase in coastal East Antarctica linked with southwest Western Australian drought;” and Vance et al. 2013 - “A Millennial Proxy Record of ENSO and Eastern Australian Rainfall from the Law Dome Ice Core, East Antarctica.”
⁴ Stager et al. 2012 - “Precipitation variability in the winter rainfall zone of South Africa during the last 1400 yr linked to the austral westerlies.”