While the Arctic ice has responded to global warming with even more rapid melting than predicted, Antarctica is responding to the planet warming in a less intuitive way.
Although there are regions of Antarctica (such as the Antarctic Peninsula) that have seen significant warming and sharp declines in sea ice cover, overall the Antarctic has actually experienced a mild increase in sea ice area over the same 33-year period.
Antarctica had record-breaking sea ice cover in 2012 and 2013, and is literally the last place on the planet where the long-term cooling trend of the past 2,000 years hasn’t yet reversed due to global warming.
Rather than poking holes in the theory of global warming, scientists believe that the growth in Antarctic sea ice is due to a combination of the effects of ozone depletion and global warming on the Southern Annular Mode.
The Southern Annular Mode
The Southern Annular Mode (SAM) is the atmospheric pressure difference between the Southern Hemisphere mid-latitudes and Antarctica. When the difference in temperatures across the Southern Hemisphere is greater, this leads to greater pressure differences as well since colder air is more dense.
Scientists also sometimes refer to the SAM as the Antarctic Oscillation, as it switches between the positive and negative phases. When SAM is in the positive phase, the temperature and pressure gradients are enhanced, and the Southern Hemisphere westerly jet stream moves poleward, closer to Antarctica.
During the negative phase of the SAM, the opposite is true: Weaker Southern Hemisphere temperature gradients create a weaker jet stream that moves away from Antarctica, further north.
The phase of the SAM is important to climate in the Southern Hemisphere because of its role as the primary pattern of climate variability; it influences temperatures and the distribution of rainfall throughout the entire hemisphere.
The Impact of the Ozone Hole on SAM
The SAM has been strongly positive over the last three decades, especially in the summer. One widely-accepted explanation for this prolonged positive phase of the SAM is that it is due to changes in atmospheric circulation caused by ozone depletion. Ozone is a strong greenhouse gas, and the ozone layer traps some of the heat in the lower atmosphere. When the ozone layer is depleted over Antarctica during Southern hemisphere spring and summer, more of that heat is released, and the Antarctic continent cools.
This cooling of the polar region in the Southern Hemisphere is the exact opposite of the situation in the Northern Hemisphere, where Arctic warming is occurring faster than anywhere else on the planet. You may have read about how the rapid warming in the Arctic is leading to a weaker temperature difference between the midlatitudes and the Arctic, causing the polar jet stream to weaken and meander in the Northern Hemisphere.
Since the Antarctic is actually cooling, the reverse is happening in the Southern Hemisphere: The temperature difference between the midlatitudes and the Antarctic is increasing, and the westerly winds forming the polar jet stream in the Southern Hemisphere are strengthening and moving poleward. This is known as the positive phase of the SAM.
However, the greater temperature difference between the Antarctic and Southern Hemisphere midlatitudes is not entirely accounted for by ozone depletion alone. Other factors are global warming, which increases the temperature in the midlatitudes, and geographic features of the Antarctic, which further prevent warming.
Geographic Differences Between the Arctic and Antarctic
I asked Antarctic sea ice expert Dr. Guy Williams from the University of Tasmania to shed some light on why Antarctica responds so differently than the Arctic to global warming. He responded:
“I think the important point here is just how different the two polar regions are. The Southern Ocean is a tremendous buffer against warming from lower latitudes transferring to Antarctica. In particular the Antarctic Circumpolar Current in the Southern Ocean, in conjunction with the belt of westerlies is strong barrier to the poleward transmission of heat than present in the Arctic. Consider the Polar Front at ~50-55 degrees South as a representation of this barrier, and then look at how much further north heat is transported (by the gulfstream etc) in the Arctic.
In addition, continents are colder than oceans (water can only go to -2°C, Vostok went to -89°C). The Arctic is essentially an ocean, and Antarctic is a continent.”
The Impact of Global warming on SAM
While Antarctica has remained cold due to geographic features and cooling from ozone depletion, global warming in all other parts of the Southern Hemisphere has not stood still. Temperatures in the midlatitudes and equatorial region have increased, causing the temperature difference between Antarctica and the midlatitudes to increase and the pressure gradient across the Southern Hemisphere to steepen, further promoting the positive phase of SAM.
Ozone layer expert Dr. Matt Hitchman from the University of Wisconsin-Madison tells Decoded Science:
“There has been a poleward migration of the Southern Hemisphere jet stream by a few degrees, not a lot, but statistically significant. It is believed to be due to the combination of a slightly warmer, moister tropics with more convection expanding the tropical volume of air somewhat poleward, together with a persistent nice and cool Antarctica. The persistence is thought to be due to the continued ozone hole. Ozone emits infrared downward near 9.6 microns, helping to warm the surface. The lack of this effect poleward of 60S is due to the anthropogenic ozone hole. This contrast of a warming tropics and still cool Antarctica can enhance the jet stream in the southern hemisphere.”
How SAM’s Positive Phase Affects Sea Ice Extent
When SAM is in the positive phase, the westerly wind belt moves poleward, where it encounters more sea ice and pushes it northward. This explains the increase in sea ice coverage in the Antarctic. SAM has been strongly positive over the last thirty years, and the index used to describe it is now at its highest long-term average level in about a thousand years.
Rather than contradicting global warming, the growth of Antarctic sea ice is another instance of an unforeseen effect of human activity. It is also important to note that the growth of Antarctic sea ice pales in comparison to the loss of sea ice in the Arctic.
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