Antarctic sea ice is one of those things in the climate system that seems to confuse people. Antarctic sea ice, on average, is increasing. How can there be global warming if sea ice is increasing in the Antarctic? Some have gone so far as to average the Arctic sea ice loss with the Antarctica sea ice gains, and imply that globally sea ice isn’t changing. That’s just silly. Even so, it’s fair to say that most of the popular explanations for Antarctic sea ice expansion haven’t been very convincing.
In this essay, I’ll try to explain where the confusion about Antarctic sea ice changes comes from, and to highlight a few recent papers in the scientific literature that add important new clarity to the picture. The bottom line is that scientific understanding Antarctic sea ice trends is actually pretty solid.
First, let’s talk about what determines how much Antarctic sea ice there is. As with Arctic sea ice, it’s a combination of thermodynamics and dynamics: the rate of cooling to the atmosphere vs. the delivery of heat from the ocean below, and the movement of sea ice by the winds and by surface ocean currents. In general, Antarctic sea ice forms near the coastline, where upwelling waters cool to the atmosphere. It melts when the winds and currents push it into areas of warmer water to the north. In the summer, it melts pretty much all the way back to the coast; the 24-hour sunlight provides plenty of energy to make that happen.
An efficient way to form lots of Antarctic sea ice during the autumn growth season is to have strong winds that push the ice away from the coastline. Pushing sea ice away leaves open water that can lose heat to the atmosphere, creating more sea ice. Near the coastline, the major source of this “push” is the katabatic winds, resulting from the flow of very cold dense air from the high elevations of Antarctic continent; katabatics are especially strong in fall and winter and conspire with the extreme cold of the polar night to cause the annual growth of sea ice. Further north, the persistent circumpolar westerlies of the “screaming sixties” help the sea ice break up, and push it into warmer waters. (Owing to the Coriolis effect, westerly winds cause northward-flowing surface ocean currents in the Southern Hemisphere). Sea ice rarely extends northward of about 55°S (the tip of South America). Note that the role of wind in the Antarctic is different from the situation in the Arctic, where sea ice movement towards warmer climes is restricted by the continents.1
The importance of the winds in controlling Antarctic sea ice leads to the obvious idea that changing winds can explain the increase that has been observed over the last several decades. In particular, it follows from the above discussion that if either the katabatic flow from the Antarctic continent, or the westerlies were to increase, we would expect Antarctic sea ice to expand. There has indeed been a substantial increase in the circumpolar westerlies; this is very well established from observations and is associated with the oft-discussed increase in the “Southern Annular Mode” (SAM) index2. Averaged over the year, the SAM index has increased nearly monotonically since the 1970s (e.g., Marshall et al., 2003).
So at first glance, it really is very simple: the westerly winds have increased, so sea ice has increased too. Furthermore, there is good evidence that the increasing westerlies are a response to anthropogenic climate forcing from CO2 and other greenhouse gas increases in the troposphere, along with ozone declines in the stratosphere (Thompson and Solomon, 2002; Thompson et al., 2011). This would suggest that the observed increase in Antarctic sea ice extent is anthropogenic in origin, just like the Arctic sea ice decline, but for very different reasons.
It sounds pretty good right? Reduced ozone in the stratosphere, and increased CO2 in the troposphere — both climate forcings that are unequivocally anthropogenic — cause increased westerly winds, which cause Antarctic sea ice to expand.
Of course, it’s not that simple. For one thing, the average increase of Antarctic sea ice is actually a small number that is the difference of two big numbers — modest increases over a large area, mostly in the Eastern Hemisphere, and very large decreases over a smaller area in the Western Hemisphere. The map below, showing change in the length of the sea ice season over the last 30 years, illustrates this point well. In spite of the average increase, there are very rapid declines in the Bellingshausen and Amundsen Seas, comparable to sea ice declines in the Arctic.
Another problem with explaining Antarctic sea ice expansion in terms of the circumpolar winds is that the only season is which there is a significant trend in the westerlies is austral summer. There is a weak positive trend in fall, but both spring and winter show no trend; the SAM trends in these seasons may even be slightly negative, depending on which data are used (Ding et al., 2012). Yet the pattern of sea ice change is quite similar in all seasons: decreasing along the Pacific coast of West Antarctica, and increasing around most of East Antarctica, and in the Ross and Weddell Seas.
Another problem is that modeling studies that have examined the relationship between the westerly winds and Antarctic sea ice have come up with results that appear to be in direct opposition to the observations. When fully coupled climate models are run with increased CO2 and decreased stratospheric ozone, the westerly winds increase as has been observed, but sea ice decreases around most of Antarctica. In fact, in at least one study (Bitz and Polvani, 2012), the pattern of trends is the mirror image of the observations, with increases, rather than decreases in the Amundsen and Bellingshausen Seas3!
So what’s really going on? Before I get to that, I want to note a couple of other ideas that have been suggested but which probably don’t provide the key answer. One idea is that model resolution might be a problem. Big climate models are generally run at relatively low spatial resolution, typically at about 2° latitude and longitude. That’s very high compared with a decade ago but is insufficient to resolve key processes such as ocean heat transport by small scale eddies. Bitz and Polvani looked at this using a very high resolution model (0.1°). Their results show only a somewhat weaker response to the ozone forcing experiment but with the same (incorrect) sign. Another idea is that changes in ocean stratification might be important. One way to increase sea ice growth is to increase the amount of fresh water getting into the Southern Ocean, which could happen from increased rainfall or from glacier melt. The latter is happening, of course, in spades (see the latest data from Sutterly et al., 2014 for example). Fresh water forms a sort of buoyant lid on the ocean, limiting the ability of heat from the warmer water below to get to the sea ice and melt it. A study by Bintanja et al. (2013) showed that it was a least plausible that this explains the Antarctic sea ice change. A basic problem, though, is that the greatest discharge of meltwater is occurring in the Amundsen Sea, exactly where sea ice is declining.
Here’s what I think is really going on.
First, comparing observations with the results of model experiments like those of Bitz and Polvani (2012) is misleading. Most such experiments are equilibrium experiments: What’s done is to run a model under “preindustrial” conditions, and then to run it again with reduced ozone and increased CO2, and to look at the difference. This provide a measure of what will eventually happen (at least in the model) after many decades or centuries. This is a sensible thing to do, because it is much more computationally expensive to do transient simulations. Also, is often reasonable to assume that the short-term (transient) response will look like the long-term (equilibrium) response; it will just be smaller in magnitude. But for this particular problem, things don’t work this way — the sign actually changes through time. When you look at the transient response to changes in the circumpolar winds, as Marshall et al (2014) have done, it turns out that two important things happen. The winds tend to push the sea ice boundary northward, as we would have expected. But also, the winds push the surface ocean northward too, and cause a slow rise in the isopycnal surfaces (surfaces of constant density). This brings relatively warm deep water closer to the surface, eventually melting sea ice after a period of a few decades, countering the initial increase in sea ice. These results explain why equilibrium model calculations find sea ice decreasing in response to ozone forced changes in the circumpolar winds, and also why observations show the opposite. Not enough time has passed for the equilibrium response to be manifested. Theses results suggest that some time in the next few decades, there will reverse, and average sea ice will begin to decline.4
Second, there’s a whole lot more going on with the winds than just “increased westerlies”. In fact, in the areas where the big sea ice losses have occurred, the concept of “circumpolar westerlies” isn’t very relevant. A far more important measure of wind variability in the Amundsen and Bellingshausen Seas is the Amundsen Sea Low (ASL).5 The ASL describes the average location of storms systems the bring heat and moisture into West Antarctica. Changes in the ASL may occur for myriad reasons, but one big hammer that can make it ring is the propagation of atmospheric planetary wave arising out of the tropics, more-or-or less associated with ENSO (El Niño-Southern Oscillation) variability. It’s been clear for many years that ENSO variability play a significant role in sea ice variability in those regions, and recent work shows that this can explain the trends pretty well too (e.g. Yuan and Li, 2008; Stammerjohn et al., 2008). Not incidentally, the adjacent land areas of the Antarctic Peninsula and the West Antarctic Ice Sheet have warmed significantly over the last few decades (Steig et al, 2009; Orsi et al., 2013; Bromwich et al, 2013), and those changes can also be attributed largely to tropical climate variability (Schneider and Steig, 2008; Ding et al., 2011; Schneider et al., 2012; Steig et al., 2013). The cause of temperature and sea ice change is the same: more warm air is being steered into West Antarctica, and the atmospheric flow tends to push sea ice against the continent, keeping it from expanding.6
So, do we get the right answer if we take into account all of the wind changes that have occurred over the last few decades? The answer is yes. This is nicely illustrated in a study by Holland and Kwok (2012), who showed that wind, ice motion, and ice concentration changes match each other remarkably well. Where the wind has been increasingly northward, concentrations are increasing; where wind and ice motion changes are toward the continent, ice concentrations are decreasing.
Moreover, this year, Holland et al. (2014), showed that when they drive an ocean and sea ice model with observed winds — not just increased westerlies, but the full range of wind changes, as calculated by the ECMWF (European Center for Medium Range Weather Forecasting) –- they correctly simulate the overall expansion of sea ice, and they also get the pattern of changes pretty much spot-on. To be sure, authors note that not all the details are explained and highlight the possibly greater importance of thermodynamic consideration (i.e. ocean temperature/stratification) in some areas than in others. Further, the period they address (1992-2010 only) is short. Still, the results are nevertheless pretty compelling. Just like the observations, their calculations show large decreases in the Amundsen and Bellinghausen seas, but increases nearly everywhere else.7
Taken as a whole, these results show that there is no significant contradiction between our understanding of Antarctic sea ice and the observation that it is, in average, expanding. We can explain sea ice trends in the Antarctic rather well if we take into account the full range of changes in winds that have occurred. The average expansion of Antarctic sea ice was not anticipated, but it hardly represents any sort of existential threat to our fundamental understanding of the climate system as a whole. It’s merely an interesting scientific challenge.
Of course, predicting how sea ice will change in the future is another matter. Turner et al. (2013) show that “hindcasts” of the late 20th and early 21st century, using the latest-generation fully coupled ocean-atmosphere models, generally get the trends wrong. Our track record, in other words, isn’t very good. But the uncertainty here lies not so much in our understanding of sea ice per se, but rather in our ability to predict the details of the wind and ocean changes that drive sea ice change. Various model imperfections, and in particular the representation of the tropical atmosphere, can result in wind biases and therefore sea-ice biases around the Antarctic continent (e.g. Song et al., 2011). We aren’t going to get projections (or hindcasts) in the Amundsen Sea area right if we don’t get details of the tropical Pacific right — and that’s a notoriously difficult problem and an active area in climate dynamics research.
Featured Image: “NASA’s DC-8 Flying Over the Weddell Sea” (11/17/2011) by NASA GISS via Flickr.
1This is not to say that winds don’t matter in the Arctic – they do. It does not, however, appear to be as dominant a factor. See for example Lindsay et al.’s (2009) paper on the 2007 Arctic sea ice anomaly.
2The SAM index can be defined a number of different ways, but essentially a measure of the pressure or geopotential height difference between the high and mid latitudes. For example, the commonly used Marshall SAM index is based on the difference of sea level pressure (SLP) between 40°S and 65°S. Since winds flow clockwise around regions of low pressure in the Southern Hemisphere, an increase in the SAM index — which means decreasing pressure at high latitudes relative to low latitudes — has to be associated with increasing westerlies.
3Whether the mirror image character of Bitz and Polvani’s (2012) results is just chance, or might tell us something fundamental, is an interesting question that, as far as I’m aware, has yet to be addressed.
4Call me a skeptic, but I think that the results of Marshall et al. (2014), which suggest that eventually sea ice will start to shrink in response to wind-driven changes in the ocean, needs further evaluation. It’s one thing to bring more warm water upwards towards the surface. It’s another thing for that heat to diffuse through the stratified upper water column, and melt sea ice.
5Some researchers talk about the variability in the Amundsen Sea as being part of the “asymmetrical response of the SAM” but I find this terminology misleading. Changes in the ASL may sometimes be associated with changes in the overall circumpolar westerlies, but they may also occur quite independently. Further, changes in the ASL can change the SAM index, even when there is no change in the circumpolar westerlies per se. Ding et al., 2012 estimated that 25% of the variability in the SAM index is due to tropically-forced variability in the ASL, entirely independent of CO2 and ozone forcing (see also L’heureux and Thompson, 2012 and Seager, 2003).
6These wind changes may also figure prominently in the thinning of West Antarctic glaciers, but that’s a subject for another essay, which I’ll write up for RealClimate.org. For those that are interested, see Steig et al. (2012) and Dutrieux et al., (2014).
7 The full magnitude of average sea ice increase modeled by Holland et al. (2014) is not quite as large (about 70% as big) as the observations suggest, but this is hardly a reason for concern. Some recent results suggest, in any case, that errors in the observations have led to an overestimate of the trend (Eisenman et al, 2014).