Why is southern sea ice increasing?
| Antarctic sea ice has been growing over the last few decades but it certainly is not due to cooling - the Southern Ocean has shown warming over same period. Increasing southern sea ice is due to a combination of complex phenomena including cyclonic winds around Antarctica and changes in ocean circulation. | |||||
Climate Myth...
Southern sea ice is increasing
'Antarctic sea ice set a new record in October 2007, as photographs distributed by the National Oceanic and Atmospheric Administration showed penguins and other cold-weather creatures able to stand farther north on Southern Hemisphere sea ice than has ever been recorded. The news of expanding Antarctic sea ice stole headlines from global warming alarmists who asserted Arctic sea ice had reached its lowest extent since 1979.' (James Taylor)
'Antarctic sea ice set a new record in October 2007, as photographs distributed by the National Oceanic and Atmospheric Administration showed penguins and other cold-weather creatures able to stand farther north on Southern Hemisphere sea ice than has ever been recorded. The news of expanding Antarctic sea ice stole headlines from global warming alarmists who asserted Arctic sea ice had reached its lowest extent since 1979.' (James Taylor)
Figure 2: Antarctic surface temperatures as observed by satellites between 1981 and 2007.
Similar trends are found when combining temperature data measured from ships and buoys. The following figure from Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions (Zhang 2007) displays trends over the ice-covered Southern Ocean - this is the region where Antarctic sea ice forms.
Figure 3: Linear trend (1979–2004) of surface air temperature over the ice-covered areas of the Southern Ocean (Zhang 2007).
We see strong warming over most of the ice-covered Southern Ocean although there is also some cooling. What is the average trend over the whole region? The overall surface temperature trend over the ice-covered regions of the Southern Ocean shows a warming trend:
Figure 4: Annual mean surface air temperature averaged over the ice-covered areas of the Southern Ocean. Straight line is trend line (Zhang 2007).
Oceanographic data also find that the waters in the Southern Ocean are warming. The waters of the Southern Ocean's Antarctic Circumpolar Current have warmed more rapidly than the global ocean as a whole. From 1960 to 2000, water temperature increased by 0.068°C per decade at depths between 300 and 1000 metres. This warming trend has increased to 0.098°C per decade since the 1980s (Boning 2008).
If the Southern Ocean is warming, why is sea ice increasing? There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). A side-effect is a strengthening of the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas leads to increased sea ice production (Turner 2009).
Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).
Antarctic sea ice is complex and counter-intuitive. Despite warming waters, complicated factors unique to the Antarctic region have combined to increase sea ice production. The simplistic interpretation that it's caused by cooling is false.
Last updated on 5 January 2011 by John Cook.
Printable Version |
Offline PDF Version |
Link to this page
Comments
-
"Goddard commits this error on several occasions."
Sorry to nitpick, but your quote at the top was from James Taylor, not Steve Goddard.Response: Please, nitpick away. I've removed the Goddard reference which was a hangover from the original post responding to several Goddard posts at Watts Up With That. -
I’m not convinced that this problem is well understood yet. My
feeling is that it is dangerous to use data from the sea off Antarctica
as evidence either for or against AGW, until there is a consistent model
covering the whole area. Unfortunately, it might not be so easy to
provide one. I’ve been trying to read the literature, and intend to give
my dumbed down version of it below, in the hope that someone with more
expertise can correct me.
OBSERVATIONS: Much of this depends on the “reanalysis” project, that tries to reconstruct climate data for the period 1957-96.
It seems clear from Boning el al that there is a measured warming trend of the water of the Southern Ocean (between 30 and 60 degrees south, down to 2000 meters). The reanalysis of temperature records seem (Zhang) to show that in the interval 1979-2004 the air surface temperature in the much smaller “ice covered area” around Antarctica has been rising. There is also a measured increase in Antarctic sea ice extent (Turner et al and references therein)
In addition to this there is also a further measured change, related to the Antarctic Oscillation (also called Southern Annular Mode). There are two modes (Thompson & Solomon): High index means cold polar temperatures, strong western winds, and low index means the opposite. This index has been rising, increasing winds and decreasing polar temperature. There is evidence that this development is driven by the “ozone hole”.
All of the above seem to comparatively safe. In particular the simplistic argument “more ice means the South Polar Sea is colder” is nonsense, we do know that the Polar Sea is getting warmer, so that the extent of ice on the Ocean is definitely a bad proxy for temperature,
But when it comes to explanations of what we see, things look a lot more murky to me. Two main players seem to be the layering of the Southern Ocean water (less dense water above denser water) and the constant western winds around 60 degree.
THE EKMAN SPIRAL: The wind possibly contributes to the mixing of layers in an interesting way: the wind drives an ocean current which runs around the Antarctic moving from West to East. This current extends down into the ocean, with the surface water moving fastest, and deeper water moving in the same direction, but slower, Because of the Coriolis effect, the current will try to veer to the left, which means towards the North. Now, since the surface water moves faster than the deeper water, the net effect is stronger at the surface, So surface water will move to the North, which forces deep water to the south. This creates a down-welling North of the current, and an upwelling South of the current. This whole business is called an Ekman spiral.
BONING: The paper by Boning et al. quotes measurements that seem to show that even if the circumpolar winds have been increasing, that “Ekman spiral” has not become stronger. They believe that the reason for this is that the increased wind also produces more eddies, which confuse the whole picture. They note the the models that has been used cannot resolve those eddies (they are too small). If they are right, the lack of enhanced mixing of layers in the ocean is not yet theoretically understood.
ZHANG: Zhang’s paper is a pure model study The main point of the article is that he can construct a model of the South Sea that agrees with two important seemingly contradictory measured factS: it has a warming ocean, but an increase in sea ice.
I think that the mechanism proposed by Zhang is slightly (but not essentially) different from what John describes. The motor driving various changes is the increase in surface temperature. This initially leads to a decrease in production of new sea ice. The top water gets warmer and less salty. Both these changes work in the same direction, they both make the top layer less dense.
The next thing that happens is that since the top layer is gets dense, there is less upwelling of warm water. This means that less ice is melted. So now, both less ice is created and less ice is melted. The model says that the change in melting is bigger than the change in the creation of new ice, so the net effect is that we get less ice.
But wait? If less ice melts in the top layer, the salinity will increase, counteracting the previous effect? Zhang says that this is so, but we still have to take the warming of the top layer into account! This warming makes the top layer lighter, decreasing mixing of layers.
So now there are lots of things going on: Since the top water gets warmer, less ice is produced. On the other hand, less ice is melted by upwelling deeper warm water. Then there is precipitation, but Zhang does not believe that the increase in precipitation is the decisive effect.
And the sum of the three effects "creating less new ice", “destroying less old ice" and "warming the water" is actually that the top layer gets less dense - driving the cycle.
This all seems a bit subtle for my taste, taking the big uncertainties into account. For instance, what happened to Boning’s eddies, which were supposed to be important? But at least this is a testable model.
TURNER: This paper seems to ignore questions of up and down convection in the oceans, the questions that dominated Boning et al. and Zhang.et al. Instead it focuses on the strengthening of the western wind – the high index of the Antarctic oscillation.
Another important point of this paper is that they break down the increase in sea ice into geographical areas and seasons. In particular, the ice in the Ross sea (close to the pole) has been increasing, while the ice in the Bellinghausen-Amundsen sea (father from the pole) has been decreasing. This is clearly important, and the regional differences should be explained.
There is some modeling going on, but the upshot seems to be unclear (they conclude that it could all be natural variability). The paper is often cited for explaining increased sea ice by the polynyas in the Ross sea. It seems to me that they only suggest this mechanism, but they don’t give strong arguments for it. Possibly I’m missing something. -
John, I swear it was somewhere on this site, but I can't find it ... There's a paper from the early 1990s where the GCM
the authors were using predicted increasing antarctic ice, which
puzzled them, and they found it was the increased precipitation
(lowering surface salinity) that was doing it (in the model). It's
really quite a coup to have predicted the increase and to have
attributed it to one of the mechanisms now believed to underlie the
observed increase.
If you know what paper I'm talking about, it deserves a shout-out from this page. Thanks!Response: I think you must've seen that paper somewhere else - the only papers I include on Antarctic sea ice analyse the trends after the event. But if you do track down this paper, please do post the URL here, thanks! -
GFW at 08:16 AM on 25 May, 2010
GFW, this isn't exactly what you're asking for. However many early models of the 80's/90's showed greatly delayed Antarctic warming compared to rapid Arctic warming. This is due (a) to the very large Southern hemisphere oceans and (b) different S and N polar ocean circulation which gives more efficient mixing of surface and deeper waters in the deep S hemisphere, transferring heat from the surface.
So, quoting from a recent review of ocean circulation modelling in which the mechanisms for hemispheric warming asymmetry are described illustrates that highly delayed Antarctic Circumpolar ocean warming has been predicted since the early 1980’s.
Here’s a bit of a summary from (direct excerpts are in blockquotes):
S. Manabe and R. J. Stouffer (2007) Role of Ocean in Global Warming J. Meterolog. Soc. Jpn. 85B 385-403.
General point about ocean modulation of surface warming:
“In response to the increase in greenhouse gas in the atmosphere, the positive temperature anomaly initially appears in the well-mixed surface layer of the ocean called the “mixed layer”. Gradually, the anomaly spreads from the mixed-layer to the deeper layers of the ocean, thereby increasing the effective heat capacity of the oceans. The increase of effective heat capacity, in turn, results in the reduction of the rate of increase in surface temperature, reducing and delaying the warming as shown by Hoffert et al (1980) and Hansen et al. (1984).”
Discussing the early models of Schneider and Thompson (1981) to evaluate the delay in the response of the sea surface temperature to gradual increase in CO2, Manabe and Stouffer say:
"Their study shows that the time-dependent response of zonal mean surface temperature differs significantly from its equilibrium response particularly in those latitude belts, where the fraction of ocean-covered area is relatively large. Based upon the study, they conjectured that the response in the Southern Hemisphere should be delayed as compared to that in the Northern Hemisphere because of the inter-hemisphere difference in the fraction of the area covered by the oceans.”
In a later model Bryan et al (1988) made the same sort of analysis, investigating the role of the oceans in modulating the response of surface warming to enhanced greenhouse gases.
"They found that the increase in surface temperature is very small in the Circumpolar Ocean of the Southern Hemisphere in contrast to high latitudes of the Northern Hemisphere where the increase is relatively large.”
It’s not just the oceans per so of course. It’s also ocean and air currents, and particularly the mechanisms governing the efficiency of surface heat transfer into the deeper oceans. If this is efficient, the deep oceans will absorb heat and there might be little measured surface warming, at least for a while. So (speaking of Bryan et al (1988)) again:
"However, the detailed analysis of the numerical experiment reveals that the absence of substantial surface warming in the Circumpolar Ocean is attributable not only to the large fraction of the area covered by the oceans but also to the deep penetration of positive temperature anomaly into the oceans.”
Later models predict the same hemispherical asymmetry that is seen in the real world. e.g. discussing the simulations of Manabe et al (1992):
“Figure 3 also reveals that there is a large asymmetry in surface warming between the two hemispheres. In the Northern Hemisphere, the surface warming increases with increasing latitude, and is particularly large in the Arctic Ocean. This is in sharp contrast to the Southern Hemisphere, where warming is relatively large in low latitudes and decreases with increasing latitudes. It becomes small in the Circumpolar Ocean of the Southern Hemisphere, particularly in the immediate vicinity of Antarctic Continent.”
Why is this, one might ask?! Here’s what Manabe and Stouffer say:
"One can ask: why the polar amplification of warming does not occur in the Southern Hemisphere, despite the existence of extensive sea ice which has a positive albedo feedback? As discussed in the following section, the absence of significant warming in the Circumpolar Ocean of the Southern hemisphere is attributable mainly to the large thermal inertia of the ocean, which results from very effective mixing between the surface layer and the deeper layers of ocean in this region. This is in sharp contrast to the Arctic Ocean, where very stable layer of halocline prevents mixing between the surface layer and the deeper layer of the ocean" ......."In view of the absence of significant surface warming, it is not surprising that the area coverage of sea ice hardly changes in the Circumpolar Ocean despite the CO2-doubling.”
n.b. remember this is a prediction from a model involving the response to [CO2] doubling; we’re nowhere near CO2 doubling yet. However these early models predicted what we're seeing in the real world today.
No comments:
Post a Comment