Sea Ice South (1): A little geography

Sea Ice South (1): A little geography

My last entry was on the decline of sea ice in the Arctic and how this is forming an entirely new environment in the Arctic. It’s an environment of open water in the summer and a freezing sea in the winter - perhaps, a little like the Great Lakes. Now I am going to start a series on trying to untangle the difficult subject of sea ice in the Southern Hemisphere.

As many know the sea ice in the Southern Hemisphere has been increasing for the past few years. Here is a picture from the National Snow and Ice Data Center.




Figure 1: Areal extent of April sea ice in the Southern Hemisphere from 1979 – 2011. (figure from National Snow and Ice Data Center)

This figure shows a plot of April monthly averages of the area of the ocean covered by ice. There is a lot of variability from year to year, and if you take an average of all the years, the amount of ocean covered by sea ice has increased in the last three decades. From 2006 to 2011 the variability is high, and it will be interesting to see if the apparent oscillation continues over the next five years.

This increase of sea ice has entered the political discussion in different ways. Most notably, it has been paired with the Arctic sea ice in plots to show that the global sea ice is remaining approximately constant. The political argument goes - hence, there is not trend; hence, the climate alarmists have isolated their attention on the Arctic to carry forward a political argument. This pairing of North and South to conclude that sea ice decrease is inconsequential is a deceptive political argument. It mixes northern summer with southern winter; hence, warm season and cold season in a way that is, from the point of view of the physical scientist, incorrect. It also dismisses the vast impact on ecosystems and regional climate that is occurring in the Arctic. The processes that determine the energy budget of sea ice in the northern and southern hemisphere are quite different. This series is my attempt to break down this complexity well enough that I can understand it.

In both my dynamics class and my climate change class, I constantly remind students of the geography of the Earth. The weather and climate of the Earth is largely determined by the energy received from the Sun, the rotation of the Earth, and the distribution of land, water, ice and air. Of special importance is the height and location of mountain ranges. Here’s an old map I like looking down on the South Pole.




Figure 2: Map of the South Pole and the Southern Ocean from the year 1894. (figure from Perry-Castañeda University of Texas Library Map Collection)

The first thing to note is that the South Pole is in the continent Antarctica, which is land (and ice). Compared with the oceanic North Pole, the ocean cannot carry heat all the way to the pole. The second thing to note is that Antarctica is high and steep. This strongly influences atmospheric storms. These two geographical facts mean that the atmosphere and ocean might carry heat to the edge of Antarctica, but the center of the continent is, perhaps, a bit isolated or protected.

There is another critically important aspect of the geography, which is suggested on the map by the dashed line labeled “average limit of floating ice.” Also note the parts of the ocean labeled “Antarctic Drift.” This section of ocean completely encircles the Earth with no land barrier. It gets narrow at the tip of South America. It is especially notable at, say, the tip of Africa the way the Agulhas Current gets swept away in the Antarctic drift. Remember, this map is from 1894 – I think it makes my points solidly.

We see here in the Southern Hemisphere, atmospheric storms that start in the warm north and propagate southward towards Antarctica. They travel through this open water around the continent, Antarctica. They are steered and broken up by the steep edge of Antarctica. The stress of these storms on the surface of the ocean causes the ocean to “drift” from west to east. This is a far different situation from the Arctic, where there is no land at the pole and a mix of land and ocean around the edge of the Arctic ocean. (see another old map at this old blog )

So this is the set up - the geography makes the northern and southern poles distinctly different places. How, then, do we think about the formation and destruction of sea ice? We have to think about energy, just like in the last blog. The atmosphere and ocean bring and take away heat. There is fresh (light) and salt (heavy) water. There is rain and snow (energy and fresh water). There is ice melting in Antarctica. And there is, in fact, a fundamental difference in the radiative forcing – ozone. In the Southern Hemisphere there is the ozone hole. Often we forget that ozone is, in fact, an important greenhouse gas. With all of this - is there any reason to expect sea ice to behave the same in the northern and southern hemisphere? With all of this - is it at all scientifically honest to mash together sea ice observations from the north and south, summer and winter, and talk about them as one?

OK – I think that is a reasonable foundation.

r


Useful links
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice