Antarctic Sea Ice Gain

We’ve looked at Arctic sea ice data, noting not only its overall decrease but changes in the annual cycle as well. As one reader suggested, let’s take a similar look at sea ice in the southern hemisphere.

Unfortunately I don’t know where to get data for Antarctic sea ice volume. But extent and area data are available, and I’ll use the monthly averages from NSIDC. This enables us to compute two derived quantities, the “spread” (extent divided by area) and “split” (extent minus area).

First of all, here are the time series for both (extent in black, area in red):

There’s no visually obvious change, but we shouldn’t expect to see it in the raw data. We can, however, transform to anomaly values (values minus the long-term average for the same month) to remove the influence of the seasonal cycle:

Both have shown increase over time which is statistically significant. The smoothed values for extent have increased by about 0.57 million km^2, area by about 0.51 million km^2. The reason for its increase is uncertain, but two possibilities which have been suggested are: 1, increased precipitation (snowfall) due to warming near the tropics; 2, intensification of the “polar vortex” perhaps due to changes in stratospheric ozone concentration.

Despite the fact that the southern ice pack is larger overall than the northern, its increases are much smaller than the decreases noted for the northern hemisphere, 1.96 million km^2 in extent and 1.92 million km^2 of area. This puts the lie to claims (oft repeated) that southern gain even “almost” balances northern loss — the northern extent loss is 3.4 times as great as the southern extent gain while northern area loss is 3.8 times as great as southern area gain. When one is nearly 4 times as big as another, they are certainly not “balanced” and anyone who claims so is either a fool or an outright liar.

Here’s the average annual cycle for extent and area:

Again, extent is in black and area in red, while the vertical bars show the difference between the two (which we’ve called “split”). Both quantities reach their annual minima in February and maxima in September.

If we look at the rate of increase (as estimated by linear regression) for each month separately, we find no real evidence of any seasonal pattern, just the kind of fluctuations which could be random:

Windowed Fourier analysis enables us to look for changes in the size, shape, and timing of the annual cycles. Although all those parameters fluctuate, there’s no sign of any trend in them. The annual cycles of southern hemisphere sea ice extent and area have not shown the dramatic changes which have been observed in the northern hemisphere.

I also looked at “spread” and “split” for the southern ice pack. Here are the raw data series:

Transforming to anomaly values, we see that spread has slightly declined:

However, this seems to be a reflection of the fact that overall ice cover has increased, since the split has shown no trend at all:

In fact, split has shown no real trend for any time of year — the one significant value (June) is actually within random fluctuation, given that we’re computing trends for 12 separate months of the year:

The average annual cycles show how these parameters vary throughout the year (again I’ve plotted the individual monthly values in time order so these are 12 monthly “time series” compressed onto a single plot):

One interesting observation is how skewed the split cycle is, rising steadily from February through November, only to take a nose-dive from December to January.

There’s very little meaningful change, if any, in the annual cycles of southern hemisphere sea ice split. There is some evidence, for instance, of a small decrease in the phase of maximum:

However, when looking for trend in many variables you should expect to find something that crosses the 95% confidence level just by chance. This may or may not be a real trend, it’s possible that the split cycle hasn’t really changed over time other than random fluctuations.

In summary, the southern hemisphere shows significant sea ice gain over the last 34 years, but it is dwarfed by the sea ice loss in the northern hemisphere (which is more than 3 times as great). The annual cycles of extent, area, and split, have shown little if any change. Spread has shown a declining trend, which seems to be due to the fact that split is stable but the overall area is increasing. All in all, other than the increase in extent and area, the southern ice pack has shown surprising stability during the satellite era.