Friday, October 09, 2009

I'll take Antarctic Melting for $10 please Alex.

Mark Francis, in comments, wrote "Why don't you take a good swing at the kerfuffle over the recent Antarctica ice study by Marco Tedesco and Andrew Monaghan?" Now, that's more like it – the easy softball questions as opposed to Peter 1’s question about models (that one has kept me up all night for that last few days ;) )

The "TEDESCO AND MONAGHAN" paper is called An updated Antarctic melt record through 2009 and its linkages to high-latitude and tropical climate variability. It has made a splash in the skeptic world because it shows the smallest "snowmelt index anomaly" ever recorded (of course in this case, ever is since 1980).

To begin with, the snowmelt index is not the same thing as snow/ice loss. The index is defined as the number of melting days multiplied by the area of melt. It is measured by satellites looking in the microwave range since liquid water provides a very strong microwave reflection. Along the coast, there is a relation between ice/snow loss and the snowmelt index, but in the interior you can have huge areas that melt under the summer sun in the day but then freeze again at night (or what passes for night). Also, keep in mind that because of the thickness of the ice cap, most of the Antarctic surface is at an elevation of 2km above sea level which tends to make it a little cooler anyway.

However this paper is not looking only at the snowmelt index, but rather the relation between a couple of well known climate phenomena and the snowmelt index. In a nutshell, the authors show that there is a relationship between the snowmelt index and the SOI and the SAM.

The SOI is the Southern Oscillation Index and is a measure of the atmospheric pressure across the Pacific Ocean (this is related to el Nino and la Nina conditions which people are probably aware of). Essentially when the SOI is positive, we are in La Nina conditions. This means that the winds blow warm water in a generally west direction across the Pacific which piles up in Northern Australia and Indonesia.

The SAM is the Southern Hemisphere Annular Mode and is another measure of atmospheric pressure difference, except this time between the southern hemisphere mid and high latitudes. When the SAM is positive you get a wind pattern forming over the Antarctic which essentially forms a “ring” of wind around 55 to 60 degrees latitude south. This wind essentially acts to insulate the Antarctic from the warmer northern areas.

Tedesco and Monaghan have shown that when both of these indices are positive then the snowmelt index is small (i.e. negative). This seems to make sense in a qualitative way since a positive SOI will mean cooler pacific waters and a positive SAM will mean a more insulated Antarctic. However this paper shows that there is a strong link in a quantitative i.e. statistical way (it is an inverse link since as the SOI and SAM go up, the snowmelt index goes down).

Now, comes the payoff! Over the last year, the SOI was the second highest for the study period and the SAM was the 4th highest for the study period. With such high values for both of these it seems expected to have a very small (smallest over the study period) snowmelt index. So rather than something new and shattering, the study provides an insight into how the climate factors interact and show that the current snowmelt is what we would expect given other climate factors and thus isn't probably the smoking gun that some make it out to be. (Although if anyone is interested, there will be a study coming out soon that looks at snow and ice loss over Greenland and the Antarctic using gravity measurements from satellites.)

In addition, the authors also point out that since the 1970’s the SAM has tended to be relatively high and it has been proposed that there is a connection between the high SAM and the loss of ozone over the South Pole. So as the ozone hole starts to heal in the future, the SAM is expected to decrease.

I will leave the final words to the authors themselves:

"Negative melting anomalies observed in recent years
do not contradict recently published results on surface
temperature trends over Antarctica [e.g., Steig et al.,
2009]. The time period used for those studies extends back
to the 1950’s, well beyond 1980, and the largest temperature
increases are found during winter and spring rather than
summer ..."

Funny how that part of the paper never gets quoted on the skeptic sites.

PS, first post so please excuse any formatting errors. I have not put a picture at the top since I didn't know how to - but more importantly I don't think I can match the Dr.'s talent for using the exactly correct graphic for each post.

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