(I was meaning to post in this thread, certainly got the two mixed up tonight 2012 snow and this one)
The AO and AAO (SAM) values are very interesting and informative numbers. This week I have learnt a lot more about them than I previously understood because I was able to look at over 60 years of monthly data for both NH and SH. Thanks to roves for posting the link to the SH data back the 1948 that was able to shed a lot of light on my understanding. The data for NH from 1950 is already on the NOA website.
The daily values themselves on a daily basis are only like the weather and difficult to interpret, its not until you start to average out the readings that you start to see what is happening. Previously I had only ever looked at the monthly average figures back to 1979 shown here.http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/month_aao_index.shtml
I could immediately how well snowy winters correlated with low values and bad winters with high values. However even at the monthly scale you can see that its only extended low values that correlate with the really good seasons like 80-81 leading to the massive 81 season and 90-91-92 leading to 3 really big snow years. Bad years you can see correlate well with high values like 82, 93.
However even at the monthly scale you still cannot see all that is happening, it was not until I plotted the values at a 10 year running mean that my eyes popped out. Suddenly you can see that the SAM has been rising quite consistently for 60 years with the last 11 years near a plateau. This is a clear sign of a contracting polar vortex. However when looking at NH AO equivalent although there is also a rise it is far from linear.
To understand this you need to realize what the SAM is. It is a temperature differential between the sub tropics and the polar region like a gradient or waterfall that can be steeper or flatter. However because the Earth is constantly going through complex temperature exchanges between the tropics and the poles the gradient is also fluctuating, from a high gradient (high SAM) to low gradient (low SAM). When there is a steep gradient for cold fronts its like they run into brick walls trying to get North and slide away, but when there is a low gradient they can much more easily depart the polar regions and slide North. Because it takes cold fronts weeks to get organized to have a chance to move North a brief drop in SAM allows only a small window of opportunity, its need to be extendedly low to have the best effect.
Because the SAM is a gradient its likely that there are limits to how high or low the value can go. What a positive value really means is a net heat gain in the lower latitudes relative to high latitudes which squashes the polar vortex. Now the interesting thing is that what the SAM shows is that Antarctica has barely been warming at all while the low latitudes have been warming much faster which is why the value is positive and the polar vortex is squashed further South than what is has been decades ago. However in the NH the AO has been able to reset itself back to zero a number of times and is more dynamic. Notice how the AO reached a peak in 1998 at the height of the large El Nino, this suggests that the El Nino was the largest heat input into the low latitudes of the NH to cause such a high value. However after the El Nino ended the SAM reset itself in the NH and this led to some of the severe winters of the 2000's in the NH.
The NH AO is able to reset itself much more easily because of the excess land masses. The heat is able to migrate to the North pole and rebalance the AO. This is exactly what we saw, large polar warming because the heat can migrate to the pole that caused the AO to drop suddenly and cause those cold winters but also low Arctic ice levels in the latter 2000's. But the AO is also struggling to get much below zero because of the excess warming in the low latitudes of the NH. The recent AO rise last winter was another rebound but without an El Nino it is not that strong yet. Once we get another El Nino more heat will be sent to the North Pole and the cycle may begin again.
It seems the SH may also be reaching a limit because the SAM is plateau-ing now. However because of the configuration of land masses in the SH it is unclear if there can be any reversal like in the NH. Because it is so difficult to get any heat down to Antarctica it may well stay colder down there relative to the low latitudes and keep the long term SAM mean high. That is why the tend looks like a one way trend for 60 years.
It is interesting that in a way something snapped even in the SH in 2000 after the El Nino but it was only brief and failed to register on the 10 years trend. Rather for us the left over's have been nearly no effect on Antarctica but a permanently elevated SAM because of the heat left over in the low latitudes from the El Nino. By rights a strong La Nina should have been enough to turn around the SAM trend, but because Antarctica is so isolated from heat and heat is still excess in low latitudes, so far we are still not making much headway.
Unless some mechanism that has not happened yet can rebalance balance the heat differential between Antarctica and lower latitudes like Australia we cannot get decent cold fronts. We need to see SAM at least reset itself like the NH AO, but just not sure if the SH oceans will ever allow it.
SAM + 10 year moving mean since 1948
NH AO + 10 year moving mean since 1950