Pacific Decadal Oscillation (PDO) Index – Back into the Negative
Posted by The Diatribe Guy on December 4, 2008
The PDO index is a measure of another one of those weird oceanic oscillation patterns. The ENSO index is a shorter-term varying event, whereas the PDO index is considered a longer-term event. Interestingly, however, when you collapse the ENSO index into longer average time periods, there does appear to be persistency in the ENSO index, as well. However, the short-term cyclical nature of the ENSO index is much more evident than the PDO index, even if the averages show similar persistency.
The PDO index is predominant in the Northern Pacific and secondarily evident in the tropics, whereas the ENSO index is the other way around. ENSO receives so much attention because of the large short-term cyclical variations, but the PDO has very significant swings as well, just usually over a longer period of time.
The index, however, is measured on a month-to-month basis and the data can be located here.
November has not yet been updated, but the October index value was -1.76. This was the single coolest reading in any month since November 1999, and was the 14th consecutive negative reading in a row. While there have been other stretches of 14 consecutive negative readings – the last being 1999-2000, it is interesting to note that the current streak of seven consecutive readings of less than -1 has not been seen since the period ending February 1976. That particular stretch ended at 7 months. If this month’s reading is below -1, it will mark the first time since the period ending June 1972. And that streak ended at 8. Beyond that point takes us to a streak in the early 1960s. So I’ll keep an eye on that.
As I did with the ENSO index, I collapsed the raw data into different average periods. Since the PDO does seem to vary over a longer period of time, I went up to a 10 year average. The persistency in the index is very clear once you take longer-term averages.
Below are a few charts of the PDO. The first is the raw data. Taking the edges off with 12-month smoothing follows that. I then present 5-year and 10-year averages.
The PDO monthly index has been negative for a bit already, as has the five-year average. But the 10-year average recently went negative, as well. Of course, all the same arguments apply as in the ENSO discussion – the persistence matters. You keep a heater running and the temperature gets warmer than the temperature an hour ago, depending on insulatory effects. Persistence in both ENSO and PDO correspond with the warming we see in the global temperature readings. The 10-year average PDO is slightly negative, and we’ve cooled slightly in the last few years. Coincidence?
The more I look at all these indices and see the same general pattern (overall average increase in sunspots during the time period of warming, overall increased persistence in ENSO index during the period of warming, and overall increased persistence in the PDO index duiring the period of warming) the more I realize how many different events have simply lined up in favor of the warming we saw from the 1970s to the late 1990s. How these things manage to get ignored in favor of simple correlation charts between temperature and CO2 (which always end around 2000 because the correlation breaks down after that) is utterly amazing.