What is Pacific Decadal Oscillation?
The PDO is a climate phenomena found primarily in the North Pacific (as opposed to El Niño which affects mostly the tropical Pacific). It has two phases that it typically alternates between; usually staying in one phase for a significant period of time (as little as 10 and as much as 40 years). However, it’s not uncommon for these long periods to be broken by intervals when it switches phases for anything between 1 and 5 years. The phases of the PDO have been called warm phases (positive values) or cool phases (negative values). Figure 1: Monthly values for PDO index: 1900 to May 2006. Figure source: Climate Impacts Group So the first lesson of PDOs is that while we talk about a 20 to 30 year period, it is not very clear cut at all. In fact, an analysis of the frequency of the events does not produce much in the way of a firm period. Incidentally back in 1999 it was predicted that we were entering a cool phase. The second lesson of PDOs is that while we talk about warm phases and cool
