I often mention the Pacific Decadal Oscillation and Interior Alaska climate, and I've written about it before, but it's so important to understanding climate variability for most of Alaska (except the North Slope), that it's worth updating.
To start with, below is the classic PDO anomaly diagram from the
University of Washington. The color field is the sea surface temperature anomalies and the arrows are the surface wind anomalies. On the left is the positive phase of the PDO and the right is the negative phase. Note that from the North Pacific-wide perspective the phases are misnamed: the positive phase features a huge area of cooler than normal temperatures from Asia to about 155W, and the negative phase a similarly place warm anomaly. The surface wind anomalies are southerly into mainland Alaska during the positive phase and northerly during the negative phase.
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| Courtesy of the U. of Washington |
Next is an updated plot of the PDO Index. I constructed this by taking the monthly PDO index page at the UW website and constructed four non-overlapping seasonal means per year (effectively a course low pass filter). The heavy black line is the ten year (40 season)
centered running mean. Using a centered mean preserves the timing of the phase transitions.
The phase changes in 1946 and 1976 are pretty clear in the ten year mean, while the early 20th and 21st century transitions are less obvious. At least the 21st century transitions is a little clearer in the even smoother annual averages (note I've used a July to June year in order to capture as a unit the Northern Hemisphere winter):
Since the 2005-06 year, only once (2009-10) has the annual mean PDO been above zero, and even that just barely, and the negative values have been repeatedly lower than any time since the 1976 transition. This is why I place the positive to negative phase PDO transition at 2007.
