I thought it would be interesting to look in a bit more detail at the seasonal variations in the PDO influence on Fairbanks temperatures - for example, does the PDO influence peak in December, or is it equally large at other times of the year?
To address this question, I first looked at monthly data and examined the outcome in months for which the PDO index was in the top or bottom quartile of the 1930-2013 historical distribution. This gives a sample size of 21 on both ends of the PDO distribution. The first chart below shows the percentage of months in which the Fairbanks temperature anomaly had the same sign as the PDO index, and the second chart shows the median monthly temperature anomalies as well as the 25th and 75th percentiles of the temperature anomalies. Note that I used the "normal" for the entire history (1930-2013) and did not adjust for long-term trends.
Looking first at the positive PDO, the first chart shows that December is indeed the month in which the positive PDO phase most reliably causes above-normal temperatures in Fairbanks (18 of 21 years), but the second chart indicates that the median anomaly is not as high in December as in November or January through March. Also, the second chart reveals that there are large and interesting differences in the width of the conditional temperature distribution during the positive PDO phase. For example, in November the positive PDO temperature distribution is quite narrow, with over 80% of years having a mean anomaly above +3 °F, but in January the temperature distribution is wide during positive PDO years. We infer that the positive PDO is a much more useful predictor for Fairbanks temperature in November than in January; but the PDO signal becomes quite robust again in February.
The temperature behavior during the negative PDO phase is largely a mirror image of the positive PDO behavior, except during June-August when the negative PDO has almost no influence on Fairbanks temperature. November is clearly the month in which the negative PDO most reliably causes below-normal temperatures in Fairbanks. The negative PDO signal is much weaker in January, then picks up again in February, although with a large amount of temperature variance. Overall, winter temperatures are more variable when the PDO is negative, leading to the conclusion that a negative PDO phase is somewhat less useful as a seasonal forecast predictor than a positive PDO phase.
Another way of looking at the data is to examine the distribution of daily temperature anomalies during positive and negative PDO phases (see charts below). Here I used the prior 30-year climatology of daily mean temperature to obtain the daily anomalies, and I ran the calculation over sliding 21-day windows throughout the year, with the PDO index interpolated from the monthly values to the center of each date window. The purpose of doing this is to see if there are any notable sub-monthly features of the climate during positive or negative PDO phases.
The charts above show quite a number of interesting features, but I'll only mention a few. First, the response of daily Fairbanks temperature to the positive PDO phase seems to be relatively stable in winter (especially early winter) and does not show either a November peak or a January dip in PDO influence, as indicated by the monthly charts. It's not necessarily surprising that daily probabilities behave very differently from monthly probabilities, but I'll have to think some more about possible explanations for the difference. Interestingly the annual peak in probability for positive temperature anomalies with the positive PDO phase falls at the end of April, and the annual minimum is less than a month later; however, the median temperature is always above normal.
The daily chart for the negative PDO clearly shows the peak in negative PDO influence in November, but then there is a fascinating spike up in December, and the median temperature anomaly is above zero on about December 20. It would be interesting to look at the historical years in more detail to see how this plays out; I suspect it is a reliable feature of the subseasonal evolution in the wake of a cold PDO-induced trough over Alaska in November.
It's also interesting to note that daily temperatures are more likely to be above normal than below normal in most of June, August, and September during the negative PDO phase.