Winter 2009 forecast for the North Atlantic - update

Summary: Our previous forecast from August 4, 2008 has been significantly revised. New data indicate that a positive North Atlantic Oscillation (NAO) phase is more likely during the winter of 2009 (CF = 25).

Fig. 1. The forecast of sea surface temperature anomalies in the North Atlantic in the winter (DJF) of 2009.  The blue (red) contours and the letter C (W) indicate colder (warmer) than normal temperatures. CL means near normal. The base period for calculating anomalies is 1971-2000. The numbers at the contours are confidence factors of the forecast.

Discussion

One of the most important factors that increased the probability of a positive phase of the NAO in the forthcoming winter is the distribution of summer-fall sea-surface temperature (SST) anomalies in the North Atlantic. When waters are anomalously warm east of Newfoundland and relatively cold in the eastern North Atlantic (as is currently observed), this creates a situation conducive to formation of a more zonal atmospheric flow in the high latitudes (rule 22).

Another important factor is Northern Hemisphere snow cover in summer, which was found to be a good predictor of the NAO index in the following winter (rule 21).  During the summer (JJA) of 2008, the area covered by snow was the lowest since 1990. When this area is low, the chances of a positive NAO phase become higher.

Rule 71 links the quasi-biennial oscillation (QBO) with the NAO index. Although the rule has a relatively low CF, it suggests that the positive NAO index is more likely. Rule 39 takes into account a combination of solar activity and QBO. It is consistent with rule 71 and indicates that the circumpolar vortex in January will probably be stronger than normal. 

Rule 38 was somewhat misinterpreted in our previously issued forecast. In fact, the rule suggests that, while the NAO index may slightly decrease from the previous year, it will likely to stay above normal.

Some of the rules still point to a possibility of a negative NAO index. These are mainly the rules that take into account solar activity. For example, rule 53 suggests that due to a very low level of solar activity, the frequency of Greenland blocking episodes may increase, which translates into a negative NAO phase. Another example is rule rule 465 that links solar activity with climatic conditions in the Barents Sea. According to this rule, during even cycles of solar activity and on its ascending branch, the Barents Sea tends to be characterized by a suppressed cyclonic activity, negative anomalies of sea and air temperature and increased ice cover. Although the temperature in the Barents Sea may cool compared to the previous winter, it will probably remain above the 1971-2000 average.

There is an indication that the Azores high may weaken during the winter of 2009 and an anomalously high pressure center may form in the east-central North Atlantic. It is interesting to note that previously rule 288 was predicting an anomalously low pressure center in that region, but due to almost a complete reversal in the North Atlantic SST anomaly field in July, the forecast has also changed. These changes mean that instead of a classical positive NAO pattern, a negative phase of the so-called Equatorward displaced NAO pattern (van den Dool et al., 2007) may develop.