Winter 2009 forecast for the North Atlantic

Summary: The North Atlantic Oscillation (NAO) index in the winter of 2009 is forecast to be negative, which is consistent with the forecast for the Northern Hemisphere. This translates into warmer than normal temperatures in the Northwest and eastern tropical North Atlantic. Colder than normal temperatures are more likely off the southeastern coast of the United States. In the Northeast Atlantic and the Nordic Seas temperatures are expected to be near the 1971-2000 average. Overall, the sea surface temperature anomaly averaged for the entire North Atlantic is forecast to be lower than in the winter of 2008, but still remain positive.

Fig. 1. 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

In the winter of 2009, a low level of solar activity (which is at the beginning of its 24th cycle), may be an important factor leading to a decrease of the North Atlantic Oscillation (NAO) index. The analysis of the relationship between solar activity and the NAO (rule 42:) reveals that during the past 50 years, the NAO index was negative in 11 of 12 years when the sunspot numbers were lower than 22. This is consistent with Lamb (1972) who found a tendency for positive SLP anomalies in Iceland during sunspot minima based on the data from 1750-1958.

The forecast of the negative NAO index is supported by the extrapolation technique described in rule 125 and the relationship with the sea-level pressure (SLP) anomaly pattern in the North Pacific as discussed in rule 38. However, the NAO forecast based on Northern Hemisphere snow cover (rule 21) remains inconclusive. Likewise, little signal is provided by the spring-summer sea-surface temperature (SST) anomaly patterns (rule 22). This is probably why the NAO forecast issued by the U.K. Met Office and based on May SST anomaly pattern in the North Atlantic calls for near-normal or slightly positive NAO values in the winter of 2009.

A low level of solar activity may increase the frequency of Greenland blocking episodes (rule 53). Frequent Greenland blocking usually translates into lighter sea-ice concentrations in the Davis Straits/Labrador Sea region and a warmer Northwest Atlantic in general. There is a strong correlation between Greenland blocking and the NAO index, with the latter being negative during most of the winters when frequency of blocking is high.

An interesting lagged relationship exists between sea-ice in Baffin Bay and a distribution of SST anomalies in the North Atlantic (rule 131). Based on this relationship, positive SST anomalies are more likely to occur east and south of Newfoundland and negative SST anomalies off the southeastern United States.

The Northeast Atlantic and Nordic Seas are forecast to be near the 1971-2000 average. This conclusion is based partially on rule 465 that takes into account solar activity. The rule is consistent with the earlier results of Bochkov (1978) and Bochkov and Seliverstov (1978) who found that during even cycles of solar activity and on its ascending branch, the Barents Sea was characterized by suppressed cyclonic activity, negative anomalies of sea and air temperature and increased ice cover. Even near-normal SSTs in the Norwegian and Barents Seas during the winter of 2009 would mean a significant cooling in this area from the level observed in the past decade. 

Overall, SST averaged over the North Atlantic is forecast to decrease in the winter of 2009 compared to the previous winter, but still remain above the 1971-2000 average. This means a continuation of the positive phase of the Atlantic Multidecadal Oscillation (AMO).