Summary: A positive phase of the North Atlantic Oscillation (NAO) is expected in the winter of 2008. Usually, this phase of the NAO is associated with warmer than normal sea surface temperatures (SST) in the Northeast Atlantic and Nordic Seas and colder than normal temperatures in the Norwest Atlantic. In the winter of 2008, however, the latter region is also forecast to be somewhat above normal. Negative SST anomalies are expected south of Greenland, south of Newfoundland and off the west African coast.

Fig. 1. The forecast of sea surface temperature anomalies in the North Atlantic in the winter (DJF) of 2008.  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

Currently, there are several indications in favor of a positive phase of the NAO during the winter of 2008. One of them is the amount of summer snow in Eurasia (Rule 23). It was found (e.g., Bojariu and Gimeno, 2003) that the mean Eurasian sow cover between April and October is significantly (at the 0.01 level) negatively correlated with the NAO index in the upcoming winter (both time series were detrended). In April 2007, the amount of snow in Eurasia was measured as 14.8 mln. sq. km., which was 1.87 mln. sq. km. below the mean value for the past ten years. If the snow cover remains below this mean value through October, the confidence factor for the positive phase of the NAO will increase. In addition, by the end of summer, an analysis of SST anomalies in the North Atlantic will provide more insight about the possible state of the NAO.

Another piece of evidence regarding the state of the NAO comes from the North Pacific. A statistical forecasting scheme was developed (Rule 38) that links the sea level pressure (SLP) anomaly pattern in the North Pacific to the NAO with the lead time of up to 1 year. Currently, the distribution of SLP anomalies strongly favors the positive phase of the NAO.

There are a number of rules, however, that indicate a possibility of a negative phase of the NAO in the upcoming winter. Among them is Rule 71 that represents a tree-structured relationship between the quasi-biennial oscillation (QBO) in stratospheric winds and the NAO index. The confidence factor for the rule is relatively small.

An interesting relationship was found between the NAO and solar activity (Rule 42). Generally speaking, the relationships between solar/geomagnetic activity and climate variables are notoriously unstable. For example, there is quite a strong relationship between sunspot numbers and the NAO index in recent decades. When the sunspot numbers were small (< 22), 11 out of 12 such years featured negative NAO index. In contrast, when sunspot numbers were greater than 22, there were 28 out 35 years with positive NAO index. However, when this relationship was tested for the data prior to 1950, it showed no skill. Rule 42 is one of the few relationships with solar activity that held through the entire period of observations since 1830. According to this rule the NAO index is more likely to be negative in the winter of 2008.

Despite some indications of a negative NAO index next winter, the confidence factor for the positive NAO index remains somewhat higher. More certain conclusion about the state of the NAO will be available in the early fall, when more rules will be triggered.

It is important to underscore one interesting aspect of the expected positive NAO phase in the winter of 2008. Usually, when the NAO index is positive, the distribution of surface air temperature (SAT) anomalies is characterized by cooling in the Northwest and warming in the Northeast Atlantic. In the winter of 2008, however, both areas are forecast to be above the mean value for the base period 1971-2000. One possible reason for that is the anticipation of a positive Greenland blocking index (Rules 53 and 59). The index is linked inversely with sea-ice anomalies in Baffin Bay (Rule 189). Another line of reasoning involves the northeastern shift of the Icelandic low, which is responsible if not for the warming, but for the lack of cooling in the region west of Greenland. This shift became particularly noticeable since the late 1980s. As a result, the northerly winds in the Labrador Sea/Davis Strait region are not as strong as during those positive phases of the NAO when the Icelandic low is located near it normal position. The northeastern shifts of the Icelandic low are usually associated with periods of Arctic warming. The previous period of Arctic warming occurred in the 1930s when the frequency of the situations with positive SAT anomalies in both the Northwest and the Northeast Atlantic was greatly enhanced (van Loon and Rogers, 1978).