Review of winter 2009 in the North Atlantic
Our first forecast issued in August 2008 indicated a higher probability of a negative NAO index during the winter of 2009. However, in October 2008 the forecast was significantly revised, and the updated forecast called for a positive NAO index. The main reason for the update was the distribution of SST anomalies in the North Atlantic during the summer and early fall of 2008. The observed winter NAO index turned out to be close to zero (Fig. 1).
It should be noted that the Icelandic low itself was quite strong. Mean winter SLP anomaly near Iceland was negative (-0.63 standard deviation), the lowest since 1999.
The Arctic Oscillation (AO) index, which is usually highly correlated with the NAO, was positive in December 2008 and January 2009 and then turned negative in February 2009. The mean winter AO index was positive.
Overall cyclonic activity during the winter of 2009 was enhanced. Storms were moving mostly along their more northern trajectories (particularly in December and February) bringing warm Atlantic air to the northern European seas. Winter temperatures in the Norwegian and Barents seas were anomalously high (Fig. 2). Although temperatures at the Kola section of the Barents Sea declined compared to the previous winter, they remained well above the long-term average (Fig. 3). Ice cover extent in the Barents Sea was below the median (Fig. 4).
The Northwest Atlantic was warmer than normal in terms of SSTs (Fig. 2) and close to normal in terms of sea-ice extent (Fig. 4). Winters with positive SST anomalies in both the Northwest and Northeast became more common in recent decade. Typically, due to a dominance of the North Atlantic Oscillation, SST anomalies in these areas are opposite to each other.
Despite the anomalously warm winter in these two areas and the central North Atlantic, the mean winter temperature averaged over the entire North Atlantic sharply decreased compared to the previous seven exceptionally warm winters. This decline occurred mainly due to a cooling in the Tropics. Nevertheless, it is probably too early to declare an end of the current positive phase of the Atlantic Multidecadal Oscillation.
