Summary: A negative phase of the Pacific Decadal Oscillation (PDO) is likely to occur in the winter of 2008. It means anomalously warm waters in the east-central North Pacific and anomalously cold waters along the North American west coast. Frequent outbreaks of cold Siberian and Arctic air are expected for the eastern Bering Sea and the Gulf of Alaska. The winter of 2008 in these areas may be colder or much colder than normal.

Fig. 1. The forecast of sea surface temperature anomalies in the North Pacific 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

The atmospheric circulation and SST patterns in the winter of 2008 will be largely determined by the developing La Nina event in the equatorial Pacific. As it usually occurs during La Nina events, the Aleutian low is expected to be weaker than normal and the number of blocking situations in the central North Pacific increased (Rule 7). The strength of the Aleutian low is also inversely related to the strength of the Icelandic low (Rule 144), although the CF for this rule is low. This is because in the past 50 years or so, there was a deepening trend in both the Aleutian and Icelandic lows. Nevertheless, several previous studies have pointed out that on the interannual time scale, a seesaw-like oscillation exists between the Aleutian and Icelandic centers of action (AL–IL seesaw). In some works (e.g., Honda and Nakamura 2001) the AL–IL seesaw was identified as the most dominant mode of interannual variability in the upper troposphere for late winter. Since the Icelandic low is expected to be stronger than normal in the winter of 2008 (see the forecast for the North Atlantic), this adds some confidence that the Aleutian low will be weaker than normal.

La Nina events are also associated with the negative phase of the West Pacific (WP) oscillation, which translates in weaker-than-normal westerly winds over the western and central North Pacific. In the past 50 years, the winter WP index features a strong positive trend. Therefore, although the WP index in 2008 will decrease, it will likely remain close to its average value for the period 1971-2000.

During La Nina events the sea level pressure (SLP) in the Gulf of Alaska (GOA) is usually above normal and the cyclonic activity is suppressed. However, during some La Niña events (for example, the La Nina of 1975/76), SLP in the GOA can be lower than normal. This situation is expected for the winter of 2008 as well. The reason for this conclusion is the state of solar activity that will still remain low during this coming winter, before starting its sharp increase in the later years (Rule 198). Although the storm activity in the GOA is expected to be increased as much as during El Niño years, the storm trajectories will be completely different. During El Niño years, storms usually enter the GOA from the southwest bringing warm tropical air. In the winter of 2008, the storms will likely be entering the GOA from the west or northwest and will be more often associated with strong northwesterly or northerly winds. This will be accompanied by frequent troughing in the mid-tropospheric level.

Overall, the negative phase of the Pacific Decadal Oscillation (PDO) is forecast. The sea surface temperature (SST) is projected to be above normal in the east-central North Pacific and below normal along the North American west coast. Due to a more western position of the upper atmospheric ridge (close to the dateline), the strip of cold waters in the East Pacific is expected to be relatively wide. Winter conditions in the eastern Bering Sea are also projected to be colder than normal.

The forecast for the Bering Sea at this time has a low confidence factor. During La Nina events winter temperatures in the Bering Sea are almost equally likely to be either below or above normal. The outcome significantly depends on the exact meridional position of the upper atmospheric ridge in the central Pacific. If the ridge is shifted west far enough (as expected in the winter 2008), the eastern Bering Sea will experience strong northwesterly winds bringing cold and dry Siberian and Arctic air. As a result, winter temperatures in the Bering Sea will be below or much below normal. More information about the possible position of the upper atmospheric ridge and, hence, the severity of the upcoming winter in the Bering Sea will be available in September of this year.