Review of winter 2009 in the North Pacific

Fig. 1. Winter (DJF) SST anomalies in the North Pacific. Base period: 1971-2000.

The distribution of SST anomalies in the North Pacific during the winter of 2009 (Fig. 1) was quite close to what was predicted. A pool of anomalously warm waters was observed in the east-central North Pacific, with colder than normal waters to the north, east and south of it. Negative SST anomalies in the Gulf of Alaska exceeded one standard deviation for the third winter in a row. 

In the western North Pacific, the distribution of SST anomalies was more mosaic, although positive SST anomalies dominated in the band from the Sea of Okhotsk to the east-central North Pacific, again in line with our forecast.

The SST anomaly pattern in Fig. 1 closely resembles the one for the cold PDO phase. Not surprisingly, the PDO index during this winter was strongly negative (Fig. 2). It had the lowest value since the winter of 2000.

Fig. 2. The winter (DJF) PDO index, 1901-2009.

The Aleutian low was extremely weak during most of the winter, except probably for a short period in January 2009. A strong blocking high was the most prominent feature of atmospheric circulation over the North Pacific during this winter (Fig. 3). The average atmospheric pressure over the basin, as measured by the North Pacific index (NPI), was the highest since 1907 (Fig. 4).

Suppressed cyclonic activity and a weak Aleutian low are characteristic features of atmospheric circulation during a negative PDO phase, especially if it is accompanied by a La Niña event in the equatorial Pacific. During the previous winter, however, when the situation was quite similar (a negative PDO plus an even stronger La Niña event), the NPI was only slightly positive (Fig. 4). This winter provides more evidence that the North Pacific climate undergoes a restructuring to a new climate regime.

Fig. 3. Winter SLP anomalies over the North Pacific.

The PDO index by itself does not show any apparent shift to a new climate regime (Fig. 2). Since 1989 it experienced rather random fluctuations around its zero value, without staying long enough in a positive or negative phase. The lack of a regime shift in this original PDO index from the University of Washington may be, at least partly, caused by the change in the underlying SST data set since January 2002. The PDO index from NCDC, which uses the same data set for the entire period, shows that the index is firmly in the negative phase since the late 1990s.

Some regional SST series also show a clear regime shift that occurred in the late 1990s. For example, SST in the central North Pacific experienced an abrupt change in 1999 Fig. 5). This new regime is significantly warmer than the previous regime of 1977-1998. For a more detailed discussion on recent regime shifts in the North Pacific click here.

Fig. 4. The North Pacific index, 1900-2009.

Fig. 5. Winter (DJF) SST anomalies in the central North Pacific, 1949-2009.