Summary: Despite positive values of the Pacific Decadal Oscillation (PDO) in July and August 2007, a negative phase of the PDO is still considered to be more likely in the winter of 2008. It translates into anomalously warm waters in the east-central North Pacific and anomalously cold waters along the North American west coast. Recent development of atmospheric and oceanic processes in the Pacific Ocean also suggests warmer, or maybe much warmer, than normal sea and air temperatures in the Sea of Okhotsk and the Bering Sea and colder than normal temperatures south of Japan.

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

Positive values of the PDO (xls, doc) were observed in July (0.78) and August (0.5). However, a recent sharp cooling of sea surface temperature (SST) along the west coast of North America suggests that the September PDO value may come out negative. The outlook for the winter of 2008 still remains a negative phase of the PDO, with positive SST anomalies in the east-central North Pacific and negative anomalies along the west coast of North America. Rule 33 (xls), which will be triggered by the end of fall 2007, will provide more clue about the phase of the PDO in the forthcoming winter.

The negative phase of the PDO is usually accompanied by a weak Aleutian low. (Note that rule 5 (xls, doc), which describes the relationship between the Icelandic and Aleutian centers of action, suggests the latter center will be stronger than normal, but the CF for this conclusion is low). A frequent upper-atmospheric ridging is expected at about 160°W-170°W, which is consistent with the atmospheric circulation pattern over the Northern Hemisphere called La_Nina_2 (see the winter 2008 outlook for the Northern Hemisphere).

The atmospheric circulation pattern over the North Pacific will likely to resemble type W1 (Rodionov et al., 2005). In this case, an enhanced advection of warm air along the eastern periphery of the upper-atmospheric ridge will result in warmer, or much warmer, than normal winters in the Bering Sea. This projection is supported by rule 98 (xls, doc) that helps to partition anomalously cold and warm winters in the Bering Sea during La Nina events. 

Winter severity in the Sea of Okhotsk tends to be opposite to that in the Bering Sea and. During the winter of 2008, however, the Sea of Okhotsk is also expected to be warmer than normal, although the CF for this forecast is relatively low. 

The winter sea level pressure (SLP) in the Gulf of Alaska is expected to be below normal and cyclonic activity in the region increased (rule 198: xls, doc). This is also consistent with rule 148 (doc), which shows that warming in the Northwest Pacific is accompanied by an increased cyclonic activity in the Gulf of Alaska. There is an overall cooling trend in the Gulf of Alaska since the summer of 2005, as expressed in the SST anomalies (xls). It is expected that SST in the GOA will be below normal in the winter of 2008.

The West Pacific (WP) teleconnection pattern (xls, doc) is one of the most typical responses of the extratropical atmospheric circulation in the North Pacific to ENSO events. In our Classification and Regression Tree (CART) analysis of the ENSO effect on NH teleconnection patterns that included the WP, Pacific/North American (PNA), and Tropical/Northern Hemisphere (TNH) indices, the WP index had a better score than the other two. During La Niña events, the WP index tends to be negative (rule 208: xls, doc). Out of 18 La Niña events since 1951, only 4 events were accompanied by a positive WP index. The winter (DJF) WP time series of the index exhibits a strong positive trend (xls). In 2007, the index was 1.27, the second highest value on record since 1951. In the forthcoming winter, the index is expected to decrease to near-zero value, or may even become negative, a conclusion supported by rule 65 (xls, doc). This will be accompanied by a cooling in the area south of Japan and warming in the Sea of Okhotsk and the western Bering Sea.