Winter 2009 forecast for the North Pacific

Summary: The distribution of sea-surface temperature anomalies in the North Pacific is forecast to be determined by a negative phase of the PDO and a negative or near-normal phase of the WP. Positive temperature anomalies are expected in the area stretching from the Sea of Okhotsk to the central North Pacific. Negative temperature anomalies are more likely in the area south of Japan and the Gulf of Alaska.

Fig. 1. The forecast of sea surface temperature anomalies in the North Pacific in the winter (DJF) of 2009.  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

Due to the lack of a clear signal from the Tropical Pacific, the winter 2009 forecast for the North Pacific is quite difficult. Nevertheless, there are a number of rules that suggest a negative phase of the Pacific Decadal Oscillation (PDO) for the winter of 2009. For example, rule 5, which links the Aleutian and Icelandic lows, indicates that the central pressure in the Aleutian low is likely to be higher than normal. When the central pressure is high, the Aleutian low is weaker than normal, which is consistent with a negative phase of the PDO.

Another measure of intensity of the mean wintertime Aleutian low is the North Pacific Index (NPI). The NPI forecast is based on rules 433 and 440, both suggesting above normal values of the index (i.e., a weak Aleutian low) for the winter of 2009. Rule 45 points to a general tendency toward a positive NPI until about 2014.

The principal modes of low-frequency atmospheric variability over the North Pacific include the Pacific North American (PNA) pattern (discussed in the forecast for North America) and the West Pacific (WP) pattern. The winter (DJF) WP index experiences a strong upward trend. For the period 1987-2007, the average value of the index was 0.46. A positive WP index translates into anomalously warm temperatures south of Japan and cold temperatures in the Northwest Pacific. A winter with a negative WP index value was last observed in 2000. Despite the positive trend, rule 450 suggests a possibility of a sharp decline in the WP index, so that its value in the winter of 2009 will be close to zero or even negative. 

The forecast map in Fig. 1 is largely a result of a negative PDO pattern and a negative or near-normal WP pattern. Some regional rules are consistent with this forecast of temperature anomaly distribution. Thus, rule 58 provides an additional support for an anomalously cold winter in the Gulf of Alaska due to the state of solar activity and quasi-biennial oscillation (QBO) of the stratospheric wind.