Negative Asia/Bering Sea/North America Index & West Pacific Oscillation Spell Possible Cold Trouble in the 11-15-day period
Fig. 1: Mega-cluster ensemble “caveat forecast” of temperature anomalies across North America in the 11-15-day period.
Discussion: During the “polar vortex” winter of 2013-14, the East Pacific oscillation (EPO) gained recognition as the leading climate signal identifying the cold central North America trough. The trough was made more intense by the compensating upstream high-pressure ridge over Alaska which reached cross-polar (known as the “ridge bridge”) and causing low-level Siberian arctic air flow over the polar region and into North America. The arctic air flowing over snow cover made the polar vortex historic in intensity during that winter season.
The West Pacific oscillation (WPO) is similar to EPO, also well-correlated to a “polar vortex” pattern across North America although with slightly westward cold bias.
A new climate signal designed to account for the presence of the “ridge bridge” is the Asia/Bering Sea/North America (ABNA) pattern. When an upper ridge pattern crests over the Bering Sea and into Alaska or Northeast Asia a negative phase of the ABNA pattern is present.
The reason we’re discussing the ABNA and WPO is that both climate indices are moderate-to-strong negative phase in the medium-range. The combination of the two negative phases could produce an Alaska ridge-bridge mild pattern and downstream polar vortex upper air regime over West-central Canada which would deliver an 11-15-day temperature anomaly forecast which is much colder in the West and Central U.S. than most operational models.
The 11-15-day forecast shown is by the mega-cluster ensemble which considers all models and their skill. The 24% risk is the least likely scenario. However, based on the moderately strong -ABNA/-WPO plus the likelihood of snow cover increase the next 10+ days the colder solution indicated in Fig. 1 is quite possible!