Winter 2008 forecast for the Northern Hemisphere
Summary: A zonal type of atmospheric circulation over the Northern hemisphere is expected during the winter of 2008. This implies stronger than normal mid-tropospheric westerly winds in the 50°-60°N latitudinal band and weaker than normal westerly wind in the subtropical latitudes. The circumpolar vortex will likely be shifted toward northern Canada. The confidence factor for this forecast is 10.
Discussion
The forecast is based primarily on the following predictors: El Niño-Southern Oscillation (ENSO), Quasi-Biennial Oscillation (QBO) in the stratospheric wind, solar and geomagnetic activity. There appears to be two types of response of winter atmospheric circulation over the Northern Hemisphere to cold ENSO events. In rule 206 they are called La_Nina_1 and La_Nina_2; they correspond to types 39 and 40 in the Dzerdzeevsky’s classification, respectively. The difference between La_Nina_1 and La_Nina_2 is particularly significant in the Pacific sector. In the case of La_Nina_1, the Bering Sea is anomalously cold, while in the case of La_Nina_2, it is anomalously warm. Another difference is a stronger and westward expanded Siberian high in the case of La_Nina_2. According to rule 206, La_Nina_2 is more likely to occur in the winter of 2008 than La_Nina_1.
A recent statistical study reveals that the response of stratospheric circulation to ENSO significantly alters between opposite phases of the 11-year solar cycle (rule 247). During solar cycle maxima, the ENSO impact turns out to be insignificant throughout the whole extratropical lower stratosphere. In contrast, the relationship becomes significant in the lower stratosphere during solar minima. Assuming that the ongoing La Niña event will reach its mature state by the winter of 2008 and the sunspot numbers will remain low, one can expect that the circumpolar vortex (CV) in the high latitude stratosphere will be significantly strengthened. At the same time, the subtropical jet and westerlies above it in the stratosphere will be weakened. An interesting aspect of this situation is a shift of the CV center toward northern Canada. The composite 500-hPa height anomaly maps for the years of La Niña and low solar activity reveal a positive anomaly center over the northern North Pacific and a negative center over northern Canada.
Rule 68 links May QBO at 30-hPa and detrended modified zonal index in the following winter. When the QBO is in its west phase and above a certain threshold, the zonal index is mostly negative, with just a few exceptions. The opposite is true when the QBO is below that threshold, but the confidence factor in this case is lower. Since the QBO in May 2007 was below the threshold, the zonal index in the winter 2008 is expected to be positive. Together with rule 247, it translates into predominantly zonal atmospheric circulation over the Northern Hemisphere, with stronger than normal westerly winds in the 50°-60°N latitudinal band.
Some other rules, such as rule 213 suggest a possibility of a meridional type of atmospheric circulation over the Northern Hemisphere. Rule 213 is based on the works of Bucha (1984, 1988, 1991) who suggests that under enhanced corpuscular (geomagnetic) radiation from the Sun, warming and an increase of pressure in the auroral zone takes place. This leads to changes in atmospheric circulation so that the zonal flow in the troposphere is intensified. At the time of low geomagnetic activity (which is expected in the winter of 2008), amplitudes of planetary waves increase again as a result of the effect of topography and surface heating prevailing over the effects of the processes in the auroral oval.
Combining the outcome of those rules that have been triggered so far, one can conclude that a zonal rather than meridional type of atmospheric circulation over the Northern hemisphere is more likely in the winter of 2008. It should be emphasized that due to the lack of a credible mechanism explaining the effect of solar activity on the Earth’s climate, the overall confidence factor for this conclusion is relatively low.