The current northern hemisphere cold spell

Negative phase of the Arctic Oscillation

These regional contrasts in temperature anomalies resulted from a strongly negative phase of the Arctic Oscillation (AO). The AO is a natural pattern of climate variability. It consists of opposing patterns of atmospheric pressure between the polar regions and middle latitudes. The positive phase of the AO exists when pressures are lower than normal over the Arctic, and higher than normal in middle latitude. In the negative phase, the opposite is true; pressures are higher than normal over the Arctic and lower than normal in middle latitudes. The negative and positive phases of the AO set up opposing temperature patterns. With the AO in its negative phase this season, the Arctic is warmer than average, while parts of the middle latitudes are colder than normal. The phase of the AO also affects patterns of precipitation, especially over Europe.

The phase of the AO is described in terms of an index value. In December 2009 the AO index value was -3.41, the most negative value since at least 1950, according to data from the NOAA Climate Prediction Center.

Blocking events, associated with persistent high-latitude
ridging and a displacement of mid-latitude westerly winds
lasting typically a week or two, are an important component of
total circulation variability on intra-seasonal time scales. In the
NH, the preferred locations for the blocking are over the Atlantic
and the Pacific (Tibaldi et al., 1994), with a spring maximum
and summer minimum in the Atlantic-European region (Andrea
et al., 1998; Trigo et al., 2004). Observations show that in the
Euro-Atlantic sector, long-lasting (>10 day) blockings are
clearly associated with the negative NAO phase (Quadrelli et
al., 2001; Barriopedro et al., 2006), whereas the blockings of 5
to 10 day duration exhibit no such relationship, pointing to the
dynamical links between the life cycles of NAO and blocking
events (Scherrer et al, 2006; Schwierz et al., 2006). Wiedenmann
et al. (2002) did not find any long-term statistically signifi cant
trends in NH blocking intensity.

Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.