The relationship between the wintertime North Atlantic Oscillation and blocking episodes in the North Atlantic

A systematic examination of the dynamical relationship between the North Atlantic Oscillation (NAO) and atmospheric blocking episodes in the North Atlantic during winter is undertaken. Employing the blocking criteria, as defined by Tibaldi and Molteni (1990), we first establish a statistical relationship, through compositing and linear regression analysis, between the two phenomena. The results show that the frequency of blocking formations in the North Atlantic region is sensitive to the phase of the NAG. Sixty-seven percent more winter blocking days are observed during the negative than the positive phase of the NAG. The lifetime of blocking episodes is also sensitive to the phase of the NAG. When the NAO is in the negative phase, the distribution of the length of blocking varies considerably. The average length of blocking during the negative phase is about 11 days, which is nearly twice as long as the 6-day length observed during the positive phase of the NAG. The NAO accounts for about 30% of the variation in the wintertime North Atlantic blocking episodes. We propose a conceptual model that strengthens the statistical association and offers an explanation for a dynamical connection between the occurrences of blocking and the NAO in the North Atlantic. Application of a low-order theoretical model by Charney and DeVore (1979) and an analysis of Northern Hemisphere observed surface temperature suggest that the NAG-related difference in blocking frequency and persistence are associated with changes in the zonally asymmetric thermal forcing which, to a large extent, is determined by the phase of the NAG. For the negative phase of the NAG, the distribution of the surface air temperature anomaly is the distinctive 'warm ocean/cold land' pattern related to the resonance forcing of topography and creates a dynamical environment favourable for the formation and persistence of blocks. For the positive phase of the NAG, on the other hand, the distribution of the surface air temperature anomalies is the distinctive 'cold ocean/warm land' pattern, which reduces or destroys the resonance forcing of topography and is unfavourable for the development and persistence of blocks. Copyright (C) 2001 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.