Distribution of major stratospheric warmings in relation to the quasi‐biennial oscillation

Data from 1953 to the present indicate that major warmings in the northern hemisphere winter have not occurred when the equatorial monthly mean zonal winds are deep westerly, i.e., westerly over more than about 5/8 of the 10–85 mb layer. Sixteen winters have had a major warming in the last 35 years. Six January–February periods had deep equatorial westerlies but did not experience a major warming. Major warmings do not require deep equatorial easterlies, nor is the occurrence of a major warming significantly correlated with the sign of the zonal mean wind at any particular level between 10 and 50 mb, although as noted by previous investigators, more than half of the observed major warmings have occurred when the equatorial flow is easterly at a given level between 10 and 50 mb. We suggest that a more relevant QBO statistic may be found in the depth of QBO wind regimes. The previously proposed connection between the equatorial quasi‐biennial oscillation and the major warmings of the northern hemisphere winter is supported by these observations.