The quasi-biennial oscillation (QBO) is a tropical stratospheric wind that travels easterly then westerly over a period for the full cycle of just over 2 years. You are right in suggesting that the westerly winds are known as positive and the easterly winds are negative. The pulse of westerly or easterly winds tend to commence in the upper regions of the tropical stratosphere and push downwards towards the troposphere. This can leave us in a state where the upper winds in the tropical stratosphere are westerly and the lower regions are easterly or vice versa depending on what stage of the QBO we are in. Sometimes these winds can push through and affect the polar stratosphere and other times they do not. The general reading for the state of the QBO mean winds is normally taken at the 30 hPA level.

Generally, it is thought that the stratospheric vortex polar is increased in a westerly QBO regime and decreased in an easterly regime. However the exact timing and positioning of these winds descending the stratosphere is going to have a big influence on polar regions.

Then we also need to take into consideration the Brewer Dobson circulation (BDC). This is the mechanism that transfers ozone from the tropical stratosphere to the polar stratosphere. Labitzke has shown that this mechanism is influenced by the state of the QBO and the amount of solar flux present. An easterly QBO in solar minimum conditions increases the BDC as does a westerly QBO in solar maximum conditions.

Increased polar ozone will lead to a warmer polar stratosphere and a reduced thermal gradient between the polar and tropical stratosphere. This in turn will lead to a reduced polar vortex and a more disrupted tropospheric jet stream.

Then we need to add in to the mix the ENSO state and how this interacts with the state of the QBO and tropical stratosphere and we begin to realise how complex all this is and that to just say a negative QBO is favourable for winter is just a little bit too simplistic!

JFM	JFM	Season	JFM	JFM
Season	AO	QBO	Quality MEI	Sunspot Count
	                
1973	0.9 	-0.31 	1	 1.35 	48
1974   -0.3 	-1.10 	1	-1.83 	25
1975	0.6 	-15.04 	2	-0.67 	10
1976	0.8 	 9.28 	1	-1.42 	11
1977   -1.8 	-15.65 	4	 0.29 	16
1978   -1.0 	 6.21 	4	 0.87 	74
1979   -1.2 	 2.29 	5	 0.32 	148
1980   -1.5 	-6.22 	4	 0.65 	147
1981   -0.7 	 7.58 	2	 0.00 	130
1982	0.4 	-14.63 	0	-0.12 	143
1983   -0.3 	 11.47 	4	 2.88 	67
1984   -0.6 	-11.66 	4	-0.25 	75
1985   -1.2 	 3.38 	4	-0.63 	17
1986   -0.5 	 10.28 	5	-0.16 	13	Hale Winter
1987   -1.5 	-11.84 	5	 1.38 	9	Hale Cycle
1988   -0.3 	 6.61 	4	 0.75 	59	
1989	2.6 	-2.69 	0	-1.16 	152	
1990	2.5 	-4.83 	0	 0.56 	150	
1991   -0.2 	 8.72 	4	 0.33 	148	
1992	0.9 	-15.04 	0	 1.87 	139	
1993	1.5 	 10.60 	3	 0.88 	73	
1994	0.2 	-9.59 	5	 0.23 	42	
1995	0.6 	 8.39 	3	 1.01 	28	
1996   -0.8 	-7.54 	4	-0.50 	8	
1997	0.8 	 1.05 	2	-0.45 	8	
1998   -0.8 	-2.91 	0	 2.68 	42	
1999   -0.3 	 5.84 	4	-1.12 	66	
2000	0.6 	 4.85 	3	-1.17 	117	
2001   -1.1 	-15.74 	5	-0.60 	97	
2002	1.2 	 7.32 	2	-0.14 	114	
2003	0.2 	-2.04 	1	 0.98 	78	
2004   -1.0 	 1.07 	2	 0.18 	50	
2005   -0.8 	-0.66 	2	 0.71 	34	
2006   -0.6 	-10.15 	3	-0.49 	19	
2007	0.6 	 2.09 	2	 0.54 	11	
2008	0.8 	-4.98 	4	-1.33 	4	
2009	0.1 	 11.49 	3	-0.71 	2	Hale Cycle
2010   -2.4 	-17.56 	5	 1.35 	11	Hale Winter?
2011	0.4 	 9.89 	4	-1.58 	34