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I googled QBO while trying to understand what WebHubTel is doing and found this: Predictability of the quasi-biennial oscillation and its northern winter teleconnection on seasonal to decadal timescales, Scaife et al
The skill of these QBO predictions remains high at very long lead times and is higher than other highly predictable modes of climate variability such as El Niño–Southern Oscillation [Fedorov et al., 2003; Barnston et al., 2012]. Correlation scores exceed 0.8 at prediction times up to 6 months. At 12 months lead time the correlations are still above 0.7, and positive correlations are maintained out to years ahead in both forecast systems (Figure 2), remaining significant out to 4 years ahead in the MiKlip forecasts [c.f. Pohlmann et al., 2013].
That's quite long but it's not very long. WebHubTel appears to be using a simple method to project the QBO back from 1953-2015 all the way to 1880. I cannot make sense of this.