The effects of the Quasi-Biennial Oscillation and the El-Nino Southern Oscillation on polar temperatures in the stratosphere

The ERA-40 dataset is used to study the effects of the Quasi-Biennial Oscillation (QBO) and the El-Nino Southern Oscillation (ENSO) on the stratosphere. In the boreal winter, Warm ENSO (WENSO) months in the Arctic are found to be significantly warmer, and Cold ENSO (CENSO) months significantly colder, than climatology. The QBO is also found to have a large effect on the Arctic stratosphere during the late fall/early winter; Westerly QBO (WQBO) poles are colder, and Easterly QBO (EQBO) poles are warmer.

ENSO and QBO have had a tendency to be correlated in time, especially in the first half of the 45 years of interest, and thus their signals are difficult to disentangle. In order to isolate each effect from the other, composites are taken of QBO months under near-neutral ENSO conditions, which show a clear effect in late fall/early winter. Due to the bimodality of QBO, producing a meaningful composite of ENSO months under near neutral QBO is impossible, as the number of available months is quite small. To distinguish ENSO from QBO, and to further study the QBO, we compare composites of months with four different combinations of QBO and ENSO anomalies, which confirms that ENSO does have an effect on the polar vortex. These groupings are also studied after removing the 2 years following one of the three major volcanic eruptions during the 45 years of data, and during the post-1979 satellite era only as well. These composites show distinct ENSO and QBO effects of comparable magnitude that are not contaminated by other influences.