Effects of the El Niño–Southern Oscillation and the Quasi‐Biennial Oscillation on polar temperatures in the stratosphere

Abstract

Reanalysis data are used to study the effects of the Quasi‐Biennial Oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) on the stratosphere. During the boreal winter in the Arctic, Warm ENSO (WENSO) months 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. In the first half of the 50 years of interest, WENSO and EQBO have had a tendency to be correlated in time, 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. Because of the bimodality of QBO, producing a meaningful composite of ENSO months under near‐neutral QBO is difficult, 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 a significant 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 50 years of data and during the post‐1979 satellite era only as well. These composites show distinct ENSO and QBO effects of comparable magnitude.