Influence of ENSO and QBO on the frequency of sudden stratospheric warmings

The Arctic polar stratosphere is highly variable during the Northern Hemisphere winter. This variability is associated with changing frequency of stratospheric sudden warmings (SSWs). SSW frequency is impacted by various forcings outside of the polar stratosphere, such as El Nino Southern Oscillation (ENSO), the Quasi-biennial oscillation (QBO), the 11-year solar cycle, and volcanic eruptions. Routine observation of the stratosphere are only available for about 50 years, hence it is difficult to statistically isolate the influence of these factors on the frequency of SSWs. Here, we present a series of experiments carried out with the Whole Atmosphere Community Climate Model, version 3.5 (WACCM 3.5) that aim to quantify the influence of ENSO and the QBO on SSW frequency. All simulations have observed solar forcing. We show the CONTROL simulation and a) simulation without the QBO, b) simulation with climatological instead of observed sea surface temperatures (SSTs), and c) simulation without a QBO and with climatological SSTs. We quantify the SSW frequency in all the simulations. We show that ENSO and QBO interact nonlinearly and only one of those factors is needed to obtain realistic SSW frequency. However, when ENSO and QBO are removed, the SSW frequency significantly decreases.