On the sensitivity of convectively coupled equatorial waves to the quasi-biennial oscillation

The seasonal-mean variance of the Madden–Julian oscillation (MJO) in austral summer has recently been shown to be significantly (p, 5%) enhanced during easterly phases of the quasi-biennial oscillation (QBO). The impact is large, with the mean MJO variance increasing by;50% compared to the QBO westerly phase. In contrast, we show using observed outgoing longwave radiation that seasonal variations for convectively coupled equatorial Kelvin, Rossby, and mixed Rossby–gravity waves are insensitive to the QBO. This insensitivity extends to all high-frequency (2–30-day period) and the non-MJO component of the intraseasonal (30–120-day period) convective variance. However, convectively coupled Kelvin wave variability shows a modest increase (;13%) that is marginally significant (p 5 10%) during easterly phases of the QBO in austral autumn, when Kelvin wave activity is seasonally strongest along the equator. The mechanism of impact on the Kelvin wave appears to be similar to what has previously been argued for the MJO during austral summer. However, the more tilted and shallower vertical structure of the Kelvin waves suggests that they cannot tap into the extra destabilization at the tropopause provided by the easterly phase of the QBO as effectively as the MJO. Lack of impact on the convectively coupled Rossby and mixed Rossby–gravity waves is argued to stem from their horizontal structure that results in weaker divergent anomalies along the equator, where the QBO impact is greatest. Our results further emphasize that the MJO in austral summer is uniquely affected by the QBO.