The Effect of ENSO and the Madden-Julian Oscillation on Stratospheric Sudden Warmings

The effect of the Madden-Julian Oscillation (MJO) and the El Nino-Southern Oscillation on the Northern Hemisphere wintertime stratospheric polar vortex and major, mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis dataset and chemistry-climate models. The same paradigm can be used to explain the impact of both ENSO and the MJO on SSWs. Both ENSO and the MJO influence the region in the tropospheric North Pacific sector that is most strongly associated with a SSW. Consistent with this, SSWs in the reanalysis record have tended to follow certain MJO and ENSO phases. The magnitudes of the influence of the MJO, ENSO, and the Quasi-Biennial Oscillation on the vortex are comparable.

There is an apparent inconsistency between the impact of opposite phases of ENSO on the seasonal mean vortex and on SSWs: El Niño leads to an anomalously warm, and La Niña leads to an anomalously cool, seasonal mean polar stratospheric state, but both phases of ENSO lead to an increased SSW frequency. A resolution to this apparent paradox is proposed. In the observational record, both La Niña and El Niño lead to similar anomalies in the region associated with precursors of SSWs and, consistent with this, there is a similar SSW frequency in La Niña and El Niño winters. A similar correspondence between the penetration of ENSO teleconnections into the SSW precursor region and SSW frequency is found in the comprehensive chemistry-climate models. The inability of some of the models to capture the observed relationship between La Niña and SSW frequency appears related to whether the modeled ENSO teleconnections result in extreme anomalies in the region most closely associated with SSWs.

Finally, the apparent impact of the MJO on SSW could be used to improve intra-seasonal projections of the Northern Hemisphere high latitude circulation, and in particular of the tropospheric Northern Annular Mode, at lags exceeding one month. in particular of the tropospheric Northern Annular Mode, at lags exceeding one month.