Extratropical Stratospheric Predictability from the MJO and ENSO in the S2S Models

The effect of the Madden‐Julian Oscillation (MJO) and El-Nino Southern Oscillation on the Northern Hemisphere wintertime stratospheric polar vortex is evaluated in operational subseasonal forecasting models.

Reforecasts which simulate stronger MJO‐related convection in the Tropical West Pacific also simulate enhanced heat flux in the lowermost stratosphere and a more realistic vortex evolution. The time scale on which vortex predictability is enhanced lies between 2 and 4 weeks for nearly all cases. Those stratospheric sudden warmings that were preceded by a strong MJO event are more predictable at ∼20 day leads than stratospheric sudden warmings not preceded by a MJO event. Hence, knowledge of the MJO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere polar stratosphere.

El Nino leads to a weakened vortex and La Nina leads to a stronger vortex in the S2S models, in contrast to the observed response to ENSO over the period covered by the S2S models but in agreement with the observed response over the past 60 years. The S2S models help provide context for interpreting the recent weakening of the ENSO-vortex relationship.