stratospheric polar vortex is evaluated in five operational subseasonal forecasting models.
Of these five models, the three with the best stratospheric resolution all indicate a weakened
vortex during the easterly phase of the QBO relative to its westerly phase, consistent with the
Holton‐Tan effect. The magnitude of this effect is well captured for initializations in late
October and November in the model with the largest ensemble size. While the QBO appears …