Identify the physics everyone is neglecting.
We wanted to understand what makes a successful sudden stratospheric warming forecast. That required inventing a new perspective on their formation.
Failure gives you a path to success
We started with simple composites of the zonal wind in the month preceding the date of a sudden stratospheric warming, and compared our ensemble forecasts to a reanalysis. We found that the stratospheric jet in our successful forecasts was consistently weaker than both the unsuccessful forecasts, and the reanalysis. Why?
Forecasts that are unsuccessful and successful at predicting a sudden stratospheric warming, and the verification (MERRA2). Here, "unsuccessful" means a false negative - a forecast should have predicted a sudden warming, but didn't. All composites relative to the sudden warming date.
To find something new, look where no one else is
Sudden stratospheric warmings are synonymous with wave drag, for good reason: the decelerating effect of breaking Rossby waves is their ultimate cause. However, the process that balances wave drag is the convergence of angular momentum by the residual circulation. When we regress these two terms (and their sum) on jet deceleration during a sudden warming period, we don’t find any differences among the forecasts, or between the forecasts and a reanalysis. Pound-for-pound, the physics are the same. So how can the forecasts be unsuccessful?
Regression of wave drag, residual mean angular momentum flux convergence, and net momentum tendency on stratospheric jet deceleration during a sudden stratospheric warming.
Let the physics speak
It’s as dead-simple as the first composite. The weaker the jet, the weaker the gradient of angular momentum on the poleward flank of the jet, so the weaker the angular momentum flux by the residual circulation and the less the atmosphere can balance the wave drag. If we “swap” the zonal mean in the momentum equation terms in the composites and recalculate the budget, we find that the stronger jet in the unsuccessful composites results in a stronger angular momentum flux convergence that tilts the balance toward acceleration, rather than deceleration.
Composites of jet-level momentum budget terms with the (black) correct zonal mean and (red) the zonal mean from the other composite.
You can read more details about this positive feedback - and how its knowledge could improve our sudden warming forecast accuracy by 44% - in Geophysical Research Letters.
Davis, N. A., Richter, J. H., Edwards, J., & Glanville, A. A. (2021). A positive zonal wind feedback on sudden stratospheric warming development revealed by CESM2 (WACCM6) reforecasts. Geophysical Research Letters, 48, e2020GL090863. https://doi.org/10.1029/2020GL090863.