A process-based evaluation of biases in extratropical stratosphere-troposphere coupling in subseasonal forecast systems

Abstract. Two-way coupling between the stratosphere and troposphere is recognized as an important source of subseasonal-to-seasonal (S2S) predictability and can open  windows of opportunity for improved forecasts. Model biases can, however, lead to a poor representation of such coupling processes;  drifts in a model’s circulation related to model biases, resolution, and parameterizations have the potential to feed back on the circulation and affect stratosphere-troposphere coupling. 

In the Northern Hemisphere, nearly all S2S forecast systems underestimate the strength of the observed upward coupling from the troposphere to the stratosphere,  downward coupling within the stratosphere, and the persistence of lower stratospheric temperature anomalies. While downward coupling from the lower stratosphere to the near surface is well represented in the multi-model ensemble mean, there is substantial inter-model spread likely related to how well each model represents tropospheric stationary waves.

In the Southern Hemisphere, the stratospheric vortex is over-sensitive to upward propagating wave flux in the forecast systems. Forecast systems generally overestimate the strength of downward coupling from the lower stratosphere to the troposphere, even as they  underestimate the radiative persistence in the lower stratosphere. In both hemispheres, models with higher lids and a better representation of tropospheric quasi-stationary waves generally perform better at simulating these coupling processes.