A note on forced versus internal variability of the stratosphere

Abstract

An ensemble of integrations of a troposphere–stratosphere general circulation model is used to distinguish between externally forced (principally by sea‐surface temperatures) and internally generated interannual variability in the zonal mean stratospheric circulation. Forced variability is identified as that evident in the ensemble mean. Internal variability is identified with differences between ensemble members. The spatial and seasonal variation in the partitioning between forced and internal variability is examined. It is argued that the significantly larger forced interannual variability seen in the tropical lower stratosphere is not simply an upward extension of the tropical tropospheric variability, but is associated with coherent interannual variability in subtropical stratospheric wave driving.