Stratosphere–Troposphere Coupling

Prior to the 1990s there was little evidence that stratospheric variability affected the troposphere. During the past 20 years evidence has mounted showing that stratospheric variability and change has substantial effects on surface weather and climate, especially on the Annular Modes, with shifts in the jet streams, storm tracks, precipitation, and likelihood of blocking events. Despite unambiguous observations of this phenomenon, as well as numerical simulations, a clear physical explanation of this downward coupling has been elusive. We lack a theory that predicts the main features of the observed relationship. I will discuss recent advances in our understanding—how pressure changes (movement of mass) in the stratosphere affects surface pressure. However, movement of mass in the stratosphere is not sufficient to fully explain the magnitude of observed surface changes—surface effects are larger than would be expected theoretically, and are larger than at the tropopause. This “tropospheric amplification” is easily quantified, and suggests a role for eddy feedbacks in response to the movement of mass. I will discuss the implications for surface climate, jets, storm tracks, etc.