Response to stratospheric forcing and its dependence on the state of the troposphere

Abstract

There is increasing evidence that changes in the stratosphere can have an impact on the surface. While observational results show a surface response of about 0.5 - 1 m/s, modeling studies can show a signal two times and in a particular extreme, four to eight times larger. In this thesis, an investigation of this extreme result revealed the model's characteristic timescale associated with the leading mode of variability was unrealistically large, which ultimately led to the exaggerated responses. Numerous experiments confirmed the tropospheric setup lay in a transition zone in the model's parameter space, teetering between an eddy-driven jet (1) coexisting with or (2) being well-separated from the subtropical jet. Modest shifts in the peak equilibrium temperature profile in either direction removed the bimodal behavior reducing the timescale associated with the internal variability. Subsequently, the response associated with a stratospheric perturbation was greatly reduced and consistent with those found in observations. Composites of the observed mid-tropospheric Northern Annular Mode (NAM) anomalies persisting much longer than normal reveal a lower stratospheric signal, while there was a much weaker signal under normal conditions, suggesting the lower stratosphere has a role in increasing the persistence of the NAM. Using this framework, the following mechanism was proposed. When the lower stratospheric winds sufficiently weaken, there is an increased wave drag in the lower stratosphere which then projects onto the annular modes.

(cont.) The negative phase of the annular mode can continue as long as both the lower stratospheric winds remain weak and the wave source is sufficient. Model runs with lower stratospheric winds that were always sufficiently weak or always too strong showed no significant tropospheric response to any extreme stratospheric events. Similarly, shifting mountains into the polar region appeared to shift the wave drag away from synoptic eddy feedback region. In either of these two cases, none of the model runs exhibited signs of a tropospheric response, consistent with the wave drag projection onto the annular mode having a key role in allowing the stratosphere to affect the tropospheric circulation.