On the relationship between stratospheric structure and tropospheric blocking patterns

Prediction of long-lived anomalous behaviour in the atmosphere is fundamental to extended range and seasonal forecasting. Prediction of changes in the climatology of such anomalous behaviour is also fundamental to regional climate modelling. Anomalous atmospheric behaviour is often related to mid-latitude tropospheric 'blocking' patterns, where the normal westerly flow associated with the temperature difference between the Poles and the Equator is disrupted. Following recent work on stratosphere-troposphere coupling, we show that the vertical structure of the atmosphere can strongly influence the climatology of tropospheric blocking. We invoke dynamical theory to argue that the development and decay of anomalous circulations is most efficient for a preferred aspect ratio of the flow, implying that the development of large-scale anomalies requires a large vertical scale. Evidence for this link comes from the observed evolution of the geopotential height. In particular, we find that the development of the large-scale tropospheric anomalies associated with blocking requires a vertical scale extending well into the stratosphere. This process is inhibited during periods of high stratospheric activity, when the vertical scale of tropospheric developments is restricted, leading to the persistence of large horizontal scales.