Atmospheric Blocking as an Evolution of Rossby Wave Packets

Atmospheric blocking often causes weather extremes in the mid- to high-latitudes, yet its first-order dynamics are still not well understood. The basic textbook theory of blocking often refers to the eddy straining mechanism of Shutts (1983) that emphasizes the generic role of eddy straining and the associated wave breaking as a positive eddy feedback. Using a large-ensemble of a QG model, we find that, however, the generic effect from straining eddies is not significant to the maintenance of the blocking pattern. Instead, we find that the evolution of coherent and propagating Rossby wave packets is critical in block life cycles. We have identified evidence of such Rossby wave packet evolution in a hierarchy of climate models, including a QG model, idealized GCMs, and CESM large-ensemble simulations.

This new wave packet theory predicts that energy dispersion should dictate blocking frequency considering the dispersive nature of Rossby waves. Using a QG model, we find that the latitude exerts the strongest influence on blocking frequency. At higher latitudes, energy dispersion is weaker, and nonlinearity of eddies is stronger, both of which offer a conducive environment for block-like wave packet to persist longer. As a result, we find an order-of-magnitude increase in blocking frequency when we place a QG model at higher latitudes. Consistently, in an idealized GCM, we also find that the blocking frequency enhances significantly when the jet shifts poleward, regardless of the change of the meridional temperature gradient. Based on the linear dispersion relation of Rossby waves, we construct a synthetic model that can resemble some critical statistics of the observed blocks. Theoretical considerations of energy dispersion and block life cycles will be discussed.