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Atmospheric Blocking as an Evolution of Rossby Wave Packets
Wednesday, 29 May 2019, 12:00
Wednesday, May 29, 2019. 12:00PM. Atmospheric Blocking as an Evolution of Rossby Wave Packets. Lei Wang, Harvard University. Sponsored by NOAA GFDL. More information here.
Atmospheric blocking is an important process for both weather and climate, yet its first-order dynamics is still not well understood. The eddy straining mechanism of Shutts (1983) has been considered as the foundation to understanding the maintenance of blocks, which is consistent with the observation that strong wave breaking concurs with blocks. Using a large-ensemble of a two-layer quasigeostrophic (QG) model, we find that, however, the generic effect from straining eddies is insignificant to the maintenance of the blocking pattern. We find that the evolution of coherent and propagating Rossby wave packets plays a vital role in block lifecycles. Evidences of such Rossby wave packets evolution, including its growing and decaying phases, have been identified in a hierarchy of climate models including a two-layer QG model, a dry GCM, and climate model simulations. Due to the dispersive nature of Rossby waves, this wave packets theory of atmospheric blocking predicts that energy dispersion should dictate blocking frequency.
Among the non-dimensional parameters in the QG model, we find that the latitude exerts 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 remain its shape beyond synoptic timescale. As a result, an order-of-magnitude increase of blocking frequency is identified when a two-layer QG model is placed at higher latitudes. Consistently, we also find that the blocking frequency in idealized GCMs enhances significantly when the jet shifts poleward, regardless the change of the meridional temperature gradient. Theoretical considerations, including a conceptual model of energy dispersion and block lifecycles, will be discussed.
Location NOAA GFDL, Smagorinsky Seminar Room, Princeton University, Princeton, NJ.