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Steven M. Cavallo

University of Oklahoma, School of Meteorology

Arctic precursors to high impact midlatitude weather

Room 811 AOSS, March 14, 2016, 3:30 PM


Does what happen near the poles stay near the poles? Arctic processes and their influence on midlatitude weather events have only recently gained notable consideration. The relative lack of knowledge in arctic dynamics in comparison to that of midlatitude and tropical dynamics contributes to headlines we often times see such as \Polar Vortex Causes Freak Winter Storm." More accurately, the headlines should have stated that a tropopause polar vortex (TPV), an often sub-synoptic cyclone born in a polar region embedded within the larger-scale tropospheric polar vortex, played a significant role in those extreme weather events. TPVs can be long-lived phenomena with monthly time scales, and have complex interactions with features such as sea ice, moisture, and surface cyclones. Unfortunately little is known regarding how polar and lower latitude processes interact, and here I will explore the hypothesis that TPVs are an important component of such interactions. This talk will begin with a synthesis of TPVs and their implications, including their characteristics and mechanisms of growth when isolated in the Arctic, and ending with their impact in the waveguide. The hypothesis is guided by the threefold tropopause framework in consideration with Rossby wave growth along the waveguide. In this framework, there are three bands of concentrated upper-level Ertel potential vorticity (EPV) gradients in subtropical, polar, and Arctic regions. It will be shown that TPVs are most closely associated with the Arctic EPV band, and from this perspective, are a coherent tropopause disturbance embedded within the tropospheric polar vortex. Diabatic heating from radiation, coupled with weak deformation, is required to maintain the structure of TPVs before they encounter increasing vertical shear associated with the polar waveguide. During the warm season, the polar waveguide is weaker, inhibiting polar to lower latitude interactions, and promoting particularly long-lived TPVs over the Arctic. Combined with reduced sea ice, this increases the potential for extreme Arctic surface cyclones. In contrast, a stronger waveguide during the cold season promotes frequent equatorward extrusions of TPVs into the polar jet stream. When TPVs move equatorward and approach the polar jet stream, jet streaks may develop from an enhanced meridional EPV gradient. The strength of jet streaks and dynamics that ensue therefore depend on the strength of TPVs. It is proposed that downstream forecast error can derive from upstream sensitivities in TPVs long before the formation of a jet streak.

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