Upper Tropospheric Jet Axis Detection and Application to the Boreal Winter 2013/14

Abstract

his study presents a detection scheme for upper tropospheric jets. The scheme identifies locations on the dynamical tropopause where the wind shear perpendicular to the wind direction vanishes, and subsequently uses a masking criterion to filter out zero-shear locations that do not belong to jets. The scheme reliably detects jet axes in ERA-Interim reanalysis data with instantaneous, weekly, or monthly averaged wind fields. The dynamical implications of the detected jet axes and their relation to objectively detected wave breaking and blocking are demonstrated for the synoptic evolution during the boreal winter 2013/14. This winter featured a remarkable episode with a stationary ridge-trough couplet over the American continent leading to anomalously cold conditions from central Canada to the eastern United States. The mean synoptic situation during this episode resembles the climatological winter mean, but featured a more spatially focused jet axis distribution in the northeast Pacific. The tight distribution suggests that a sequence of similar weather events lead to the mean synoptic conditions. Although the distribution of jet axes and wave breaking events together with the persistence of the anomalous ridge over the northeastern Pacific indicate a blocked situation, the block is not detected with common conventional methods due to the lack of a persistent gradient reversal of potential temperature on the dynamical tropopause. In addition, we demonstrate the importance of sub-seasonal variations in this winter by pointing out a period in which the jet configuration deviated considerably from the seasonal mean.