Pathways to synoptic scale buildups in available potential energy

Zonal available potential energy (ZAPE) is an estimate of the amount of potential energy in the atmosphere available for conversion to kinetic energy, providing a good proxy for the overall strength of the jet stream and its associated cyclones and anticyclones. Large short term depletions of ZAPE have been attributed to synoptic scale processes including intense mid-latitude cyclones which are often associated with cyclonic (LC2) wave break events on the dynamic tropopause; such events have included the 1993 March Superstorm amongst others. Little effort, however, has been put into understanding how ZAPE is built up on synoptic (3-10 day) time scales. By further understanding the processes which lead to such buildup events in the past (1979-2011), we may be able to increase the medium range predictability for high-impact weather events in Canada caused by significant depletions.

Our current research is focusing on the importance of anticyclonic (LC1) wave break events in the upper troposphere in helping to rapidly generate ZAPE on synoptic time scales. These wave break events are often responsible for significant troposphere-deep anticyclones, which can substantially perturb the jet stream from its climatological mean. In particular, we’re focusing on the importance of such events eastern portions of the North Pacific and North Atlantic Ocean basins. In nearly all winter (December-January-February) events in which ZAPE is built up to an anomalously high state, LC1 wave break events can be found in both of these regions, resulting in an extension of the jet stream into the west coasts of North America and Western Europe. The ensuing scenario leads to rapid regional cold air generation over Alaska and Northwestern Canada and a reduction in storm activity along the west coast of North America is reduced. Such processes can result in amplified atmospheric patterns capable of producing high-impact regional weather events over North America and elsewhere in the Northern Hemisphere.

Composite mean 500 hPa heights (thin black contour, m, 60 m contour intervals), 500 hPa height anomaly from weighted DJF climatology (colored, m), and significance criteria from 2-sided students t-test (thick black contour (95th percentile), hatched (99th percentile)) for lag day -9 (A, prior to peak ZAPE buildup day), lag day -6 (B), lag day -3 (C), and lag day 0 (D, peak ZAPE buildup day). Low height anomalies (blue) over Alaska and Northwestern Canada are in response to rapid regional cooling, while high height anomalies (red) over the eastern North Pacific basin are in response to anticyclonic wave break activity and reduced storm activity.

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