The Importance of Wave Break Events for Synoptic Scale Buildups of Zonal 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 general circulation. Previous studies have estimated total hemispheric ZAPE, ZAPE generation, and conversion to kinetic energy, and proposed physical mechanisms to describe the annual ZAPE cycle as well as short term (sub-seasonal to synoptic) APE depletion events.  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.  However, increases in ZAPE on similar time scales have not been examined in detail. 

In this study, we examine the association of significant synoptic time-scale increases in ZAPE with dynamic tropopause wave break events.   ZAPE buildup events are determined using a 1979-2011 daily Northern Hemisphere (20°-85° N) ZAPE climatology calculated from the NCEP Reanalysis-2 dataset in an isobaric framework.  Build-up events are identified using a standardized anomaly (SA) technique, where the standardized anomaly of ZAPE must increase by at least 2 SA in a near-continuous manner for a 3-10 day period.  To diagnose the importance of wave breaks in the troposphere, we objectively identify wave breaks using potential temperature on the dynamic tropopause, identifying both anti-cyclonic (LC1) and cyclonic (LC2) wave breaks during the 1979-2011 period.

Our results indicate that LC1 wave break events appear to play an important role in ZAPE buildup events.  These anti-cyclonic wave break events are associated with significant poleward surges of warm tropical air.  In particular, the analysis of winter buildup events indicate that LC1 wave break events occur more frequently in the equatorward exit regions of the North Pacific and North Atlantic jet streams when compared with climatology.  The formation of these wave break events result in the development of statistically significant warm-core high pressure anomalies in these regions, acting to zonally elongate the jet, reduce baroclinic conversions to kinetic energy, and to regionally generate ZAPE.  Finally, the importance of ZAPE buildup events in creating an environment prone to significant cyclogenesis will be examined by exploring the frequency of LC2 (cyclonic) wave break events in the days following a ZAPE buildup event, providing insight into the use of ZAPE buildup events in medium-range weather prediction.