Identifying tropospheric sources of synoptic time-scale increases in Zonal Available Potential Energy (APE)
Available Potential Energy (APE), which approximates the overall strength of the general circulation in the troposphere, has been shown to be modulated on time scales ranging from synoptic time scales (3-10 days) up to annual time scales. Though previous studies have established a strong framework for the physical mechanisms modulating APE on annual time scales and depleting APE on sub-seasonal to synoptic time scale, little has been done to examine the ability for APE to increase on synoptic time scales. Applying an objective, standardized anomaly-based identification scheme to identify synoptic time scale increases in daily (1979-2011) zonal APE (ZAPE), we find 160 buildup events which are sub-divided both by season and into two categories based on how anomalous the final ZAPE anomaly is. Using these categories, we attempt to examine sources of increase in ZAPE based using several zonal mean approaches: energetic conversion terms, energetic generation terms, thermodynamic budget terms, and perturbations to the U, V, and omega decomposition of the wind. Our results suggest that rapid buildups in ZAPE are not so much a function of ZAPE generation as they are a function of a large-scale reduction in baroclinic conversions from ZAPE to eddy APE (EAPE). Furthermore, there are statistically significant changes to the zonal mean tropospheric circulation, which will be commented on in the context of impacts on thermally direct circulations.