Synoptic-Scale Zonal Available Potential Energy Increases in the Northern Hemisphere

Abstract

Abstract Available potential energy (APE), a measure of the energy available for conversion to kinetic energy, has been previously applied to examine changes in baroclinic instability and seasonal changes in the general circulation. Here, pathways in which the troposphere can build the reservoir of zonal available potential energy AZ on synoptic (3–10 day) time scales are explored. A climatology of AZ and its generation GZ and conversion terms are calculated from the National Centers for Environmental Prediction–Department of Energy Reanalysis 2 dataset from 1979 to 2011 for 20°–85°N. A standardized anomaly-based identification technique identifies 183 AZ buildup events, which are grouped into two event types based upon their final AZ standardized anomaly (σ) value: 1) buildup anomalous (BA) events, which exceed 1.5σ, and 2) buildup neutral (BN) events, which do not exceed 1.5σ. Increases in GZ and reductions in baroclinic conversion CA, source and sink terms for AZ, are shown to equally contribute toward increasing AZ in most seasons. A synoptic analysis of composited mass fields for winter BA events (n = 18 events) and winter BN events (n = 28 events) is performed to identify contributions to anomalously low CA and high GZ. A process of high-latitude cooling near 160°E–120°W is found for both composite event types. The cooling processes are characterized by a period of poleward moisture flux and ascent followed by an isolation of the Arctic from the midlatitude flow, resulting in enhanced GZ. Negative anomalies in CA are also diagnosed, which generally occur in regions with northerly dynamic tropopause wind anomalies and neutral to positive thickness anomalies.