Anomalous Temperature Regimes during the Cool Season: Triggers and Physical Connections to Low Frequency Modes

Even in the context of warming background temperatures, cool season anomalous temperature regimes (ATRs) will remain of interest to U.S. weather forecasters because of their numerous socioeconomic impacts. However, the trigger mechanisms for ATRs are not fully understood. Our study identifies and quantifies the primary synoptic and dynamic triggers for ATR events in order to better understand the physical nature of ATRs and their linkage to prominent low frequency modes. This research is pursued via a detailed diagnostic assessment of the three-dimensional synoptic and dynamic circulation structures of ATR events using NCEP/NCAR Reanalysis data from 1948-2011.

Discrete ATR events are first identified using magnitude, duration and separation criteria. The ATR events are then categorized based on the sign and magnitude of monthly low frequency mode indices. Composite fields of geopotential height, winds, air temperature and potential vorticity (PV) are then constructed using these categorizations, to isolate the basic synoptic and dynamic structural distinctions among different classes of ATR events. An evaluation of the time evolution of these composite fields is used to clarify the nature of the modulation of ATRs by the low frequency modes. In addition to composite analyses, detailed synoptic analyses of individual case studies are performed to assess the role of case-to-case variability.

Our composite analyses serve to isolate systematic dynamic features of ATR evolution, phrased in terms of PV anomaly structures. Additional PV inversion diagnoses are performed to assess the relative role of distinct dynamic features in enacting the lower tropospheric advective temperature changes leading to ATR onset. Upon identifying the essential dynamic features (PV anomaly structures), additional dynamic diagnostic analyses are employed to discern their proximate sources. For example, wave activity fluxes are used to identify associated wave propagation characteristics along with source and sink regions of anomalous wave activity. One main focus in this latter research is to infer the mechanistic role of low frequency modes in ATR evolution. Our results provide new information about the physical mechanisms leading to ATR events with a particular focus on the role of natural modes of low frequency variability.