Extreme temperature regimes during the cool season: Their trends, variability, triggers and physical connections to low frequency modes
Westby, Rebecca Marie
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During the boreal cool season (December – February) extreme temperature regimes (ETRs), including cold air outbreaks (CAOs) and warm waves (WWs), affect regional economies and human safety via their significant impacts on energy consumption, local agriculture and human health. This work aims to improve our understanding of the trends and variability in ETRs, their physical connections to low frequency modes, and the dynamical mechanisms leading to ETR onset. Earlier studies on ETR trends and variability do not consider the last decade. Further, little is known about the physical and dynamical nature of ETR onset. These unknowns motivate this dissertation and are particularly important for WWs, which have rarely been studied. This study begins with an updated analysis of the long-term trends and interannual variability in ETRs. Even with the inclusion of the last decade, no significant trends in either WW or CAO occurrence are identified over the continental United States between 1949-2011. The accompanying correlation analysis reveals that on interannual time scales, ETRs in specific regions of the U.S. tend to be modulated by certain low frequency modes. This analysis highlights an important regional asymmetry in the low frequency mode modulation of ETRs, and also indicates that the influence of ENSO upon ATRs is mainly limited to a modest modulation of WWs over the southeast US. Further, a multiple linear regression analysis reveals that the regional collective influence of low-frequency modes accounts for as much as 50% of interannual ETR variability. A synoptic-dynamic characterization of ETR onset over the southeast US is then performed using composite time-evolution analyses of events occurring between 1949-2011 to provide a qualitative indication of the role of low frequency modes. During CAO (WW) onset, negative (positive) geopotential height anomalies are observed in the upper troposphere over the Southeast with oppositely-signed anomalies in the lower troposphere over the central US. In most cases, there is a surface east-west height anomaly dipole, with anomalous northerly (CAO) or southerly (WW) flow into the Southeast leading to cold or warm surface air temperature anomalies, respectively. Companion potential vorticity anomaly analyses reveal prominent features in the mid- to upper-troposphere consistent with the geopotential height anomaly patterns. The composite analyses reveal significant roles for both synoptic and large-scale disturbances in ETR development. Synoptic-scale disturbances serve as dynamic triggers for ETR events, while low-frequency modes can provide a favorable environment for ETR onset. A suite of diagnostic analyses is conducted next and aims to identify the primary thermodynamic processes and dynamical mechanisms responsible for ETR development over the Southeast US. Heat budget analyses implicate linear temperature advection as the primary contributor to ETR development, while nonlinear advection plays a smaller role. Both the linear and the nonlinear terms contribute positively to the temperature tendencies of interest, while the adiabatic and diabatic terms offset some of the advection contributions. Piecewise PV inversion analyses are then conducted to assess which dynamical features directly contribute to the local temperature changes that occur in association with ETRs. A novel result is the discovery of the potential pathway through which the low frequency mode modulation of ETRs takes place. An upper-tropospheric PV feature first induces near-surface temperature advection, which then creates a near-surface temperature anomaly and a corresponding circulation that further enhances the initial temperature advection and ultimately leads to the ETR event.