Uncovering Potential Dynamical Pathways from Planetary Scales to Regional Extreme Weather Events

Weather extremes (WEs) such as cold waves, heat waves, floods and droughts have major regional impacts. Although planetary-scale climate modes (PCMs) such as El-Nino Southern Oscillation or the Pacific Decadal Oscillation affect WE likelihood, planetary-scale forcing alone is largely inadequate in creating WEs and intermediate large-scale pathways (e.g., via atmospheric blocking, cyclones, or polar anticyclones) are required for WE occurrence. We review a new diagnostic research approach to comprehensively assess the phenomenological nature and large-scale organization of, and mechanistic pathways for, temperature and precipitation extremes.

This new diagnostic approach is two-pronged and merges a “bottom-up” analysis of large-scale circulation structures associated with WEs with a new parallel objective analysis of planetary-wave structures. A key distinction with prior research is the lack of a-priori assumptions on how the larger-scale circulation arises (e.g., via external forcing).  To illustrate this approach, we focus on the behavior of winter cold waves. Reanalysis data is used to construct an updated analysis of US cold waves stressing (a) regional frequency, intensity and structure, (b) objective characterization of associated large-scale meteorological patterns (LMPs) and (c) isolation of mechanistic pathways among PCMs, LMPs and WEs. In the latter, dynamical inferences are derived from the application of multi-scale diagnostic tools based upon wave activity and energy. Processes considered include anomalous boundary forcing, large-scale flow instability and multi-scale interactions. Our observational analysis provides statistical, synoptic and dynamic metrics to be used for the purpose of model validation. To this end, preliminary results for parallel historical model simulations are also presented.