The Weather-Climate Nexus: Large-Scale Organization of Extreme Events

Tuesday, May 13, 2014 - 07:00
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Extreme weather events (EWEs) due to variations in the energy and water cycles have important societal consequences. EWEs include temperature events, cold air outbreaks (CAOs) and warm waves (WWs), and precipitation events such as floods and droughts. The frequency of EWEs is strongly impacted by planetary-scale climate modes (PCMs) such as ENSO, PDO and NAO. We first summarize our prior DOE-supported research on EWE behavior and modulation by PCMs: The annual frequency of winter CAOs and WWs in the US is found to exhibit no significant long-term trends and is regionally modulated by several PCMs. Polar Annular Mode events (linked to north-south movements in the stratospheric polar vortex) are found to be initiated by tropospheric planetary waves and provide long-lasting regional impacts upon surface air temperature and midlatitude storm activity. Dynamical triggers for CAO and WW onset were diagnosed via piecewise potential vorticity inversion, separating the large-scale circulation into parts related to distinct dynamical features. Validation studies reveal that CMIP5 models have deficiencies in replicating some PCMs, with important consequences for representing associated regional climate variability. Although CMIP5 models qualitatively replicate observed EWE behavior, WW (CAO) frequency is over (under) estimated and the collective influence of PCMs upon EWEs is underestimated. Thus, predictions of future EWE behavior are currently limited by model deficiencies in representing large-scale influences upon EWEs. Parallel research on the role of the stratosphere in determining CMIP5 climate behavior was pursued in separate collaborative research efforts. Although PCMs modulate EWE behavior, they are insufficient for producing EWEs by themselves. Intermediary large-scale meteorological patterns (LMPs) such as atmospheric blocks, cyclones, and polar anticyclones are additionally required. A recent US CLIVAR workshop on EWEs revealed a pressing need for the coordinated study of the behavior and physics of LMPs related to EWEs. We will discuss a formalism for revealing the dynamical pathway for large-scale organization and scale interaction among PCMs, LMPs and EWEs. This includes fully characterizing critical LMPs and PCM-LMP-EWE linkages, diagnosing LMP-related physics, constructing circulation and dynamical LMP metrics (for model assessment) and studying LMP behavior and physics in global models. The approach is illustrated via a pilot synoptic, diagnostic and modeling study of a prototypical LMP responsible for WW, CAO and drought conditions over North America during January 2014. We show that this event mainly arose from the combined contributions of anomalous forcing by warm SST anomalies over the North Pacific along with synoptic eddy feedback.

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