Collaborative Research: An Observational and Modeling Study of Spring Onset in the Northern Hemisphere Circulation

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The spring onset (i.e., yearly transition from winter to spring) and its interannual variability are important components of regional climate. These two features influence the hydrological cycle, through variations in clouds, sea ice, air temperature and ozone in the troposphere affecting humanity, vegetation growth, and ecosystem productivity. Recent observational studies have indicated that spring onset is strongly linked to stratospheric final warming (SFW). Past numerical studies have also indicated that the interannual variability in spring onset is strongly coupled to concomitant variations in the timing of the SFW events. The PIs' focus on SFW events provides a research setting for studying the physics and predictability of spring onset along with stratosphere-troposphere dynamical coupling that influences regional climate and its variability. Intellectual merit: The PIs will use a blend of synoptic and dynamic observational diagnoses and idealized modeling studies. First, a thorough phenomenological characterization of spring onset will be developed using observations. Then, the composite observational fields will be used to address various fundamental dynamical questions for spring onset. Guided by the phenomenological characterization and exploratory linear model calculations, several numerical experiments will be carried out to study stratosphere-troposphere coupling during spring onset. Completion of this work will offer new insights into:1. Effects of wave-mean flow interactions on the evolution of spring onset,2. Roles of the stratosphere in spring onset,3. Physical mechanisms responsible for noted changes in tropospheric climate and predictability.Broader impacts: The research provides a dynamical framework for understanding and interpreting regional climate changes associated with spring onset. It will have important implications for regional climate predictions. The research fosters collaboration between two universities and fills in gaps between tropospheric and stratospheric research communities and may lead to a comprehensive study of spring onset and its modulations by SFW events.