Analyses of Large-scale Extratropical Climate Variability and Change

Colorado State University, Fort Collins CO

Investigators

Abstract

This project considers the atmospheric circulation in middle and high latitudes of the Northern and Southern Hemisphere, regions referred to as the extratropics. The project has two components, one of which is to improve understanding of the internal variability of the extratropical circulation, and the other is to examine the processes through which thermal forcing drives externally forced north-south shifts in the extratropical storm tracks and their associated eddy fluxes (which occur in part through the air movements associated with frontal weather systems). Research in the first component is based on the premise that large-scale variability in the extratropical flow arises in the context of two primary types of structures: 1) those that dominate the variance of zonal-mean kinetic energy and the conversion between eddy and zonal-mean kinetic energy (which occurs through eddy momentum fluxes); and 2) those that dominate the variance in eddy potential energy and the conversion between zonal-mean and eddy potential energy (which occurs through eddy heat fluxes). In the Southern Hemisphere (SH), which would be the focus of research in the first component, the structure that dominates variance of zonal-mean kinetic energy is the well-known Southern Annular Mode (SAM), which constitutes a north-south shift of the jet stream and accompanying storm track. Research in the second component seeks to understand how the extratropical circulation responds to a variety of external forcings including the depletion of stratospheric ozone (the "ozone hole"), reductions in Arctic snow cover and sea ice extent, increases in greenhouse gases, and solar irradiance. The research addresses the fundamental dynamics which govern the response of jet streams and their attendant storm tracks using simplified atmospheric circulation models, guided by the hypothesis that key aspects of the response can be understood through a diffusive framework in which eddy fluxes of heat and potential vorticity shift to match, in a down-gradient sense, the changes in their respective mean gradients. The work has broader impacts due to the importance of the fundamental dynamics addressed in the project for understanding and predicting the sensible weather and climate experienced at the Earth's surface. In addition, the work will support and train two graduate students from underrepresented groups, thereby broadening participation in the sciences and contributing to the development of the workforce in this research area.