Analyses of Large-scale Climate Variability: Understanding Periodicity in the Extratropical Storm Tracks

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Weather systems appearing on daily maps are driven by the temperature contrast between the tropics and the poles, which is expressed dynamically as the baroclinicity of the atmosphere, or the variation of pressure on surfaces of constant density. Pressure differences on density surfaces produce a pressure gradient force, which provides the kinetic energy that powers weather systems and the winds associated with them. Recently the PI demonstrated that the interplay between baroclinicity and kinetic energy (in this case referred to as eddy kinetic energy or EKE), results in variations in the vigor of weather activity with a period between 20 and 25 days (see AGS-1343080). The 20-25 day oscillation was documented in the longitudinally averaged EKE of the Southern Hemisphere and has an annular structure with peak amplitude near the center of the storm track. The mode is thus referred to as the Southern Baroclinic Annular Mode (SBAM), and the PI has identified a comparable although weaker counterpart, the NBAM, in the Northern Hemisphere.Work funded here seeks to develop a physical understanding of the modes, using a combination of observational data analysis and model simulations to understand why the modes exist and what determines their periodicity, spatial structure, relationship to individual weather systems, and other characteristics. The work also considers the implications of the modes for practical weather forecasting, in part by examining their predictability in the forecasts archived as part of the Seasonal to Subseasonal Prediction Project (performed under the auspices of the World Meteorological Organization).The research is of societal as well as scientific interest due to the possible implications of predictable oscillations in weather activity for long-range forecasting. The PI and his group also perform outreach to the local school district through presentations to students and teachers on the science of weather and climate. In addition, the project supports two graduate students, thereby providing for the future workforce in this research area.