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

出现在每日地图上的天气系统是由热带和两极之间的温差驱动的，这一温差动态地表示为大气的斜压性，或恒定密度表面上的气压变化。密度面上的压力差产生压力梯度力，为天气系统和与之相关的风提供动能。最近，PI表明，斜压性和动能(在这种情况下称为涡动能或EKE)之间的相互作用导致天气活动的活力变化，周期在20至25天之间(见AGS-1343080)。20-25天的振荡记录在南半球经向平均的EKE中，在风暴路径中心附近具有峰值幅度的环状结构。因此，这种模式被称为南方斜压环状模式(SBAM)，PI在北半球发现了一个可比但较弱的对应模式-NBAM。这里资助的工作试图通过结合观测数据分析和模式模拟来发展对这些模式的物理理解，以了解为什么这些模式存在，是什么决定了它们的周期性、空间结构、与个别天气系统的关系以及其他特征。这项工作还考虑了这些模式对实际天气预报的影响，部分是通过检查它们在季节性到亚季节预测项目(由世界气象组织赞助下进行)存档的预报中的可预测性。由于可预测的天气活动振荡可能对长期预报产生影响，这项研究具有社会和科学意义。PI和他的小组还通过向学生和教师介绍天气和气候科学，向当地学区进行外联。此外，该项目还资助了两名研究生，从而为这一研究领域未来的劳动力提供了条件。

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.