Dynamics of the Jet Stream Anomalies Across Scales

About This Grant

The meandering variability of upper-tropospheric jet streams have a strong influence on the surface weather of the middle latitudes, and very large meanders of the jet streams lead to extreme surface weather including heat waves, cold air outbreaks, and heavy precipitation. Classical linear wave theory can account for jet meanders but the theory assumes that the meanders have small amplitude, an unfortunate limitation since the meanders that matter for extreme weather are quite large. In earlier NSF-funded work the Principal Investigator (PI) pioneered a theory of wave dynamics which overcomes the small-amplitude constraint of linear theory and is thus better suited to real-world weather. The theory is based on a conserved quantity called Local Wave Activity (LWA), a measure of the waviness of the atmospheric circulation that can be used to identify the dynamical mechanisms responsible for important forms of wave behavior in the jet streams. Work performed here uses the LWA framework to address three longstanding problems in middle-latitude atmospheric circulation dynamics, the first of which is the suppression of cyclonic weather activity in the storm zones of the North Pacific and North Atlantic, which is typically weaker in midwinter than in October and March despite seemingly more favorable conditions for cyclogenesis. The work is guided by the hypothesis that the key issue is the exchange of LWA between weather systems and lower-frequency flow variations. The second problem is the unexplained dynamics that leads to Sudden Stratospheric Warmings (SSWs), in which the stratosphere over the North Pole warms dramatically as the stratospheric circulation flowing around it breaks down. The work focuses on the role of wave resonance, with different resonant modes responsible for different forms of SSWs (splitting versus displacement). The third problem is explaining the duration of blocking events, in which a high-pressure center forms in the upper troposphere and persists for many days as the air flow moves around it. The LWA framework is used to understand the lifecycle of the block and the various factors, including condensational heating in clouds, that could contribute to its persistence. The work is of practical as well as scientific interest given the potential for all of the above phenomena to create extreme weather. In addition, the work contributes to the efforts of the Model Diagnostics Task force of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory. The project also has educational value through its support of graduate students and a postdoctoral research fellow. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.