During the northern hemisphere cool-season, variability in the stratospheric flow can lead changes in the frequency and distribution of extreme weather events. These changes to the occurrence of extreme events can linger for extended periods of weeks to a season, which means that knowledge of stratospheric variability can be used as a tool in the creation of extended range weather outlooks. Stratospheric variability is largely driven by the vertical propagation of the largest scale of atmospheric waves, known as Rossby waves, from the troposphere into the stratosphere. This project investigated weather systems as potential sources of this large-scale wave forcing for periods of stratospheric variability. 

The research identified and investigated more than 250 atmospheric blocking events, more than 2,800 rapidly deepening cyclones, known as bomb cyclones, and about 150 extratropical transition of tropical cyclone events. The results showed that each type of event has a specific region where it is more likely to increase the forcing that produces stratospheric variability. The analysis showed that northern European blocking events and Western North Pacific rapidly deepening cyclones, were associated with anomalous upward wave forcing that tended to weaken the stratospheric flow, but the analysis showed that Atlantic cyclones and Western North Pacific blocking events were more likely to produce a reduction in wave forcing and support periods of stability for the stratospheric flow. The analysis showed that alone, the synoptic evets were not enough to produce the wave forcing from the troposphere into the stratosphere, but when events occurred in these specific regions, they enhanced the preexisting background flow to produced anomalous periods of wave forcing on the stratosphere. These results support the idea that the stratosphere is not a passive actor in the periods leading to stratospheric variability as well as the idea that synoptic events do contribute in a significant way to periods of wave forcing for stratospheric variability.

Using ensemble forecasts with large uncertainty in the stratospheric flow, the project also investigated the processes that enhanced or suppressed the likelihood that a synoptic-scale event will have an impact on stratospheric variability. The results showed that the diabatic processes within the synoptic-scale event can reconfigure the tropopause-level flow. When a poleward directed warm conveyor belt existed within the synoptic scale event, diabatic outflow typically amplified the flow in the upper troposphere but also lower stratosphere and promoted upward wave forcing in the region from the tropopause into the stratosphere.

The results of this project can be applied in combination with an assessment with other large-scale climate modes of variability, especially those that impact the extratropical storm tracks, to evaluate changes in the likelihood of stratospheric variability at longer lead times.

This project supported the PI in developing a new graduate-level course on troposphere-stratosphere coupling at the University at Albany, SUNY. Over the course of the project, roughly 35 graduate students had the opportunity to complete the course which included a final research project. One student led research project from this course is now in the peer-reviewed literature at the time of the completion of this project, with another in preparation for submission. Additionally, a real-time tropopause and stratosphere analysis webpage was developed by a project supported graduate student based on the themes of this project for community use. The results of this project were decimated via peer-reviewed literature and conference presentations, which resulted in two outstanding student presentation awards for project supported graduate students.

Last Modified: 11/02/2021
Modified by: Andrea Lang