Stratosphere-Troposphere Coupling

Studying weather patterns and medium- and long-term forecasting requires examining multiple systems interacting with each other in the Earth climate system. The weather we experience occurs in the troposphere, and the atmosphere producing this weather responds to multiple things including land-sea contrasts, radiative and non-radiative (e.g., sensible and latent heat) fluxes, and other forcings. However, weather patterns, especially during the late fall through early spring months, can also be influenced by changes in the atmosphere from above – i.e., by changes in the circulation of the stratosphere. One of these changes come from alterations in the stratospheric polar vortex. In fact, the two layers (troposphere and stratosphere) actually “talk” to each other and thus can be considered coupled.

A main focus of the Applied Climate Dynamics Research Group is on extratropical stratosphere – troposphere coupling (i.e., coupling that happens poleward of about 35°N). At these latitudes, the troposphere talks to the stratosphere via vertical Rossby wave propagation. These waves can be planetary-scale longwaves (e.g., those that form because of air flow over mountains and because of land-sea contrast) or sometimes shorter waves (e.g., extratropical cyclones). In either case, once these waves move into parts of the lower and middle stratosphere, they will eventually break and deposit heat and momentum that they carry with them into the stratosphere, thus dynamically changing the circulation in the stratosphere. As the stratospheric circulation begins to be changed, these changes are then communicated downward through the column and can then impact the circulation in the troposphere (i.e., alter the strength and position of the jet stream). Because this communication occurs only during the (extended) cold season, using the stratosphere as a way to advance predictions of tropospheric weather is a real move forward to improve winter weather forecasts across the middle and high latitudes.

In the Applied Climate Dynamics Research Group, we study multiple facets of stratosphere – troposphere coupling, its utility in extending wintertime subseasonal-to-seasonal forecast skill, and also evaluating how well models resolve these dynamical connections. Some specific projects and interests in the groups include:

      • Forecast skill of sudden stratospheric warming events (i.e., complete breakdowns of the polar vortex) in operational subseasonal prediction systems.
      • Building statistical models for subseasonal prediction of temperature, precipitation, and the strength of the jet stream using tropical and extratropical predictors.
      • Evaluating the role that the stratospheric polar vortex plays when coupled with different phases of tropical convection.
      • Changes in strength and mean position of the Northern Hemisphere polar vortex under future climate change.
      • Dynamics of vertical wave propagating waves and their sensitivity to background tropospheric and stratospheric conditions.

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