Quantifying Sources and Sinks of Rossby Wave Activity in the Atmosphere

It is useful to think of the atmospheric circulation above the middle latitudes as a progression of waves, with alternating northward and southward air flow, superimposed on an eastward-flowing mean jet stream. The theory based on this wave-mean flow framework has been successful in explaining many aspects of the general circulation including the evolution of frontal weather systems and the effect of mountains on the upper-tropospheric flow.

The Prpincipal Investigator (PI) has contributed to this theory by developing a measure of waviness which captures a trade-off in which wave activity increases at the expense of mean jet speed and vice versa, a consequence of the fluid analog of angular momentum conservation known as Kelvin's circulation theorem. The measure is called Finite Amplitude Wave Activity (FAWA), where finite amplitude refers to the fact that the measure can be used even when the waves distort the flow enough to create large circulation features which break off from the prevailing flow.

FAWA represents the bulk wave activity around a latitude circle, and the PI has developed a related quantity called Local Wave Activity (LWA) to express waviness at each longitude along a latitude circle. Previous work by the PI's team has used the FAWA/LWA framework to explain the breakdown of the polar vortex during Sudden Stratospheric Warmings (SSWs) and the formation of blocking anticyclones (see AGS-1909522).

The application of FAWA and LWA to problems like SSWs and blocking relies on their ability to capture interactions between waves and the eastward mean flow, which can be understood without knowing how the waves are generated or what eventually happens to them. But the generation and dissipation of wave activity are also represented in the evolution equations for FAWA and LWA, and the equations can be used to study wave sources and sinks and their role in atmospheric circulation.

For example the dissipation of wave activity through diffusive mixing matters for the strength of the low-latitude meridional overturning cells (the Hadley cells), thus analysis of the sink term could offer insights into the expansion of the cells and the associated aridification seen in climate change simulations.

Analysis of sources and sinks is promising but also challenging due to the need for intricate calculations involving small spatial scales and quantities which are hard to observe. Work under this award takes up the challenge, using a variety of techniques applied to idealized models, archived climate model output, and observational datasets (the ERA5 reanalysis in particular).

Topics to be addressed through source/sink analysis include the seasonal cycle of wave activity, the role of the atmospheric boundary layer in generating and damping wave activity, and the generation of wave activity by condensational heating in convective clouds.

The work has societal value due to the connection between upper-tropospheric waviness and surface weather, for instance blocking events are associated with extreme precipitation and SSWs can lead to severe cold air outbreaks. The software used to generate FAWA, LWA, and source/sink terms will be made openly available to the public via github, along with documentation on their use.

Education and outreach are conducted through a biennial summer school and a local community science center. In addition, the project supports two graduate students and a postdoctoral fellow, thereby fostering the next generation of scientists in this research area.

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.