The interaction between atmospheric gravity waves and the polar vortex

Atmospheric gravity waves are wind and temperature perturbations in the atmosphere.

They can propagate upwards from their sources in the troposphere and carry energy and momentum all the way up to the mesosphere. This transport has a very strong impact on winds and air circulation in the stratosphere and above.

Despite their importance, many effects that these waves have on the large-scale dynamics of the atmosphere still cannot be accurately quantified, largely due to their relatively small scale, which makes them hard to observe.The Swedish MATS satellite was launched on 4 November 2022. The main goal of this mission is observation of gravity waves in the altitude range of 65 km to 110 km.

MATS observes air glow in the limb of the atmosphere and uses these measurements to perform tomography: determination of 3-D structure of air temperature, from which the relevant parameters of individual gravity waves in the atmosphere can be derived.

MATS provides more complete and detailed information on gravity waves than previously available from any global data sets and allows to accurately calculate how much energy and momentum these waves carry thus quantifying their effects on the large scale atmosphere dynamics.The polar vortex is a large vortex in the stratosphere and mesosphere that forms over each pole in winter and is characterised by strong zonal winds at its edges .

Every spring the vortex breaks down.

The timing and the exact nature of this process has major effect on tropospheric weather and weather extremes in the polar regions and middle latitudes.

This break-up is typically caused by excitation of strong, planetary (large-scale) waves that disturb the vortex and lead to sudden decrease in wind speed at its edges and increase in temperature (sudden stratospheric warming).

However many modelling studies show that gravity waves play a major role in this process, and the momentum they deposit into the vortex edge has a strong influence on the start of the breakup process.Here we propose to use MATS data to study the role of gravity waves in the polar vortex dynamics and break-up.

This work will consist of the following parts:MATS 3-D temperature data will be processed using newly developed techniques that will allow to isolate gravity wave and planetary wave signalsChanges in the amount of momentum carried by gravity waves in the vortex area during its disruption will be investigated to learn how strongly gravity waves can affect the vortex and which physical processes control it.

Correlations between gravity wave and planetary wave activity will be studied to to quantify their interdependence.

The longitudinal structure of the gravity wave signal will be used to determine if and how strongly the vortex is affected by the distribution of gravity wave sources in the troposphere and by planetary waves.Ray tracing will be used to identify the sources of gravity waves that interact with the vortex.

This will allow to determine the relative importance of tropospheric weather and purely stratospheric processes for the vortex disruption and learn more about gravity wave generation in the stratosphere and mesosphere.

Ray tracing results will also be used to determine how far do gravity waves observed by MATS propagate in the horizontal direction, which will provide constraints for gravity wave modelling efforts and help to determine which gravity wave source regions, in terms of latitude as well as altitude, have an effect on the vortex.This work should allow to improve the fundamental knowledge of the dynamics of polar vortex, which is essential for understanding extreme events and weather patterns at high altitudes in the changing climate.

This study would also provide data necessary for improving weather forecasting, since lack of data about gravity wave propagation is currently a major source of uncertainty in the models.