Baroclinic and barotropic aspects of extratropical wave-mean flow interaction

• Main Author: Boljka, Lina
• Published: University of Reading 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.766309

Baroclinic and barotropic processes are the key components of midlatitude tropospheric dynamics. Baroclinic processes are involved in the growth of extratropical storms, whereas barotropic processes are involved in their decay, suggesting the two processes are closely linked. Their links are conventionally studied through wavemean flow interaction theory and through modes of variability, and both planetary and synoptic scale waves play an important role in interacting with the baroclinic and barotropic mean flow. These processes are studied using multiscale asymptotic methods, which provide a framework for studying wave-mean flow interactions on different spatial and temporal scales. This framework is used to derive the full set of equations for small amplitude planetary and synoptic scale waves and for the zonal mean flow and its interactions with planetary and synoptic waves. In a zonally inhomogeneous framework (planetary-wave amplitudes comparable to synoptic-wave amplitudes) this theory predicts a coupling of baroclinic and barotropic processes through the planetary scale waves, and the interactions between the planetary and synoptic waves only occurring via the zonal mean flow or diabatic and frictional processes. However, in a zonally homogeneous framework (negligible planetary waves) baroclinic and barotropic processes are decoupled, with eddy momentum fluxes only affecting the barotropic flow and eddy heat fluxes only affecting the baroclinic flow, consistent with some recent observational studies. The latter somewhat counterintuitive result is studied in a zonally homogeneous idealized model and in Southern Hemisphere observations, using the baroclinic and barotropic annular modes of variability at different timescales. This shows that the decoupling of the two processes can indeed occur, but is frequency-dependent. The important role of planetary scale waves is explored in a zonally inhomogeneous idealized model and in Northern Hemisphere observations through the variability in the barotropic and baroclinic mean flows in storm track regions, and links with teleconnection patterns are established.