This project seeks to improve our understanding of dynamical mechanisms responsible for the formation and maintenance of zonal jets and their implications for atmospheric mixing and transport. The approach is underpinned by the observation that potential vorticity (PV) is a fundamental dynamical quantity that can unify questions of small-scale eddy mixing, wave propagation on jets, and mixing due to wave breaking in their vicinity. In particular, the project will address: (i) fundamental constraints on zonal jets arising from angular momentum conservation (natural, since zonal mean structures such as zonal jets contain most of the atmosphere's angular momentum) and flow stability; (ii) wave dynamics (and associated eddy momentum fluxes) and the role of critical levels; the role also of edge waves propagating on a PV jump in contrast to waves propagating on a smooth background PV gradient; (iii) PV mixing as a route to zonal jet formation, independently of the direction of energy cascade; (iv) tracer mixing and transport and the role of jets as transport barriers; (v) jet evolution and mechanisms of jet merger; (vi) secondary circulations and the development of steep tracer gradients through vertical motions; alignment of tracer and PV gradients. The dynamical processes involved govern zonal structures observed in the terrestrial atmosphere and oceans as well as in the atmospheres of the gas giants.
The work will impinge on several areas of climate science, including variability of the mid-latitude storm tracks, subtropical jet, and Arctic oscillation, the hydrological cycle, stratospheric ozone recovery, the dynamical coupling of the stratosphere and troposphere, and transport and mixing across the extratropical tropopause.
This project has focused on the large-scale circulation patterns of the troposphere and stratosphere---the lowermost fifty kilometers of the atmosphere---where the effects of increasing carbon dioxide concentrations, depleted stratospheric ozone levels, and the complex distribution of water vapor play leading roles in the determination of the climate we live in. Most of the research has involved an investigation of the key dynamical processes that control the formation and maintenance of the strong, predominantly eastward jets, such as the tropospheric jet-stream and the winter stratospheric polar vortex. These jets rapidly transport air and trace gases in the east-west direction while inhibiting such transport in latitude. While strong jets inhibit latitudinal transport, they are simultaneously associated with the nearly complete mixing of the air masses that lie between jets; the jets thus separate regions of quasi-uniform chemical composition. The project has developed our understanding of how the forcing of atmospheric wave and eddy motions influence the development of these jets, in particular identifying the conditions that favor the development of strong jets and robust transport barriers. It was found that strong jets are favored when wave and eddy forcing is in a certain sense weak, a result that is at first sight paradoxical in view of the need for complete mixing between jets. The resolution of the paradox lies in the nature of the inter-jet regions, which are dynamically susceptible to mixing, however weak, and in the fact that only weak eddy forcing allows the jet formation processes to proceed unhindered. Two distinct mixing regimes active in the inter-jet regions were identified, which arise according to the dominant length scales of the forced wave and eddy motions, and which give rise to different jet characteristics and distributions. Finally, the project has revealed important insights into how one jet configuration may transform into another without a significant change in the total energy or angular momentum of the atmosphere as a whole. The possibility of such transitions has important implications for our understanding of the behavior of the winter stratospheric polar vortex and its dynamical signature in the troposphere; the latter is closely related to the Arctic oscillation and is a key controlling factor in mid-latitude weather regimes on seasonal timescales.
