סמינר בחוג למדעי כדור הארץ: Wave-Mean Flow Interactions Maintaining the Different Flow Regimes of the Jet Stream

Abstract:

The type of the tropospheric jet stream dominating the flow changes with the seasonal cycle and the geographic location. A subtropical jet is characterized by strong and steady upper tropospheric winds located at the subtropical edge of the Hadley cell. A jet called "eddy driven jet", "midlatitude jet", or "polar front jet" is characterized by large meanders located above the surface westerlies at the midlatitudes. Sometimes one type of jet dominates, sometimes the two jets coexist, and sometimes they merge to one mixed subtropical-eddy driven jet. These different states can be viewed as different flow regimes, which depend on the interaction of the jet with the atmospheric waves and with the mean meridional circulation.

In this work, the wave-mean flow interactions leading to the different jet regimes are studied using a new numerical model, designed to reproduce the observed regimes in a qualitatively realistic manner, while keeping the simplest possible framework. As the model parameters are varied to allow for faster growth of the waves, the flow transitions from a subtropical to a merged and then to an eddy driven jet regime. Each regime is characterized by different properties of the flow, including the structure of the jet, its location relative to the circluation cells, its time variability patterns and time scales, the strength of the waves, the structure of the wave fluxes and the waves’ spectral properties.

The maintenance of the mean flow and the waves, as well as the feedbacks responsible for the time variability properties, are analyzed in detail for each regime. The dynamical mechanisms responsible for the regime transitions are revealed, and a comparison with observations implies that they hold for the atmospheric flow over large longitudinal sectors. This research provides a physical picture of the balances and temporal variations of all observed regimes under one coherent framework, allowing us to relate different dynamical mechanisms to each other and to the jet stream state.

מארגנת הסמינר: ד"ר רוית חלד