2018年4月19, 24, 27日 Yohai Kaspi 准教授セミナー終了しました
学振招聘研究者として東大・先端研に滞在するイスラエルWeizmann科学研究所の Yohai Kaspi 准教授を迎えて，stormtrackと渦駆動ジェットに関するセミナーを3件開催します．
なお，先端研3号館へのアクセスは こちら をご参照下さい．
【セミナー1】日 時：2018年4月19日(木) 17:15-18:45
場 所：東京大学先端科学技術研究センター3号館 M2階 セミナー室
題 目：Dynamics of eddy-driven jets and macroturbulent scales in the atmosphere
East-west atmospheric jet streams dominate the dynamics of Earth and the other planets. We study the dynamics of eddy-driven jets using a high-resolution idealized atmospheric GCM, where we systematically vary Earth’s rotation rate. This allows for a clear separation between the subtropical and eddy-driven jets, which in Earth's atmosphere, due to Earth’s size and rotation rate, are often merged together. We find that due to asymmetries in baroclinicity across the width of the jet, eddy-driven jets in general tend to migrate poleward. The migrating jets allow a latitude-by-latitude analysis of the macroturbulent scales controlling the jets. It is found that over a wide range of eddy scales and latitudes the jet width scales with the Rhines scale. This is analyzed through partitioning of the barotropic energy spectrum to the role of eddy-mean and eddy-eddy interactions, in transferring energy up- and down-scale. We find that this can explain the observed energy spectrum in Earth's atmosphere. Finally, the role of eddy-eddy interactions is analyzed through comparison to simulations where they are omitted. It is found that eddy-eddy interactions are not a prerequisite for formation of jets, and in fact limit the meridional extent of multiple eddy-driven jets.
【セミナー2】日 時：2018年4月24日(火) 10:15-11:45
場 所：東京大学先端科学技術研究センター3号館 207号室
題 目：Mechanisms controlling the spatial structure of midlatitude storm tracks and their dependence on climate change
Midlatitude storm tracks dominate the transport of momentum, heat and water-vapor in the extratropics. Therefore, identifying the mechanisms governing their temporal and spatial structure is vital for understanding weather and climate. In this talk, we analyze the spatial structure of midlatitude storm tracks by tracking transient cyclonic eddies in an idealized GCM with several levels of complexity and in CMIP5 simulations. The localized atmospheric response is decomposed in terms of a time-zonal mean background flow, a stationary wave and a transient eddy field. The Lagrangian tracks are used to construct cyclone composites and perform a spatially varying PV budget. Three distinct mechanisms that contribute to the spatial structure emerge: transient nonlinear advection, latent heat release and stationary advection. The downstream evolution of the PV composites shows the different role of these mechanisms as the storm tracks evolve downstream. We demonstrate that the poleward motion of individual cyclones increases with increasing global mean temperatures, and by this provide an alternate quantitative explanation to the poleward shift of storm tracks under climate change.
【セミナー3】日 時：2018年4月27日(金) 16:30-18:00
題 目：The relation between jet characteristics and a midwinter minimum in eddy activity over the Pacific and Atlantic basins
The existence of a midwinter minimum in eddy activity over the Pacific has been a key puzzle ever since its identification by Nakamura (1992). Here we show that a similar phenomenon to the well known Pacific midwinter minimum can be found over the Atlantic in years when the jet is on average more equatorward in winter. The reduction in eddy activity over the Atlantic is smaller than the Pacific case and persists for a shorter duration. It is suggested that this difference is due to the Atlantic jet being overall more eddy-driven and climatologically farther poleward than the Pacific jet. To further evaluate the relation between the character of the jet and eddy activity, an idealized GCM that is designed to resemble different climatological jet characteristics is used. We find that differences in jet characteristics between a purely eddy-driven jet (as often occurs in the Atlantic and during the Pacific shoulder seasons), and a mixed eddy-driven-subtropical jet (as often occurs in midwinter in the Pacific) play an important role in the relation between jet strength and eddy activity. It is therefore suggested that the seasonal difference in jet characteristics plays an important role in the existence of the Pacific midwinter minimum. This is demonstrated both in reanalysis data and in idealized GCM simulations.