报告题目：A weather-feature perspective on jet stream dynamics
报告人：Prof. Thomas Spengler (University of Bergen)
邀请人：张洋 教授
时间：2025年2月28日（周五）上午 10:00 - 11:30
地点：仙林大气楼 D103
Abstract：
We explore the link between day-to-day weather and the climatological zonal mean perspective of jet stream dynamics, which commonly focuses on eddy momentum flux convergence to explain jet maintenance. Specifically, we decompose the full momentum flux divergence into contributions from mean flow and perturbations both in the time and zonal direction as well as their combinations, and then systematically relate synoptic jets, cyclones, and Rossby wave breaking events to the instantaneous momentum fluxes. We also investigate the sensitivity of the interpretation of the role of diabatic heating on time and spatial filtering on the interpretation of the feedback of diabatic heating on the jet stream.
We show that averaging in time and/or in the zonal direction masks large amounts of compensation of momentum flux convergence and divergence. Thus, the time and zonal mean must be regarded as a residual that is at least an order of magnitude smaller than the local or instantaneous values. Further, a large fraction of eddy momentum flux convergence and divergence occurs in association with weather features. Synoptic jets alone account for 60-80% of the convergence from the subtropics throughout the mid-latitudes. The link between weather and momentum fluxes is thus unaffected by the presence or absence of stationary waves.
Regarding the role of diabatic heating, we highlight a dichotomy between the commonly used time-zonal mean framework and the instantaneous synoptic framework. In the former, it is commonly argued that diabatic heating is located poleward of the time-zonal mean jet and hence that diabatic heating weakens the jet. In the synoptic framework, however, the heating resides on the warm equatorward side of the jet, thereby contributing to an increase in thermal wind and thereby jet intensity. We provide an explanation for this discrepancy, which is due to statistically averaging highly asymmetric states.
报告人简介：
Thomas Spengler focuses on the combination of theory, observations, and modelling, specialized on scales ranging from meso, synoptic, to large-scale flow and participated and coordinated several field campaigns. He did his PhD at ETH Zurich, Switzerland, followed by a postdoc at Princeton University, USA, before accepting his professorship at the University of Bergen, Norway. Since 2015, he is the director of the RCN funded Norwegian Research School on Changing Climates in the Coupled Earth System. He is currently leading research projects focusing on atmosphere-ocean-ice interactions in higher latitudes as well as air-sea interactions and cyclone development in the midlatitude storm tracks.
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