资源环境科技发展态势分析平台

Using the Weather Research and Forecasting model, numerical experiments with reconstructed ideal zonally uniform sea surface temperatures were designed to discuss the midlatitude upper-level zonal wind response to the change of subtropical front strength over the North Pacific in winter. The results show that the enhancement of the subtropical frontal zone (STFZ) of the North Pacific leads to the stronger upper zonal wind, without considering the lateral boundary barotropic and baroclinic disturbances in this regional model. The enhanced STFZ first results in positive meridional temperature gradient anomalies by affecting the sea surface sensible heat flux. Then the low-level wind increases because of the increased thermal wind. In addition, the temperature gradient anomalies spread to middle and upper troposphere by baroclinic waves, causing the increase of Eady growth rate and enhanced storm tracks on the southern side of the STFZ and the decrease on the northern side. The strengthened storm tracks give rise to a stronger midlatitude westerly jet accompanied with a southward movement through wave-mean flow interaction, including both barotropical energy conversion and the Rossby wave breaking.

Plain Language Summary The related dynamical mechanism through which the subtropical frontal zone (STFZ) strength influences upper zonal wind in winter is not yet clear and is the key issue of this manuscript. In order to discuss the interrelated dynamical process, five numerical experiments were designed with the Weather Research and Forecasting model. The results of simulation experiments without considering the atmospheric lateral boundary suggest that lower atmospheric meridional temperature gradients increased rapidly via the anomalies of upward sea surface sensible heat flux after the STFZ enhancement. Then, the anomalies spread to the middle and upper troposphere, causing the Eady growth rate of the troposphere to strengthen on the southern side and to decrease on the northern side of the STFZ, causing stronger storm tracks on the southern side. As a result, more energy was conveyed from transient eddies to the mean flow in the southern side of the subtropical region, leading to stronger zonal wind in the south of the western Pacific jet stream main body. The enhancement of transient eddies arouses stronger Rossby wave breaking events in the upper troposphere, resulting in more transient perturbation kinetic energy transported to the western Pacific jet stream, finally accelerating the westerly. This study may be beneficial for weather forecast and precipitation predication of North America.