Numerical Simulation of Midlatitude Upper-Level Zonal Wind Response to the Change of North Pacific Subtropical Front Strength

Abstract

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.