Understanding the Seasonality of Orographically Forced Stationary Waves: Interaction between Mechanical and Thermal Forcing

Abstract

Idealized simulations of the atmosphere?s stationary response to the Rockies, Tibetan Plateau, and the Greenland Ice Sheet are made using a nonlinear, quasigeostrophic model and are compared to observations. Observational data indicate low-level heating (cooling) occurs above the Rockies and Tibet in the summer (winter). Low-level cooling is found above Greenland in both seasons. The atmosphere responds to both diabatic heating (termed thermal forcing) and low-level flow being obstructed by the mountain?s presence (termed mechanical forcing). The response to thermal and mechanical forcing together can be very different from the response to either forcing individually. The presence of modest low-level heating or cooling (±1.5 K day?1) causes significant changes to the mechanical forcing and, thereby, to the stationary wave response. For example, while the nonlinear response to mechanical forcing and low-level heating is characterized by a cyclone over the orography, the response to mechanical forcing and low-level cooling consists of an anticyclone over the orography. These differences cannot be fully explained using linear theory. The presence of heating (cooling) tends to reduce (amplify) both the mechanical forcing and the far-field stationary wave response. In addition, the presence of low-level heating or cooling lowers the critical mountain height below which the response is essentially linear;including nonlinear temperature advection at the surface is especially important for obtaining an accurate response.