Quantifying Eddy Generation and Dissipation in the Jet Response to Upper- versus Lower-Level Thermal Forcing

Abstract

Abstract The relative roles of upper- and lower-level thermal forcing in shifting the eddy-driven jet are investigated using a multilevel nonlinear quasigeostrophic channel model. The numerical experiments show that the upper-level thermal forcing is more efficient in shifting the eddy-driven jet. The finite-amplitude wave activity diagnostics of numerical results show that the dominance of the upper-level thermal forcing over the lower-level thermal forcing can be understood from their different influence on eddy generation and dissipation that affects the jet shift. The upper-level thermal forcing shifts the jet primarily by affecting the baroclinic generation of eddies. The lower-level thermal forcing influences the jet mainly by affecting the wave breaking and dissipation. The former eddy response turns out to be more efficient for the thermal forcing to shift the eddy-driven jet. Furthermore, two quantitative relationships based on the imposed thermal forcing are proposed to quantify the response of both eddy generation and eddy dissipation, and thus to help predict the shift of eddy-driven jet in response to the vertically nonuniform thermal forcing. By conducting the overriding experiments in which the response of barotropic zonal wind is locked in the model and a multiwavenumber theory in which the eddy diffusivity is decomposed to contributions from eddies and mean flow, we find that the eddy generation response is sensitive to the vertical structure of the thermal forcing and can be quantified by the imposed temperature gradient in the upper troposphere. In contrast, the response of eddy diffusivity is almost vertically independent of the imposed forcing, and can be quantified by the imposed vertically averaged thermal wind. Significance Statement Climate models predict enhanced warming over tropical upper troposphere and Arctic surface in response to greenhouse gas increases, which has competing effects on the latitudinal shift of eddy-driven jet and thus requires a better understanding of the relative roles of upper- and lower-tropospheric thermal forcing for future climate projection. We make a new quantitative comparison on responses of eddy generation and dissipation that sustain the jet shift and relate them to the imposed thermal forcing. This approach extends the fundamental eddy closure topic from vertically uniform situation to vertically nonuniform forcing. These quantitative relationships are also helpful for better understanding and predicting the jet and storm-track variabilities under other forms of thermal forcing (e.g., SST front, aerosols, latent heat release).