Connecting the Energy and Momentum Flux Response to Climate Change Using the Eliassen–Palm Relation

Abstract

Coupled climate models project that extratropical storm tracks and eddy-driven jets generally shift poleward in response to increased CO2 concentration. Here the connection between the storm-track and jet responses to climate change is examined using the Eliassen–Palm (EP) relation. The EP relation states that the eddy potential energy flux is equal to the eddy momentum flux times the Doppler-shifted phase speed. The EP relation can be used to connect the storm-track and eddy-driven jet responses to climate change assuming 1) the storm-track and eddy potential energy flux responses are consistent and 2) the response of the Doppler-shifted phase speed is negligible. We examine the extent to which the EP relation connects the eddy-driven jet (eddy momentum flux convergence) response to climate change with the storm-track (eddy potential energy flux) response in two idealized aquaplanet model experiments. The two experiments, which differ in their radiation schemes, both show a poleward shift of the storm track in response to climate change. However, the eddy-driven jet shifts poleward using the sophisticated radiation scheme but equatorward using the gray radiation scheme. The EP relation gives a good approximation of the momentum flux response and the eddy-driven jet shift, given the eddy potential energy flux response, because the Doppler-shifted phase speed response is negligible. According to the EP relation, the opposite shift of the eddy-driven jet for the different radiation schemes is associated with dividing the eddy potential energy flux response by the climatological Doppler-shifted phase speed, which is dominated by the zonal-mean zonal wind.

The Sensitivity of the Jet Stream Response to Climate Change to Radiative Assumptions

“…Figure C1 with the right panel of Figure 4). The eddy heat flux metric is particularly similar to the DSE flux metric, because the eddy heat flux dominates the eddy DSE flux and the eddy potential energy flux is an order of magnitude weaker (Lachmy & Shaw, 2018). The storm track positions based on the near-surface EKE metric are close to those based on the eddy DSE flux metric in the control climates, but they shift more strongly poleward between the control and the representative warm climates.…”

“…This study and Lachmy and Shaw () both demonstrate that a radiation hierarchy combined with a dynamical theory can help improve our understanding of the shift of the storm track and eddy‐driven jet in response to warming. More generally, this combined radiative‐dynamical perspective will likely be useful for understanding other radiation‐induced circulation responses, such as the response of extratropical circulation variability (e.g., the annular mode patterns and time scales) and the tropical circulation (e.g., the Hadley cell).…”

“…In contrast, the critical level mechanism does not apply: as the critical level on the poleward flank of the jet shifts equatorward (first column of Figure , compare the magenta lines), the EMF pattern does not simply shift equatorward, but it switches sign from positive to negative at latitudes poleward of 40° (not shown). Lachmy and Shaw () also found that the change in the critical line does not explain the different eddy‐driven jet and storm track responses in the GR model.…”

“…This poleward shift also occurs for alternative storm track metrics such as the eddy heat flux or the near‐surface EKE (see Appendix ). Lachmy and Shaw () provided a more detailed physical interpretation of the different responses of the eddy‐driven jet and storm track in GRaM. For all other radiation schemes, the storm tracks shift robustly poleward as climate warms (Figure , right panel, colored lines).…”

“…Following Lachmy and Shaw (2018), we identify the eddy-driven jet as the latitude of maximum surface westerlies (which aligns with the maximum surface zonal wind stress and maximum vertically integrated eddy meridional momentum flux convergence, see Appendix C) and the latitude of maximum vertically…”