Using the Moist Static Energy Budget to Understand Storm Track Shifts across a Range of Timescales

Storm tracks shift meridionally in response to forcing across a range of time scales. Here we formulate a moist static energy (MSE) framework for zonal-mean storm track position and use it to understand storm track shifts in response to seasonal insolation, El Nino minus La Nina conditions and CO₂ direct (increased CO₂ with fixed sea surface temperature) and indirect (fixed CO₂ and increased sea surface temperature) effects. We define storm track position as the latitude of zero transient eddy MSE flux divergence. An equation for storm track shift is derived by linearizing about the storm track position. According to this linearized framework any storm track shift can be decomposed into contributions from changes in net energy (MSE input to the atmosphere minus atmospheric MSE storage) and MSE flux divergence by the mean meridional circulation and stationary eddies. This decomposition reveals two dominant shift regimes. The first regime involves stationary-transient eddy compensation and it dominates the response to seasonal insolation, El Nino minus La Nina conditions and CO₂ direct effect in the Northern Hemisphere. The second regime involves mean meridional circulation-transient eddy compensation and it dominates the shift in response to CO₂ indirect effect during Northern winter and in the Southern Hemisphere. Overall, the MSE framework explains why the seasonal storm track shift in the Northern Hemisphere is larger than that in the Southern Hemisphere. Furthermore, it shows that the equatorward shift of wintertime storm track in response to El Nino minus La Nina condition involves a different shift regime than the poleward shift in response to increased CO₂, even though the tropical upper troposphere warms in both cases.