Using the Moist Static Energy Budget to Understand Storm-Track Shifts across a Range of Time Scales

Storm tracks shift meridionally in response to forcing across a range of time scales. Here the authors formulate a moist static energy (MSE) framework for storm-track position and use it to understand storm-track shifts in response to seasonal insolation, El Nino minus La Nina conditions, and direct (increased CO2 over land) and indirect (increased sea surface temperature) effects of increased CO2. Two methods (linearized Taylor series and imposed MSE flux divergence) are developed to quantify storm-track shifts and decompose them into contributions from net energy (MSE input to the atmosphere minus atmospheric storage) and MSE flux divergence by the mean meridional circulation and stationary eddies. Net energy is not a dominant contribution across the time scales considered. The stationary eddy contribution dominates the storm-track shift in response to seasonal insolation, El Nino minus La Nina conditions, and CO2 direct effect in the Northern Hemisphere, whereas the mean meridional circulation contribution dominates the shift in response to CO2 indirect effect during northern winter and in the Southern Hemisphere during May and October. Overall, the MSE framework shows the seasonal storm-track shift in the Northern Hemisphere is connected to the stationary eddy MSE flux evolution. Furthermore, the equatorward storm-track shift during northern winter in response to El Nino minus La Nina conditions involves a different regime than the poleward shift in response to increased CO2 even though the tropical upper troposphere warms in both cases.