Spectral Energy Budget Analysis in the Frequency Domain

A challenging, but crucial component of air-sea interaction is deciphering whether the atmosphere or the ocean drives the other fluid. In this work, we use a frequency-domain approach to classify the contributions into and out of the energy budget of the ocean and the atmosphere, with the goal of determining the dominant fluid at each timescale. While it is common to study the fluxes of energy across spatial scales, this technique instead focuses on the exchange of processes across temporal scales. This novel approach is applied to the output of an idealized, North-Atlantic, coupled ocean-atmosphere model by calculating the spectral energy and temperature budget in each fluid. The spectral energy budget diagnostic is well-suited to study energy and heat transfer, as it identifies the terms that contribute to the input and removal of energy across timescales, from one day to 100 years. Our findings suggest that the fluids are primarily intrinsically-driven by the nonlinear advection terms at all timescales. The spatial distribution of the ocean’s spectral energy budget shows that the behavior varies across regions, and we infer that the majority of eddy energy on interannual to interdecadal timescales is input in the western boundary current separation region, and that this energy input is balanced by the extraction of energy along the western edge, at the bottom of the basin.