Contributions to the zonal-mean zonal momentum balance in the suite of two-level runs with increasing k = 1 heterogeneities in thermal forcing. Top row: zonal-mean zonal momentum balance (30), as in Figures 6(b) and (e). Bottom row: spectral decomposition of the EMFC due to transients K_k,ω(ϕ) (see text and Appendix E), averaged for ϕ ∈  [10°S, 10°N]. Plus signs indicate RK modes, and stars indicate westward-propagating MRG-like waves. Gaussian smoothing with standard deviation 5 × 10⁻³ is applied along the frequency axis. Note that the color scale of the left three panels differs from that of the right three panels by a factor 4.