Spectral Exactness and Quantitative Instability Approximation in the Euler-Zeitlin Model for Zonal Equilibria
Building on finite-time convergence results for geometric matrix hydrodynamics, we establish global spectral exactness for Zeitlin's approximation of the two-dimensional Euler equation on the sphere linearized about polynomial zonal vorticities. By separating each azimuthal block into a skew-adjoint transport operator and a compact perturbation, we obtain a dimension-uniform estimate $\|K_N-K\|=O(N^{-1})$ and the sharper fixed-block estimate $\|K_{N,m}-K_m\|=O(N^{-2})$. These bounds, together with strong-resolvent convergence of transport, yield exact approximation of the isolated spectrum off the imaginary axis: algebraic multiplicities are preserved, Riesz projections converge in norm, and spurious off-axis spectral limits are excluded. Every algebraically simple off-axis eigenvalue has an $O(N^{-2})$ approximation error. For eigenvalues of higher multiplicity we establish second-order convergence of arithmetic means of eigenvalue clusters and $O(N^{-2/α})$ individual eigenvalue bounds, where $α$ is the maximal Jordan-block size. For any prescribed positive growth margin, the numbers of unstable and stable eigenvalues beyond that margin are eventually correct. The argument combines an exact matrix-harmonic coordinate recurrence, uniform bounds for high azimuthal modes, and resolvent factorizations involving compact operators. An explicit degree-three zonal equilibrium has a rigorously enclosed simple unstable eigenvalue, providing a concrete instance of the quantitative theory.