= Melnikov splitting of a periodically perturbed cellular flow
{title2=$M(X_0,t_0)=\frac{\pi\omega}{\sinh(\pi\omega/2)}\sin[\omega(t_0-\log\tan(X_0/2))]$}
Perturb the <sinusoidal cellular flow> by $\epsilon\cos x\cos y\sin(\omega t)$ in its <stream function>. On $y=0$, parameterizing the unperturbed orbit with $q_0(0)=(X_0,0)$ gives $x(s)=2\arctan e^{s+a}$, $a=\log\tan(X_0/2)$. The <heteroclinic Melnikov function for a periodic planar flow> has integrand $\operatorname{sech}^2(t-t_0+a)\sin(\omega t)$. Its odd part vanishes and the Fourier transform of $\operatorname{sech}^2$ gives the displayed result. For each fixed nonzero frequency its simple zeros produce transverse intersections at sufficiently small perturbation, hence exchange of fluid lobes across the former cell boundary. The leading splitting is exponentially small at high frequency; that estimate is not uniform in frequency and perturbation size.
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