We consider Vallis’ symmetric and asymmetric Lorenz models for El Niño—systems of autonomous
ordinary differential equations in 3D—with the usual parameters and, in both cases,
by using rigorous numerics, we locate topological horseshoes in iterates of Poincaré return
maps. The computer-assisted proofs follow the standard Mischaikow–Mrozek–Zgliczynski
approach. The novelty is a dimension reduction method, a direct exploitation of numerical
Lorenz-like maps associated to the two components of the Poincaré section.