The general connection is that the Tropical Instability Waves appear stronger during La Nina phases.

Precisely along the equator there is no preferred vortex behavior as Coriolis cancels, which forms the premise for an ENSO standing wave solution. Jan was asking about vortices, so that's why I mentioned the TIW.

My ENSO+QBO equatorial solution is [here]( which is a Google books excerpt. Start with equations 11.1, then apply the 2-equator topological ansatz of 11.2 when simplifying, and the analytical standing wave solution of 11.3 results.

I propose that high wave-number solutions when perturbed off-equator will lead to the TIW.