From Pierre's link:

The distinction between a teleconnection and a common-mode forcing is important. Why would a behavior in one ocean basin remotely impact a behavior in a geographically separated one? I know it's possible to create a mathematical network connection between the two, but is that physically plausible? (note: this was in fact the motivation for the Azimuth Project ENSO study if you look back to much earlier discussion threads)

Much more plausible that a common-mode forcing mechanism links the two. The forcing is much the same, but the difference in the basin structure creates a different standing wave pattern. Now, and imho, it is possible that 2nd order factors are influencing the North Wall, which would presumably then impact the standing wave boundary conditions.