Here's a great illustration of what happened in 2015. Versus what occurred in December of this season. In a single image from the following paper. But it's also important to keep in mind other factors which may be present. One such example could be the PDO. Which was positive in 2015. Or the QBO. Nothing works in a vacuum. It's all interconnected. That's also important. It can get really extremely complicated. That's what makes seasonal forecasting so very difficult to this day. This image shows what effects could be expected from 2 different areas of focus via tropical convection. You can see how different the effects can be just from some subtle difference in location. Because of the rossby waves originating from these events. Indonesian activity drives a -PNA. Which we witnessed in December, in an anomalous way. However, move that just a bit east into the west/central Pacific. You see something like a 2015. Which is what happened that season. Hope this can be somewhat helpful. This is why we look to the MJO for clues.
https://www.nature.com/articles/s41598-019-49449-7