Thanks for the response.

That said, I was still really curious about this topic after posting my question, so I did a web search about it (search term: efectos de El Niño en Costa Rica).  And, by looking at trustworthy sites, I got an answer: 

During El Niño, the trade winds across the Caribbean get stronger, which results in the Caribbean coast getting more rainfall and the Pacific coast being drier.  From looking at NOAA satellite images, it looks like this is what is happening, especially in Costa Rica and Panama (as well as Nicaragua).

I know that El Niño is just one factor with this, and that there are others (such as global warming and the MJO) that are probably influencing this.  But, I thought it was interesting, so I figured I'd share it with you.