I will also add that even if the weekly was homogenized, we still wouldn't use it to define El Nino.  That's because El Nino is a seasonal (3 month average) climate phenomenon that is best identified on those timescales.  More on this topic is here:  https://www.climate.gov/news-features/blogs/enso/keep-calm-and-stop-obsessing-over-weekly-changes-enso