ERA Interim problem with Waves
I haven't looked into ERA-Interim wave data a huge amount but I know around 1991 it started to assimilate ocean wave data from satellite altimeters (ERS-1).
See the first figure here
https://climatedataguide.ucar.edu/climate-data/era-interim

I remember Jean-Raymond Bidlot who runs WAM at ECWMF showing me so experiments with and without the assimilation into the wave. I think the Hs differed by ~1m in large events so not insignificant.