Yes, its worth checking out Relative Nino Index. However, this should be historically verified for a long period (30years).

The base period changes every 5 years for the ERSSTv5 dataset, as outlined in Huang et al. (2017, J. Climate).

At present the base period for calculating SST anomalies is updated every 5 years to account for the long-term warming trend in the ocean. This ensures that El Niño and La Niña anomalies are reflective of current climate conditions rather than being skewed by historical temperature averages. The current base period used is from 1991 to 2020.

By updating the base period, the calculation avoids the issue where a fixed historical base period would increasingly define conditions as El Niño due to the gradual warming of the ocean. For instance, using an outdated base like 1856-1885 would result in almost perpetual El Niño conditions under current sea temperatures.

The base period from 1991-2020 is used and this will change to 1996-2025 in a years time.

This method has ensured that the ONI remains relevant and accurate in diagnosing ENSO phases amidst a changing climate.