I am using the ONI index to classify the years of high rainfall seasons (December–February) as El Niño, Neutral or La Niña. El Niño year is classified as one if the ONI index value is +0.5 °C for 5 consecutive overlapping seasons.
My question since my season in between two Gregorian calendar years, what seasons should I check for the +0.5 °C threshold? Is it the preceding seasons or should I check since the beginning of year. I ask this because I am interested in investigating how El Niño might affect rainfall during a season.