alt Hacker NewsI'm not an authority on this, but here's is my understanding - I'd appreciate if someone could correct my mistakes.
The baseline of 0.0 represents the average of all years. Anything above / below the baseline is a (standard) deviation from the average. The blue lines are the individual years since 1991 [1] while the red line is the year 2026.
If a line is above the baseline, then the sea-surface temperature was hotter on that day than average. If below, it was cooler than average.
The year 2026 is an outlier, dwarfing all the others starting around June / July. The Nino 3.4 sea-surface temperature is significantly hotter than any previous year during that time. New record, I guess?
[1]: I'm confused about the two date ranges given: 1982-2026 and 1991-2020. I'm assuming this graph is based on measurements from 1982-2026 to calculate the average, but the lines shown are only from 1991-2020, for some statistical reason I don't understand.
I think it is the other way around: SD is calculated from 1982-2020, while all measurement readings in the plot are 1982-2026. I believe this is meant to not introduce an unwanted shift but compare to sort of a 'stable process'. However, that should have been described and argued somewhere.