The X axis is the day of the year. The y axis is the average temperature across the entire surface area of the the sea (between -60 to 60 degrees) on that day.

The X axis is the day of the year. The y axis is the average temperature across the entire surface area of the the sea (between -60 to 60 degrees) on that day. Each line is a different year. It sounds like you're interpreting "average" as being an aggregate over time, but it is actually an aggregate over space at a given point in time. The graph you're getting the 2015 number from is not comparable. It is only about the temperatures in the limited area in the Pacific where the El Nino effect happens.