Seasonal mean 500mb geopotential height of IFS hindcasts. Each hindcast is 13-months long integration with 01Nov initial conditions.
Data under each year consist of November-December of that year and January-November of the next year. For example, DJF
under 1960 is average of Dec(1960), Jan(1961) and Feb(1961), SON under 1960 is average of Sep(1961), Oct(1961) and
Nov(1961) and so on.
For technical reasons, Nov 30 is omitted in SON means of leap years.