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The ER-M-Climate is derived from a set of extended range re-forecasts created using the same calendar start dates over several years for data times either side of the time of the extended ENS run itself.  The re-forecast runs are at the same resolution as the extended medium range run itself (currently ~18km up to day15 and ~36km thereafter) and run over the 46-day extended range ENS period.   

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• 2m temperature.
• soil temperature.
• sea-surface temperature.
• mean sea level pressure.
• precipitation. 

Different reference periods for M-Climate and ER-M-Climate

ECMWF uses different reference periods but essentially the same re-forecast runs

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to build the M-Climate and the ER-M-Climate.   The key difference is that those runs are grouped and used in different ways. This is because they are used in different ways. 

• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails.  It has been shown that using 1980 realisations (spanning 4 weeks) achieves this much better than using 660 (spanning 1 week). 
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by spanning 1 week rather than 4.  The extended range uses week-long averaging and the tails should not be so prone to having a reduced sample size.