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• the mean of the anomalies of a forecast product relative to a reference model climate and
• the mean of the anomalies of observations or reanalysis relative to the same reference model climate.
• For medium range forecasts the anomaly correlation coefficient is evaluated between:
  
  • the mean of the anomaly of the forecast product relative to the medium range model climate (M-climate) and
  • the mean of the anomaly of the verifying control analysis relative to the medium range model climate (M-climate).
• For sub-seasonal range products the correlation is evaluated between:
  • the mean of the anomaly of the forecast product measured relative to the sub-seasonal range model climate (SUBS-M-climate) and
  • the mean of the anomaly of the verifying control analysis relative to the sub-seasonal range model climate (SUBS-M-climate).
• For seasonal products the correlation is evaluated between:
  • the mean of the anomaly of the forecast product relative to the a model climatology based on the ERA-interim re-analysis (based on the period 1993-2016) and
  • the mean of the anomaly of the verifying observations or reanalysis relative to the seasonal model climate (S-M-Climate). 

The medium range model climate (M-climate) and sub-seasonal range model climate (SUBS-M-climate) are based on re-forecasts spanning the last 20 years, which used the ERA5 reanalysis for their initialisation

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