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All data is received from the contributing centres on a common 1x1 degree grid prescribed by C3S; the regridding from the original model resolution is done by each contributing centre. This does not apply to the data from JMA, which is provided on a 1.25x1.25 degree grid. The original grids of the modelling system components are listed here: Description of the C3S seasonal multi-system.

In general, the common hindcast period, 1993 - 2016, is used as the reference period for C3S data and graphical products, regardless of the hindcast period available for each individual component system (unless stated otherwise). 

Sub-daily/daily data on single or pressure levels

• Seasonal forecast daily and subdaily data on single levels
• Seasonal forecast subdaily data on pressure levels

This is the data at the original time resolution received from the contributing centres (once a day, or once every 6 hours for single levels, depending on the variable and once every 12 hours for pressure levels), and the data at this resolution is used to derive the monthly statistics.

Monthly Statistics

• Seasonal forecast monthly statistics on single levels
• Seasonal forecast monthly statistics on pressure levels

Monthly statistics are made available, in addition to the sub-daily/daily original time resolution. These include

1. Ensemble: ensemble mean, hindcast climate mean
2. Individual members: monthly mean, monthly minimum, monthly maximum, monthly standard deviation (note that this does not apply to all variables in all cases, E.g. no minimum for precipitation, or minimum, maximum and standard deviation for pressure level variables)

These monthly aggregations are calculated from the original sub-daily/daily data provided, for each calendar month and lead-time in the hindcasts and real-time forecasts. In the case of wind, the speed is first calculated from the instantaneous u and v components of the wind at the original time resolution, then aggregated into monthly statistics. The common hindcast period is used for estimating the hindcast climate mean. The ensemble mean for individual systems is an equal-weights average of all 'qualifying ensemble members' used for the graphical products (not all members are included for lagged start ensembles).

-qualifying ensemble members (= C3S ensemble of real-time forecasts): the subset of ensemble members used in the C3S graphical products; in some cases (lagged-start systems) this is a subset of the total forecast ensemble in the catalogue. The same ensemble members are used for all variables for which anomalies are provided, even though not all variables are part of C3S graphical products. The sub-selection is chosen as a compromise between ensemble size (which should be large) and the start/lead times used in the ensemble (which should not span too long a period). Details on the qualifying members for each origin are available in the documentation.

Anomalies (of monthly means)

• Seasonal forecast anomalies on single levels
• Seasonal forecast anomalies on pressure levels

Monthly anomalies are calculated for real-time forecasts, using the model hindcast mean as a reference. They are calculated separately for each system and each ensemble member, for each nominal start time and lead time, by subtracting the respective hindcast mean from the corresponding real time forecast. The period used as reference for the estimation of the model climatology is the common hindcast period.

The ensemble mean anomaly for individual systems is an equal-weights average of all 'qualifying' ensemble members (explained above).

Ocean variables

• Monthly averages of ocean variables

This data is provided by the contributing centres aggregated to a monthly time resolution.