We use a reanalysis and hindcast dataset from NorCPM, which combines the Norwegian Earth system model (Bentsen et al. 2013) and an ensemble Kalman filter (Evensen 2003). This system version is comparable to the one providing operational forecast, namely, it assimilates sea-surface temperature and hydrographic profiles (temperature and salinity) using 60 members and strongly coupled data assimilation between the ocean and sea ice component (Bethke et al. 2021) - meaning that the ocean data correct also the sea ice component. We perform anomaly assimilation, meaning that the climatological monthly mean of the observations and the model are removed before comparing the two. The monthly climatology is constructed from the 60-member historical ensemble run (without assimilation) over the period 1982--2010. For the hydrographic profiles, it is constructed from EN4 objective analysis (Good et al. 2013). Only the ocean and sea ice are directly updated by the data assimilation. The other components of the model (atmosphere, land) are adjusting dynamically through the coupling in between the monthly assimilation steps. The initial ensemble at the start of the reanalysis in 1980 is constructed by selecting 60 random initial conditions from a stable pre-industrial simulation and integrating the ensemble from 1850 to 1980 using historical forcings from the Coupled Model Intercomparison Project version 5 (Taylor et al. 2012). The seasonal hindcasts start on the 15th January, April, July, and October each year from 1985--2010, i.e., in total 104 hindcasts (26 years with four hindcasts per year). Each hindcast runs 60 realizations (ensemble members) for 13 months, initialized from the corresponding member in the reanalysis. Data is organized with a separate folder for each hindcast start date. There is a file for each member.
