For SIP:
My model predicts SIC and I have an ensemble, so I computed SIP as requested: I first converted September mean SIC into SIE for each ensemble member. Then I averaged the ensemble across the September mean SIE. Hence, my SIP is the probability of sea ice cover in my ensemble, and it ranges from 0 to 100%
For IFD
I have daily SIC output for each ensemble member, so for each ensemble, I computed the first ice-free day when SIC falls below 15% for all points where there is at least 15% SIC on the day I initialized the model. If the point is ice free (SIC<15%) at initialization, I let IFD be 182 (Jul 1). If the point is always covered in ice (SIC>=15%), I let IFD be 273 (Sep 31). I then computed the average and standard deviation of IFD across the ensemble.