# Flotation criterion, mask, and sea level¶

The most basic decision about marine ice sheet dynamics made internally by PISM is whether a ice-filled grid cell is floating. That is, PISM applies the “flotation criterion” [30] at every time step and at every grid location to determine whether the ice is floating on the ocean or not. The result is stored in the mask variable. The mask variable has pism_intent = diagnostic, and thus it does not need to be included in the input file set using the -i option.

The possible values of the mask are given in Table 23. The mask does not by itself determine ice dynamics. For instance, even when ice is floating (mask value MASK_FLOATING), the user must turn on the usual choice for ice shelf dynamics, namely the SSA stress balance, by using options -stress_balance ssa or -stress_balance ssa+sia.

Table 23 The PISM mask, in combination with user options, determines the dynamical model.
0 = MASK_ICE_FREE_BEDROCK ice free bedrock
2 = MASK_GROUNDED ice is grounded
3 = MASK_FLOATING ice is floating (the SIA is never applied; the SSA is applied if the ssa or ssa+sia stress balance model is selected
4 = MASK_ICE_FREE_OCEAN ice-free ocean
Assuming that the geometry of the ice is allowed to evolve (which can be turned off by option -no_mass), and assuming an ocean exists so that a sea level is used in the flotation criterion (which can be turned off by option -dry), then at each time step the mask will be updated.