dt is the timestep in seconds for the ice model thermodynamics. The thermodynamics component is stable but not necessarily accurate for any value of the timestep. The value chosen for dt depends on the stability of the transport and the grid resolution. A conservative estimate of dt for the transport using the upwind advection scheme is:
min(Δx, Δy)  
Δt < 

(1) 
4 max(u, v) 
Maximum values for dt for the two standard CESM POP grids, assuming
max(u, v) = 0.5m/s
Grid 
min(Δx, Δy)

maxΔt

gx3v5  28845.9 m  4.0 hr 
gx1v3  8558.2 m  1.2 hr 
Occasionally, ice velocities are calculated that are larger than what is assumed when the model timestep is chosen. This causes a CFL violation in the transport scheme. A namelist option was added (xndt_dyn) to subcycle the dynamics to get through these instabilities that arise during long integrations. The default value for this variable is one, and is typically increased to two when the ice model reaches an instability. The value in the namelist should be returned to one by the user when the model integrates past that point.