Both wind and seasonal CO2 ice sculpt the dunes of Mars in today's climate. The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter has returned extensive temporal coverage of changes on the north polar dunes for nearly four Mars years. The processes driving dune morphology changes such as the formation of new alcoves have been investigated. Considerable interannual variability has been observed. Most changes occur in the period of time when HiRISE cannot image: late summer and fall when light levels are too low to see subtle changes on the dunes and the polar hood obscures the surface, and winter when the cap is in polar night. This is consistent with seasonal control but does not allow us to directly differentiate between eolian processes vs. CO2 ice as the driving agent for alcove formation. Circumstantial evidence and observations of analog processes in the southern mid-latitudes however implicates processes associated with frost emplacement and removal.