Graduation of Stefanie Nanninga

Floating ice shelves regulate Antarctic ice sheet mass loss by buttressing upstream land ice discharge towards the ocean. Frontal iceberg calving following rift propagation and basal thinning by oceanic melting both result in a reduction of this buttressing effect, and an acceleration in global sea level rise. The extend of this ice shelf change is however poorly understood, also because km-wide rift propagation is unpredictable and generally not resolved in coarse climate models. The sub-shelf ocean circulation is believed to play an important role in rift propagation as it is closely related to ice shelf melting and freezing. A rift filled with marine ice trough oceanic freezing could become mechanically resistant to calving stresses, while shelf thinning by oceanic melting possibly could enhance iceberg formation. Recent studies suggest that the sub-shelf circulation of an intact ice shelf is altered by the presence of km-wide basal channels, indicating that a comparable process might occur beneath a rifted shelf. As it could impact freezing and melting while presumably altering rift propagation, the potential effect of basal channels on a rifted Antarctic ice shelf should be explored.
 
This study applies the Massachusetts Institute of Technology general circulation model to, for the first time, explore the potential effect of basal channels on the melting and freezing of a rifted Antarctic ice shelf. As it is the first to investigate such a physical process, this research is limited to only varying the inclusion of basal channels and a rift in the ice-ocean interface of an idealized high-resolution ice shelf model while performing four sensitivity runs: channels, rift, both, and none. As previous studies showed that basal channels alter melting, freezing and sub-shelf circulation; these physical aspects of the sensitivity runs are compared. Results show that, in accordance with previous research, basal channels decrease basal melting. In addition to this knowledge, this study finds that basal channels increase the freezing inside a rift. A sub-shelf boundary current on the Coriolis favoured side of the domain is reformed to a clockwise circulation in each channel, resulting in an adjusted flow pattern inside the rift from one single large clockwise return flow to a smaller one behind each channel. Buoyant cold shelf meltwater then does not only enter the rift in the boundary current but after every topographic incision, thereby increasing the thermal forcing. Furthermore, the multiple return flow pattern enlarges the average frictional velocity inside the rift, which is positively related to the freezing rate intensity. The influence of the thermal forcing is approximately three times larger than the friction velocity.
 
The presence of basal channels thus decreases rifted ice shelf melting and increases freezing inside rifts. These observations possibly imply that fracture propagation and iceberg formation are delayed through a potential increase in marine ice accretion, and ice shelf thinning and front retreat could be counteracted. Given these relations, this study stresses the importance of including basal channels and rifts in ice shelf models to robustly reproduce Antarctic sub-shelf circulation and basal melt.