The Effect of Ice Shelf Meltwater on Antarctic Sea Ice and the Southern Ocean in an Earth System Model

Abstract:

It has been suggested that recent Antarctic sea ice expansion resulted from an increase in fresh water reaching the Southern Ocean. This presentation investigates this conjecture in an Earth System Model. The freshwater flux from ice sheet and ice shelf mass imbalance is largely missing in models that participated in the Fifth Coupled Model Intercomparison Project (CMIP5). However, CMIP5 models do account for the fresh water from precipitation minus evaporation (P-E). On average in CMIP5 models P- E reaching the Southern Ocean has increased to a present value of about 2600 Gt yr^-1 greater than pre-industrial times and 3-8 times larger than estimates of the mass imbalance of Antarctic ice sheets and shelves. Two sets of model experiments were conducted from 1980-2013 in CESM1-CAM5 artificially distributing fresh water either at the ocean surface according to an estimate of iceberg melt, or at the ice shelf fronts at depth. An anomalous reduction in vertical advection of heat into the surface mixed layer resulted in sea surface cooling at high southern latitudes, and an associated increase in sea ice area. A freshwater enhancement of 1780 Gt yr^-1 (approximately 1.3 times either present day basal melt or iceberg calving freshwater fluxes) raised the sea ice total area by 1×10⁶ km². Yet, even a freshwater enhancement up to 2670 Gt yr^-1 was insufficient to offset the sea ice decline due to anthropogenic forcing for any period of 20 years or longer. Further, the sea ice response was found to be insensitive to the depth of fresh water injection.