Representing the break-up of tabular icebergs in models

Approximately half of the freshwater flux from the Antarctic Ice Sheet into the Southern Ocean occurs in the form of icebergs that calve off the ice sheet’s glaciers and ice shelves. A major shortcoming in current iceberg models is that they typically do not capture the decay of tabular icebergs, even though these giant icebergs account for the vast majority of the iceberg freshwater flux in the Southern Ocean. This shortcoming is partially due to the difficulty of adequately modelling breakup due to brittle fracturing, which is the dominant decay process for tabular icebergs. While the general drift trajectories of small and large icebergs have been modelled successfully in the past for both hemispheres, large Antarctic icebergs in models consistently survive too long and travel too far north compared with observations. Here, we address this issue by including an idealized representation of fracturing using a stochastic breakup scheme. We optimize the parameters of this breakup scheme by forcing the iceberg model with an ocean state estimate and comparing the distribution of iceberg trajectories and areas with the Antarctic Iceberg Tracking Database, which contains satellite-derived data for over 500 tabular icebergs during the period 1992-2019. The iceberg model is computationally inexpensive, and we simulate the trajectory of each individual iceberg a large number of times to resolve the effects of the stochastic breakup scheme. We show how including a representation of stochastic iceberg fracturing affects the meltwater distribution in the Southern Ocean. The results also have implications for how iceberg distributions may change in a warming climate.