Abstract. We compare the mass budget of the Arctic sea ice for 15 models
submitted to the latest Coupled Model Intercomparison Project (CMIP6),
using new diagnostics that have not been available for previous model
inter-comparisons. These diagnostics allow us to look beyond the standard
metrics of ice cover and thickness to compare the processes of sea ice
growth and loss in climate models in a more detailed way than has previously
been possible. For the 1960–1989 multi-model mean, the dominant processes causing annual ice
growth are basal growth and frazil ice formation, which both occur during
the winter. The main processes by which ice is lost are basal melting, top
melting and advection of ice out of the Arctic. The first two processes
occur in summer, while the latter process is present all year. The sea ice
budgets for individual models are strikingly similar overall in terms of the
major processes causing ice growth and loss and in terms of the time of
year during which each process is important. However, there are also some
key differences between the models, and we have found a number of
relationships between model formulation and components of the ice budget
that hold for all or most of the CMIP6 models considered here. The relative
amounts of frazil and basal ice formation vary between the models, and the
amount of frazil ice formation is strongly dependent on the value chosen for
the minimum frazil ice thickness. There are also differences in the relative
amounts of top and basal melting, potentially dependent on how much
shortwave radiation can penetrate through the sea ice into the ocean. For
models with prognostic melt ponds, the choice of scheme may affect the
amount of basal growth, basal melt and top melt, and the choice of
thermodynamic scheme is important in determining the amount of basal growth
and top melt. As the ice cover and mass decline during the 21st century, we see a
shift in the timing of the top and basal melting in the multi-model mean,
with more melt occurring earlier in the year and less melt later in the
summer. The amount of basal growth reduces in the autumn, but it increases
in the winter due to thinner sea ice over the course of the 21st
century. Overall, extra ice loss in May–June and reduced ice growth in
October–November are partially offset by reduced ice melt in August and
increased ice growth in January–February. For the individual models, changes
in the budget components vary considerably in terms of magnitude and timing
of change. However, when the evolving budget terms are considered as a
function of the changing ice state itself, behaviours common to all the
models emerge, suggesting that the sea ice components of the models are
fundamentally responding in a broadly consistent way to the warming climate. It is possible that this similarity in the model budgets may represent a
lack of diversity in the model physics of the CMIP6 models considered here.
The development of new observational datasets for validating the budget
terms would help to clarify this.
Grease ice in basin-scale sea-ice ocean models