On the discretization of the ice thickness distribution in the NEMO3.6-LIM3 global ocean–sea ice model

Abstract

The ice thickness distribution (ITD) is one of the core constituents of modern sea ice models. The ITD accounts for the unresolved spatial variability of sea ice thickness within each model grid cell. While there is a general consensus on the added physical realism brought by the ITD, how to discretize it remains an open question. Here, we use the ocean-sea ice general circulation model, Nucleus for European Modelling of the Ocean (NEMO) version 3.6 and Louvain-la-Neuve sea Ice Model (LIM) version 3 (NEMO3.6-LIM3), forced by atmospheric reanalyses to test how the ITD discretization (number of ice thickness categories, positions of the category boundaries) impacts the simulated mean Arctic and Antarctic sea ice states. We find that winter ice volumes in both hemispheres increase with the number of categories and attribute that increase to a net enhancement of basal ice growth rates. The range of simulated mean winter volumes in the various experiments amounts to ∼ 30 % and ∼ 10 % of the reference values (run with five categories) in the Arctic and Antarctic, respectively. This suggests that the way the ITD is discretized has a significant influence on the model mean state, all other things being equal. We also find that the existence of a thick category with lower bounds at ∼ 4 and ∼ 2 m for the Arctic and Antarctic, respectively, is a prerequisite for allowing the storage of deformed ice and therefore for fostering thermodynamic growth in thinner categories. Our analysis finally suggests that increasing the resolution of the ITD without changing the lower limit of the upper category results in small but not negligible variations of ice volume and extent. Our study proposes for the first time a bi-polar process-based explanation of the origin of mean sea ice state changes when the ITD discretization is modified. The sensitivity experiments conducted in this study, based on one model, emphasize that the choice of category positions, especially of thickest categories, has a primary influence on the simulated mean sea ice states while the number of categories and resolution have only a secondary influence. It is also found that the current default discretization of the NEMO3.6-LIM3 model is sufficient for large-scale present-day climate applications. In all cases, the role of the ITD discretization on the simulated mean sea ice state has to be appreciated relative to other influences (parameter uncertainty, forcing uncertainty, internal climate variability).

Resolving and Parameterising the Ocean Mesoscale in Earth System Models

“…This can be considered akin to a resolution increase for sea ice models and can have a considerable impact on heat exchanges over, and through, sea ice (Komuro and Suzuki, 2013). Inclusion of a prognostic ITD has been shown to have a considerable impact on sea ice evolution and feedback within climate models [128]. For example, enhancement of the (positive) ice-albedo feedback, coupled with suppression of the (negative) thickness-growth and thickness-strength (i.e.…”

Impact of the ice thickness distribution discretization on the sea ice concentration variability in the NEMO3.6–LIM3 global ocean–sea ice model

“…In the third set, the lower boundary of the thickest category is set as 4 m thick, and the ITD resolution is increased or reduced by merging or splitting existing categories. The upper limit at 4 m thick corresponds to the maximum thickness that thermodynamic ice growth can sustain in the Arctic (Maykut and Untersteiner, 1971) and therefore allows the thickest category to host the deformed ice produced in the model. For more details of the ITD and these experiments we refer to Massonnet et al (2019).…”

PARASO, a circum-Antarctic fully coupled ice-sheet–ocean–sea-ice–atmosphere–land model involving f.ETISh1.7, NEMO3.6, LIM3.6, COSMO5.0 and CLM4.5

“…Subgrid-scale sea-ice heterogeneity is rendered with a five-category ice-thickness distribution (Bitz et al, 2001); i.e., each sea-ice field is five-fold, with distinct values over different ice-thickness ranges. The sea-ice-thickness category redistribution follows Lipscomb (2001), with intercategory boundaries located around 0.5, 1.15, 2 and 3.8 m. The choice of five categories comes from a trade-off between computational constraints and physical realism in sea-ice mean state and variability (Massonnet et al, 2019;Moreno-Chamarro et al, 2020). The sea-ice thermodynamics is based on an energy conserving scheme (Bitz and Lipscomb, 1999) and includes an explicit representation of the salt content of the sea ice .…”