An inter-comparison of the mass budget of the Arctic sea ice in CMIP6 models

Abstract

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.

Impacts of Sea Ice Mushy Thermodynamics in the Antarctic on the Coupled Earth System

“…Instead, the ice‐ocean salt and freshwater exchange are directly related to the changes in ice volume. This is an important limitation in the current model, but this approach to the ice‐ocean coupling is consistent with other state‐of‐the‐art Earth‐system‐models (see model descriptions in Keen et al., 2021). However, it is clearly unrealistic and does not take full advantage of the improved sea ice salinity processes available in MUSHY.…”

An improved regional coupled modeling system for Arctic sea ice simulation and prediction: a case study for 2018

“…The initialization of melt pond based on the observations (e.g., Ding et al, 2020) in CAPS is a direction to improve the representation of the ice surface properties. Second, the mass budget analysis by both Keen et al (2021) and this study show that the contribution of a lateral melting term is relatively small, which might be due to the fact that CMIP6 models and CAPS assume constant floe size (i.e., 300 m in CICE), which is a critical value to determine the strength of lateral melting (e.g., Horvat et al, 2016;Steele, 1992). Recently, several studies have proposed floe size distribution models (e.g., Bateson et al, 2020;Bennetts et al, 2017;Boutin et al, 2020;Horvat and Tziperman, 2015;Roach et al, 2018Roach et al, , 2019Zhang et al, 2015Zhang et al, , 2016.…”

“…The excessive basal growth may be partly due to the fact that the bias of the CFS data propagates into the model domain through lateral boundary conditions and its accumulated effect influences Arctic sea ice simulation during the freeze-up period. Keen et al (2021) analyzed the Arctic mass budget of 15 models participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). We notice that, first, the top melting and the basal melting terms in CMIP6 models have comparable contributions in July, while the top melting term only has ∼ 50 % contribution relative to the basal melting term in CAPS.…”

An energy budget approach to understand the Arctic warming during the Last Interglacial

“…It is the consequence of the substancial loss of sea ice during the previous summer which is exacerbated by the poor recovery of the sea ice cover in autumn. On figure 17, we can also note that sea ice advection is surprisingly low, while Keen et al (2021) show that it is one of the main factor of the mass budget variations.…”

“…To better understand the causes of this decline, we compute the mass budget terms anomalies using new diagnostics developed for the lastest Coupled Model Intercomparison Project CMIP6 (Notz et al, 2016;Keen et al, 2021). The mass budget terms analysed here are the following: the basal growth, the ice formation in supercooled open water (or frazil), the melting at the top surface of the ice, the melting at the base of the ice, ice formation due to the transformation of snow to sea ice, the change in ice mass due to evaporation and sublimation and the ice advection into or out of the Arctic domain.…”