Anatomy of Arctic and Antarctic sea ice lows in an ocean–sea ice model

Sea ice has exhibited a number of record lows in both hemispheres over the past two decades. While the causes of individual
sea ice lows have already been investigated, no systematic comparison across events and hemispheres has been conducted
in a consistent framework yet. Here, the global standalone ocean–sea ice model NEMO4.2.2-SI³ at 1/4° resolution is used
to decompose the sea ice mass budget. We separate the relative contributions of ice melt/growth and thermodynamic/dynamic
processes, both from a climatological perspective and for selected individual years. The seasonal cycles of Arctic and Antarctic
ice mass fluxes show similarities, such as the prevalence of basal growth and melt in the mass budget. The long-term evolution
of the mass budget terms reveals an increased importance of basal melt in both hemispheres, at the expense of surface and
lateral melt. Regarding sea ice lows, the model indicates that the Arctic summer 2007 anomaly was chiefly caused by dynamic
factors, while the Arctic summer 2012 event was rather explained by thermodynamic factors. The Antarctic summer 2022 event
was driven by dynamic processes transporting ice towards sectors where more melt than usual occurred. The Antarctic winter
2023 event was characterized by a lack of basal growth. This study emphasises the dominance of processes at the ice-ocean
interface in driving the ice mass evolution at all time scales considered here, and highlights the potential of the ice mass budget
decomposition to disentangle oceanic and atmospheric contributions in the evolution of the ice state in a changing climate.