Comparison of ERA5 and ERA-Interim near surface air temperature and precipitation over Arctic sea ice: Effects on sea ice thermodynamics and evolution

Abstract

Abstract. Rapid changes are occurring in the Arctic, including a reduction in sea ice thickness and coverage and a shift towards younger and thinner sea ice. Snow and sea ice models are often used to study these ongoing changes in the Arctic, and are typically forced by atmospheric reanalyses in absence of observations. ERA5 is a new global reanalysis that will replace the widely used ERA-Interim (ERA-I). In this study, we compare the 2 m air temperature (T2M) and precipitation between ERA I and ERA5, and evaluate these products using buoy observations from Arctic sea ice. We further assess how biases in reanalyses influence the snow and sea ice evolution in the Arctic, when used to force a thermodynamic sea ice model. We find that both reanalyses have a warm bias over Arctic sea ice in relation to the buoy observations. The warm bias is smaller in the warm season, and larger in the cold season, especially when the T2M is lower than −25 °C. Interestingly, the warm bias in the new ERA5 is on average 2.1 °C (daily mean) larger than ERA-I during the cold season. While ERA-I is drier than most modern reanalyses in the Arctic, the total precipitation along the buoy trajectories is often lower in ERA5 than in ERA-I. Nonetheless, the snowfall products are broadly similar for both ERA I and ERA5. ERA-I had substantial anomalous Arctic rainfall, which is greatly reduced in ERA5. Simulations with a freezing degree days (FDD) model and a 1D thermodynamic sea ice model demonstrate that the warm bias in ERA5 acts to reduce thermodynamic ice growth. However, the lower precipitation in ERA5 results in a thinner snow pack that allows more heat loss to the atmosphere. Thus, the larger warm bias and lower precipitation in ERA5, compared with ERA I, compensate in terms of the effect on winter ice growth. Ultimately, we find slightly thicker ice at the end of growth season when using ERA5 forcing, compared with ERA-I. Thus differences in the precipitation fields of the two reanalyses have a larger influence on the sea ice evolution than the T2M.