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. Rapid changes are occurring in the Arctic, including areduction in sea ice thickness and coverage and a shift towards younger andthinner sea ice. Snow and sea ice models are often used to study theseongoing changes in the Arctic, and are typically forced by atmosphericreanalyses in absence of observations. ERA5 is a new global reanalysis thatwill replace the widely used ERA-Interim (ERA-I). In this study, we comparethe 2 m air temperature (T2M), snowfall (SF) and total precipitation (TP)from ERA-I and ERA5, and evaluate these products using buoy observationsfrom Arctic sea ice for the years 2010 to 2016. We further assess how biases inreanalyses can influence the snow and sea ice evolution in the Arctic, whenused to force a thermodynamic sea ice model. We find that ERA5 is generallywarmer than ERA-I in winter and spring (0–1.2  ∘ C), but colderthan ERA-I in summer and autumn (0–0.6  ∘ C) over Arctic sea ice.Both reanalyses have a warm bias over Arctic sea ice relative to buoyobservations. The warm bias is smaller in the warm season, and larger in thecold season, especially when the T2M is below −25 ∘ C in theAtlantic and Pacific sectors. Interestingly, the warm bias for ERA-I and newERA5 is on average 3.4 and 5.4  ∘ C (daily mean),respectively, when T2M is lower than −25 ∘ C. The TP and SF alongthe buoy trajectories and over Arctic sea ice are consistently higher in ERA5than in ERA-I. Over Arctic sea ice, the TP in ERA5 is typically less than 10 mm snow water equivalent (SWE) greater than in ERA-I in any of the seasons, while the SF in ERA5 canbe 50 mm SWE higher than in ERA-I in a season. The largest increase inannual TP (40–100 mm) and SF (100–200 mm) in ERA5 occurs in the Atlanticsector. The SF to TP ratio is larger in ERA5 than in ERA-I, on average 0.6for ERA-I and 0.8 for ERA5 along the buoy trajectories. Thus, thesubstantial anomalous Arctic rainfall in ERA-I is reduced in ERA5,especially in summer and autumn. Simulations with a 1-D thermodynamic sea icemodel demonstrate that the warm bias in ERA5 acts to reduce thermodynamicice growth. The higher precipitation and snowfall in ERA5 results in athicker snowpack that allows less heat loss to the atmosphere. Thus, thelarger winter warm bias and higher precipitation in ERA5, compared withERA-I, result in thinner ice thickness at the end of the growthseason when using ERA5; however the effect is small during the freezingperiod.

the cryosphere

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