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), snowfall (SF) and total precipitation (TP)
from ERA-I and ERA5, and evaluate these products using buoy observations
from Arctic sea ice for the years 2010 to 2016. We further assess how biases in
reanalyses can influence the snow and sea ice evolution in the Arctic, when
used to force a thermodynamic sea ice model. We find that ERA5 is generally
warmer than ERA-I in winter and spring (0–1.2 ∘C), but colder
than 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 buoy
observations. The warm bias is smaller in the warm season, and larger in the
cold season, especially when the T2M is below −25 ∘C in the
Atlantic and Pacific sectors. Interestingly, the warm bias for ERA-I and new
ERA5 is on average 3.4 and 5.4 ∘C (daily mean),
respectively, when T2M is lower than −25 ∘C. The TP and SF along
the buoy trajectories and over Arctic sea ice are consistently higher in ERA5
than 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 can
be 50 mm SWE higher than in ERA-I in a season. The largest increase in
annual TP (40–100 mm) and SF (100–200 mm) in ERA5 occurs in the Atlantic
sector. The SF to TP ratio is larger in ERA5 than in ERA-I, on average 0.6
for ERA-I and 0.8 for ERA5 along the buoy trajectories. Thus, the
substantial anomalous Arctic rainfall in ERA-I is reduced in ERA5,
especially in summer and autumn. Simulations with a 1-D thermodynamic sea ice
model demonstrate that the warm bias in ERA5 acts to reduce thermodynamic
ice growth. The higher precipitation and snowfall in ERA5 results in a
thicker snowpack that allows less heat loss to the atmosphere. Thus, the
larger winter warm bias and higher precipitation in ERA5, compared with
ERA-I, result in thinner ice thickness at the end of the growth
season when using ERA5; however the effect is small during the freezing
period.
Evaluation of the Sea-Ice Simulation in the Upgraded Version of the Coupled Regional Atmosphere-Ocean- Sea Ice Model HIRHAM–NAOSIM 2.0
