I was going to blog about this cool new paper that my colleagues at DMI have produced, but John Kennedy has as always done such a good job I will just point you over there…
Wondering whether a warm bias in the Arctic in ERA5 affects our estimates of global temperature change.
When is an Arctic bias not an Arctic bias?
Sterna Paradisaea 
I recall the ERA5 warm but decreasing bias over the Arctic Ocean being discussed shortly after ERA5 came online, and the though was the ERA5’s simplistic slab sea ice thickness was the likely culprit, which is much thinner than 20th century thickness but closer to reality nowadays.
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