Figure 2.

Change in selected 20CR uncertainty metrics through time

(Top) The inter-region mean ensemble spread (uncertainty in daily reconstruction of weather) for the tropics and the ACRE Antarctica domain in the 20CR version 3 (20CRv3) January to March. It shows progressive improvement for both regions through time from the mid-19^th to mid-20^th century. (Bottom) The mean ensemble spread ratio (ACRE Antarctica mean ensemble spread divided by the tropics mean ensemble spread) is a dimensionless index indicating that, despite overall 20CR improvement, there is still lower uncertainty for past daily weather in the tropics (possibly a result of greater density or greater consistency of observations in that region) relative to the high southern latitudes. It also shows that distinct seasonal differences for the mean ensemble spread uncertainty are lowest for the southern high latitudes in summer and worst for autumn and winter.