Was the 2015 Subpolar North Atlantic Cold Blob Predictable?

Sea surface temperature (SST) in the subpolar North Atlantic reached near-record cold conditions during 2015 before rapidly warming to normal in 2016. 2015 was also remarkable for the persistently positive conditions of North Atlantic Oscillation (NAO), which was necessary for this cold blob’s evolution. Here, we examine how predictable this extreme event was using the Community Earth System Model Decadal Prediction Large Ensemble (CESM-DPLE). The 1-year prediction from the CESM-DPLE reproduces the multidecadal swings in observed SST and its 40 ensemble members encompass observed SST for every year – except for the 2015 cold blob. This predictive failure occurs within 6 months of initialization and despite already cold initial conditions. In addition, no ensemble members from the 2014 hindcast set have a persistently positive NAO. Analysis of observations and the uninitialized CESM Large Ensemble indicates that 60 ensemble members should be sufficient to produce 1-2 ensemble members with atmosphere and ocean conditions similar to those in 2015. An additional 20 ensemble members are produced for the 2014 hindcast to test if ensemble size is too small. None of the new ensemble members have a strong cold blob or persistently positive NAO conditions, suggesting that ensemble size is likely not the problem. We suspect that large biases in sea ice initial conditions may have impacted this prediction.