Abstract/Summary

Worldwide sea surface temperatures (SST) have increased on average by about 1oC since 1900 with the exception of a region of the North Atlantic subpolar gyre (SPG) near 50oN which has cooled by up to 0.9oC over the same period, generating the negative feature on temperature anomaly maps which has been colloquially described by Rahmstorf et al. (2015) as the ‘cold blob’ (abbreviated here CB). This unique long term surface cooling trend is most evident in February but in August net warming is observed even at CB epicentre and the CB itself is reduced to a mere ‘warming hole’. These seasonal changes in the intensity of the CB are the product of two separate factors: (1) a long term winter cooling specific for the CB region which appears to be associated with cooling of Greenland coastal waters in autumn, plausibly linked to summer meltwater from icebergs and sea ice and (2) summer warming effects which derive from (a) dramatic reductions in summer sea ice cover in the sub‐Arctic over the last 30 years that allows enhanced absorption of sunlight by the new open water in summer and (b) an unusual period of increased summer sub‐Arctic ice cover in the early 20th century which lowers the SST baseline measured from 1900, thus increasing the calculated linear rate of change of SST with time. Both of these effects could contribute to the observed Arctic amplification of warming.