Since AR4, instrumental biases in upper - ocean temperature records have been identified and reduced, enhancing confidence in the assessment of change. (ossfoundation.us)
Reference: D. E. Parker, C. K. Folland and M. Jackson, 1995, MARINE SURFACE TEMPERATURE: OBSERVED VARIATIONS AND DATA REQUIREMENTS, Climatic Change 31: 559 - 600 here C. K. Folland and D. E. Parker, 1995, CORRECTION OF INSTRUMENTAL BIASES IN HISTORICAL SEA SURFACE TEMPERATURE DATA, Q.J.R. Meteorolol. (climateaudit.org)
We argue that the abrupt temperature drop of ~ 0.3 °C in 1945 is the apparent result of uncorrected instrumental biases in the sea surface temperature record [since it is only apparent in SSTs]. (skepticalscience.com)