Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5): Upgrades, Validations, and Intercomparisons

The monthly global 2°×2° Extended Reconstructed Sea Surface Temperature (ERSST) for 1854–2015 has been comprehensively revised and updated from version 4 to version 5. This update incorporates a new release of ICOADS R3.0, a decade of near-surface data from Argo floats, and a new estimate of centennial sea-ice from HadISST2. A number of choices in aspects of quality control, bias adjustment and interpolation have been substantively revised. The resulting ERSST estimates are less spatio-temporally smoothed, have better representation of high latitude SSTs, and ship SST biases are now calculated relative to more recent buoy measurements.

Progressive experiments have been undertaken to highlight the effects of each change in data source and/or analysis technique upon the final product. The single largest impact is that the reconstructed SST absolute series is systematically decreased by 0.077°C over the global oceans throughout the record owing to the change in ship SST final bias correction to be relative to modern buoys. Furthermore, high latitude SSTs are decreased by 0.1°–0.2°C by using sea-ice concentration from HadISST2. Changes arising from remaining innovations are mostly important at small space and time scales, primarily having an impact where and when input observations are sparse. Cross-validations and verifications with independent observation data show that the updates incorporated in ERSSTv5 have improved the representation of spatial variability over the global oceans, the magnitude of El Niño and La Niña events, and the decadal nature of SST changes in 1930s–40s when observation instruments changed rapidly. Both long (1900–2015) and short (2000–2015) term SST trends in ERSSTv5 remain significant as in ERSSTv4. Global mean anomaly series behavior, used in climate monitoring, remains largely unchanged between the two versions across a range of timescales and changes fall well within quantified uncertainties in the v4 product.