Extended Reconstructed Sea Surface Temperature Version 4 (ERSST.v4), Part I. Upgrades and Intercomparisons

The monthly Extended Reconstructed Sea Surface Temperature (ERSST) dataset has been revised herein to version 4 (v4) from v3b. Major revisions include: updated and substantially more complete input data from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) Release 2.5; revised Empirical Orthogonal Teleconnections (EOTs) and EOT acceptance criterion; updated sea surface temperature (SST) quality control procedures; revised SST anomaly (SSTA) evaluation methods; updated bias adjustments of ship SSTs using Hadley Nighttime Marine Air Temperature version 2 (HadNMAT2); and a buoy SST bias adjustment not previously made in v3b.

Tests showed that the revisions to ship SST bias adjustment in ERSST.v4 are dominant among all revisions and updates. The effect is to make SST 0.1°C-0.2°C lower north of 30°S but 0.1°C-0.2°C higher south of 30°S in ERSST.v4 than in ERSST.v3b before 1940. In comparison with the UK Met Office SST product (HadSST3), the ship SST bias adjustment in ERSST.v4 is 0.1°C-0.2°C lower in the tropics but 0.1°C-0.2°C higher in the mid-latitude oceans both before 1940 and from 1945 to 1970. The 1901-2013 global SST trend changed from 0.71°C/century in v3b to 0.73°C/century in ERSST.v4. The tests also showed that, when compared with v3b, SSTAs in ERSST.v4 can substantially better represent the El Niño/La Niña behavior when observations are sparse before 1940.