According to a NOAA page "Major improvements include updated and substantially more complete input data from the 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, revised low-frequency data filing in data sparse regions using nearby available observations, updated bias adjustments of ship SSTs using Hadley Nighttime Marine Air Temperature version 2 (HadNMAT2), and buoy SST bias adjustments not previously made in v3b."
The two versions are compared in some graphs and maps below.
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