Why Did Large Differences Arise in the Sea Surface Temperature Datasets across the Tropical Pacific during 2012?

During June-November 2012, pronounced differences in tropical Pacific sea surface temperature (SST) anomalies were observed between three widely used SST products: the extended reconstructed SST version 3b (ERSSTv3b), and the optimum interpolation SST version 2 analyses (OISST), produced weekly (OISSTwk) and daily (OISSTdy). During June-August 2012, the Nino-3.4 SST anomaly (SSTA) index was 0.2 degrees-0.3 degrees C lower in ERSSTv3b than in OISSTwk and OISSTdy, while it was 0.3 degrees-0.4 degrees C higher from September to November 2012. Such differences in the Nino-3.4 SSTA index can impact the assessment of the status of the El Nino-Southern Oscillation, which is determined using a threshold of 0.5 degrees C in the Nino-3.4 SSTA index.To investigate the reasons for the differences between ERSSTv3b and OISSTdy/OISSTwk, an experimental analysis (called ERSSTsat) is created that is similar to ERSSTv3b but includes satellite-derived SSTs. However, significant differences in the Nino-3.4 SSTA index remained between ERSSTsat and OISSTdy/OISSTwk. Comparisons of ERSSTsat and OISSTdy indicate that their differences are mostly associated with the different schemes for bias adjustment applied to the satellite-based SSTs. It is therefore suggested that the differences in the Nino-3.4 SSTA index between ERSSTv3b and OISSTdy cannot be solely due to the inclusion of but by the bias adjustment methodology of satellite data in OISSTdy.Finally, the SST products are compared with observations from ships, buoys, and satellites. On the monthly time scale, the area-averaged Nino-3.4 SSTA index in the tropical Pacific is more consistent with in situ observations in ERSSTv3b than in OISSTdy. In contrast, pointwise observations across the tropical Pacific are more consistent with OISSTdy than ERSSTv3b. It is therefore suggested that the differences among SST products are partially due to a structural uncertainty of various SST estimates.