Correcting Observational Biases in Sea Surface Temperature Observations Removes Anomalous Warmth during World War II

Most historical sea surface temperature (SST) estimates indicate warmer World War II SSTs than expected from forcing and internal climate variability. If real, this World War II warm anomaly (WW2WA) has important implications for decadal variability, but the WW2WA may also arise from incomplete corrections of biases associated with bucket and engine room intake (ERI) measurements. To better assess the origins of the WW2WA, we develop five different historical SST estimates (reconstructions R1-R5). Using uncorrected SST measurements from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) version 3.0 (R1) gives a WW2WA of 0.41 degrees C. In contrast, using only buckets (R2) or ERI observations (R3) gives WW2WAs of 0.18 degrees and 0.08 degrees C, respectively, implying that uncorrected biases are the primary source of the WW2WA. We then use an extended linear-mixed-effect method to quantify systematic differences between subsets of SSTs and develop groupwise SST adjustments based on differences between pairs of nearby SST measurements. Using all measurements after applying groupwise adjustments (R4) gives a WW2WA of 0.13 degrees C [95% confidence interval (c.i.): 0.01 degrees-0.26 degrees C] and indicates that U.S. and U.K. naval observations are the primary cause of the WW2WA. Finally, nighttime bucket SSTs are found to be warmer than their daytime counterparts during WW2, prompting a daytime-only reconstruction using groupwise adjustments (R5) that has a WW2WA of 0.09 degrees C (95% c.i.: -0.01 degrees to 0.18 degrees C). R5 is consistent with the range of internal variability found in either the CMIP5 (95% c.i.: -0.10 degrees to 0.10 degrees C) or CMIP6 ensembles (95% c.i.: -0.11 degrees to 0.10 degrees C). These results support the hypothesis that the WW2WA is an artifact of observational biases, although further data and metadata analyses will be important for confirmation. SIGNIFICANCE STATEMENT Major observational sea surface temperature (SST) estimates show a warm anomaly during World War II (WW2) that exceeds the warming expected from internal variability and known climate forcing. We systematically intercompare different groups of SST observations and trace the origin of the WW2 warmth foremost to anomalously warm U.S. and U.K. naval observations. We also find that nighttime bucket SSTs are anomalously warm, likely because of being measured inboard to avoid light pollution. SST estimates adjusted for these systematic biases give a more stable and smoothly evolving record of historical warming with a WW2 SST anomaly within the 95% range of internal variability found in an ensemble of general circulation model simulations.

Automated summary

The study suggests that the warm anomaly in sea surface temperature during World War II may be attributed to observational biases, particularly data from U.S. and U.K. naval observations. Adjusted SST estimates after systematic corrections provide a more stable record of historical warming trends, with the WW2 SST anomaly falling within the range of internal variability found in model simulations.