Global and Regional Discrepancies between Early 20th Century Coastal Air and Sea-Surface Temperature Detected by a Coupled Energy-Balance Analysis


A major uncertainty in reconstructing historical sea-surface temperature (SST) before the 1990s involves correcting for systematic offsets associated with bucket and engine-room intake temperature measurements. A recent study used a linear scaling of coastal station-based air temperatures (SATs) to infer nearby SSTs, but the physics in the coupling between SATs and SSTs generally gives rise to more complex regional air-sea temperature differences. In this study, an energy-balance model (EBM) of air-sea thermal coupling is adapted for predicting near-coast SSTs from coastal SATs. The model is shown to be more skillful than linear-scaling approaches through cross-validation analyses using instrumental records after the 1960s and CMIP6 simulations between 1880-2020. Improved skill primarily comes from capturing features reflecting air-sea heat fluxes dominating temperature variability at high latitudes, including damping high-frequency wintertime SAT variability and reproducing the phase lag between SSTs and SATs. Inferred near-coast SSTs allow for inter-calibrating coastal SAT and SST measurements at a variety of spatial scales. The 1900–1940 mean offset between the latest SST estimates available from the UK Met Office, HadSST4, and SAT-inferred SSTs range between −1.6 ([−1.7, 1.4]; 95% c.i.)°C to 1.2 ([0.8, 1.6])°C across 10°×10° grids. When further averaged along the global coastline, HadSST4 is significantly colder than SAT-inferred SSTs by 0.20 ([0.07, 0.35])°C over 1900–1940. These results indicate that historical SATs and SSTs involve substantial inconsistencies at both regional and global scales. Major outstanding questions involve the distribution of errors between our inter-calibration model and instrumental records of SAT and SST as well as the degree to which coastal inter-calibrations are informative of global trends.