Effects of instrumentation changes on ocean surface temperature measured in situ

Measurements of ocean surface temperature are an important climate record, complementing terrestrial air temperature observations, records of marine air temperature, ocean subsurface temperatures and ocean heat content. Sea-surface temperature (SST) has been measured since the mid-18th Century, although observations are sparse in the early period. Historically, marine observing systems relied on observations made by seafarers and necessary information on measurement methods is often not available. There are many historical descriptions of observing practice and instrumentation, some including quantification of biases between different methods. This documentation has been used, with the available observations, to develop models for the expected biases which vary according to how the measurement were made, over time and with the environmental conditions. Adjustments have been developed for these biases and some gridded SST datasets adjust for these differences and provide uncertainty estimates, including uncertainties in the bias adjustments. The modern in situ SST observing system continues to evolve and now includes many observations from moored and drifting buoys which must be characterised relative to earlier observations to provide a consistent record of multi-decadal changes in SST.