Assessing Air–Sea Surface Turbulent Heat Fluxes from Reanalyses Using Concurrent Shipboard Measurements

Abstract Air–sea surface turbulent heat fluxes are crucial to forming accurate weather and climate predictions. However, there is considerable disagreement among current reanalysis products. The study assesses biases in these products using concurrent shipboard measurements of air temperature, barometric pressure, relative humidity, and upper-ocean temperature taken along two transbasin expendable bathythermograph (XBT) transects in the North Atlantic between 2020 and 2022. Following a quality control procedure, the data from shipboard measurements and hourly ERA5 and 6-hourly NCEP-2 reanalysis data were collocated to the locations and time of the XBT profile data. Our analyses show that mean biases among the three products are not statistically significant for all variables, but the biases for individual cases can be large. XBT could reliably estimate SST in the mixed layer, with a good comparison with the reanalysis products. Additionally, the correlation between the shipboard measurements and reanalyses is high ( r > 0.9) for all variables except relative humidity (RH). Specifically, RH shows a correlation slightly lower between shipboard and ERA5 data ( r = 0.84) and much lower between NCEP-2 and the other products ( r = 0.42 for shipboard and r = 0.46 for ERA5 data). The collocated data were then used to compute the surface latent and sensible heat fluxes in four scenarios using the Coupled Ocean–Atmosphere Response Experiment flux algorithm version 3.6: in situ data with satellite winds, in situ data with ERA5 winds, ERA5 data, and NCEP-2 data. Overall, ERA5-based fluxes agree better with the shipboard estimates of both latent and sensible heat fluxes than NCEP-2-based fluxes, pointing to the importance of higher-resolution reanalysis to improve air–sea flux estimates.