Observed subdaily variations in air–sea turbulent heat fluxes under different marine atmospheric boundary layer stability conditions in the Gulf Steam

Based on data collected from 14 air–sea buoys in the Gulf Stream, this study presents an examination of how the hourly air–sea turbulent heat fluxes vary on subdaily timescales under persistent marine atmospheric boundary layer (MABL) stability conditions. The annual mean magnitudes of the subdaily variations in the latent heat (LH) and sensible heat (SH) at all stations are determined to be 40 and 15 Wm^-2, respectively. Under near-neutral conditions, the hourly anomalies of the air–sea humidity and temperature differences are the major drivers of the subdaily variations in LH and SH, respectively, followed by nonlinear effects and wind anomalies. Wind anomalies play dominant roles in determining the subdaily variations in LH and SH when the MABL is stable. In contrast, the contributions of the hourly anomalies of the air–sea differences in humidity and temperature are secondary but also significant. For a convectively unstable MABL, the wind anomalies control the subdaily variations in LH, whereas the subdaily variations in SH are dominated by the air–sea temperature anomalies. Accordingly, the above mechanism also controls the subdaily magnitudes. Quantitative estimates of the above relations are given in this study. However, compared to the observations when using daily mean SST, the subdaily variations in the reanalysis are found to be underestimated on average by 17% and 5% for LH and SH, respectively. Resolving the subdaily variations contributes significantly to the mean LH/SH estimates. For near-neutral and unstable MABLs, the subdaily contributions are O(100) and O(20) Wm^-2 for LH and SH, respectively, while they are O(10) Wm^-2 for LH/SH under stable conditions.