An oceanic boundary layer model to understand near-surface temperature gradients for satellite temperature retrievals

Because of their spatial coverage and time history extending back over four decades, satellite retrievals of sea surface temperature (SST) are important to benchmark climate warming. However, the task of homogenization of satellite climate records is complicated by artefacts linked with stratified boundary layers on different depth and time scales, and also by the characteristic depth scales of the measuring instruments. To first order, the oceanic skin temperatures measured from earth orbit are biased cool by a few tenths of Kelvin with respect to in situ measurements at depths of centimeters-to-metres that have traditionally been used for comparison and calibration. This ‘cool skin’ effect is due to the fast response of the surface skin layer to surface heat flux loss compared with the vertical heat transport of mixing processes that are driven by wind speed. An additional consideration for satellite SST retrievals is the ‘warm layer effect’: the near surface stratification caused by solar heating during daytime, especially during light winds. This contribution presents an analysis of the modified Kantha and Clayson one-dimensional diffusion model to simulate the formation and evolution of the oceanic boundary layer on diurnal time scales with the aim of quantifying the ‘cool skin’ and ‘warm layer’ effects for satellite SST applications. The upper ocean model is forced by ERA5 reanalysis data over short time segments of two days, i.e., long enough to spin up the geophysical mixing processes and obtain a complete diurnal temperature cycle. Preliminary results illustrate how near surface diurnal warming effects vary as a function of wind speed and surface heat flux. The model is used estimate satellite SST bias with respect to in situ measurements from a global match-up data base from January 2024.