Diurnal Variability of Upper Ocean Temperature from the Surface to the Base of the Mixed Layer

Diurnal forcing has a significant impact on sea surface temperature (SST), and the effects of diurnal forcing can propagate downward through the mixed layer. In this study, diurnal temperature variability from the ocean surface to the base of the mixed layer is evaluated by pairing Argo temperature profiles with geographically co-located satellite sea surface temperature (SST) measurements from the Advanced Microwave Scanning Radiometer for Earth Observation System (AMSR-E). The satellite SSTs serve as a reference against which to assess diurnal variability within the Argo profiles. Results show significant diurnal variability at all latitudes near the ocean surface, with largest diurnal variations in the tropics. Amplitude attenuates with depth, and the time of maximum temperature is consistent with downward propagation of the signal from the surface. Somewhat surprisingly, diurnal temperature amplitudes are larger at the base of the mixed layer than they are at 5 m depth. Numerical experiments with a one-dimensional version of the K-Profile Parameterization (KPP) model indicate that the large diurnal variability below the mixed layer results from diurnal variation in the depth of the mixed layer. The effect of the diurnal cycle is significant, and because it governs entrainment across the base of the mixed layer, it influences the seasonal cycle of SST. These results have implications for mixed layer physics and for strategies used to calibrate and validate both in situ near-surface temperature measurements and satellite SST observations.