Synergistic Impact of Diurnal Warm Layers and Inertial Wave Mixing on Sea Surface Temperature Warming and Upper Ocean Stratification

We study two sea surface temperature (SST) warming events and upper ocean stratification changes in the northern South China Sea in 2022 using data from an EM-APEX float and satellite observations. The diurnal warm layers (DWLs) and the increasing buoyancy frequency N² above the top of the thermocline can restrict the penetration depth of nighttime convection and wind-driven mixing, which prevents cooler water from mixing upward, allowing solar heating to increase the SST by more than 1°C in a few days. The stratification budget approach is used to reproduce observations below 40 m despite some uncertainties in estimating variables such as horizontal gradient. After the first SST warming event, the stratification changes in the subsurface layers constituted by an increase in N² above 70 m and a decrease below this depth can be attributed to the combined effects of turbulent diffusion and vertical advection rather than to horizontal advection or penetrative solar radiation. This ocean interior mixing is likely caused by the shear of near-inertial waves at ∼50 m, when the nighttime convection could not penetrate through the DWL's base around 20 m. The stratification budget approach fails to simulate the changes above 40 m after the second SST warming event partly due to the presence of a near-surface freshwater layer. Our observations offer insights into the effect of inertial wave-induced mixing in the ocean interior when near-surface stratified layers are present, which can lead to changes in upper ocean stratification and SST.