Diurnal cycle effects of the surface heat flux on the mixed layer depth and sea surface temperature under the stabilizing heat flux

The mixed layer depth (MLD) has large impacts on sea surface temperature (SST), air-sea fluxes, and large-scale climate. The impacts are especially large in summer because the SST response to a given surface heat flux is large due to small MLD. Here, diurnal cycle effects of the surface heat flux on summer MLD and SST, that are suggested large by previous GCM studies but have not been quantified, are investigated through large-eddy simulations (LESs). The LES results show that the diurnal cycle makes MLD greater (smaller) at lower (higher) latitudes. Nighttime cooling comes into play in shaping the latitudinal dependence of MLD. The wind-induced shear-driven turbulence becomes largest at the half inertial period (T _i /2), while the cooling keeps reducing stratification till the end of nighttime (T ₂₄ = 24h). At lower latitudes where T_ {i} /2 > T ₂₄, T_i/2 determines the time at which MLD becomes greatest. This holds true for MLD without diurnal cycle though the convective turbulence induced by surface cooling helps increasing MLD. At higher latitudes, on the other hand, the nighttime cooling continues to reduce stratification after T_i/2 and determines the greatest MLD. The MLD change by the diurnal cycle induces the corresponding change in the SST-increasing rate. At higher latitudes over the North Pacific in June, the diurnal cycle makes MLD smaller by 10% and SST higher by an order of 0.1 K than those without diurnal cycle. This might have an impact on longer-scale climate variability.