Abstract
In the present study, large-eddy simulations (LESs) were performed to investigate mixed layer depth (MLD) and sea surface warming (SSW) under diurnally cycling surface heat flux in the heating season, in which a mixed layer (ML) is shoaling on intraseasonal time scales. The LES results showed that the diurnal cycle makes the MLD greater (smaller) at lower (higher) latitudes than the MLD without the cycle. Time scales of the wind-induced shear and the surface heat are a key to understand this latitudinal dependence of the diurnal cycle effects. The wind-induced shear-driven turbulence developed from early morning and became strongest at half the inertial period (Ti/2), while nighttime cooling weakened the ML stratification until the end of the nighttime (T24 = 24 h). At lower latitudes where Ti/2 > T24 (lower than 15°), the shear-driven turbulence continued to grow after T24 and determined the time of the greatest MLD. Thus, the shear-driven turbulence shaped the latitudinal dependence of the MLD, though convective turbulence helped further deepening of the ML. At higher latitudes (Ti/2 < T24), on the other hand, the shear-driven turbulence ceased growing before the nighttime cooling ended. However, reduced stratification due to the nighttime cooling supported the shear-driven turbulence to continue deepening the ML. Thus, the nighttime cooling shaped the latitudinal dependence of the MLD at higher latitudes. The MLD change induced by the diurnal cycle altered the SSW rate. At higher latitudes, the diurnal cycle is expected to reduce the MLD and increase the SSW by 10% in the heating season.
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