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Abstract: This study explores the influence of Stokes drift and the thermal effects on the upper ocean bias which occurs in the summer with overestimated sea surface temperature (SST) and shallower mixed layer depth (MLD) using Mellor-Yamada turbulence closure scheme. The upper ocean thermal structures through Princeton ocean model are examined by experiments in the cases of idealized forcing and real observational situation. The results suggest that Stokes drift can generally enhance turbulence kinetic energy and deepen MLD either in summer or in winter. This effect will improve the simulation results in summer, but it will lead to much deeper MLD in winter compared to observational data. It is found that MLD can be correctly simulated by combining Stokes drift and the thermal effects of the cool skin layer and diurnal warm layer on the upper mixing layer. In the case of high shortwave radiation and weak wind speed, which usually occurs in summer, the heat absorbed from sun is blocked in the warm layer and prevented from being transferred downwards. As a result, the thermal effects in summer nearly has no influence on dynamic effect of Stokes drift that leads to deepening MLD. However, when the stratification is weak in winter, the thermal effects will counteract the dynamic effect of Stokes drift through enhancing the strength of stratification and suppress mixing impact. Therefore, the dynamic and thermal effects should be considered simultaneously in order to correctly simulate upper ocean thermal structures in both summer and winter.
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Key words:
- mixed layer /
- cool skin layer /
- diurnal warm layer /
- Stokes drift
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Figure 1. The difference between experiment EXP1-S and EXP1 in the vertical structure of simulated. a, f. Temperature profile (T); b, c, g, h. horizontal velocities (U and V); d, i. turbulence kinetic energy (TKE, q2); and e, j. vertical kinematic viscosity (KM). The colors represent different wind speed, respectively. The top row shows the result forcing by high shortwave radiation 800 W/m2 (a–e), the other row shows that forcing by low short wave radiation 200 W/m2 (f–j).
Figure 2. The evolution of sea temperature profile with 2 m/s wind in high vertical resolution model result (EXP2) are shown. The colors represent temperature profile per 1 h, respectively.
Figure 3. The evolution of sea temperature profile per 3 h in high vertical resolution model result (EXP2) are shown. The colors represent the control experiments with wind speed from 0 to 10 m/s, respectively.
Figure 4. The evolution of sea temperature profile in different control model result. The colors represent temperature profile per 2 h, respectively. Line: EXP1; and line with points: EXP2.
Figure 5. The evolution of MLD and SST in two days with idealized forcing: 800 W/m2 solar radiation and wind speeds from weak to strong. Solid black line: EXP1; solid red line: EXP2; black line with points: EXP1-S; and red line with points: EXP2-S.
Figure 6. The evolution of MLD and SST in two days with idealized forcing: 200 W/m2 solar radiation and wind speeds from weak to strong. Solid black line: EXP1; solid red line: EXP2; black line with points: EXP1-S; and red line with points: EXP2-S.
Figure 7. Observed and simulated SST and MLD at Station Papa for 2012. Black line: observation; blue line: EXP1; green line: EXP1-S; yellow line: EXP2; and red line: EXP2-S.
Figure 8. Observed and simulated isotherm depth at Station Papa for 2012. a. EXP1, b. EXP2, c. EXP1-S, d. EXP2-S, and e. observation.
Figure 9. Simulated temperature bias profile at Station Papa for 2012. a. EXP1, b. EXP1-S, c. EXP2, and d. EXP2-S.
Figure 10. The difference of the net heat flux released into the atmosphere compared with EXP1. Yellow line: EXP2; green line: EXP1-S; and red line: EXP2-S.
Table 1. Summery of the experiments used in this study
Experiment Vertical resolution Stokes drifts Idealized forcing OWS Papa Wind speed
/m·s–1Amplitudes of the solar radiation
/W·m–2Wind speed
/m·s–1Solar radiation
/W·m–2EXP1 low no 0–20 200/800 observed observed EXP2 high no 0–20 200/800 observed observed EXP1-S low yes 0–20 200/800 observed observed EXP2-S high yes 0–20 200/800 observed observed 
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Table 2. Difference between model and observed SST and MLD at Papa for 2012
Bias EXP1 EXP2 EXP1-S EXP2-S Annual SST/°C 1.18 0.96 0.57 0.53 Summer SST/°C 1.98 1.21 0.64 0.48 Winter MLD/m 9.03 8.36 28.94 7.65 Summer MLD/m 13.11 12.11 8.05 6.17
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