Air Sea Exchange and Ocean Mixing at the Diurnal and Intraseasonal Scales

Lau and Sui (1997) and Sui et al. (1997b) analyzed the COARE data and found that the dry and wet phases in the atmospheric precip-itable water are associated with the passage of the intraseasonal oscillations during the TOGA COARE. The passage of the intrareasonal oscillations also affects the surface radiative and heat fluxes that impact the ocean mixing processes and thus the upper-ocean stratification. The 2-3-day disturbances and diurnal cycles are phenomena with the dominant time scales during the wet phase, whereas the diurnal cycles in the SST are regularly forced by the diurnal solar radiative flux in the day and outgoing radiative and heat fluxes in the night during the dry phase (Fig. 2). This implies that the vertical solar absorption profile in the upper ocean plays an important role in determining the mixed-layer depth and the amount of solar absorption in the mixed layer, which eventually control the evolution of ocean mixed-layer temperature.

Sui et al. (1997b) employed a mixed-layer model to study the role of the vertical solar absorption profile in the diurnal ocean temperature simulations and their impacts on the intraseasonal variability. Owing to the asymmetric diurnal variation for shoaling and deepening

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Diurnal Ocean
Figure 2. Time series of observed SST during the TOGA COARE (upper panel) and the simulated heating rate (solid) and mixed-layer depth (dashed) during selected COARE periods (lower panel).

of the mixed layer, the cumulative effect of diurnal mixing cycles is essential to maintaining a stable upper-ocean thermal stratification and to simulating a realistic evolution of mixed-layer and temperature at the intraseasonal time scale. Further sensitivity tests of mixed layer to diurnal cycles indicate that the inclusion of diurnal convective-radiative processes in the atmosphere-ocean system in the coupled models affects the capability of simulating intraseasonal variability.

The salinity contributes more to the density stratification than does the temperature, which is responsible for upper-ocean stability. Li et al. (1998) further included the salinity in the ocean mixed-layer modeling to examine the impacts of the precipitation and associated upper-ocean salinity stratification on the ocean mixed layer. The inclusion of salinity and precipitation-induced fresh water flux in the simulation shows that much deeper mixing occurs when rainfall appears during the night since the fresh water flux induces a much shallower mixed layer with a large deepening rate, which is consistent with the observations. The inclusion of the salinity stratification could cause warmer water entrained into the ocean mixed layer since the salinity stratification maintains the upper-ocean stability, whereas the exclusion of salinity in the simulation only shows entrainment of cold water into the ocean mixed layer since the thermal stratification accounts for the upper-ocean stability. Because the Kraus-Tuner mixing parametrization scheme (Niiler and Kraus, 1977) requires both thermal and saline stratifications to determine the mixed-layer depth, Li et al. carried out decoupled salinity experiments to examine the effect of thermal stratification on the saline structure. The experiments reveal that when the fresh water input is large, the salinity variations simulated with and without the thermal stratification can be significantly different. The difference in the salinity could be 0.2 PSU. The simulations indicate that the inclusion of precipitation-induced fresh water flux and salinity stratification improves the simulation of thermal evolution in the ocean mixed layer.

Sui et al. (1998b) used a mixed-layer model along with the estimate of surface forcing obtained from the TOGA COARE to estimate the amount of heat absorbed in the observed mixed layer to maintain the observed amplitudes of SST which is largely dependent on the depth-dependent solar heating. The simulated amplitude of the diurnal cycle of mixed-layer temperature (< 1°C) is significantly smaller than the observed amplitude (1-3°C), implying that the mixed layer in the simulation does not absorb enough solar heat. They found that more than 39% of the net surface solar irradiance is absorbed within the first 0.45 meters in order to maintain the observed SST, which is higher than previous estimates. The vertical profile of solar absorption is then modified, and the simulation with the modified solar profile yields more realistic amplitudes of the SST at both the diurnal and intraseasonal time scales.

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