Coupled Boundary Layer and Forced Oceanic Responses

Sui et al. (1991) developed an equilibrium model to study the coupled ocean-atmosphere boundary layer in the tropics, which consists of a one-dimensional thermodynamic atmospheric model for a partially mixed, partially cloudy convective boundary layer (CBL), Betts and Ridgway, 1988; 1989 and an oceanic mixed-layer (OML) model. Two experiments were performed with sea surface temperature (SST) specified. They solve the equilibrium state of the coupled system as a function of SST for a given surface wind, and as a function of surface wind for a given SST. The increases in SST lead to the increase in the depth of the con-vective boundary layer owing to the increase in the water vapor. The moistened and deepened CBL leads to a reduced net surface heat flux which is balanced by weakened upwelling and causes a deepened ocean mixed layer. The increase in surface wind also causes the increase in the depth of the ocean mixed layer and the decrease in the upwelling below the ocean mixed layer. But this is due to the generation of turbulence kinetic energy and the decrease in the net downward heat flux. The latter is due to the nonlinear change of air-sea humidity difference with increasing surface wind, such that a deepening CBL reduces the downward solar radiation and increases the downward longwave radiation at the surface. In another two experiments, the coupled model was solved iteratively as a function of surface wind for a fixed upwelling, and for a fixed mixed-layer depth (h). SST falls with increasing wind in both experiments, but the fall is gradual in the fixed upwelling condition because the depth is allowed to deepen and the cooling is spread over a larger mass of water, while the fall is steeper in the other experiment because h is fixed. The decrease of evaporation with increasing wind in fixed h condition leads to a very dry and shallow CBL. More experiments with surface wind and SST (upwelling) prescribed as a function of longitude similar to the observed values across the Pacific give realistic gradients of mixed-layer depth and upwelling (SST). The work quantifies the sensitivity of the equilibrium state of the coupled system to the coupling of the boundary layers, and provides a framework for understanding physical processes in the CBL and OML in coupled models.

Li et al. (2000) developed a coupled ocean-cloud-resolving atmosphere model to study effects of precipitation on ocean mixed-layer temperature and salinity. When the effects of fresh water flux and salinity were included in the coupled model, differences in the horizontal-mean mixed-layer temperature and salinity between 1D and 2D experiments were about 0.4°C and 0.3 PSU, respectively. The mean salinity difference was larger than the mean temperature difference in terms of their contributions to the mean density difference. In the 2D experiment, the surface heat flux showed a significant diurnal signal with the dominance of downward solar radiation during daytime and upward flux (longwave, sensible and latent heat fluxes) during nighttime at each grid, although the amplitude was affected by precipitation. Thus, there was a strong thermal correlation between grids. Narrow cloudy areas were surrounded by broad, cloud-free areas. Horizontal-mean precipitation could occur, whereas the precipitation may not occur in most of the integration period. Thus, there is a very low correlation between horizontal-mean and grid values of the fresh water fluxes. Since the rain rates have significant spatial variations, the fresh water flux has much larger spatial fluctuations than the saline entrainment. Therefore, the fresh water flux determines large spatial salinity fluctuations, which contributes to a large mean salinity difference between the 1D ocean model experiment and the 2D ocean model experiment.

Sui et al. (2003) investigated the impacts of high-frequency surface forcing on the upper ocean over the equatorial Pacific by conducting a nonlinear reduced-gravity isopycnal ocean circulation simulations with the daily and monthly mean forcings, respectively, and found that the daily-forcing experiment produces a colder SST than does the monthly-forcing experiment, and the difference in the SST between the two experiments is generated in the first year integration. The negative difference in the SST between the daily-forcing and monthly-forcing experiments in the western Pacific is primarily caused by enhanced latent heat loss due to the transient winds. Over the eastern Pacific, the zonal thermal advection accounts for the difference, while other terms are large but cancel each other out. Relative to the monthly forcing, the effect of daily forcing is found to (1) enhance vertical mixing and reduce vertical shear in the upper ocean; (2) reduce net heat into the ocean through two contrasting processes — increased surface latent heat loss induced by transient winds, and a colder SST (due to stronger mixing) reducing surface heat loss; (3) weaken meridional thermal advection through more active instability waves; (4) change mixed-layer depth so that the temperature in the simulation with the daily forcing is warmer around the ther-mocline.

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