Numerical Study On The Ocean Interior Turbulent Mixing Based On Argo Observations

Mixing in the ocean interior has been one of the hot issues in physical oceanography study for many years. Ocean mixing parameterization plays a vital role in the ocean numerical models, directly affects the reliability of simulation results. Inaccurate mixing parameterization scheme in ocean models will lead to unrealistic advection, meridional overturning circulation, temperature and salinity distribution. Therefore, the development of rational ocean mixing parameterization scheme can effectively improve the numerical ocean modeling and forecasting capabilities.Ocean interior mixing coefficient is a parameter with spatial variability. However, due to the difficulty of in-situ observation, there is not yet the observation-based mixing coefficient covering the global ocean fitting for model application. In practice, the defining of it mainly relies on some sparsely distributed local observations or modeler’s experiences. In some ocean models, constant is usually used in representing both horizontal mixing and vertical (diapycnal) mixing, which is obviously lack of physics. When using the mixing tensor in each direction to describe ocean mixing interior, such as in MOM4, neutral surface slope is one of the important parameters. In numerical simulation, the neutral surface slope is currently calculated using the model outputs (temperature, salt, etc.). Due to the inherent error of model, the calculated neutral surface slope will probably deviate from the actual one. Therefore, it is necessary to calculate the slope based on in-situ observation to improve ocean interior mixing parameterization scheme. Another category of ocean models (e.g. HYCOM) hire a contant background mixing coefficient together with a mixed layer parameterization scheme (e.g. KPP) to set the diapycnal mixing. Similarly, the background mixing coefficient here should have spatial variability therefore preferably be calculated with in-situ observation.In the past, observing over the global ocean is relatively sparse, with poor spatial and temporal continuity. Information required to calculate the neutral surface slope and diapycnal mixing coefficient in ocean interior was hardly obtained at global scale. Argo profiling float’s appearance to a large extent compensates this deficiency. It can provide the upper 2000m ocean temperature and salinity in real-time. Based on the valuable Argo data, this study will estimate the more realistic neutral surface slope and diapycnal mixing coefficient to improve parameterization reasonableness and accuracy in the ocean interior, therefore improve global ocean climate models and predictive ability for ocean climate research.This study collects the Argo data since the beginning of the Program to December 2012. On the basis of comparing two delayed mode calibration methods for salinity, all profiles went through effective quality control. In particular, the oceanic phenomenons that need to be paid attention to during the visual check are studied. Global gridded datasets for temperature, salinity as well as pressure are derived. According to the new international thermodynamic equation of seawater, TEOS-2010, multi-year averaged density on standard levels are calculated for global upper 2000m ocean.Here, local reference potential density surface is used as the approximation of neutral surface. The neutral surface slope of the global ocean is calculated using Argo derived density and its magnitude and pattern are analyzed. Then the global ocean gridded diapycnal mixing coefficients Ad that has the spatial variation closely related to the latitude and depth is derived based on the Argo data. The magnitude of AD is from about 0.05×10-5 to 2.5×10-5 m2/s, varying with space significantly. Horizontally, AD increases with latitude; vertically, it increases with the depth. In the area of large wind energy input, such as the Antarctic Circumpolar Current region, patchy region with dramatically increased mixing coefficient will show up. The average AD in Southern Hemisphere is higher than that in Northern Hemisphere. The enhanced mixing induced by strong westerlies could reach the depth of Argo observation. Comparison with previous studies indicates that the results in this study are basically reliable both in the magnitude and distribution pattern. Then diapycnal mixing coefficient is applied in HYCOM to test the effects induced by the change of background mixing. Two sets of experiment, the Atlantic experiments and global experiments are designed. Both of them indicate that the introduction of diapycnal mixing coefficient with spatial variation and more physical meaning leads to significant change of meridional overturning circulation, temperature and salinity in the simulation results. Especially in the equatorial and high-latitude regions, two crucial areas for the global climate, these changes are more obvious.Moreover, internal wave induced mixing tensor, K, fitting for the Z coordinate model is obtained on the basis of neutral surface slope and AD.Its components in x, y, z direction are discussed respectively in terms of magnitude and distribution pattern. Kxx and Kyy have the similar magnitude and spatial pattern. Kzz varies between 10-7～10-5m2/s, with mangnitude much smaller than that of Kxx and Kyy. It is believed that this mixing tensor which with the spatial variation is a step forward in improving the physical meaning of the ocean numerical model.