Numerical Studies Of Convective Instability In The Ocean

Convective instability processes in the ocean are mainly divided into two categories:thermal hydrothermal plume rising from the bottom of the sea and heavy plume,caused by brine rejection at high latitudes,sinking from the surface.Hydrothermal plume is a key channel exchanging momentum,heat flux and matter between the Earths crust and seawater,and the number of hydrothermal zones is large and their distribution range is wide.Brine rejection is one of the mechanisms forming a sinking plume.A sinking plume belongs to the deep convection process,which is in turn an important mechanism for the formation of world's deep ocean water masses,CO2 transport and storage and thermohaline circulation.Two types of plume are highly turbulent convective instability processes and their locations are very special,so it is difficult to observe or simulate them.Therefore,further research is needed to understand the details of their three-dimensional structure,mixing characteristics and related influence factors.Then the parameterizations of their characteristic parameters can be concluded which have important reference value for improving the parameterization in general models,and also have good theoretical support for the study of global thermohaline circulation changes.In this study,convective instability processes in the ocean are analyzed from theoretical analysis and numerical simulation.For the theoretical analysis,conditions for convective instability to occur are discussed.While for the numerical simulation,Large eddy simulation(LES)is used to model the above two types of convective instability processes.Mainly the following aspects:(1)Theoretical analysis of convective instability shows that one of the necessary conditions for convective instability to occur in the case of viscous is that the Rayleigh number is greater than a certain threshold.By calculating the Rayleigh number(Ra)of all tests,it is found that all the Ra are much larger than the critical values necessary for the convective instability process,indicating that the configuration of hydrothermal rise plume and dense sink plume in this study is reasonable.(2)The three-dimensional structure and development process of these two convective instability processes are first analyzed and found that,at the root of hydrothermal plume,it mainly presented upward flow,which induced a robust clockwise vortex in the southern hemisphere(it is reverse in the northern hemisphere).At the cap region,it mainly distributed a divergence field,flowing from the plume core to all sides.As a whole,hydrothermal plume display an umbrella-shaped distribution,which is consistent with previous studies.The distribution of sinking plume is resemble to hydrothermal plume,however,the difference is that sinking plume display a descending cone-shaped distribution with a upward flow along the central axis,which is caused by the strong rotation effect.(3)By analyzing the eddy viscosity(K)which characterizes the vertical transport of the plume,it is found that the eddy viscosity of both the hydrothermal plume and the sinking plume reach the maximum at the location of 0.5?0.6 Zmax,indicating that the vertical transport of plume is the strongest at this location.(4)A series sensitivity experiments are tested to figure out the impact of environmental factors on above two types of plumes,such as heat flux from the hydrothermal vent(HF),salinity flux supposed on the surface(SaF),rotation rate(f)and background stratification(N).It is found that,HF,SaF and f have a significant effect on the plumes:the increasement of HF and SaF facilitate the vertical transport of plumes,while the increasement of f inhibate the vertical transport of plumes.The effect of N is not sensitive.(5)Employing the sensitivity results,parameterizations of K and Zmax with respect to HF,SaF,fand N are concluded,which have important reference value for improving the parameterization of the general models.