| Internal waves are a kind of fluctuating process that occurs in stratified oceans.As a special kind of internal waves,internal solitary waves(ISWs)are also called nonlinear internal waves.The research of early ISWs mainly focused on low and middle latitudes regions,and a systematic understanding has been formed.However,the understanding of the generation and dissipation process of ISWs in high latitudes is still remain insufficient.In high-latitude seas,the existence of sea ice is a very unique environmental factor.This unique environmental feature makes the generation,propagation and dissipation of ISWs in the Polar Regions demonstrate some mechanisms and properties that are different from those of middle and low latitudes.Investigating the generation,propagation and dissipation processes of ISWs in Polar Regions under sea ice is very important to promote the understanding of ISWs in high latitudes.Low-mode ISWs are the main object of previous studies on the generation and dissipation of ISWs in low and mid-latitudes,and the understanding of their breaking and dissipation processes is now relatively comprehensive.In recent years,as highmode ISWs represented by mode-2 ISWs have been observed more frequently,the ephemeral existence and highly dissipated nature of mode-2 ISWs draw much attention.But the current understanding of the specific dissipation mechanism of high-mode ISWs is still insufficient,especially the dissipation process of mode-2 ISWs in the background shear flow.Therefore,reproducing the modulation and dissipation process of the mode-2 ISWs in the background shear flow is of great significance for revealing the local turbulent dissipation process of the mode-2 ISWs and improving the theoretical system of ISWs.In summary,this paper combines nonlinear and non-hydrostatic numerical models and observations to focus on high-latitude regions and the high-mode ISWs,the following work has been carried out:Firstly,it reviews the research results of ISWs in middle and low latitudes,as well as the nonlinear,non-hydrostatic MITgcm model commonly used in the numerical simulation of ISWs and its application in ISW related work.It briefly explained its modular structure and introduced the functions of several modules that are important for the numerical simulations in this paper.After that,the interaction of barotropic currents and ice keels in high-latitude seas was simulated,and it was found that multi-modal ISWs can be excited in this process,accompanied by a strong mixing process.At the same time,the energy of the barotropic tides is also transferred to the ISWs and radiated outward.Based on this,a unique generation mechanism of ISWs in high-latitude sea areas covered by sea ice is proposed for the first time,which provides a new idea for studying the source of ISWs under ice keels.With the sensitivity experiments,it is proved that the amplitude of barotropic tidal current and the depth of ice keel can significantly modulate the generation process of ISWs under ice keel.Then,the propagation and dissipation process of ISWs under ice keels was studied.Combined with the theoretical characteristic model of the ice keel,it is found that under the modulation of the ice keel,the ISW will disintegrate into multiple secondary waves near the ice keel,accompanied by the process of energy cascade to the secondary waves.When the ISW disintegrate near the ice keel,it will also significantly strengthen the turbulent mixing and vertical heat transfer near the ice keel,which will have a certain impact on the growing and melting of the ice keel.Sensitivity experiments show that the depth and width of the ice keel will significantly modulate the generation of secondary waves,change the energy distribution process of ISWs near the ice keel,and directly affect the strength of turbulent mixing near the ice keel.Finally,the dissipation process of the mode-2 ISWs in the background shear flow is discussed.The modulated and disintegrated mechanism of the mode-2 ISWs in the background shear flow are studied.The existence of background shear flow will stimulate shear instability,which will destroy the structure of the mode-2 ISWs and cause the destabilization and disintegration of the ISW,and generate special structures such as internal breathers.Experiments have confirmed that the existence of background shear flow is the reason for the ephemeral existence and highly dissipated nature of mode-2 ISWs that cannot be ignored.The amplitude of the background shear flow and the degree of offset relative to the pycnocline of the background shear flow can also make significant impact on the disintegration and dissipation of the mode-2ISWs.As a strong nonlinear dynamic process,ISWs have attracted much attention because of their ability to transport materials,transfer energy across scales,and adjust vertical heat balance.The highlights of this paper is that for the first time,a mechanism for generating ISWs triggered by the interaction of barotropic tides and ice keels beneath sea ice covered areas is proposed.Based on this,combined with observations and statistical models of ice keel revealed propagation of ISWs is modulated by the ice keel and the process can enhanced local mixing.At the same time,the thermal effect in this process is quantitatively estimated,showing that the generation,propagation,evolution and breaking of ISWs under the ice keel have an impact on the vertical heat transport and heat balance.Noted that this influence cannot be ignored.On the other hand,this paper focused on the ephemeral existence of the mode-2 ISWs in the background shear currents.The results confirmed that the background shear current is an important factor causing the deformation,disintegration and strong dissipation of the mode-2 ISWs.It is closely related to the characteristics of background shear current.This is very important for explaining the contribution of mode-2 ISWs to local mixing.Therefore,the work of this paper not only expanded the research on the generation and propagation mechanism of ISWs in high latitudes,but also supplemented and improved the dissipation mechanism of high mode ISWs in mid latitudes.In the meantime,it has application value for the exploiting of marine ecological resources,marine environmental protection,and the safety of high latitude underwater navigation as well as deployment of offshore engineering. |
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