At present,studies on large-amplitude internal solitary waves mostly adopt strong stratification models,such as the twoand three-layer Miyata–Choi–Camassa(MCC)internal wave models,which omit the pycnocline or treat it as another fluid layer with a constant density.Because the pycnocline exists in real oceans and cannot be omitted sometimes,the computational error of a large-amplitude internal solitary wave within the pycnocline introduced by the strong stratification approximation is unclear.In this study,the two-and three-layer MCC internal wave models are used to calculate the wave profile and wave speed of large-amplitude internal solitary waves.By comparing these results with the results provided by the Dubreil–Jacotin–Long(DJL)equation,which accurately describes large-amplitude internal solitary waves in a continuous density stratification,the computational errors of large-amplitude internal solitary waves at different pycnocline depths introduced by the strong stratification approximation are assessed.Although the pycnocline thicknesses are relatively large(accounting for 8%–10%of the total water depth),the error is much smaller under the three-layer approximation than under the two-layer approximation.
An internal gravity wave model was employed to simulate the generation of internal solitary waves(ISWs)over a sill by tidal flows.A westward shoaling pycnocline parameterization scheme derived from a three-parameter model was adopted,and then 14 numerical experiments were designed to investigate the influence of the pycnocline thickness,density difference across the pycnocline,westward shoaling isopycnal slope angle and pycnocline depth on the ISWs.When the pycnocline thickness on both sides of the sill increases,the total barotropic kinetic energy,total baroclinic energy and ratio of baroclinic kinetic energy(KE)to available potential energy(APE)decrease,whilst the depth of isopycnal undergoing maximum displacement and ratio of baroclinic energy to barotropic energy increase.When the density difference on both sides of the sill decreases synchronously,the total barotropic kinetic energy,ratio of baroclinic energy to barotropic energy and total baroclinic energy decrease,whilst the depth of isopycnal undergoing maximum displacement increases.When the westward shoaling isopycnal slope angle increases,the total baroclinic energy increases whilst the depth of turning point almost remains unchanged.When the depth of westward shoaling pycnocline on both sides of the sill reduces,the ratio of baroclinic energy to barotropic energy and total baroclinic energy decrease,whilst the total barotropic kinetic energy and ratio of KE to APE increase.When one of the above four different influencing factors was increased by 10%while the other factors keep unchanged,the amplitude of the leading soliton in ISW Packet A was decreased by 2.80%,7.47%,3.21%and 6.42%respectively.The density difference across the pycnocline and the pycnocline depth are the two most important factors in affecting the characteristics and energetics of ISWs.
Haibin LliYujun LiuXiaokang ChenGuozhen ZhaShuqun Cai
Seven operative thermal infrared(TIR)spectrometers launched at sun-synchronous polar orbits supply huge amounts of information about Arctic methane(CH4)year-round,day and night.TIR data are unique for estimating CH4 emissions from a warming Arctic,both terrestrial and marine.This report is based on publicly available CH4 concentrations retrieved by NOAA and NASA from spectra of TIR radiation delivered by EU IASI and US AIRS sounders.Data were filtered for high thermal contrast in the troposphere.Validation versus aircraft measurements at three US continental sites reveal a reduced,but still significant sensitivity to CH4 anomalies in the troposphere below 4 km of altitude.The focus area is the Barents and Kara seas(BKS).BKS is impacted with warm Atlantic water and mostly free of sea ice.It is a shelf area with vast deposits of oil and natural gas(~90%CH4),as well as methane hydrates and submarine permafrost.Although in summer AIRS and IASI observe no significant difference in CH4 between BKS and N.Atlantic,a strong,monthly positive CH4 spatial anomaly of up to 30 ppb occurs during late autumn–winter.One of explanations of this increase is a fall/winter pycnocline breakdown after a period of blocked mixing caused by a stable density seawater stratification in summer:enhanced mixing lets CH4 to reach the sea surface and atmosphere.