Spectral modification and geographic redistribution of the semi-diurnal internal tide

详细信息    查看全文

• 作者：Harper L. Simmons
• 关键词：Global baroclinic tidal model ; Single tidal constituent ; Radiated baroclinic energy flux
• 刊名：Ocean Modelling
• 出版年：2008
• 出版时间：2008
• 年：2008
• 卷：21
• 期：3-4
• 页码：126-138
• 全文大小：2267 K

文摘

A realistic-geometry global baroclinic tidal model forced with a single tidal constituent (M₂) is used to investigate the generation of the internal tide and the associated radiated baroclinic energy flux. The model internal wave spectrum is populated at discrete frequency multiples (1/2,1,3/2,2,5/2,…) of M₂. The <img src=""http://www.sciencedirect.com/cache/MiamiImageURL/B6VPS-4RP0MW2-2-1B/0?wchp=dGLbVtz-zSkWz"" alt=""View the MathML source"" title=""View the MathML source"" align=""absbottom"" border=""0"" height=17 width=""48""/> subharmonic is particularly energetic at its turning latitude of ±28.8°. Poleward only integer superharmonics of M₂ are significantly excited. The subharmonic turning latitude (SHTL) disturbance has high vertical wavenumber and shear, provided that internal tide energy level exceeds a threshold value. Under these circumstances, Richardson numbers smaller than 1/4 occur in the upper few hundred meters in both the realistic-geometry model and in a complimentary idealized geometry two-dimensional (2D) model. In the 2D model, the disturbance enables Richardson number dependent diapycnal entrainment to effect a modification of the stratification of the upper 400 m of the ocean, and poleward cross-SHTL energy flux falls to 10 % of its pre-instability value due to energy transfer to the non-propagating (i.e., inertial) subharmonic. Realistic-geometry simulations suggest a more modest 40 % decrease in net flux, although the strongest beams are almost entirely shut down. The predicted energy flux-convergence implies a thermocline dissipation rate in the 28.5–30.0°N latitude band of <img src=""http://www.sciencedirect.com/cache/MiamiImageURL/B6VPS-4RP0MW2-2-1F/0?wchp=dGLbVtz-zSkWz"" alt=""View the MathML source"" title=""View the MathML source"" align=""absbottom"" border=""0"" height=20 width=""114""/>, with an associated diapycnal diffusivity of <img src=""http://www.sciencedirect.com/cache/MiamiImageURL/B6VPS-4RP0MW2-2-1G/0?wchp=dGLbVtz-zSkWz"" alt=""View the MathML source"" title=""View the MathML source"" align=""absbottom"" border=""0"" height=16 width=""80""/>. North of Hawaii the implied regional dissipation rate reaches <img src=""http://www.sciencedirect.com/cache/MiamiImageURL/B6VPS-4RP0MW2-2-1H/0?wchp=dGLbVtz-zSkWz"" alt=""View the MathML source"" title=""View the MathML source"" align=""absbottom"" border=""0"" height=20 width=""115""/> with an associated thermocline diffusivity of <img src=""http://www.sciencedirect.com/cache/MiamiImageURL/B6VPS-4RP0MW2-2-1K/0?wchp=dGLbVtz-zSkWz"" alt=""View the MathML source"" title=""View the MathML source"" align=""absbottom"" border=""0"" height=16 width=""107""/>. Investigations of subgridscale parameterization and resolution sensitivity suggest that the basic character and magnitude of the predictions are robust to details of the numerical solutions. The present results are taken as further evidence that an increase in shear-driven turbulent mixing in the upper ocean is predicted at special latitudes. It is suggested that the search should be directed to regions where intense low-mode internal tide beams cross their subharmonic turning latitude.