A mechanism for establishment and maintenance of the meridional overturning in the upper ocean

A two-dimensional analytical residual-mean model of the meridional overturning in the upper ocean is presented which illustrates dynamics of the interaction between the Northern and Southern hemispheres. The theory is based on the semi-adiabatic approximation in which all diabatic processes are confined to the upper mixed layer. The overturning circulation is driven directly by the wind forcing which, in our model, is affected by the sea-surface temperature distribution. The surface boundary conditions are symmetric with respect to the equator, and therefore one of the steady state solutions represents a symmetric flow characterized by the absence of the inter-hemispheric buoyancy fluxes. However, linear stability analysis, which takes into account both mechanical and thermodynamic forcing at the sea surface, indicates that the symmetric configuration such as this is unstable. The instability results in transition to the asymmetric regime with finite cross-equatorial exchange flows and heat transfer. Weakly nonlinear instability theory makes it possible to estimate the equilibrium fluxes in the new asymmetric steady states; for the oceanographically relevant range of parameters our model predicts the meridional overturning of about 10 Sv. While earlier studies considered the role of salt advection in spontaneous symmetry breaking, our study relies on a positive feedback between atmospheric winds and the oceanic meridional circulation

Hydrodynamically-based Detection of the Surface and Subsurface Wakes

NPS NRP Executive SummaryHydrodynamically-based Detection of the Surface and Subsurface WakesN2/N6 - Information WarfareThis research is supported by funding from the Naval Postgraduate School, Naval Research Program (PE 0605853N/2098). https://nps.edu/nrpChief of Naval Operations (CNO)Approved for public release. Distribution is unlimited.

Direct Numerical Simulation of 3D Salt Fingers: From Secondary Instability to Chaotic Convection

The amplification and equilibration of three-dimensional salt fingers in unbounded uniform vertical gradients of temperature and salinity is modeled with a Direct Numerical Simulation in a triply periodic computational domain. A fluid dynamics video of the simulation shows that the secondary instability of the fastest growing square-planform finger mode is a combination of the well-known vertical shear instability of two-dimensional fingers [Holyer, 1984] and a new horizontal shear mode.Comment: APS DFD Gallery of Fluid Motion 200

Stabilization of Isolated Vortices in a Rotating Stratified Fluid

The key element of Geophysical Fluid Dynamics—reorganization of potential vorticity (PV) by nonlinear processes—is studied numerically for isolated vortices in a uniform environment. Many theoretical studies and laboratory experiments suggest that axisymmetric vortices with a Gaussian shape are not able to remain circular owing to the growth of small perturbations in the typical parameter range of abundant long-lived vortices. An example of vortex destabilization and the eventual formation of more intense self-propagating structures is presented using a 3D rotating stratified Boussinesq numerical model. The peak vorticity growth found during the stages of strong elongation and fragmentation is related to the transfer of available potential energy into kinetic energy of vortices. In order to develop a theoretical model of a stable circular vortex with a small Burger number compatible with observations, we suggest a simple stabilizing procedure involving the modification of peripheral PV gradients. The results have important implications for better understanding of real-ocean eddies

The salt finger amplitude in unbounded T-S gradient layers

Finite amplitude numerical calculations are made for a completely unbounded salt finger domain whose overall vertical property gradients (Tz and Sz) are uniform and remain unaltered in time. For diffusivity ratio τ = κS/κT = O (1), Prandtl number ν/κT \u3e\u3e 1, and density ratio R = Tz/Sz \u3e 1 this regime corresponds to a double gradient sugar (S)—salt (T) experiment. Two-dimensional pseudo-spectral calculations are made in the vicinity of the minimum critical condition for salt finger instability, viz., small ε ≡ (Rτ)-1 - 1 \u3e 0; the allowed spectrum includes the fastest growing wave of linear theory. When the vertical wavelength of the fundamental Fourier component is systematically increased the solution changes from a single steady vertical mode to a multi-modal statistically steady chaotic state. Each of the long vertical modes can be amplified by the (unchanging overall) gradient Sz, and can be stabilized by the induced vertical T, S gradients on the same scale as the modes; nonlinear triad interactions in the T - S equations can also lead to amplitude equilibration even though ε, κT/ν, and the Reynolds number are extremely small. When subharmonics of the horizonal wavelength of maximum growth are introduced into the numerical calculations the new wave amplifies (via Sz) and produces a quantitative change in the time average fluxes. Experimentally testable values of heat flux and rms horizontal T-fluctuations are computed in the range 2.8 \u3e R \u3e1.6 for τ = 1/3. Asymptotic similarity laws ε → 0 are also presented

Salt fingers in three dimensions

Three dimensional (3D) numerical calculations are made for a vertically unbounded fluid with initially uniform vertical gradients of sugar ( S ) and salt ( T ), where τ = κS/κT = 1/3 is the diffusivity ratio, and the molecular viscosity is ν \u3e\u3e κT. The latter inequality allows us to neglect the nonlinear term in the momentum equation, while retaining such terms in the T-S equations. The discrete 3D Fourier spectrum resolves the fastest growing horizontal wavelength, as well as the depth independent Fourier component. Unlike previous calculations for the pure 2D case the finite amplitude equilibration in 3D is primarily due to the instability of the lateral S-gradients in the fingers, and the consequent transfer of energy to vertical scales comparable with the finger width. It is shown that finite amplitude two-dimensional disturbances are unstable and give way to three dimensional fingers with much larger fluxes. Calculations are also made for rigid boundary conditions at z = (0,L) in order to make a rough quantitative comparison with previous lab experiments wherein a finger layer of finite thickness is sandwiched between two well-mixed (T,S) reservoirs. The flux ratio is in good agreement, and the fluxes agree within a factor of two even though the thin interfacial boundary layer between the reservoir and the fingers is not quite rigid because sheared fingers pass through it. It is suggested that future experiments be directed toward the much simpler unbounded gradient model, for which flux and variance laws are given herein

Understanding the Sources of Illicit Drug Bale Wash-up

NPS NRP Project PosterUnderstanding the Sources of Illicit Drug Bale Wash-upN2/N6 - Information WarfareUS Coast Guard Research and Development CenterThis research is supported by funding from the Naval Postgraduate School, Naval Research Program (PE 0605853N/2098). https://nps.edu/nrpChief of Naval Operations (CNO)Approved for public release. Distribution is unlimited.

Understanding the Sources of Illicit Drug Bale Wash-up

NPS NRP Executive SummaryUnderstanding the Sources of Illicit Drug Bale Wash-upN2/N6 - Information WarfareUS Coast Guard Research and Development CenterThis research is supported by funding from the Naval Postgraduate School, Naval Research Program (PE 0605853N/2098). https://nps.edu/nrpChief of Naval Operations (CNO)Approved for public release. Distribution is unlimited.

Inferring the Pattern of the Oceanic Meridional Transport from the Air-Sea Density Flux

The article of record as published may be found at http://dx.doi.org/10.1175/2008JPO3748.1An extension of Walin’s water mass transformation analysis is proposed that would make it possible to assess the strength of the adiabatic along-isopycnal component of the meridional overturning circulation (MOC). It is hypothesized that the substantial fraction of the adiabatic MOC component can be attributed to the difference in subduction rates at the northern and southern outcrops of each density layer—the “push–pull” mechanism. The GCM-generated data are examined and it is shown that the push–pull mode accounts for approximately two-thirds of the isopycnal water mass transport in the global budget and dominates the Atlantic transport. Much of the difference between the actual interhemispheric flux and the push–pull mode can be ascribed to the influence of the Antarctic Circumpolar Current, characterized by the elevated (at least in the GCM) values of the diapycnal transport. When the diagnostic model is applied to observations, it is discovered that the reconstructed MOC is consistent, in terms of the magnitude and sense of overturning, with earlier observational and modeling studies. The findings support the notion that the dynamics of the meridional overturning are largely controlled by the adiabatic processes—time-mean and eddy-induced advection of buoyancy

Salt fingers in an unbounded thermocline

Numerical solutions for salt fingers in an unbounded thermocline with uniform overall vertical temperature-salinity gradients are obtained from the Navier-Stokes-Boussinesq equations in a finite computational domain with periodic boundary conditions on the velocity. First we extend previous two-dimensional (2D) heat-salt calculations [Prandtl number Pr = ν/kT = 7 and molecular diffusivity ratio τ = kS/kT = 0.01] for density ratio R = 2; as R decreases we show that the average heat and salt fluxes increase rapidly. Then three-dimensional (3D) calculations for R = 2.0, Pr = 7, and the numerically accessible values of τ = 1/6, 1/12 show that the ratio of these 3D fluxes to the corresponding 2D values [at the same (τ, R, Pr)] is approximately two. This ratio is then extrapolated to τ = 0.01 and multiplied by the directly computed 2D fluxes to obtain a first estimate for the 3D heat-salt fluxes, and for the eddy salt diffusivity (defined in terms of the overall vertical salinity gradient). Since these calculations are for relatively small domains [O (10) finger pairs], we then consider much larger scales, such as will include a slowly varying internal gravity wave. An analytic theory which assumes that the finger flux is given parametrically by the small domain flux laws shows that if a critical number A is exceeded, the wave-strain modulates the finger flux divergence in a way which amplifies the wave. This linear theoretical result is confirmed, and the finite amplitude of the wave is obtained, in a 2D numerical calculation which resolves both waves and fingers. For highly supercritical A (small R) it is shown that the temporally increasing wave shear does not reduce the fluxes until the wave Richardson number drops to ~0.5, whereupon the wave starts to overturn. The onset of density inversions suggests that at later time (not calculated), and in a sufficiently large 3D domain, strong convective turbulence will occur in patches