Seasonal and regional characterization of horizontal stirring in the global ocean (original) (raw)

Seasonal and regional characterization of horizontal mixing in the global ocean

2011

Recent work on Lagrangian descriptors has shown that Lyapunov Exponents can be applied to observed or simulated data to characterize the horizontal stirring and transport properties of the oceanic flow. However, a more detailed analysis of regional dependence and seasonal variability was still lacking. In this paper, we analyze the near-surface velocity field obtained from the Ocean general circulation model For the Earth Simulator (OFES) using Finite-Size Lyapunov Exponents (FSLE). We have characterized regional and seasonal variability. Our results show that horizontal stirring, as measured by FSLEs, is seasonally-varying, with maximum values in Summer time. FSLEs also strongly vary depending on the region: we have first characterized the stirring properties of Northern and Southern Hemispheres, then the main oceanic basins and currents. We have finally studied the relation between averages of FSLE and some Eulerian descriptors such as Eddy Kinetic Energy (EKE) and vorticity (ω) over the different regions.

VERTICAL MIXING, ENERGY, AND THE GENERAL CIRCULATION OF THE OCEANS

Annual Review of Fluid Mechanics, 2004

Key Words ocean circulation, ocean mixing, ocean circulation energy s Abstract The coexistence in the deep ocean of a finite, stable stratification, a strong meridional overturning circulation, and mesoscale eddies raises complex questions concerning the circulation energetics. In particular, small-scale mixing processes are necessary to resupply the potential energy removed in the interior by the overturning and eddy-generating process. A number of lines of evidence, none complete, suggest that the oceanic general circulation, far from being a heat engine, is almost wholly governed by the forcing of the wind field and secondarily by deep water tides. In detail however, the budget of mechanical energy input into the ocean is poorly constrained. The now inescapable conclusion that over most of the ocean significant "vertical" mixing is confined to topographically complex boundary areas implies a potentially radically different interior circulation than is possible with uniform mixing. Whether ocean circulation models, either simple box or full numerical ones, neither explicitly accounting for the energy input into the system nor providing for spatial variability in the mixing, have any physical relevance under changed climate conditions is at issue.

Submesoscale Routes to Lateral Mixing in the Ocean

2010

Abstract: Long-term goals are to determine whether lateral mixing at O (1-10 km) scales is due to a balanced or unbalanced downscale cascade from the mesoscale, or due to local vertical mixing by internal waves and surface forcing. Our work will test hypothesis 3 of the white paper" Scalable Lateral Mixing and Coherent Turbulence": Non-QG, submesoscale instabilities feed a forward cascade of energy, scalar and Ertel PV variance, which enhances both isopycnal and diapycnal mixing.

Chaotic transport in a double gyre ocean

Geophysical Research Letters, 1994

Lagrangian trajectories in a deterministic simple flow can exhibit complex behavior. This is called the chaotic advection, which can induce the chaotic transport and mixing (which oceanographers conventionally call stirring) in the fluid particles and line structure in the tracer fields. We propose the chaotic transport as a tyre-scale transport mechansim in oceans.

Mesoscale vortices, ageostrophic motions and vertical mixing in the ocean

2010

We investigate the role played by wind-forced eddies in the vertical transport and mixing at the ocean mesoscale with a high-resolution primitive-equation model. Both idealized and realistic configurations are implemented. With a detailed analysis of the vertical velocity field in the idealized experiments we find that in the vortex cores and inside intense vorticity filaments, the motion is strongly ageostrophic,

Chaotic advection, mixing, and property exchange in three-dimensional ocean eddies and gyres

This work investigates how a Lagrangian perspective applies to models of two oceanographic flows: an overturning submesoscale eddy and the Western Alboran Gyre. In the first case, I focus on the importance of diffusion as compared to chaotic advection for tracers in this system. Three methods are used to quantify the relative contributions: scaling arguments including a Lagrangian Batchelor scale, statistical analysis of ensembles of trajectories, and Nakamura effective diffusivity from numerical simulations of dye release. Through these complementary methods, I find that chaotic advection dominates over turbulent diffusion in the widest chaotic regions, which always occur near the center and outer rim of the cylinder and sometimes occur in interior regions for Ekman numbers near 0.01. In thin chaotic regions, diffusion is at least as important as chaotic advection. From this analysis, it is clear that identified Lagrangian coherent structures will be barriers to transport for long times if they are much larger than the Batchelor scale. The second case is a model of the Western Alboran Gyre with realistic forcing and bathymetry. I examine its transport properties from both an Eulerian and Lagrangian perspective. I find that advection is most often the dominant term in Eulerian budgets for volume, salt, and heat in the gyre, with diffusion and surface fluxes playing a smaller role. In the vorticity budget, advection is as large as the effects of wind and viscous diffusion, but not dominant. For the Lagrangian analysis, I construct a moving gyre boundary from segments of the stable and unstable manifolds emanating from two persistent hyperbolic trajectories on the coast at the eastern and western extent of the gyre. These manifolds are computed on several isopycnals and stacked vertically to construct a three-dimensional Lagrangian gyre boundary. The regions these manifolds cover is the stirring region, where there is a path for water to reach the gyre. On timescales of days to weeks, water from the Atlantic Jet and the northern coast can enter the outer parts of the gyre, but there is a core region in the interior that is separate. Using a gate, I calculate the continuous advective transport across the Lagrangian boundary in three dimensions for the first time. A Lagrangian volume budget is calculated, and challenges in its closure are described. Lagrangian and Eulerian advective transports are found to be of similar magnitudes.

Dynamics and mixing in the Upper Ocean Layer

2005

The response of the mixed layer depth (MLD) to short-term (synoptic) variations of atmospheric forcing in the North Atlantic Ocean was analysed using CTD data, microstructure pro…ling, ADCP velocities and atmospheric measurements taken during the last 12 days of April 2001 at 42 stations located close to 53 N. The transect followed the climatological position of zero annually-averaged wind stress curl (WSC), crossing the Labrador Current and several branches and meanders of the North Atlantic Current. Atmospheric forcing was characterized by relatively high wind speeds (mean value being 10.7 ms 1) and negative surface heat balance. Three strong storms were encountered during the measurements and the wind stress at drift stations reached 0:2 0:4 Nm 2. The averaged amplitude of Ekman transport hM E i was about 1 m 2 s 1 , but during the storms the magnitude of hM E i went up to 3:4 3:5 m 2 s 1. The meridional transport across 53 N in the Atlantic is mainly limited to depths below the upper 200-meter layer. The ageostrophic ‡ow in the upper layer was mainly southward and eastward. The meridional ageostrophic transports, M Ey , were usually greater than those calculated using the residuals between ADCP and geostrophic velocities, M AG , but were in the same direction as M Ey when jM AG j exceeded 0:3 m 2 s 1. The MLD (h D) at each station was identi…ed. The deepest observed h D was 110 m, the mean hh D i = 45 m. The MLD was compared with the current reversal depth (CRD is the shallowest depth where the current vector changes the sign of its rotation). The mean and median estimates for CRD appeared to be very close to those for MLD, suggesting that the drift currents were mostly con…ned to the upper mixed layer. It was found that the MLD is correlated with the friction velocity u , and the correlation coe¢ cient is 0:71 when u data were time advanced by 12 hrs. The correlation of MLD with the surface buoyancy ‡ux J b was weak. The ratio between h D and the Monin-Obukhov length scale L mo = u 3 =J b , based on the time shifted u , indicates that at almost all stations h D =L mo < 1, suggesting the dominance of wind-induced mixing over convection. Parameterization of MLD in terms of the xiii TABLE OF CONTENTS

When complexity leads to simplicity: Ocean surface mixing simplified by vertical convection

2012

The effect of weak vertical motion on the dynamics of materials that are limited to move on the ocean surface is an unresolved problem with important environmental and ecological implications (e.g., oil spills and larvae dispersion). We investigate this effect by introducing into the classical horizontal time-periodic double-gyre model vertical motion associated with diurnal convection. The classical model produces chaotic advection on the surface. In contrast, the weak vertical motion simplifies this chaotic surface mixing pattern for a wide range of parameters. Melnikov analysis is employed to demonstrate that these conclusions are general and may be applicable to realistic cases. This counter intuitive result that the very weak nocturnal convection simplifies ocean surface mixing has significant outcomes.