Mixing by ocean eddies

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 163-175).Mesoscale eddies mix and transport tracers such as heat and potential vorticity laterally in the ocean. While this transport plays an important role in the climate system, especially in the Southern Ocean, we lack a, comprehensive understanding of what sets mixing rates. This thesis seeks to advance this understanding through three related studies. First, mixing rates are diagnosed from an eddy-resolving state estimate of the Southern Ocean, revealing a meridional cross-section of effective diffusivity shaped by the interplay between eddy propagation and mean flow. Effective diffusivity diagnostics are then applied to quantify surface mixing rates globally, using a, kinematic model with velocities derived from satellite observations; the diagnosed mixing rates show a rich spatial structure, with especially strong mixing in the tropics and western-boundary-current regions. Finally, an idealized numerical model of the Southern Ocean is analyzed, focusing on the response to changes in win( stress. The sensitivity of the meridional overturning circulation to the wind changes demonstrates the importance of properly capturing eddy mixing rates for large-scale climate problems.by Ryan Abernathey.Ph.D

Global surface eddy diffusivities derived from satellite altimetry

[1] Velocities derived from AVISO sea-surface height observations, adjusted to be nondivergent, are used to simulate the evolution of passive tracers at the ocean surface. Eddy mixing rates are derived from the tracer fields in two ways. First, the method of Nakamura is applied to a sector in the East Pacific. Second, the Osborn-Cox diffusivity is calculated globally to yield estimates of diffusivity in two dimensions. The results from the East Pacific show weak meridional mixing at the surface in the Southern Ocean (&1000 m2 s−1, consistent with previous results) but higher mixing rates (~3000–5000 m2 s−1) in the tropical ocean. The Osborn-Cox diagnostic provides a global picture of mixing rates and agrees reasonably well with the results from the East Pacific. It also shows extremely high mixing rates (~104 m2 s−1) in western boundary current regions. The Osborn-Cox diffusivity is sensitive to the tracer initialization, which we attribute to the presence of anisotropic mixing processes. The mixing rates are strongly influenced by the presence of a mean flow nearly everywhere, as shown by comparison with an eddy-only calculation, with the mean flow absent. Finally, results are compared with other recent estimates of mixing rates using Lagrangian and inverse methods

Phase Speed Cross Spectra of Eddy Heat Fluxes in the Eastern Pacific

This study investigates the observed spectral character of eddy heat fluxes near the ocean surface, focusing on the distribution in wavenumber and phase speed space. Eddy heat fluxes in the eastern Pacific are calculated from concurrent satellite sea surface height and sea surface temperature data. A high-resolution coupled climate model is also analyzed in order to verify the physical mechanisms involved and to validate the model against observations. Wavenumber, frequency, and phase speed power spectra and cross spectra are constructed and presented as a function of latitude. These spectra reveal the dominance of coherent mesoscale eddies in both the length scale and phase speed of eddy heat fluxes. The breadths of the spectra are characterized via spectral moments; these moments show that the eddy fluxes are relatively concentrated around the dominant wavenumber and phase speed. Good agreement is found between the model and the observed spectra. The integrated heat transport and corresponding eddy diffusivity are shown to compare well with previous studies, but the results give a deeper insight into what determines the heat flux. Implications for eddy parameterization are discussed

Global Patterns of Mesoscale Eddy Properties and Diffusivities

Mesoscale eddies play a major role in the transport of tracers in the ocean. Focusing on a sector in the east Pacific, the authors present estimates of eddy diffusivities derived from kinematic tracer simulations using satellite-observed velocity fields. Meridional diffusivities are diagnosed, and how they are related to eddy properties through the mixing length formulation of Ferrari and Nikurashin, which accounts for the suppression of diffusivity due to eddy propagation relative to the mean flow, is shown. The uniqueness of this study is that, through systematically varying the zonal-mean flow, a hypothetical “unsuppressed” diffusivity is diagnosed. At a given latitude, the unsuppressed diffusivity occurs when the zonal-mean flow equals the eddy phase speed. This provides an independent estimate of eddy phase propagation, which agrees well with theoretical arguments. It is also shown that the unsuppressed diffusivity is predicted very well by classical mixing length theory, that is, that it is proportional to the rms eddy velocity times the observed eddy size, with a spatially constant mixing efficiency of 0.35. Then, the suppression factor is estimated and it is shown that it too can be understood quantitatively in terms of easily observed mean flow properties. The authors then extrapolate from these sector experiments to the global scale, making predictions for the global surface eddy diffusivity. Together with a prognostic equation for eddy kinetic energy and a theory explaining observed eddy sizes, these concepts could potentially be used in a closure for eddy diffusivities in coarse-resolution ocean climate models

Topographic Enhancement of Eddy Efficiency in Baroclinic Equilibration

The processes that determine the depth of the Southern Ocean thermocline are considered. In existing conceptual frameworks, the thermocline depth is determined by competition between the mean and eddy heat transport, with a contribution from the interaction with the stratification in the enclosed portion of the ocean. Using numerical simulations, this study examines the equilibration of an idealized circumpolar current with and without topography. The authors find that eddies are much more efficient when topography is present, leading to a shallower thermocline than in the flat case. A simple quasigeostrophic analytical model shows that the topographically induced standing wave increases the effective eddy diffusivity by increasing the local buoyancy gradients and lengthening the buoyancy contours across which the eddies transport heat. In addition to this local heat flux intensification, transient eddy heat fluxes are suppressed away from the topography, especially upstream, indicating that localized topography leads to local (absolute) baroclinic instability and its subsequent finite-amplitude equilibration, which extracts available potential energy very efficiently from the time-mean flow

The impact of ozone depleting substances on the circulation, temperature, and salinity of the Southern Ocean: An attribution study with CESM1(WACCM)

Observations show robust changes in the circulation, temperature, and salinity of the Southern Ocean in recent decades. To what extent these changes are related to the formation of the ozone hole in the late twentieth century is an open question. Using a comprehensive chemistry-climate Earth system model, we contrast model runs with varying and with fixed surface concentrations of ozone depleting substances (ODS) from 1955 to 2005. In our model, ODS cause the majority of the summertime changes in surface wind stress which, in turn, induce a clear poleward shift of the ocean's meridional overturning circulation. In addition, more than 30% of the model changes in the temperature and salinity of the Southern Ocean are caused by ODS. These findings offer unambiguous evidence that increased concentrations of ODS in the late twentieth century are likely to have been been an important driver of changes in the Southern Ocean

The Dependence of Southern Ocean Meridional Overturning on Wind Stress

An eddy-resolving numerical model of a zonal flow, meant to resemble the Antarctic Circumpolar Current, is described and analyzed using the framework of J. Marshall and T. Radko. In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation (MOC) to exist at depth. The strength of the residual MOC is diagnosed for different strengths of surface wind stress. It is found that the eddy circulation largely compensates for the changes in Ekman circulation. The extent of the compensation and thus the sensitivity of the MOC to the winds depend on the surface boundary condition. A fixed-heat-flux surface boundary severely limits the ability of the MOC to change. An interactive heat flux leads to greater sensitivity. To explain the MOC sensitivity to the wind strength under the interactive heat flux, transformed Eulerian-mean theory is applied, in which the eddy diffusivity plays a central role in determining the eddy response. A scaling theory for the eddy diffusivity, based on the mechanical energy balance, is developed and tested; the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress. The MOC sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

Diagnostics of isopycnal mixing in a circumpolar channel

Mesoscale eddies mix tracers along isopycnals and horizontally at the sea surface. This paper compares different methods of diagnosing eddy mixing rates in an idealized, eddy-resolving model of a channel flow meant to resemble the Antarctic Circumpolar Current. The first set of methods, the “perfect” diagnostics, are techniques suitable only to numerical models, in which detailed synoptic data is available. The perfect diagnostic include flux-gradient diffusivities of buoyancy, QGPV, and Ertel PV; Nakamura effective diffusivity; and the four-element diffusivity tensor calculated from an ensemble of passive tracers. These diagnostics reveal a consistent picture of isopycnal mixing by eddies, with a pronounced maximum near 1000 m depth. The isopycnal diffusivity differs from the buoyancy diffusivity, a.k.a. the Gent–McWilliams transfer coefficient, which is weaker and peaks near the surface and bottom. The second set of methods are observationally “practical” diagnostics. They involve monitoring the spreading of tracers or Lagrangian particles in ways that are plausible in the field. We show how, with sufficient ensemble size, the practical diagnostics agree with the perfect diagnostics in an average sense. Some implications for eddy parameterization are discussed

Exploring the isopycnal mixing and helium–heat paradoxes in a suite of Earth system models

This paper uses a suite of Earth system models which simulate the distribution of He isotopes and radiocarbon to examine two paradoxes in Earth science, each of which results from an inconsistency between theoretically motivated global energy balances and direct observations. The helium–heat paradox refers to the fact that helium emissions to the deep ocean are far lower than would be expected given the rate of geothermal heating, since both are thought to be the result of radioactive decay in Earth's interior. The isopycnal mixing paradox comes from the fact that many theoretical parameterizations of the isopycnal mixing coefficient ARedi that link it to baroclinic instability project it to be small (of order a few hundred m² s⁻¹) in the ocean interior away from boundary currents. However, direct observations using tracers and floats (largely in the upper ocean) suggest that values of this coefficient are an order of magnitude higher. Helium isotopes equilibrate rapidly with the atmosphere and thus exhibit large gradients along isopycnals while radiocarbon equilibrates slowly and thus exhibits smaller gradients along isopycnals. Thus it might be thought that resolving the isopycnal mixing paradox in favor of the higher observational estimates of ARedi might also solve the helium paradox, by increasing the transport of mantle helium to the surface more than it would radiocarbon. In this paper we show that this is not the case. In a suite of models with different spatially constant and spatially varying values of ARedi the distribution of radiocarbon and helium isotopes is sensitive to the value of ARedi. However, away from strong helium sources in the southeastern Pacific, the relationship between the two is not sensitive, indicating that large-scale advection is the limiting process for removing helium and radiocarbon from the deep ocean. The helium isotopes, in turn, suggest a higher value of ARedi below the thermocline than is seen in theoretical parameterizations based on baroclinic growth rates. We argue that a key part of resolving the isopycnal mixing paradox is to abandon the idea that ARedi has a direct relationship to local baroclinic instability and to the so-called "thickness" mixing coefficient AGM

Gateways to the Ocean: A Symposium Celebrating Arnold Gordon's Contributions to Physical Oceanography

The global ocean circulation affects our climate in myriad ways and plays a central role in mediating the planet’s response to climate change. A key aspect of this circulation is the importance of specific localized gateways (or choke points) which control the strength, structure and basin connectivity of the circulation. Examples of such gateways are the Indonesian Throughflow, the Agulhas retroflection, and outflows from the Ross and Weddell Seas to name a few. These systems often consist of multiple strong jets that vary over a range of space and time scales and so remain extremely challenging to observe, model and predict. Given the rapid development of new observing technologies, and the emergence of a new class of high-resolution global ocean models that are critical for understanding the accelerating rate of climate change, the time is ripe to assess the state of knowledge in the field of ocean gateways and define key challenges for future research. A two-day symposium to bring together distinguished and early-career researchers from across the world, including observationalists, theoreticians and modelers to address these topics was convened at Scripps Institution of Oceanography in San Diego, 13-14 February, the week prior to the 2020 AGU Ocean Science meeting. The symposium coincided with the 80th birthday of Prof. Arnold L. Gordon, a pioneer in the field of ocean gateways. Professor Gordon’s singular research career, spanning over 50 years, has transformed our understanding of ocean gateways and their role in climate. The conveners and attendees celebrated Prof. Gordon’s contributions over the two-day symposium, held in John Martin House, an intimate forum on the SIO campus that fostered interaction and lively discussion among attendees. Attendance was limited to about 50 participants per day, in keeping with the occupancy limit of the venue. In an effort to cultivate the next generation of leaders in the field of ocean gateways, invitations were extended to early career researchers and members of underrepresented groups in addition to well-established researchers in the field. The symposium presentations are archived here as pdf files, organized by session. An introduction, including the two-day agenda, is included in the Session_01 compilation of files. Symposium Organizing Committee: Ryan Abernathey, Bruce Huber (Lamont-Doherty Earth Observatory/Columbia University) Janet Sprintall (Scripps Institution of Oceanography) Martin Visbeck (GEOMAR Helmholtz Centre for Ocean Research Kiel) The symposium was funded by the US National Science Foundation, grant number OCE 2006148