Meiman Lee - Academia.edu (original) (raw)

Papers by Meiman Lee

Ocean Modelling, 2003

The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ... more The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ocean model (OCCAM). The focus is on the transport by transient eddies in the deep ocean. The transport streamfunction is calculated in four different combinations of coordinate system. Depending on the coordinate system employed, the strength of transient eddy transport varies from 6 Sv meridional transport in latitude-density coordinates to 20 Sv across-streamline transport in streamline-depth coordinates. It is shown that transient eddies as well as standing eddies are necessary for cancelling the Deacon cell.In the Antarctic bottom water density layer, the major contribution of the transient eddies towards net equatorward transport occurs (a) as a strong transport over the narrow Drake Passage and (b) as a weaker but systematic transport over a broader region in the southeast Pacific where the Antarctic circumpolar current breaks up into multiple jets. In contrast, in the North Atlantic deep water density layer the net poleward eddy transport is spread out almost everywhere. This suggests that attention to eddies should not be restricted to places where the eddy transport has large magnitude.

Journal of Physical Oceanography, 2002

Abstract This study looks at the behavior of fluid particles/parcels that move along isopycnic su... more Abstract This study looks at the behavior of fluid particles/parcels that move along isopycnic surfaces. The aim is to show that the diffusive process of mixing by eddies causes such particles to move toward regions of greater isopycnic layer thickness or, equivalently, weaker stratification. This is illustrated using a wind-driven eddy-resolving isopycnic layer model in a zonal channel configuration. Particles and tracers are integrated to demonstrate that their centers of mass move up the layer thickness gradient. In particular, a field of uniformly distributed particles is seen to move toward regions of large layer thickness, so the distribution of particles becomes asymmetric. The asymmetry is most obvious when the gradient of layer thickness is large in comparison to the volume of the layer. In the ocean, in the absence of other influences such as advection and varying diffusivity, one might expect isopycnic floats released in the upper thermocline to show a similar asymmetric behavior.

Journal of Physical Oceanography, 2004

In Part I , the “vertical” transport streamfunction was defined as resulting from isopycnic avera... more In Part I , the “vertical” transport streamfunction was defined as resulting from isopycnic averaging at constant height in the same way that the meridional streamfunction results from averaging at constant latitude. Part II here discusses the relationship between these two isopycnic streamfunctions and the Eulerian residual streamfunction that arises from the transformed Eulerian mean (TEM). It is known that the meridional isopycnic streamfunction can be approximated by a Taylor expansion to give an Eulerian residual streamfunction involving the horizontal eddy flux. This Taylor expansion approximation works well in the interior, removing the spurious mixing associated with the simple Eulerian-averaged streamfunction. However, it fails near the surface where isopycnals outcrop to the surface. It can be shown in a similar way that the vertical isopycnic streamfunction can formally be approximated by a residual streamfunction involving the vertical eddy flux. However, if horizontal i...

Journal of Physical Oceanography, 2009

This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnost... more This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnostic methods for inferring effective eddy isopycnic diffusivity from point release tracers. The first method is based on the growth rate of the area occupied by the tracers (the equivalent variance). During the period when tracer dispersion is dominated by stirring, the equivalent variance is found to increase at a rate between the second power law (for a pure shearing flow regime) and the exponential law (for a pure stretching flow regime). The second method is based on the length of the tracer contours. In the framework of equivalent radius, the two methods of inferring eddy diffusivity can be understood as two different averagings over the tracer patch. Over a shorter period of tracer dispersion the two methods give different eddy diffusivities, and only over a longer time when tracer dispersion approaches the final stage of diffusion do they give a similar value of diffusivity. A new d...

Journal of Physical Oceanography, 2002

Recent idealized studies have shown that both explicit horizontal diffusion and the implicit diff... more Recent idealized studies have shown that both explicit horizontal diffusion and the implicit diffusion associated with the advection scheme in high-resolution z-coordinate models may drive unrealistically high rates of diapycnal mixing. The aim here is to see whether the diapycnal mixing associated with the advection scheme in a global eddy-permitting (¼Њ by ¼Њ) z-level model is sufficiently strong to corrupt the thermohaline circulation. This paper diagnoses the diapycnal fluxes by using the ideas of water mass transformation. In the Southern Ocean, the model deep and bottom waters drift rapidly away from the Levitus climatology, with dense isopycnals moving downward at rates of up to 35 m yr Ϫ1. The strong upward flux (up to 50 Sv) through the dense isopycnals cannot be explained by the incorrect surface forcing (as a result of poor surface fluxes and no ice model) as most of the anomalous diapycnal fluxes are occurring in the deep ocean far from surface forcing. Hence, the excessive diapycnal flux is driven by diffusion in the model, both explicit and implicit. The ''effective'' diapycnic diffusivity driven by the numerical diffusion (associated with the horizontal advection scheme) is found to be the same order, 1-10 cm 2 s Ϫ1 , as that driven by the explicit horizontal diffusion. For strong vertical velocities (ϳ20 m day Ϫ1) as in models forced by high frequency winds, the vertical advection scheme also gives similar effective diffusivities. These effective diffusivities are considerably greater than suggested by observations. To alleviate these problems, it is suggested that eddy-resolving z-level climate models will require 1) less diffusive horizontal advection schemes and 2) better vertical resolution throughout much of the water column.

Journal of Physical Oceanography, 2012

Subduction—the transport of fluid across the base of mixed layer—exchanges water masses and trace... more Subduction—the transport of fluid across the base of mixed layer—exchanges water masses and tracers between the ocean surface and interior. Eddies can affect subduction in a variety of ways. First, eddies shoal the mixed layer by restratifying water columns through baroclinic instabilities. Second, eddies induce an isopycnic transport that leads to the entrainment of warm waters and subduction of cold waters, which effectively counters the wind-driven overturning circulation. In this study, the authors use an idealized model to examine these two mechanisms by which eddies influence subduction and to discuss how eddy subduction may be better approximated using the concept of vertical transport streamfunction than the conventional meridional transport streamfunction.

Journal of Marine Research, 1997

Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses a... more Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses and tracers in a down-gradient manner. However, eddies also have an advective role that may lead to an up-gradient transfer of tracers, as has been recognized in atmospheric tracer studies and recent eddy parameterizations developed for the ocean. Eddies provide an advective transfer or "bolus" velocity through the secondary circulation formed by the slumping of density surfaces in baroclinic instability. Here we use an eddy-resolving isopycnal ocean model to investigate the meridional transfer across a zonal jet. The jet undergoes baroclinic instability, forming a vibrant eddy field and inducing a meridional bolus velocity. The bolus velocity is found to be correlated with gradients of potential vorticity rather than thickness. A transient tracer is released with high and low values at the southern and northern boundaries respectively. Over the first few years, the tracer spreads diffusively in a down-gradient manner. The implied eddy diffusivity of the passive tracer is found to be reassuringly similar to that of the dynamic tracer, potential vorticity. On the decadal time scale, however, the eddy-induced advection dominates and leads to a poleward spreading of tracer in the upper layer, and equatorward spreading of tracer in the lower layer. This eddy-induced advection is likely to be important in controlling the water-mass distribution wherever the time-mean meridional flow is weak. Observationally, the transport velocity is difficult to measure directly, but we argue might be inferred from the spreading of transient tracers, such as CFCs, before they reach a statistically-steady state.

Journal of Marine Research, 2000

Eddies provide a systematic, large-scale transfer of tracers and nutrients along isopycnals throu... more Eddies provide a systematic, large-scale transfer of tracers and nutrients along isopycnals through a combination of eddy-induced diffusion and advection. The nutrient distribution is controlled by advection, rather than diffusion, when the nutrient lifetime exceeds a timescale given by /V 2 where is the lateral diffusivity and V is the characteristic transport velocity. Idealized, eddy-resolving experiments are conducted using an isopycnic model configured for a zonal channel. Whether the eddies act to enhance or inhibit biological production depends on the location of the nutrient source and the nutrient lifetime. For a subtropical gyre and the Southern Ocean, the eddy-induced diffusion and advection are likely to oppose each other over the euphotic zone but reinforce each other at depth. Over the euphotic zone, the combination of eddy diffusion and Ekman advection of nutrients should dominate the opposing eddy-induced advection. This lateral transfer becomes more important for long-lived nutrients, such as dissolved organic nitrogen, within the euphotic zone.

Journal of Climate, 2011

This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean... more This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of di...

Journal of Physical Oceanography, 2004

Simple Eulerian averaging of velocities, density, and tracers at constant position is the most na... more Simple Eulerian averaging of velocities, density, and tracers at constant position is the most natural way of averaging. However, Eulerian averaging gives incorrect watermass distributions and properties as well as spurious diabatic circulations such as the Deacon cell. Instead of averaging at constant height, averaging along isopycnals removes such fictitious mixing and diabatic circulations. Such isopycnal averaging is normally performed at constant latitude, that is, averaging along isopynals as they heave up and down. As a result, height information is lost and the sea surface becomes much warmer (or lighter) than with simple Eulerian averaging. In fact, averaging can be performed along arbitrarily aligned surfaces. This study considers a particular case in which isopycnal averaging is performed at constant height. Thus, this new isopycnal averaging follows isopycnals as they meander horizontally at constant z. Height information is now retained at the cost of losing latitudinal...

Ocean Modelling, 2003

The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ... more The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ocean model (OCCAM). The focus is on the transport by transient eddies in the deep ocean. The transport streamfunction is calculated in four different combinations of coordinate system. Depending on the coordinate system employed, the strength of transient eddy transport varies from 6 Sv meridional transport in latitude-density coordinates to 20 Sv across-streamline transport in streamline-depth coordinates. It is shown that transient eddies as well as standing eddies are necessary for cancelling the Deacon cell.In the Antarctic bottom water density layer, the major contribution of the transient eddies towards net equatorward transport occurs (a) as a strong transport over the narrow Drake Passage and (b) as a weaker but systematic transport over a broader region in the southeast Pacific where the Antarctic circumpolar current breaks up into multiple jets. In contrast, in the North Atlantic deep water density layer the net poleward eddy transport is spread out almost everywhere. This suggests that attention to eddies should not be restricted to places where the eddy transport has large magnitude.

Journal of Physical Oceanography, 2002

Abstract This study looks at the behavior of fluid particles/parcels that move along isopycnic su... more Abstract This study looks at the behavior of fluid particles/parcels that move along isopycnic surfaces. The aim is to show that the diffusive process of mixing by eddies causes such particles to move toward regions of greater isopycnic layer thickness or, equivalently, weaker stratification. This is illustrated using a wind-driven eddy-resolving isopycnic layer model in a zonal channel configuration. Particles and tracers are integrated to demonstrate that their centers of mass move up the layer thickness gradient. In particular, a field of uniformly distributed particles is seen to move toward regions of large layer thickness, so the distribution of particles becomes asymmetric. The asymmetry is most obvious when the gradient of layer thickness is large in comparison to the volume of the layer. In the ocean, in the absence of other influences such as advection and varying diffusivity, one might expect isopycnic floats released in the upper thermocline to show a similar asymmetric behavior.

Journal of Physical Oceanography, 2004

In Part I , the “vertical” transport streamfunction was defined as resulting from isopycnic avera... more In Part I , the “vertical” transport streamfunction was defined as resulting from isopycnic averaging at constant height in the same way that the meridional streamfunction results from averaging at constant latitude. Part II here discusses the relationship between these two isopycnic streamfunctions and the Eulerian residual streamfunction that arises from the transformed Eulerian mean (TEM). It is known that the meridional isopycnic streamfunction can be approximated by a Taylor expansion to give an Eulerian residual streamfunction involving the horizontal eddy flux. This Taylor expansion approximation works well in the interior, removing the spurious mixing associated with the simple Eulerian-averaged streamfunction. However, it fails near the surface where isopycnals outcrop to the surface. It can be shown in a similar way that the vertical isopycnic streamfunction can formally be approximated by a residual streamfunction involving the vertical eddy flux. However, if horizontal i...

Journal of Physical Oceanography, 2009

This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnost... more This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnostic methods for inferring effective eddy isopycnic diffusivity from point release tracers. The first method is based on the growth rate of the area occupied by the tracers (the equivalent variance). During the period when tracer dispersion is dominated by stirring, the equivalent variance is found to increase at a rate between the second power law (for a pure shearing flow regime) and the exponential law (for a pure stretching flow regime). The second method is based on the length of the tracer contours. In the framework of equivalent radius, the two methods of inferring eddy diffusivity can be understood as two different averagings over the tracer patch. Over a shorter period of tracer dispersion the two methods give different eddy diffusivities, and only over a longer time when tracer dispersion approaches the final stage of diffusion do they give a similar value of diffusivity. A new d...

Journal of Physical Oceanography, 2002

Recent idealized studies have shown that both explicit horizontal diffusion and the implicit diff... more Recent idealized studies have shown that both explicit horizontal diffusion and the implicit diffusion associated with the advection scheme in high-resolution z-coordinate models may drive unrealistically high rates of diapycnal mixing. The aim here is to see whether the diapycnal mixing associated with the advection scheme in a global eddy-permitting (¼Њ by ¼Њ) z-level model is sufficiently strong to corrupt the thermohaline circulation. This paper diagnoses the diapycnal fluxes by using the ideas of water mass transformation. In the Southern Ocean, the model deep and bottom waters drift rapidly away from the Levitus climatology, with dense isopycnals moving downward at rates of up to 35 m yr Ϫ1. The strong upward flux (up to 50 Sv) through the dense isopycnals cannot be explained by the incorrect surface forcing (as a result of poor surface fluxes and no ice model) as most of the anomalous diapycnal fluxes are occurring in the deep ocean far from surface forcing. Hence, the excessive diapycnal flux is driven by diffusion in the model, both explicit and implicit. The ''effective'' diapycnic diffusivity driven by the numerical diffusion (associated with the horizontal advection scheme) is found to be the same order, 1-10 cm 2 s Ϫ1 , as that driven by the explicit horizontal diffusion. For strong vertical velocities (ϳ20 m day Ϫ1) as in models forced by high frequency winds, the vertical advection scheme also gives similar effective diffusivities. These effective diffusivities are considerably greater than suggested by observations. To alleviate these problems, it is suggested that eddy-resolving z-level climate models will require 1) less diffusive horizontal advection schemes and 2) better vertical resolution throughout much of the water column.

Journal of Physical Oceanography, 2012

Subduction—the transport of fluid across the base of mixed layer—exchanges water masses and trace... more Subduction—the transport of fluid across the base of mixed layer—exchanges water masses and tracers between the ocean surface and interior. Eddies can affect subduction in a variety of ways. First, eddies shoal the mixed layer by restratifying water columns through baroclinic instabilities. Second, eddies induce an isopycnic transport that leads to the entrainment of warm waters and subduction of cold waters, which effectively counters the wind-driven overturning circulation. In this study, the authors use an idealized model to examine these two mechanisms by which eddies influence subduction and to discuss how eddy subduction may be better approximated using the concept of vertical transport streamfunction than the conventional meridional transport streamfunction.

Journal of Marine Research, 1997

Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses a... more Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses and tracers in a down-gradient manner. However, eddies also have an advective role that may lead to an up-gradient transfer of tracers, as has been recognized in atmospheric tracer studies and recent eddy parameterizations developed for the ocean. Eddies provide an advective transfer or "bolus" velocity through the secondary circulation formed by the slumping of density surfaces in baroclinic instability. Here we use an eddy-resolving isopycnal ocean model to investigate the meridional transfer across a zonal jet. The jet undergoes baroclinic instability, forming a vibrant eddy field and inducing a meridional bolus velocity. The bolus velocity is found to be correlated with gradients of potential vorticity rather than thickness. A transient tracer is released with high and low values at the southern and northern boundaries respectively. Over the first few years, the tracer spreads diffusively in a down-gradient manner. The implied eddy diffusivity of the passive tracer is found to be reassuringly similar to that of the dynamic tracer, potential vorticity. On the decadal time scale, however, the eddy-induced advection dominates and leads to a poleward spreading of tracer in the upper layer, and equatorward spreading of tracer in the lower layer. This eddy-induced advection is likely to be important in controlling the water-mass distribution wherever the time-mean meridional flow is weak. Observationally, the transport velocity is difficult to measure directly, but we argue might be inferred from the spreading of transient tracers, such as CFCs, before they reach a statistically-steady state.

Journal of Marine Research, 2000

Eddies provide a systematic, large-scale transfer of tracers and nutrients along isopycnals throu... more Eddies provide a systematic, large-scale transfer of tracers and nutrients along isopycnals through a combination of eddy-induced diffusion and advection. The nutrient distribution is controlled by advection, rather than diffusion, when the nutrient lifetime exceeds a timescale given by /V 2 where is the lateral diffusivity and V is the characteristic transport velocity. Idealized, eddy-resolving experiments are conducted using an isopycnic model configured for a zonal channel. Whether the eddies act to enhance or inhibit biological production depends on the location of the nutrient source and the nutrient lifetime. For a subtropical gyre and the Southern Ocean, the eddy-induced diffusion and advection are likely to oppose each other over the euphotic zone but reinforce each other at depth. Over the euphotic zone, the combination of eddy diffusion and Ekman advection of nutrients should dominate the opposing eddy-induced advection. This lateral transfer becomes more important for long-lived nutrients, such as dissolved organic nitrogen, within the euphotic zone.

Journal of Climate, 2011

This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean... more This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of di...

Journal of Physical Oceanography, 2004

Simple Eulerian averaging of velocities, density, and tracers at constant position is the most na... more Simple Eulerian averaging of velocities, density, and tracers at constant position is the most natural way of averaging. However, Eulerian averaging gives incorrect watermass distributions and properties as well as spurious diabatic circulations such as the Deacon cell. Instead of averaging at constant height, averaging along isopycnals removes such fictitious mixing and diabatic circulations. Such isopycnal averaging is normally performed at constant latitude, that is, averaging along isopynals as they heave up and down. As a result, height information is lost and the sea surface becomes much warmer (or lighter) than with simple Eulerian averaging. In fact, averaging can be performed along arbitrarily aligned surfaces. This study considers a particular case in which isopycnal averaging is performed at constant height. Thus, this new isopycnal averaging follows isopycnals as they meander horizontally at constant z. Height information is now retained at the cost of losing latitudinal...