Lagrangian diffusivity estimated from coherent mesoscale eddies in an idealized basin circulation model

Tracer Diffusivity ◽

<div> <div> <div> <p>Lagrangian methods have been used to estimate the lateral eddy diffusivity in the ocean using surface drifter and subsurface float tracks and using the numerical particles advected by satellite-derived velocity fields. The diffusivity is estimated from the rate of dispersion of these particles. Accurate point-wise estimates of diffusivity generally require averages over a large number of drifters or floats, but the distribution of drifters and floats is generally sparse and many tracks of drifters are contaminated by winds. On the other hand, the convergence time for the particle-based diffusivity is on the order of a month for both in situ and numerical particles, which makes the estimates inefficient and allows for the accumulation of measurement error. Studies of vortex-dominated 2D turbulence have found that particle dispersion is dominated by the movement of coherent eddies, and that the dispersion rate of coherent eddies themselves can provide accurate estimates of the Lagrangian diffusivity. We found that the potential vorticity diffusivity in two-layer quasigeostrophic turbulence can also be accurately estimated by the rate of dispersion of coherent eddies, and this estimate converges more than four times faster than the diffusivity estimated from particles inserted uniformly in the flow. If this result also holds for oceanic mesoscale turbulence, it can form the basis for a potentially useful technique for diagnosing mesoscale diffusivity based on the tracks of coherent mesoscale eddies.</p> <p>This presentation examines the relation between the dispersion of coherent eddies and tracer diffusivity in an idealized configuration of Massachusetts Institute of Technology general circulation model which contains multiple gyres, boundary currents, and a zonally reentrant channel flow analogous to the Antarctic Circumpolar Current. The coherent eddies are identified and tracked from the sea surface height snapshots, and the diffusivity estimated from coherent eddies is compared to the tracer diffusivity diagnosed by a tracer inversion method. The diffusivity inferred from dispersion of coherent eddies generally converges within 15 days. Direct comparison of two diffusivity estimates is not straightforward, since the tracer-based diffusivity varies vertically. Approaches for reconciling the two estimates are discussed. This study shows the possibility of relating the Lagrangian movement of coherent eddies to the Eulerian tracer diffusivity.</p> </div> </div> </div>

Baroclinic Instability of the Faroe Bank Channel Overflow*

10.1175/jpo-d-14-0080.1 ◽

2014 ◽

Vol 44 (10) ◽

pp. 2698-2717 ◽

Cited By ~ 12

Author(s):

Chuncheng Guo ◽

Mehmet Ilicak ◽

Ilker Fer ◽

Elin Darelius ◽

Mats Bentsen

Keyword(s):

General Circulation ◽

Baroclinic Instability ◽

Lower Boundary ◽

Circulation Model ◽

Mesoscale Eddies ◽

Volume Transport ◽

Eddy Heat Flux ◽

Eddy Characteristics ◽

Abstract The generation mechanism of mesoscale eddies in the Faroe Bank Channel (FBC) overflow region and their spatiotemporal characteristics are examined using the high-resolution regional Massachusetts Institute of Technology general circulation model (MITgcm). From the modeled overflow, it is found that the volume transport downstream of the FBC sill exhibits strong variability with a distinct period of ~4 days. Energetic, alternating cyclonic and anticyclonic eddies appear at ~40 km downstream of the sill. They grow side by side in the nascent stage, but later the cyclones migrate along the 800-m isobath to the south of Iceland, whereas the anticyclones descend downslope across the isobath and gradually dissipate. Analysis of the eddy characteristics shows that the cyclones are associated with a larger plume thickness and width, larger volume transport, colder and denser water, and a plume core located farther downslope, whereas the opposite is true for the anticyclones. The oscillatory structure developed at the lower boundary of the mean plume and the following generation of alternating cyclones and anticyclones are typical features of baroclinic instability. A linear instability analysis of a two-layer analytical baroclinic model yields a most unstable mode that agrees favorably with the simulations. The calculation of the divergent eddy heat flux shows a substantial rightward (upslope)-directed component downstream of the FBC sill. This region is also associated with a strong baroclinic conversion rate. The above arguments constitute evidence for the generation of unstable plume and mesoscale eddies in the FBC region by baroclinic instability.

Cyclonic Eddies in the Gulf of Mexico: Observations by Underwater Gliders and Simulations by Numerical Model

10.1175/jpo-d-14-0138.1 ◽

2015 ◽

Vol 45 (1) ◽

pp. 313-326 ◽

Cited By ~ 30

Author(s):

Daniel L. Rudnick ◽

Ganesh Gopalakrishnan ◽

Bruce D. Cornuelle

Keyword(s):

Gulf Of Mexico ◽

General Circulation ◽

Circulation Model ◽

Atlantic Water ◽

Momentum Balance ◽

Loop Current ◽

Underwater Gliders ◽

Flow Curvature ◽

AbstractCirculation in the Gulf of Mexico (GoM) is dominated by the Loop Current (LC) and by Loop Current eddies (LCEs) that form at irregular multimonth intervals by separation from the LC. Comparatively small cyclonic eddies (CEs) are thought to have a controlling influence on the LCE, including its separation from the LC. Because the CEs are so dynamic and short-lived, lasting only a few weeks, they have proved a challenge to observe. This study addresses that challenge using underwater gliders. These gliders’ data and satellite sea surface height (SSH) are used in a four-dimensional variational (4DVAR) assimilation in the Massachusetts Institute of Technology (MIT) general circulation model (MITgcm). The model serves two purposes: first, the model’s estimate of ocean state allows the analysis of four-dimensional fields, and second, the model forecasts are examined to determine the value of glider data. CEs have a Rossby number of about 0.2, implying that the effects of flow curvature, cyclostrophy, to modify the geostrophic momentum balance are slight. The velocity field in CEs is nearly depth independent, while LCEs are more baroclinic, consistent with the CEs origin on the less stratified, dense side of the LCE. CEs are formed from water in the GoM, rather than the Atlantic water that distinguishes the LCE. Model forecasts are improved by glider data, using a quality metric based on satellite SSH, with the best 2-month GoM forecast rivaling the accuracy of a global hindcast.

Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Ocean Science ◽

10.5194/os-13-61-2017 ◽

2017 ◽

Vol 13 (1) ◽

pp. 61-75 ◽

Cited By ~ 4

Author(s):

Arash Bigdeli ◽

Brice Loose ◽

An T. Nguyen ◽

Sylvia T. Cole

Keyword(s):

Gas Exchange ◽

Sea Ice ◽

Mixed Layer ◽

General Circulation ◽

Mixed Layer Depth ◽

Circulation Model ◽

Horizontal Resolution ◽

The Arctic ◽

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

On modeling the Southern Ocean Phytoplankton Functional Types

10.5194/bg-2019-289 ◽

2019 ◽

Author(s):

Svetlana N. Losa ◽

Stephanie Dutkiewicz ◽

Martin Losch ◽

Julia Oelker ◽

Mariana A. Soppa ◽

...

Keyword(s):

Southern Ocean ◽

General Circulation ◽

Circulation Model ◽

Biogeochemical Model ◽

Pigment Analysis ◽

Functional Types ◽

Model Configuration ◽

Abstract. This study highlights recent advances and challenges of applying coupled physical-biogeochemical modeling for investigating the distribution of the key phytoplankton groups in the Southern Ocean, an area of strong interest for understanding biogeochemical cycling and ecosystem functioning under present climate change. Our simulations of the phenology of various Phytoplankton Functional Types (PFTs) are based on a version of the Darwin biogeochemical model coupled to the Massachusetts Institute of Technology (MIT) general circulation model (Darwin-MITgcm). The ecological module version was adapted for the Southern Ocean by: 1) improving coccolithophores abundance relative to the original model by introducing a high affinity for nutrients and an ability to escape grazing control for coccolithophores; 2) including two different (small vs. large) size classes of diatoms; and 3) accounting for two distinct life stages for Phaeocystis (single cell vs. colonial). This new model configuration describes best the competition and co-occurrence of the PFTs in the Southern Ocean. It improves significantly relative to an older version the agreement of the simulated abundance of the coccolithophores and diatoms with in situ scanning electron microscopy observations in the Subantarctic Zone as well as with in situ diatoms and haptophytes (including coccolithophores and Phaeocystis) chlorophyll a concentrations within the Patagonian Shelf and along the Western Antarctic Peninsula obtained by diagnostic pigment analysis. The modeled Southern Ocean PFT dominance also agrees well with satellite-based PFT information.

GEOS-MITgcm coupled atmosphere-ocean simulation for DYAMOND

10.5194/egusphere-egu21-14947 ◽

2021 ◽

Author(s):

Ehud Strobach ◽

Andrea Molod ◽

Atanas Trayanov ◽

William Putman ◽

Dimitris Menemenlis ◽

...

Keyword(s):

General Circulation ◽

Circulation Model ◽

Atmospheric Model ◽

Ocean Model ◽

Second Phase ◽

Grid Spacing ◽

Institute Of Technology ◽

Horizontal Grid ◽

Ocean Simulation

<p>During the past few years, the Goddard Earth Observing System (GEOS) and Massachusetts Institute of Technology general circulation model (MITgcm) groups have produced, respectively, global atmosphere-only and ocean-only simulations with km-scale grid spacing. These simulations have proved invaluable for process studies and the development of satellite and in-situ sampling strategies. Nevertheless, a key limitation of these simulations is the lack of feedback between the ocean and the atmosphere, limiting their usefulness for studying air-sea interactions and designing observing missions to study these interactions. To remove this limitation, we have coupled the km-scale GEOS atmospheric model with the km-scale MITgcm ocean model. We will present preliminary results from the GEOS-MITgcm contribution to the second phase of the DYAMOND (DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains) initiative.</p><p>The coupled atmosphere-ocean simulation was integrated using a cubed-sphere-1440 (~6-7 km horizontal grid spacing) configuration of GEOS and a lat-lon-cap-2160 (2&#8211;5-km horizontal grid spacing) configuration of MITgcm. We will show results from a preliminary analysis of air-sea interactions between Sea Surface Temperature (SST) and surface winds. In particular, we will discuss non-local atmospheric overturning circulation formed above the Gulf Stream SST front with characteristic sub-mesoscale width. This formation of a secondary circulation above the front suggests that capturing such air-sea interaction phenomena requires high-resolution capabilities in both the models' oceanic and atmospheric components.</p>

State estimates and forecasts of the eddy field in the Subtropical Countercurrent in the Northern Philippine Sea

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-20-0083.1 ◽

2021 ◽

Author(s):

Ganesh Gopalakrishnan ◽

Bruce D. Cornuelle ◽

Matthew R. Mazloff ◽

Peter F. Worcester ◽

Matthew A. Dzieciuch

Keyword(s):

General Circulation ◽

Circulation Model ◽

Ocean Model ◽

The State ◽

Philippine Sea ◽

Subtropical Countercurrent ◽

Continuous State ◽

Eddy Field ◽

AbstractA strongly nonlinear eddy field is present in and around the Subtropical Countercurrent in the Northern Philippine Sea (NPS). A regional implementation of the Massachusetts Institute of Technology general circulation model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system is found to be able to produce a series of two-month-long dynamically-consistent optimized state estimates between April 2010 and April 2011 for the eddy-rich NPS region. The assimilation provides a stringent dynamical test of the model, showing that a free run of the model forced using adjusted controls remains consistent with the observations for two months. The 4DVAR iterative optimization reduced the total cost function for the observations and controls by 40–50% from the reference solution, initialized using the Hybrid Coordinate Ocean Model 1/12° global daily analysis, achieving residuals approximately equal to the assumed uncertainties for the assimilated observations. The state estimates are assessed by comparing with assimilated and withheld observations and also by comparing one-month model forecasts with future data. The state estimates and forecasts were more skillful than model persistence and the reference solutions. Finally, the continuous state estimates were used to detect and track the eddies, analyze their structure, and quantify their vertically-integrated meridional heat and salt transports.

State estimates and forecasts of the northern Philippine Sea circulation including ocean acoustic travel times

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-20-0178.1 ◽

2021 ◽

Author(s):

Ganesh Gopalakrishnan ◽

Bruce D. Cornuelle ◽

Matthew R. Mazloff ◽

Peter F. Worcester ◽

Matthew A. Dzieciuch

Keyword(s):

General Circulation ◽

Mesoscale Eddy ◽

Low Frequency ◽

Circulation Model ◽

The State ◽

Sea Surface ◽

Travel Times ◽

Philippine Sea ◽

AbstractThe 2010–2011 North Pacific Acoustic Laboratory (NPAL) Philippine Sea experiment measured travel times between six acoustic transceiver moorings in a 660–km diameter ocean acoustic tomography array in the Northern Philippine Sea (NPS). The travel-time series compare favorably with travel times computed for a yearlong series of state estimates produced for this region using the Massachusetts Institute of Technology general circulation model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system constrained by satellite sea surface height and sea surface temperature observations and by Argo temperature and salinity profiles. Fluctuations in the computed travel times largely match the fluctuations in the measurements caused by the intense mesoscale eddy field in the NPS, providing a powerful test of the observations and state estimates. The computed travel times tend to be shorter than the measured travel times, however, reflecting a warm bias in the state estimates. After processing the travel times to remove tidal signals and extract the low-frequency variability, the differences between the measured and computed travel times were used in addition to SSH, SST, and Argo temperature and salinity observations to further constrain the model and generate improved state estimates. The assimilation of the travel times reduced the misfit between the measured and computed travel times, while not increasing the misfits with the other assimilated observations. The state estimates that used the travel times are more consistent with temperature measurements from an independent oceanographic mooring than the state estimates that did not incorporate the travel times.

Float-Derived Isopycnal Diffusivities in the DIMES Experiment

10.1175/jpo-d-13-0175.1 ◽

2014 ◽

Vol 44 (2) ◽

pp. 764-780 ◽

Cited By ~ 37

Author(s):

J. H. LaCasce ◽

R. Ferrari ◽

J. Marshall ◽

R. Tulloch ◽

D. Balwada ◽

...

Keyword(s):

General Circulation ◽

Circulation Model ◽

Drake Passage ◽

Small Range ◽

Consistent Estimate ◽

Density Surface ◽

Temperature Records ◽

Institute Of Technology ◽

Circumpolar Current

Abstract As part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), 210 subsurface floats were deployed west of the Drake Passage on two targeted density surfaces. Absolute (single particle) diffusivities are calculated for the floats. The focus is on the meridional component, which is less affected by the mean shear. The diffusivities are estimated in several ways, including a novel method based on the probability density function of the meridional displacements. This allows the determination of the range of possible lateral diffusivities, as well as the period over which the spreading can be said to be diffusive. The method is applied to the float data and to synthetic trajectories generated with the Massachusetts Institute of Technology General Circulation Model (MITgcm). Because of ballasting problems, many of the floats did not remain on their targeted density surface. However, the float temperature records suggest that most occupied a small range of densities, so the floats were grouped together for the analysis. The latter focuses on a subset of 109 of the floats, launched near 105°W. The different methods yield a consistent estimate for the diffusivity of 800 ± 200 m2 s−1. The same calculations were made with model particles deployed on 20 different density surfaces and the result for the particles deployed on the neutral density surface γ = 27.7 surface was the same within the errors. The model was then used to map the variation of the diffusivity in the vertical, near the core of the Antarctic Circumpolar Current (ACC). The results suggest mixing is intensified at middepths, between 1500 and 2000 m, consistent with several previous studies.

Double-Ridge Internal Tide Interference and Its Effect on Dissipation in Luzon Strait

10.1175/jpo-d-11-0210.1 ◽

2012 ◽

Vol 42 (8) ◽

pp. 1337-1356 ◽

Cited By ~ 73

Author(s):

Maarten C. Buijsman ◽

Sonya Legg ◽

Jody Klymak

Keyword(s):

Internal Waves ◽

General Circulation ◽

Circulation Model ◽

The Philippines ◽

Luzon Strait ◽

Flux Divergence ◽

Wave Fields ◽

Lee Waves ◽

Abstract Luzon Strait between Taiwan and the Philippines features two parallel north–south-oriented ridges. The barotropic tides that propagate over these ridges cause strong internal waves and dissipation. The energy dissipation mechanisms and the role of the baroclinic wave fields in this dissipation are investigated using numerical simulations with the Massachusetts Institute of Technology general circulation model (MITgcm). The model is integrated over two-dimensional configurations along a zonal transect at 20.6°N for a maximum duration of a spring–neap cycle. Nearly all dissipation occurs at the steep ridge crests due to high-mode turbulent lee waves with horizontal scales of several kilometers and vertical scales of hundreds of meters. The spatial structure and timing of the predicted velocities and dissipation agree with observations and confirm the existence of these lee waves. The lee wave strength is greatly affected by the internal waves generated at the other ridge. When semidiurnal barotropic tides are dominant, the internal wave beams from both ridges nearly superpose after one surface reflection. The remotely generated internal waves from both ridges are therefore in phase with each other and the barotropic tides at the ridges. The barotropic-to-baroclinic energy conversion, energy flux divergence, ridge top velocities, and dissipation are stronger compared to the sum of the single east ridge and single west ridge cases. When diurnal tides are dominant, the wave fields are more out of phase and the conversion, divergence, and dissipation are less than or equal to the single ridge cases combined.

Numerical experiments on subaqueous melting of Greenland tidewater glaciers in response to ocean warming and enhanced subglacial discharge

Annals of Glaciology ◽

10.3189/2012aog60a139 ◽

2012 ◽

Vol 53 (60) ◽

pp. 229-234 ◽

Cited By ~ 98

Author(s):

Yun Xu ◽

Eric Rignot ◽

Dimitris Menemenlis ◽

Michele Koppes

Keyword(s):

General Circulation ◽

Sea Water ◽

Water Flux ◽

Circulation Model ◽

Melting Process ◽

Ocean Model ◽

Thermal Forcing ◽

Tidewater Glaciers ◽

AbstractThe largest dischargers of ice in Greenland are glaciers that terminate in the ocean and melt in contact with sea water. Studies of ice-sheet/ocean interactions have mostly focused on melting beneath near-horizontal floating ice shelves. For tidewater glaciers, melting instead takes place along the vertical face of the calving front. Here we modify the Massachusetts Institute of Technology general circulation model (MITgcm) to include ice melting from a calving face with the freshwater outflow at the glacier grounding line. We use the model to predict melt rates and their sensitivity to ocean thermal forcing and to subglacial discharge. We find that melt rates increase with approximately the one-third power of the subglacial water flux, and increase linearly with ocean thermal forcing. Our simulations indicate that, consistent with limited field data, melting ceases when subglacial discharge is shut off, and reaches several meters per day when subglacial discharge is high in the summer. These results are a first step toward a more realistic representation of subglacial discharge and of ocean thermal forcing on the subaqueous melting of tidewater glaciers in a numerical ocean model. Our results illustrate that the ice-front melting process is both complex and strongly time-dependent.