The Effects of Mesoscale Eddies on the Main Subtropical Thermocline

2004 ◽  
Vol 34 (11) ◽  
pp. 2428-2443 ◽  
Author(s):  
Cara C. Henning ◽  
Geoffrey K. Vallis
Keyword(s):  
Mixed Layer ◽  
Mesoscale Eddy ◽  
Mesoscale Eddies ◽  
Western Boundary ◽  
Residual Circulation ◽  
Mean Flow ◽  
Vertical Diffusion ◽  
Mode Water ◽  
Water Region ◽  
The Mean

Abstract The effects of mesoscale eddies on the main subtropical thermocline are explored using a simply configured wind- and buoyancy-driven primitive equation numerical model in conjunction with transformed Eulerian mean diagnostics and simple scaling ideas and closure schemes. If eddies are suppressed by a modest but nonnegligible horizontal diffusion and vertical diffusion is kept realistically small, the model thermocline exhibits a familiar two-regime structure with an upper, advectively dominated ventilated thermocline and a lower, advective– diffusive internal thermocline, and together these compose the main thermocline. If the horizontal resolution is sufficiently high and the horizontal diffusivity is sufficiently low, then a vigorous mesoscale eddy field emerges. In the mixed layer and upper-mode-water regions, the divergent eddy fluxes are manifestly across isopycnals and so have a diabatic effect. Beneath the mixed layer, the mean structure of the upper (i.e., ventilated) thermocline is still found to be dominated by mean advective terms, except in the “mode water” region and close to the western boundary current. The eddies are particularly strong in the mode-water region, and the low-potential-vorticity pool of the noneddying case is partially eroded away as the eddies try to flatten the isopycnals and reduce available potential energy. The intensity of the eddies decays with depth more slowly than does the mean flow, leading to a three-way balance among eddy flux convergence, mean flow advection, and diffusion in the internal thermocline. Eddies subduct water along isopycnals from the surface into the internal thermocline, replenishing its water masses and maintaining its thickness. Just as in the noneddying case, the dynamics of the internal thermocline can be usefully expressed as an advective–diffusive balance, but where advection is now by the residual (eddy-induced plus Eulerian mean) circulation. The eddy-induced advection partially balances the mean upwelling through the base of the thermocline, and this leads to a slightly thicker thermocline than in the noneddying case. The results suggest that as the diffusivity goes to zero, the residual circulation will go to zero but the thickness of the internal thermocline may remain finite, provided eddy activity persists.

scholarly journals Diagnosing the Influence of Mesoscale Eddy Fluxes on the Deep Western Boundary Current in the 1/10° STORM/NCEP Simulation

2019 ◽  
Vol 49 (3) ◽  
pp. 751-764 ◽  
Author(s):  
Veit Lüschow ◽  
Jin-Song von Storch ◽  
Jochem Marotzke
Keyword(s):  
Potential Energy ◽  
Mesoscale Eddy ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Coriolis Parameter ◽  
Deep Western Boundary Current ◽  
Eddy Fluxes ◽  
The Mean

AbstractUsing a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.

scholarly journals Characteristics, Energetics, and Origins of Agulhas Current Meanders and Their Limited Influence on Ring Shedding

2015 ◽  
Vol 45 (9) ◽  
pp. 2294-2314 ◽  
Author(s):  
Shane Elipot ◽  
Lisa M. Beal
Keyword(s):  
Kinetic Energy ◽  
Single Mode ◽  
Cyclonic Circulation ◽  
Recent Analysis ◽  
Altimeter Data ◽  
Western Boundary ◽  
Mean Flow ◽  
Current Time ◽  
Agulhas Current ◽  
The Mean

AbstractThe Agulhas Current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or Natal pulses. New observations and analyses are presented of the variability of the current and its meanders using mooring observations from the Agulhas Current Time-Series Experiment (ACT) near 34°S. Using a new rotary EOF method, mesoscale meanders and smaller-scale meanders are differentiated and each captured in a single mode of variance. During mesoscale meanders, an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore, leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions. Smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow. Synthesizing in situ observations with altimeter data leads to an account of the number of mesoscale meanders at 34°S: 1.6 yr−1 on average, in agreement with a recent analysis by Rouault and Penven (2011) and significantly less than previously understood. The links between meanders and the arrival of Mozambique Channel eddies or Madagascar dipoles at the western boundary upstream are found to be robust in the 20-yr altimeter record. Yet, only a small fraction of anomalies arriving at the western boundary result in meanders, and of those, two-thirds can be related to ring shedding. Most Agulhas rings are shed independently of meanders.

scholarly journals Mesoscale Eddy Energy Locality in an Idealized Ocean Model

2013 ◽  
Vol 43 (9) ◽  
pp. 1911-1923 ◽  
Author(s):  
Ian Grooms ◽  
Louis-Philippe Nadeau ◽  
K. Shafer Smith
Keyword(s):  
Energy Budget ◽  
Mesoscale Eddy ◽  
Mesoscale Eddies ◽  
Spatial Filter ◽  
Ocean Model ◽  
Energy Budgets ◽  
Subgrid Scale ◽  
Common Time ◽  
Local Functions ◽  
The Mean

Abstract This paper investigates the energy budget of mesoscale eddies in wind-driven two-layer quasigeostrophic simulations. Intuitively, eddy energy can be generated, dissipated, and fluxed from place to place; regions where the budget balances generation and dissipation are “local” and regions that export or import large amounts of eddy energy are “nonlocal.” Many mesoscale parameterizations assume that statistics of the unresolved eddies behave as local functions of the resolved large scales, and studies that relate doubly periodic simulations to ocean patches must assume that the ocean patches have local energetics. This study derives and diagnoses the eddy energy budget in simulations of wind-driven gyres. To more closely approximate the ideas of subgrid-scale parameterization, the authors define the mean and eddies using a spatial filter rather than the more common time average. The eddy energy budget is strongly nonlocal over nearly half the domain in the simulations. In particular, in the intergyre region the eddies lose energy through interactions with the mean, and this energy loss can only be compensated by nonlocal flux of energy from elsewhere in the domain. This study also runs doubly periodic simulations corresponding to ocean patches from basin simulations. The eddy energy level of ocean patches in the basin simulations matches the level in the periodic simulations only in regions with local eddy energy budgets.

Investigating the Impact of Reemerging Sea Surface Temperature Anomalies on the Winter Atmospheric Circulation over the North Atlantic

2007 ◽  
Vol 20 (14) ◽  
pp. 3510-3526 ◽  
Author(s):  
Christophe Cassou ◽  
Clara Deser ◽  
Michael A. Alexander
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Mean Flow ◽  
Thermal Anomalies ◽  
Temperature Anomalies ◽  
The North ◽  
The Mean ◽  
The North Atlantic ◽  
The Impact ◽  
Control Integration

Abstract Extratropical SSTs can be influenced by the “reemergence mechanism,” whereby thermal anomalies in the deep winter mixed layer persist at depth through summer and are then reentrained into the mixed layer in the following winter. The impact of reemergence in the North Atlantic Ocean (NAO) upon the climate system is investigated using an atmospheric general circulation model coupled to a mixed layer ocean/thermodynamic sea ice model. The dominant pattern of thermal anomalies below the mixed layer in summer in a 150-yr control integration is associated with the North Atlantic SST tripole forced by the NAO in the previous winter as indicated by singular value decomposition (SVD). To isolate the reemerging signal, two additional 60-member ensemble experiments were conducted in which temperature anomalies below 40 m obtained from the SVD analysis are added to or subtracted from the control integration. The reemerging signal, given by the mean difference between the two 60-member ensembles, causes the SST anomaly tripole to recur, beginning in fall, amplifying through January, and persisting through the following spring. The atmospheric response to these SST anomalies resembles the circulation that created them the previous winter but with reduced amplitude (10–20 m at 500 mb per °C), modestly enhancing the winter-to-winter persistence of the NAO. Changes in the transient eddies and their interactions with the mean flow contribute to the large-scale equivalent barotropic response throughout the troposphere. The latter can also be attributed to the change in occurrence of intrinsic weather regimes.

scholarly journals Frontal Circulation and Submesoscale Variability during the Formation of a Southern Ocean Mesoscale Eddy

2017 ◽  
Vol 47 (7) ◽  
pp. 1737-1753 ◽  
Author(s):  
Katherine A. Adams ◽  
Philip Hosegood ◽  
John R. Taylor ◽  
Jean-Baptiste Sallée ◽  
Scott Bachman ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Mesoscale Eddy ◽  
Drake Passage ◽  
Mathematical Framework ◽  
Intermediate Water ◽  
Surface Mixed Layer ◽  
Mode Water ◽  
Scotia Sea ◽  
Simplifying Assumptions ◽  
Circumpolar Current

AbstractObservations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.

scholarly journals Mixed layer sub-mesoscale parameterization – Part 1: Derivation and assessment

Ocean Science ◽  
2010 ◽  
Vol 6 (3) ◽  
pp. 679-693 ◽  
Author(s):  
V. M. Canuto ◽  
M. S. Dubovikov
Keyword(s):  
High Resolution ◽  
Mixed Layer ◽  
Wind Direction ◽  
Small Scale ◽  
Mean Flow ◽  
Mean Velocity ◽  
Geostrophic Velocity ◽  
Global Circulation Models ◽  
The Mean ◽  
Strong Winds

Abstract. Several studies have shown that sub-mesoscales (SM ~1 km horizontal scale) play an important role in mixed layer dynamics. In particular, high resolution simulations have shown that in the case of strong down-front wind, the re-stratification induced by the SM is of the same order of the de-stratification induced by small scale turbulence, as well as of that induced by the Ekman velocity. These studies have further concluded that it has become necessary to include SM in ocean global circulation models (OGCMs), especially those used in climate studies. The goal of our work is to derive and assess an analytic parameterization of the vertical tracer flux under baroclinic instabilities and wind of arbitrary directions and strength. To achieve this goal, we have divided the problem into two parts: first, in this work we derive and assess a parameterization of the SM vertical flux of an arbitrary tracer for ocean codes that resolve mesoscales, M, but not sub-mesoscales, SM. In Part 2, presented elsewhere, we have used the results of this work to derive a parameterization of SM fluxes for ocean codes that do not resolve either M or SM. To carry out the first part of our work, we solve the SM dynamic equations including the non-linear terms for which we employ a closure developed and assessed in previous work. We present a detailed analysis for down-front and up-front winds with the following results: (a) down-front wind (blowing in the direction of the surface geostrophic velocity) is the most favorable condition for generating vigorous SM eddies; the de-stratifying effect of the mean flow and re-stratifying effect of SM almost cancel each other out, (b) in the up-front wind case (blowing in the direction opposite to the surface geostrophic velocity), strong winds prevents the SM generation while weak winds hinder the process but the eddies amplify the re-stratifying effect of the mean velocity, (c) wind orthogonal to the geostrophic velocity. In this case, which was not considered in numerical simulations, we show that when the wind direction coincides with that of the horizontal buoyancy gradient, SM eddies are generated and their re-stratifying effect partly cancels the de-stratifying effect of the mean velocity. The case when wind direction is opposite to that of the horizontal buoyancy gradient, is analogous to the case of up-front winds. In conclusion, the new multifaceted implications on the mixed layer stratification caused by the interplay of both strength and directions of the wind in relation to the buoyancy gradient disclosed by high resolution simulations have been reproduced by the present model. The present results can be used in OGCMs that resolve M but not SM.

The Antarctic Circumpolar Current in Three Dimensions

2006 ◽  
Vol 36 (4) ◽  
pp. 651-669 ◽  
Author(s):  
Timour Radko ◽  
John Marshall
Keyword(s):  
Antarctic Circumpolar Current ◽  
Three Dimensional ◽  
Perturbation Expansion ◽  
Dimensional Structure ◽  
Residual Circulation ◽  
Mean Flow ◽  
Overturning Circulation ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract A simple theory is developed for the large-scale three-dimensional structure of the Antarctic Circumpolar Current and the upper cell of its overturning circulation. The model is based on a perturbation expansion about the zonal-average residual-mean model developed previously by Marshall and Radko. The problem is solved using the method of characteristics for idealized patterns of wind and buoyancy forcing constructed from observations. The equilibrium solutions found represent a balance between the Eulerian meridional overturning, eddy-induced circulation, and downstream advection by the mean flow. Depth and stratification of the model thermocline increase in the Atlantic–Indian Oceans sector where the mean wind stress is large. Residual circulation in the model is characterized by intensification of the overturning circulation in the Atlantic–Indian sector and reduction in strength in the Pacific Ocean region. Predicted three-dimensional patterns of stratification and residual circulation in the interior of the ACC are compared with observations.

scholarly journals New Perspectives on the Generation and Maintenance of the Kuroshio Large Meander

2019 ◽  
Vol 49 (8) ◽  
pp. 2095-2113 ◽  
Author(s):  
Yang Yang ◽  
X. San Liang
Keyword(s):  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Low Frequency ◽  
Analysis Tool ◽  
Mean Flow ◽  
Large Meander ◽  
Kuroshio Large Meander ◽  
The Mean ◽  
The Kuroshio ◽  
Canonical Transfer

AbstractThe internal dynamical processes underlying the Kuroshio large meander are investigated using a recently developed analysis tool, multiscale window transform (MWT), and the MWT-based canonical transfer theory. Oceanic fields are reconstructed on a low-frequency mean flow window, a mesoscale eddy window, and a high-frequency synoptic window with reference to the three typical path states south of Japan, that is, the typical large meander (tLM), nearshore non-large meander (nNLM), and offshore non-large meander (oNLM) path states. The interactions between the scale windows are quantitatively evaluated in terms of canonical transfer, which bears a Lie bracket form and conserves energy in the space of scale. In general, baroclinic (barotropic) instability is strengthened (weakened) during the tLM state. For the first time we found a spatially coherent inverse cascade of kinetic energy (KE) from the synoptic eddies to the slowly varying mean flow; it occupies the whole large meander region but exists only in the tLM state. By the time-varying multiscale energetics, a typical large meander is preceded by a strong influx of mesoscale eddy energy from upstream with a cyclonic eddy, which subsequently triggers a strong inverse KE cascade from the mesoscale window to the mean flow window to build up the KE reservoir for the meander. Synoptic frontal eddies are episodically intensified due to the baroclinic instability of the meander, but they immediately feed back to the mean flow window through inverse KE cascade. These results highlight the important role played by inverse KE cascades in generating and maintaining the Kuroshio large meander.

scholarly journals Intermediate-Depth Circulation of the Indian and South Pacific Oceans Measured by Autonomous Floats

2005 ◽  
Vol 35 (5) ◽  
pp. 683-707 ◽  
Author(s):  
Russ E. Davis
Keyword(s):  
Indian Ocean ◽  
Seasonal Variability ◽  
World Ocean ◽  
South Pacific ◽  
Western Boundary ◽  
Mean Flow ◽  
The North ◽  
The Indian Ocean ◽  
The Mean ◽  
Mean Circulation

Abstract As part of the World Ocean Circulation Experiment, 306 autonomous floats were deployed in the tropical and South Pacific Ocean and 228 were deployed in the Indian Ocean to observe the basinwide circulation near 900-m depth. Mean velocities, seasonal variability, and lateral eddy diffusivity from the resultant 2583 float-years of data are presented. Area averages, local function fits, and a novel application of objective mapping are used to estimate the mean circulation. Patterns of mean circulation resemble those at the surface in both basins. Well-developed subtropical gyres, twice as strong in the Indian Ocean as in the Pacific, feed western boundary currents. Tropical gyres are separated by eastward flow along the equator in both hemispheres of both basins, although the Indian subcontinent splits the north Indian tropical gyre. The Antarctic Circumpolar Current (ACC) and west wind drifts are prominent in both basins, generally tending slightly southward but deviating to the north behind the Del Cano, Kerguelen, and Campbell Plateaus and, of course, South America. Remarkably, the eastern boundaries of the southern subtropical gyres in all three basins apparently occur in the ocean interior, away from land. The Indian Ocean’s subtropical gyre, and perhaps part of the South Atlantic’s, reaches east to a retroflection just upstream of the Campbell Plateau south of New Zealand. Seasonal variability at 900 m is focused around the equator with weaker variability found near certain bathymetric features. There is a remarkable agreement between the observed seasonable variability and that predicted by the Jet Propulsion Laboratory (JPL)–Estimating the Circulation and Climate of the Ocean (ECCO) data-assimilating numerical model. Aside from seasonal effects, eddy variability is greatest along the equator, in tropical and subtropical western basins, and along the ACC. Integrals of velocity across regional passages (Tasman Sea, Mozambique Channel) provide useful reference for hydrographic analyses of transport. Across whole ocean basins, however, the uncertainty associated with the appropriate continuity relation for horizontal flow (e.g., geostrophy vs nondivergence) is comparable to the mean flow.

scholarly journals Eddy Shape, Orientation, Propagation, and Mean Flow Feedback in Western Boundary Current Jets

2013 ◽  
Vol 43 (8) ◽  
pp. 1666-1690 ◽  
Author(s):  
Stephanie Waterman ◽  
Brian J. Hoskins
Keyword(s):  
Western Boundary Current ◽  
Upstream Region ◽  
Wave Radiation ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Idealized Model ◽  
Recirculation Gyres ◽  
Flow Interactions ◽  
The Mean

Abstract This manuscript revisits a study of eddy–mean flow interactions in an idealized model of a western boundary current extension jet using properties of the horizontal velocity correlation tensor to diagnose characteristics of average eddy shape, orientation, propagation, and mean flow feedback. These eddy characteristics are then used to provide a new description of the eddy–mean flow interactions observed in terms of different ingredients of the eddy motion. The diagnostics show patterns in average eddy shape, orientation, and propagation that are consistent with the signatures of jet instability in the upstream region and wave radiation in the downstream region. Together they give a feedback onto the mean flow that gives the downstream character of the jet and drives the jet's recirculation gyres. A breakdown of the eddy forcing into contributions from individual terms confirms the expected role of cross-jet gradients in meridional eddy tilt in stabilizing the jet to its barotropic instability; however, it also reveals important roles played by the along-jet evolution of eddy zonal–meridional elongation. It is the mean flow forcing derived from these patterns that acts to strengthen and extend the jet downstream and forces the time-mean recirculation gyres. This understanding of the dependence of mean flow forcing on eddy structural properties suggests that failure to adequately resolve eddy elongation could underlie the weakened jet strength, extent, and changed recirculation structure seen in this idealized model for reduced spatial resolutions. Further, it may suggest new ideas for the parameterization of this forcing.

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