Intrathermocline Eddies Research Articles

Formation of intrathermocline eddies at ocean fronts by wind-driven destruction of potential vorticity

A mechanism for the generation of intrathermocline eddies (ITEs) at wind-forced fronts is examined using a high resolution numerical simulation. Favorable conditions for ITE formation result at fronts forced by “down-front” winds, i.e. winds blowing in the direction of the frontal jet. Down-front winds exert frictional forces that reduce the potential vorticity (PV) within the surface boundary in the frontal outcrop, providing a source for the low-PV water that is the materia prima of ITEs. Meandering of the front drives vertical motions that subduct the low-PV water into the pycnocline, pooling it into the coherent anticyclonic vortex of a submesoscale ITE. As the fluid is subducted along the outcropping frontal isopycnal, the low-PV water, which at the surface is associated with strongly baroclinic flow, re-expresses itself as water with nearly zero absolute vorticity. This generation of strong anticyclonic vorticity results from the tilting of the horizontal vorticity of the frontal jet, not from vortex squashing. During the formation of the ITE, high-PV water from the pycnocline is upwelled alongside the subducting low-PV surface water. The positive correlation between the ITE’s velocity and PV fields results in an upward, along-isopycnal eddy PV flux that scales with the surface frictional PV flux driven by the wind. The relationship between the eddy and wind-induced frictional PV flux is nonlocal in time, as the eddy PV flux persists long after the wind forcing is shut off. The ITE’s PV flux affects the large-scale flow by driving an eddy-induced transport or bolus velocity down the outcropping isopycnal layer with a magnitude that scales with the Ekman velocity.

Why do Intrathermocline Eddies Form in the Japan/East Sea? A Modeling Perspective

Abstract : Intrathermocline eddies (ITEs) are characterized by a subsurface lens of relatively homogeneous water. By definition, they are situated within the thermocline and therefore split the stratified water column, taking the form of a dome in the upper part of the thermocline and a bowl in the lower part. Observations of ITEs in diverse regions of the world ocean indicate typical spatial scales of 10-100 km horizontally and 100 m vertically. In the Japan/East Sea (JES) there are at least three mechanisms for the formation of ITEs from pre-existing non-ITE eddies based on results from the HYbrid Coordinate Ocean Model (HYCOM). Those mechanisms include advection of the stratified seasonal variations of temperature and salinity through the Tsushima Strait, restratification of the upper water column due to seasonal heating and cooling of the upper ocean, and subduction of ITE water originating from the Tsushima Strait beneath the wintertime Subpolar Front. The formation mechanisms are not mutually exclusive. Indeed, all three are shown to be interactively affecting the formation of an ITE in at least one case. Gordon et al. (2002) reported the existence of ITEs in the JES based on observations from SeaSoar instrumentation, conductivity-temperature depth (CTD) sensors, and airborne expendable bathythermographs (AXBTs). Their paper contains extensive analysis of ITEs in the JES and observational evidence of formation mechanisms based on cruise data collected during 1999-2000 as part of the Office of Naval Research (ONR) JES Department Research Initiative as well as results from earlier studies. The Gordon et al. work inspired a numerical modeling study to examine whether or not similar features could be simulated. If they could be simulated, could the ocean model be used as a tool to elucidate the formation mechanisms of the ITEs? This study uses HYCOM to simulate JES ITEs that have domed stratification at the top, forming a lens-shaped interior of nearly unstratified water.

Intrathermocline eddies in the Southern Indian Ocean

In 2001, two relatively saline intrathermocline eddies (ITEs) were observed southeast of Madagascar at 200 m depth. They are characterized by a subsurface salinity maximum of over 35.8 at potential temperatures between 18° and 22°C. The oxygen concentrations within the high salinity cores are slightly elevated compared with those of the surrounding water. Their horizontal extent is about 180 km, several times the Rossby deformation radius, while their thickness is about 150 m. The observed circulation around the ITEs is anticyclonic and maximum velocities of 20 to 30 cm/s are observed at 200 m depth. In these cores the potential density anomaly (25.0 < γ < 25.9 kg/m3) has a relatively low vertical gradient and therefore a low planetary potential vorticity. The hydrographic properties of these ITEs are distinctly different from those of the surrounding thermocline water, and especially from the much fresher water mass in the East Madagascar Current. Strong evidence has been found that the distant formation area of the water mass in the ITEs is the subtropical Southern Indian Ocean east of 90°E and south of 25°S, where Subtropical Underwater (STUW) is formed with similar characteristics. Similar high‐salinity cores as the ITEs are also found in the thermocline around 200 m depth along an almost zonal section between Madagascar and 100°E. Differences between the hydrographic properties of these cores and the ITEs near Madagascar may partly be explained by interannual variations in the temperature and salinity of the surface mixed layer water in the possible formation area.

Japan/East Sea Intrathermocline Eddies

Intrathermocline eddies (ITE) with diameters of 100 km and of thickness greater than 100 m are observed within each of the three quasi-stationary meanders of the Tsushima Current of the Japan/East Sea. Within the ITE homogenous, anticyclonic flowing core, the temperature is near 10°C with a salinity of 34.12 psu. Because of compensatory baroclinicity of the upper and lower boundaries of the ITE core, the ITE has minor sea level expression. The ITE core displays positive oxygen and negative salinity anomalies in comparison to the surrounding thermocline water, indicative of formation from winter mixed layer water along the southern side of the Japan/East Sea subpolar front. The winter mixing layer is then overridden, or slips below, the regional upper thermocline stratification with its characteristic salinity maximum layer. The winter mixed layer off the coast of Korea closely matches the ITE core characteristics, and is considered as a potential source region. Other sources may be present along the southern boundary of the subpolar front, including a frequently observed warm eddy over the western side of Yamato Rise.

Subduction along a Midocean Front and the Generation of Intrathermocline Eddies: A Theoretical Study*

Through an idealized model, the authors consider the dynamics of subduction along a midocean front and its linkage to the intrathermocline eddies (ITEs). The subduction is necessitated by advective–diffusive balance of potential vorticity (PV), with its flux mainly a function of the mixed layer depth over the normal range of the horizontal diffusivity. The mismatch of PV impedes the entry of the subducted water into the interior, resulting in an excess flux that peaks at some intermediate mixed layer depth. This mismatch also causes the generation of anticyclonic ITEs, whose radius contains no lower bound, and a maximum limited by the entrainment rate. Through entrainment cooling, ITEs may leave their imprints in the surface temperature, giving rise to a meandering appearance of the front, even in the absence of instability.

Hydrological and dynamical characterization of Meddies in the Azores region: A paradigm for baroclinic vortex dynamics

The Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Expérimentale (SEMAPHORE) oceanographic experiment surveyed a 500 × 500 km2 domain south of the Azores from June to November 1993 and collected hydrological data, float trajectories, and current meter recordings. This data exhibited three intrathermocline eddies of Mediterranean water (Meddies), two of them being repeatedly sampled. Their hydrological and dynamical properties are quantified here by an isopycnic analysis. For the three Meddies, intense temperature and salinity anomalies (up to 4°C and 1.1 practical salinity units (psu)) are observed extending vertically over up to 1000 m and centered around 1000 m. Horizontally, these anomalies spread out to radii of 50–60 km, while the maximum azimuthal velocities (30 cm s−1, as computed by geostrophy) lie only at 35–40 km from the central axis. These Meddies followed curved trajectories, with drift velocities up to 7.5 cm s−1, under the influence of the neighboring mesoscale features (cyclonic vortices or Azores Current meanders). The three‐dimensional structure of potential vorticity in and around these features evidences their complex interactions. Northwest of the domain, a Meddy was coupled to a subsurface anticyclone, forming an “aligned” vortex. It later interacted with the Azores Current, creating a large‐amplitude northward meander by vertical alignment of vorticity. In the southeastern part of the domain, another Meddy was vertically aligned with an anticyclonic meander of the Azores Current and horizontally coupled with a cyclone of large vertical extent. These two features, as well as a small warm and salty fragment in their vicinity, seem to result from the southward crossing of the Meddy under the Azores Current. These observations illustrate previous theoretical studies of baroclinic vortex dynamics.

Wintertime convection in warm‐core rings: Thermocline ventilation and the formation of mesoscale lenses

Large heat losses from warm‐core ring 82B during February, March, and April 1982 formed a 400‐m‐thick thermostad in the center of the ring. Two different models were constructed to simulate the formation of the thermostad water mass properties: a one‐dimensional vertical model forced with a meteorological time series and a volumetric mixing model with climatological input. Both models indicate that a flux of slope water into the ring is necessary to yield the observed thermostad salinities. The subsequent slow decay of the ring core in the slope water suggests that such winter‐convected rings can easily maintain the “warm band” water identified by Wright and Parker (1976). The large loss of core water during interactions with the Gulf Stream suggests that the reabsorption of such winter‐cooled warm rings is an important source of high‐salinity, high‐oxygen anomalies in the main thermocline of the Sargasso Sea. The volume of water in a typical ring reabsorption is sufficient to generate many (>20) intrathermocline eddies with T‐S properties close to those of recently reported lenses. This suggests that many of these small eddies are much more local in origin and may be more highly dissipative than has previously been suggested. An estimate of the flux of high‐salinity, high‐oxygen water into the main thermocline by this mechanism leads to ventilation at a rate that is a significant fraction of the Ekman pumping at these density levels. Thus the reabsorption of winter‐cooled warm‐core rings by the Gulf Stream is an important eddy ventilation mechanism for the Sargasso Sea.