Anticyclonic Vorticity Research Articles

Balanced ellipsoidal vortex at finite Rossby number

ABSTRACT Here we examine the motion of an isolated ellipsoidal vortex in a rotating stratified fluid. We derive an analytical solution to a set of balanced equations at the next order to quasi-geostrophic theory, providing insights into geophysical vortices at finite Rossby number ε. This is achieved through the solution of a set of complicated Poisson equations. Though complicated, the analytical solution give rise to a velocity field that depends linearly on the spatial coordinates inside the vortex, and, thus preserves the ellipsoidal form. From this general solution, we determine a number of equilibria where the vortex rotates steadily about the vertical axis and examine their stability. At the next order to QG, one finds asymmetry in the behaviour of cyclonic and anti-cyclonic vortices, with anti-cyclonic vortices rotating faster and generally more unstable than cyclonic vortices.

Spatio‐temporal stability analysis applied to monsoon anticyclone flow

AbstractFlow on a beta‐plane driven by a steady localised anticyclonic forcing of potential vorticity (or equivalently a mass source) is considered as a simple model of the Asian monsoon flow in the upper troposphere. Previous authors have noted that the response may be steady, or unsteady, according to the magnitude of the forcing, with the unsteadiness manifested as westward eddy shedding. A detailed study of the transition between steady and eddy‐shedding regimes reveals a third regime ('break up'), for intermediate forcing magnitude, where the flow is steady in the neighbourhood of the forcing, but the westward extending plume of low potential vorticity breaks up into isolated anticyclonic vortices some distance away from the forcing region. A related spatio‐temporal instability problem for flow on a beta plane is specified and analysed. The flow can be stable, convectively unstable or absolutely unstable. It is argued that these three stability regimes correspond to the steady, break‐up and eddy‐shedding regimes for the forced flow and good quantitative correspondence between the regimes is demonstrated by explicit solution of the spatio‐temporal stability problem.

Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges

AbstractMesoscale features present at the Denmark Strait sill regularly enhance the volume transport of the Denmark Strait overflow (DSO). They are important for the Atlantic Meridional Overturning Circulation and ultimately, for the global climate system. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high‐transport events) and DSO cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high‐transport events start collapsing. Regardless of their formation mechanism, DSO cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.

On the vortex evolution in non-isothermal protoplanetary discs

ABSTRACT It is believed that large-scale horseshoe-like brightness asymmetries found in dozens of transitional protoplanetary discs are caused by anticyclonic vortices. These vortices can play a key role in planet formation, as mm-sized dust – the building blocks of planets – can be accumulated inside them. Anticyclonic vortices are formed by the Rossby wave instability, which can be excited at the gap edges opened by a giant planet or at sharp viscosity transitions of accretionally inactive regions. It is known that vortices are prone to stretching and subsequent dissolution due to disc self-gravity for canonical disc masses in the isothermal approximation. To improve the hydrodynamic model of protoplanetary discs, we include the disc thermodynamics in our model. In this paper, we present our results on the evolution of the vortices formed at the outer edge of an accretionally inactive region (dead zone) assuming an ideal equation of state and taking PdV work, disc cooling in the β-approximation, and disc self-gravity into account. Thermodynamics affects the offset and the mode number (referring to the number of small vortices at the early phase) of the RWI excitation, as well as the strength, shape, and lifetime of the large-scale vortex formed through merging of the initial small vortices. We found that the inclusion of gas thermodynamics results in stronger, however decreased lifetime vortices. Our results suggest that a hypothetical vortex-aided planet formation scenario favours effectively cooling discs.

Strengthened Connection between Springtime North Atlantic Oscillation and North Atlantic Tripole SST Pattern since the Late 1980s

AbstractThis study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic.

Flow structures and kinetic-potential exchange in forced rotating stratified turbulence

Flow structures and the kinetic-potential energy exchange in rotating stratified turbulence are studied by simulations. Differences of cyclonic and anticyclonic vortices are shown. The intense kinetic-potential energy exchange is found to be associated with the cyclonic structures.

Near-inertial internal waves and multiple-inertial oscillations trapped by negative vorticity anomaly in the central Sea of Japan

This study aims to reveal the interaction between wind-induced internal waves and mesoscale features in a frontal region of the central Sea of Japan. A mooring system with an upward-looking acoustic Doppler current profiler was deployed over a meandering pathway of the Tsushima Warm Current. The annual record of horizontal current showed an extraordinary event of near-inertial internal waves (NIWs) that were trapped in a mesoscale warm-core eddy, characterized by negative vorticity anomaly. Near the critical depth inside the eddy, where background vorticity vanishes, the amplitude of downward-traveling NIW reached 50–70 cm s−1 in root-mean-square current magnitude. The amplified NIWs were concurrent with the emergence of superinertial oscillations, peaking at multiple inertial (MI) frequencies, i.e., double, triple, and quadruple inertial. The MI oscillations are regarded as arising through nonlinear resonance of NIW. Examining local surface winds suggest that a fast-moving cyclone passing over the site initially generated the NIW event. The trapping and amplification processes of wind-generated NIW are examined by a series of ray-tracing simulations, where background vorticity is reconstructed from in situ hydrographic observations. Based on the results above, a specific picture model of kinetic energy circulation around the Sea of Japan can be presented: the atmospheric event provides momentum energy into the ocean surface, resulting in the generation of NIWs and trapping within anticyclonic vortices. A part of the near-inertial kinetic energy is then transferred into the MI oscillations if NIW is vigorously amplified near the critical depth; then it likely dissipates into turbulence.

Generation of a large-scale vorticity in a fast-rotating density-stratified turbulence or turbulent convection.

We find an instability resulting in generation of large-scale vorticity in a fast-rotating small-scale turbulence or turbulent convection with inhomogeneous fluid density along the rotational axis in anelastic approximation. The large-scale instability causes excitation of two modes: (i) the mode with dominant vertical vorticity and with the mean velocity being independent of the vertical coordinate; (ii) the mode with dominant horizontal vorticity and with the mean momentum being independent of the vertical coordinate. The mode with the dominant vertical vorticity can be excited in a fast-rotating density-stratified hydrodynamic turbulence or turbulent convection. For this mode, the mean entropy is depleted inside the cyclonic vortices, while it is enhanced inside the anticyclonic vortices. The mode with the dominant horizontal vorticity can be excited only in a fast-rotating density-stratified turbulent convection. The developed theory may be relevant for explanation of an origin of large spots observed as immense storms in great planets, e.g., the Great Red Spot in Jupiter and large spots in Saturn. It may be also useful for explanation of an origin of high-latitude spots in rapidly rotating late-type stars.

О РОЛИ МЕЗОМАСШТАБНЫХ ВИХРЕЙ В ПЕРЕМЕЩЕНИИ ПЯТЕН НЕФТЕПРОДУКТОВ В ЮГО-ВОСТОЧНОЙ БАЛТИКЕ (СПУТНИКОВАЯ ИНФОРМАЦИЯ)

In this paper, on the basis of an analysis of the successive satellite optical images (MODISAqua, TIRS Landsat-8, AVHRR NOAA-18) and radar images (SAR-C Sentinel-1A, SAR-C Radarsat-2) on June 8–11, 2015, the effect of the mesoscale vortex movement (anticyclone with diameter of 35 km and associated cyclone) on the transport of oil spots in the northern part of the Gdansk Bay was demonstrated for the first time. The velocities of this transport are estimated; the observed picture of the movement of the spots is compared with their transfer according to the Seatrack Web model. The largest (about 20 cm/s) drift velocity corresponded to the spot that appeared near the periphery of the anticyclonic vortex (the region of maximum velocities), the smallest one was at the spot near the center of the vortex. At a wind speed of not more than 5 m/s on June 10 and an assumed orbital velocity of the anticyclone of the order of 20 cm/s, the contribution of the vortex motion to the total transport of the spots under the influence of wind and vortex should be decisive. The observed drift of the spots did not correspond to the forecast of their movement by the Seatrack Web numerical model, which did not take into account the vortex dynamics of the waters.

Observations of Island Wakes at High Rossby Numbers: Evolution of Submesoscale Vortices and Free Shear Layers

AbstractOceanic vortex evolution on the lee side of Taiwan’s Green Island (~7 km in diameter), where the Kuroshio flows at a speed of 1–1.5 m s−1, is observationally examined and compared to theories and the preceding results of laboratory experiments. In the near wake, recirculation occurs with a relative vorticity of ζ ~ 20f (where f is the planetary vorticity) and subsequently sheds at a combination of periods resulting from the tidal oscillations and the intrinsic time scale of eddy evolution. The tidal oscillations are the predominant processes. Our analysis suggests that an island positioned in the Kuroshio with periodic and cross-stream tidal excursions is analogous to a cross-stream oscillating cylinder. Consequently, the shedding period of the vortex is synchronized to a tidal period occurring close to the intrinsic period. The free shear layer, which is characterized by an ~30f relative vorticity band (2 km wide) and a wavy thermal front, develops between the Kuroshio and recirculation. The frontal wave occurring over a time period of 0.5–2 h resembles Kelvin–Helmholtz instability corresponding to high Re values. For the far wake, repeated cross-wake surveys suggest that cyclonic and anticyclonic vortices are alternatively present at a period close to the period of M2 tides in agreement with near-wake measurements. Repeated along-wake surveys reveal a cyclonic eddy shedding downstream at a speed of 0.35 m s−1, 1/3 of the upstream current speed, from the near wake. In comparing our observations with the results of previous water tank experiments, an Re value of O(103) for the submesoscale wake regime is expected.

The Zonal Dipole Pattern of Tropical Cyclone Genesis in the Indian Ocean Influenced by the Tropical Indo-Pacific Ocean Sea Surface Temperature Anomalies

AbstractFrom a basinwide perspective, the dominant mode of Indian Ocean tropical cyclone genesis (TCG) in September–November (SON) shows an equatorially symmetric east–west zonal dipole pattern, which can explain approximately 13% of the SON TCG variance. This zonal dipole TCG pattern is significantly related to the tripole pattern of the sea surface temperature anomalies (SSTAs) in the tropical Indo-Pacific Ocean (IPT). The IPT, which is a combined interbasin mode and presents a dipole pattern of SSTAs in the tropical Indian Ocean and El Niño–like SSTAs in the tropical Pacific Ocean, can influence the local Walker circulation and zonal dipole TCG pattern over the tropical Indian Ocean. Associated with a positive IPT phase, abnormal ascending (descending) motions are induced and favorable for more (less) water vapor transport to the lower–middle level in the western (eastern) tropical Indian Ocean; significant anticyclonic vorticity anomalies are evoked in the lower level over the eastern tropical Indian Ocean, and weak easterly vertical wind shear appears over the tropical Indian Ocean. Thus, abnormally strong upward motion, abundant water vapor in the lower–middle level, and weak vertical wind shear are favorable for more TCG in the western tropical Indian Ocean, while the combined negative contributions of the vertical motion, lower-level vorticity, and humidity terms result in less TCG in the eastern tropical Indian Ocean.

Feature of formation of the Black Sea surface layer temperature during air cold intrusions

Application of the results of the drifter observations performed in the western Black Sea permitted to show that during air cold intrusions (CI) in winter accompanied by the wind (10 m/c and more), the upper mixed layer (UML) was cooled by 0,1-0,2 °C in course of a day. At that the seasonal thermocline (ST) and the cold intermediate water (CIW) sink deeper; after CI is over these layers rise to the depths smaller than their previous ones. It results in decrease of temperature in UML and its increase, as compared to the period preceding CI, in the layer below ST. The process of the sea upper layer mixing is explained by the fact that anti-cyclonic mesoscale vortices are involved in it.

Dynamics of summer monsoon current around Sri Lanka

From June–September, the summer monsoon current (SMC) flows eastward south of Sri Lanka and bends northeastward to form a swift jet that enters the Bay of Bengal (BoB). This part of SMC serves as the major component of the water exchange between the Arabian Sea and the BoB, maintaining the salt and freshwater of the North Indian Ocean. The processes that determine the evolution, intensification, and meandering of the SMC involve both local and remote forcing by winds. Interactions of the SMC with westward-propagating Rossby waves and eddies are only partly understood. In this study, we investigate these processes using an Indian Ocean general circulation model (MOM4p1) that is capable of simulating the SMC realistically. Because eddies and meanders are smoothed out in the climatology, our analyses focus on a single year of 2009, a period when a strong anticyclonic bend in the SMC was observed. An eddy-kinetic-energy budget analysis shows the region to be a zone of significant eddy activity, where both barotropic and baroclinic instabilities are active. Based on the analysis, we classify the evolution of SMC into stages of onset, intensification, anticyclonic bend, anticyclonic vortex formation, meandering, and termination. In addition, analysis of eddy-potential-vorticity flux and eddy-enstrophy decay reveal when, where, and how the eddies tend to drive the mean flow. Rossby waves and westward-propagating eddies arriving from the east energize the SMC in June and accelerate the mean flow through an up-gradient eddy-potential-vorticity flux. At the same time, local winds also strengthen the flow, by increasing its mean near-surface kinetic energy and raising isopycnals, the latter building up available potential energy (APE). The baroclinic instability that takes place in late July and early August releases APE, thereby generating the SMC meanders.

Long-term tracking of circumpolar cyclones on Jupiter from polar observations with JunoCam

We use observations from the JunoCam instrument on the Juno spacecraft to map the polar regions of Jupiter between 2016 and 2018. These polar maps track the long-term evolution of the pentagonal and octagonal structures of circumpolar cyclones (CPCs). The south-polar pentagonal and north-polar octagonal arrangements of the CPCs have remained largely stable over the last 2 years of observation. The morphologies of individual cyclones also remain largely stable for 1–2 years, although some changes are observed, so that the location of each CPC is uniquely identifiable. At both north and south poles, individual cyclones move around the center to a limited extent, and there is a gap of variable width between one pair of south polar CPCs. Measurements at consecutive closest approaches (perijoves, 53 days apart) show small movements of individual cyclones both eastward and westward. However, the south-polar CPCs have a long-term systematic westward drift rate of 1.5 ± 0.2° per orbit or 0.040 ± 0.005 m/s, whereas north polar CPCs do not show a consistent drift in longitude. Around the perimeter of the southern pentagon, we detect no rapid jet in short-term animations, and we provide evidence that anticyclonic white ovals may have a range of longitudinal drifts between −44 and +15° per orbit (+1.8 to −0.7 m/s). Measurement of the angular velocities within the cyclones shows increasing angular velocity with decreasing radius, except in the inner bright cloud deck of one class of northern CPCs, which are inferred to have smaller or even negative angular velocities very near the center. Tangential wind velocities within the cyclones range up to 100 m/s (and even >100 m/s in the southern polar cyclone) and are maximal between radii of ~900–1600 km. Overall, we find that the behavior of the polar cyclones constellations compares well to vortex crystal theory, even though the theory does not include anticyclonic vorticity.

Subsurface Mesoscale Eddy Generation in the Ocean off Central Chile

AbstractOff the coast of central Chile, subsurface anticyclonic eddies are a salient feature of the oceanic circulation, transporting a significant fraction of coastal water that is rich in nutrients and poor in dissolved oxygen offshore. In this study, the formation mechanism of these eddies is analyzed through a high‐resolution (~0.3 km) and low‐resolution (~3 km) oceanic model that realistically simulate the regional mean circulation, including the Peru‐Chile Undercurrent (PCUC). An analysis of the vorticity and eddy kinetic energy in both simulations indicated that the subsurface eddies can be triggered through a combination of processes that are associated with instabilities of the PCUC. In the high‐resolution simulation, we observed that the interaction between the PCUC and topographic slope generates anticyclonic vorticity and potential vorticity close to zero in the bottom boundary layer. The separation of the undercurrent from the slope favors the intensification of anticyclonic vorticity. It reaches magnitudes that are larger than the planetary vorticity while kinetic energy is converted from the PCUC to the eddy flow. These processes set the necessary conditions for the development of centrifugal instabilities, which can form submesoscale structures. The coalescence of submesoscale structures generates a subsurface anticyclonic mesoscale eddy. In the low‐resolution simulations (>3 km) centrifugal instabilities are not simulated, and the barotropic conversion of the mean kinetic energy into eddy kinetic energy appears as the main process of eddy formation. We showed that the vertical structure of these eddies is sensitive to the spatial resolution of the model.

Thermographic visualization of a flow instability in an electromagnetically driven electrolyte layer

Visualization of complex flows is one of the most challenging problems in fluid dynamics. In this paper, we analyze experimentally the free surface swirling flow of a shallow layer of a weak electrolyte driven electromagnetically in a cylindrical container using a thermographic camera. The flow is produced by a force created by the interaction of a radial electric current and the vertical magnetic field of a permanent magnet. For given values of the applied current and fluid layer thickness, the azimuthal flow develops an instability that leads to the formation of long-lived, traveling anticyclonic vortices close to the walls of the container. Under isothermal conditions, a colorant is used to visualize the onset of the flow instability. Thermographic images provide evidence that the instability is also present under non-isothermal conditions and reveal the thermal effect of the vortices in the global flow. Our results encourage the use of non-intrusive electromagnetic and thermographic methods to explore complex phenomena in fluid dynamics and heat transfer.

A study on causes and recurrence of the Mid-Mediterranean Jet from 2003 to 2015 using satellite thermal and altimetry data and CTD casts

ABSTRACT The existence of the Mid-Mediterranean Jet (MMJ) as part of the pathway of the Atlantic Water (AW) in Eastern Mediterranean is still debated and the reasons for its formation are unknown. The MMJ was investigated over the 2000–2015 summers using thermal and altimetry satellite data and ‘in situ’ casts. In the Levantine basin, the AW flowing along the Libyan-Egyptian coast splits into an along-shore and an offshore branch, the latter following the northern border of the warm anti-cyclones located near the African coast (Libyan-Egyptian Eddies, LEEs). The MMJ is triggered when a surface cold water mass formed south-east of Crete, in part outflowing the Aegean Sea and in part generated within the Rhodes Gyre, moves southward and intrudes the middle Levantine basin. The surface cold water overlays the AW flowing along the northern periphery of LEEs that sinks dozens of metres below increasing its budget of anti-cyclonic vorticity and causing the formation of the jet. Satellite data show that the MMJ formed every summer from 2003 to 2015, sometimes for limited periods of time and at different latitudes, with the exception of summers 2005–2006, probably due to the weakness of the African anti-cyclones that in those years did not advect AW offshore.

A numerical study of island wakes in the Xisha Archipelago associated with mesoscale eddies in the spring

Eddies caused by island wakes often appear downstream of an island where upwelling currents are coupled with nutrient transport. Several island wake eddies downstream of the Yongle atoll (YA), Huaguang Reef (HR) and Yuzhuo Reef (YR) in the Xisha Archipelago (XA) are found based on SAR and GF-1 satellite images taken on April 14, 2009 and May 30, 2014, respectively. The present study aims to explore the features and mechanisms of spring island wakes based on a high-resolution numerical model as well as satellite data. Our results are: 1) The cyclonic wake eddies (CWEs) are surface-intensified, and introduce a 20 – 40 m uplift of the isopycnal for the most intense eddies; While the anticyclonic wake eddies (AWEs) reach a subsurface maximum at a depth of 25 – 50 m, which leads to the weak isopycnal downwelling. The CWEs are generally more abundant than the AWEs in different radius classes under different incoming flows, which is due to the inertial-centrifugal instability (ICI) of the AWEs. 2) In the XA, lower SST features observed in the downstream side of the reefs demonstrate the presence of current-induced island wakes. In the near wakes of YA, HR and YR, the periodic vorticity generation is mainly caused by shear flow instabilities arising from the side boundaries of the reefs. Anticyclonic vorticity generation is slightly larger than that of the cyclonic for these three reefs. 3) For the submesoscale wake dynamics, the barotropic instability is more significant than the baroclinic in the XA. Strong toroidal vortices of alternating sign, which strongly perturb the density, were observed as the existence of the ICI in the transects of an unstable intense AWE. But these could hardly be found in a weak AWE. The unstable AWEs may have a larger decay of the velocity at their eddy edges and large persistent vorticities at their cores, which lead to an increase of the ratio between the core vorticity and the Rossby number Γ. While stable AWEs have longer lifetimes with nearly constant Γ values that are similar to those of the stable CWEs.

Coastal summer eddies in the Peter the Great Bay of the Japan sea: In situ data, numerical modeling and Lagrangian analysis

Coastal mesoscale anticyclonic eddies are regularly generated by the Primorsky Current flowing past the capes in the largest bay of the Japan Sea, the Peter the Great Bay. They were observed in 18 of 21 CTD summer surveys performed from 2001 to 2017. The regional ROMS model is used to study the evolution of the eddy that has been sampled in the oceanographic survey in August 2009. The model is able to reproduce the observed eddy on the days of sampling including its location, size and vertical structure. The Lagrangian maps are computed to investigate and document the generation, deformation and decay of the eddy, to identify the vortex contour and origin of water masses inside it. Such coastal eddies are typical for the summer period in the Bay regulating water exchange between the continental shelf and open sea. They are shown to be generated after intensification of the eastern wind which is sufficiently strong to produce downwelling and increased horizontal density gradients and to generate an anticyclonic vorticity past the capes.

Vortex competition in coastal outflows

Experiments and field observations have shown that there are at least two modes of behavior for river plumes. In many cases, the plume turns to the right (in the Northern Hemisphere) on leaving the river mouth and follows the direction of Kelvin-wave propagation. Alternatively, a “bulge” can form in the plume and a fraction of the outflow volume becomes trapped near the mouth. This paper discusses how bulge formation can be affected by the vorticity profile at the river mouth. Due to the image effect, regions of cyclonic vorticity tend to propagate rightwards, whereas regions of anticyclonic vorticity propagate leftward upon exit from the source. If an outflow consists of regions of cyclonic vorticity to the left of regions of anticyclonic vorticity, the two image effects are in competition. We explore this phenomenon using a quasi-geostrophic model with piecewise-constant potential vorticity, which allows the vorticity profile at the source to be set as part of the problem. We present analytic solutions valid in the source region and at the head of the plume and show that all of the outflow travels rightwards if and only if the region of cyclonic vorticity is dominant. The initial-value problem for the model is integrated numerically using the method of contour dynamics, and the full parameter space is explored. We find that if the cyclonic and anticyclonic contributions cancel, as in the experiments of Avicola and Huq (2003), then steady solutions are unstable and a bulge can form downstream of the river mouth.