Anticyclonic Vorticity Research Articles

An Extreme Tropical Cyclone Silence in the Western North Pacific in July 2020

ABSTRACT The western North Pacific (WNP) tropical cyclone (TC) number in July 2020 hit a record low since 1949, with no TC formation throughout July. It is shown that this record-breaking TC event can be largely attributed to the extremely strong lower-level anticyclone and subsidence over the TC-genesis-prone region (GPR), concurrent with an intensified western Pacific subtropical high (WPSH) there. This configuration is closely linked to both the air–sea interaction in the tropical Indo-western Pacific and the vigorous wave activities over the mid-high latitude Eurasia. The warming western Indian Ocean and the resultant active convection excited an anomalous lower-level anticyclone and descending atmospheric Kelvin wave over the GPR, together with a strengthened WPSH. Accompanied with this monthly background field, long-lasting Madden-Julian Oscillation was confined west of 90°E, which intensified the proposed anticyclonic vorticity and decreased the moisture condition in the South China Sea and tropical WNP. Meanwhile, two active wave fluxes prevailed across the jet streams over the mid-high latitude Eurasia with a quasi-barotropic cyclonic abnormity dominating from Northeast China to Japan. The resultant positive potential vorticity on its south excited the anomalous local ascending around 35°N, which favoured the upper-level convergence in the tropical WNP and reinforced the local subsidence through modulating the meridional circulation. This extratropical impact further aggravated the suppressed circulation condition for TC formation and increased the chance that the extreme tropical cyclone silence would happen.

On the evolution of vortex in locally isothermal self-gravitating discs: a parameter study

ABSTRACT Gas-rich dusty circumstellar discs observed around young stellar objects are believed to be the birthplace of planets and planetary systems. Recent observations revealed that large-scale horseshoe-like brightness asymmetries are present in dozens of transitional protoplanetary discs. Theoretical studies suggest that these brightness asymmetries bf could be caused by large-scale anticyclonic vortices triggered by the Rossby wave Instability (RWI), which can be excited at the edges of the accretionally inactive region, the dead zone edge. Since vortices may play a key role in planet formation, investigating the conditions of the onset of RWI and the long-term evolution of vortices is inevitable. The aim of our work was to explore the effect of disc geometry (the vertical thickness of the disc), viscosity, the width of the transition region at the dead zone edge, and the disc mass on the onset, lifetime, strength and evolution of vortices formed in the disc. We performed a parametric study assuming different properties for the disc and the viscosity transition by running 1980 2D hydrodynamic simulations in the locally isothermal assumption with disc self-gravity included. Our results revealed that long-lived, large-scale vortex formation favours a shallow surface density slope and low- or moderate-disc masses with Toomre Q ≲ 1 h-1, where h is the geometric aspect ratio of the disc. In general, in low viscosity models, stronger vortices form. However, rapid vortex decay and re-formation is more widespread in these discs.

Flow structures, nonlinear inertial waves and energy transfer in rotating spheres

We investigate flow structures, nonlinear inertial waves and energy transfer in a rotating fluid sphere, using a Galerkin spectral method based on helical-wave decomposition (HWD). Numerical simulations of flows in a sphere are performed with different system rotation rates, where a large-scale forcing is employed. For the case without system rotation, the intense vortex structures are tube-like. When a weak rotation is introduced, small-scale structures are reduced and vortex tubes tend to align with the rotation axis. As the rotation rate increases, a large-scale anticyclonic vortex structure is formed near the rotation axis. The structure is shown to be led by certain geostrophic modes. When the rotation rate further increases, a cyclone and an anticyclone emerge from the top and bottom of the boundary, respectively, where two quasi-geostrophic equatorially symmetric inertial waves dominate the flow. Based on HWD, effects of spherical confinement on rotating turbulence are systematically studied. It is found that the forward cascade becomes weaker as the rotation increases. When the rotation rate becomes larger than some critical value, dual energy cascades emerge, with an inverse cascade at large scales and a forward cascade at small scales. Finally, the flow behavior near the boundary is studied, where the average boundary layer thickness gets smaller when system rotation increases. The flow behavior in the boundary layer is closely related to the interior flow structures, which create significant mass flux between the boundary layer and the interior fluid through Ekman pumping.

Characteristics of the Wind Field in Three Nontornadic Low-Level Mesocyclones Observed by the Doppler On Wheels Radars

The three-dimensional wind fields within three nontornadic supercell thunderstorms are retrieved from dual-Doppler radar observations obtained by a pair of Doppler on Wheels (DOW) radars. The observations focus on the low-level mesocyclone regions of the storms near the time of strongest low-level rotation. All three storms display strong low-level rotation (e.g., the vertical vorticity maxima exceed 0.05 s-1 in the lowest 1000 m AGL in each storm). A principal finding is that the nontornadic mesocyclones possess many of the same signatures found in tornadic supercells, even those viewed in similarly fine resolution; e.g., rear-flank gust fronts wrapping around the circulation centers, multiple cyclonic vertical vorticity maxima along the gust front that spiral inward toward the circulation center, and arching vortex lines joining the cyclonic vorticity maxima to regions of anticyclonic vertical vorticity on the opposite side of the hook echo. The nontornadic mesocyclones possess less circulation than most of the tornadic mesocyclones that have been observed by the DOW radars, particularly within 1 km of the axis of rotation. Another finding is that the trajectories of air parcels passing through the near-surface vertical vorticity maxima have relatively shallow upward vertical excursions, suggesting that these parcels do not enter the overlying midlevel updraft and mesocyclone.

Circulation conservation in the outflow of warm conveyor belts and consequences for Rossby wave evolution

AbstractRossby waves on the jet stream are associated with meridional motions, displacing air and the strong potential vorticity (PV) gradient on isentropic surfaces. Poleward motion along sloping isentropic surfaces typically results in ascent and a ridge of air with low PV values. Latent heating in the ascending warm conveyor belt (WCB) enables air to cross isentropic surfaces so that the WCB outflow into a ridge occurs in a higher isentropic layer than the inflow. However, the PV impermeability theorem states that there can be no PV flux across isentropic surfaces, so how can heating alter the PV pattern of a Rossby wave? Here, the ways in which heating in WCBs can influence Rossby wave evolution at tropopause level are explained in the context of the PV impermeability theorem. First, a WCB outflow volume is defined by the upper tropospheric air in a ridge that has experienced net heating over the last few days, using a tracer within short global model forecasts. Second, the boundary of this outflow volume is tracked backwards using isentropic trajectories allowing quantification of the degree to which circulation is conserved, as predicted by theory, even though the WCB transports mass into the volume from lower isentropic layers. This diabatic flux of mass into the outflow volume results in an increase in density and expansion in the outflow area, the partition being determined approximately by PV inversion. The area expansion, combined with conservation of circulation, implies stronger anticyclonic vorticity. The relative vorticity change from divergent outflow can be as large as the decrease relative to the background planetary vorticity associated with poleward displacement of the circuit. The additional anticyclonic relative motion results in enhanced anticyclonic overturning of PV contours on the eastern flank of the ridge, altering qualitatively the nonlinear evolution of the Rossby wave.

The Evolution of Hail Production in Simulated Supercell Storms

AbstractHailstorms pose a significant socioeconomic risk, necessitating detailed assessments of how the hail threat changes throughout their lifetimes. Hail production involves the favorable juxtaposition of ingredients, but how storm evolution affects these ingredients is unknown, limiting understanding of how hail production evolves. Unfortunately, neither surface hail reports nor radar-based swath estimates have adequate resolution or details needed to assess evolving hail production. Instead, we use a novel approach of coupling a detailed hail trajectory model to idealized convective storm simulations to better understand storm evolution’s influence on hail production. Hail production varies substantially throughout storms’ mature phases: maximum sizes vary by a factor of two, and the concentration of severe hail more than fivefold during 45-60-min periods. This variability arises from changes in updraft properties, which come from (i) changes in low-level convergence, and (ii) internal storm dynamics, including anticyclonic vortex shedding/storm splitting, and the response of the updraft’s airflow and supercooled liquid water content to these events. Hodograph shape strongly affects such behaviors. Straighter hodographs lead to more prolific hail production through wider updrafts and weaker mesocyclones, and a periodicity in hail size metrics associated with anticyclonic vortex shedding and/or storm splitting. In contrast, a curved hodograph (favorable for tornadoes) led to a storm with a stronger but more compact updraft, which occasionally produced giant (10-cm) hail, but that was a less-prolific severe hail producer overall. Unless storms are adequately sampled throughout their lifecycles, snapshots from ground reports will insufficiently resolve the true nature of hail production.

On the Structure and Kinematics of an Algerian Eddy in the Southwestern Mediterranean Sea

An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it.

The Sandwich Mode for Vertical Shear Instability in Protoplanetary Disks

Abstract Turbulence has a profound impact on the evolution of gas and dust in protoplanetary disks (PPDs), from driving the collisions and the diffusion of dust grains, to the concentration of pebbles in giant vortices, thus, facilitating planetesimal formation. The vertical shear instability (VSI) is a hydrodynamic mechanism, operating in PPDs if the local rate of thermal relaxation is high enough. Previous studies of the VSI have, however, relied on the assumption of constant cooling rates, or neglected the finite coupling time between the gas particles and the dust grains. Here, we present the results of hydrodynamic simulations of PPDs with the PLUTO code that include a more realistic thermal relaxation prescription, which enables us to study the VSI in the optically thick and optically thin parts of the disk under consideration of the thermal dust-gas coupling. We show the VSI to cause turbulence even in the optically thick inner regions of PPDs in our two- and three-dimensional simulations. The collisional decoupling of dust and gas particles in the upper atmosphere and the correspondingly inefficient thermal relaxation rates lead to the damping of the VSI turbulence. Long-lived anticyclonic vortices form in our three-dimensional simulation. These structures emerge from the turbulence in the VSI-active layer, persist over hundreds of orbits and extend vertically over the whole extent of the turbulent region. We conclude that the VSI leads to turbulence and the formation of long-lived dust traps within ±3 pressure scale heights distance from the disk midplane.

Cyclonic and anticyclonic contributions to atmospheric energetics

Migratory cyclones and anticyclones account for most of the day-to-day weather variability in the extratropics. These transient eddies act to maintain the midlatitude jet streams by systematically transporting westerly momentum and heat. Yet, little is known about the separate contributions of cyclones and anticyclones to their interaction with the westerlies. Here, using a novel methodology for identifying cyclonic and anticyclonic vortices based on curvature, we quantify their separate contributions to atmospheric energetics and their feedback on the westerly jet streams as represented in Eulerian statistics. We show that climatological westerly acceleration by cyclonic vortices acts to dominantly reinforce the wintertime eddy-driven near-surface westerlies and associated cyclonic shear. Though less baroclinic and energetic, anticyclones still play an important role in transporting westerly momentum toward midlatitudes from the upper-tropospheric thermally driven jet core and carrying eddy energy downstream. These new findings have uncovered essential characteristics of atmospheric energetics, storm track dynamics and eddy-mean flow interaction.

Vorticity and moisture budget analyses on a plateau vortex that cause an intense rainfall event within the Qaidam Basin

AbstractAs one of the most seriously arid areas in China, the Qaidam Basin (QB) features a notable growing probability of intense rainfall under global warming. Compared to a normal/humid region, intense rainfall usually results in more severe disasters in an arid area. Considering that few studies focused on intense rainfall within the QB, there is an urgent need to understand the mechanisms governing intense rainfall in this region. A type of Tibetan Plateau vortex (TPV) associated intense rainfall within the QB was investigated in this study, which partly fills the existing research gaps in the field. The main findings are as follows: (a) the intense‐rainfall‐producing TPV formed and maintained in a favorable background environment which was characterized by a notable upper‐level divergence north of a strong upper‐level jet and a strong middle‐level warm advection ahead of a shortwave trough over the Tibetan Plateau. (b) Vorticity budget indicates that two factors affected the vortex's formation notably, one was the convergence‐related vertical stretching, which dominated the vortex's formation and the other was the import of the horizontal transport of anticyclonic vorticity which was the most detrimental factor for the formation of the TPV. Tilting and vertical transport only exerted weak effects on the TPV's formation, since convective activities were relatively weak in this event. (c) Moisture budget shows that the southwestern and southern moisture transport channels, which were mainly driven by the wind field associated with the shortwave trough over the Tibetan Plateau, contributed ~70% to the total moisture income of the intense rainfall within the QB. The transport was accomplished primarily through the southern boundary of the QB, with the moisture mainly coming from the Indian Peninsula and Indochina Peninsula.

South African drought, deconstructed

Drought is a slow onset, recurring and inevitable feature of South Africa's climate. This research deconstructs the meteorological processes underlying drought and its impacts on surface temperature and vegetation in the more productive eastern half of South Africa. We use an index area 22–31°S, 22–32°E and extract monthly satellite and reanalysis data in the period 1979–2019. Drought intensity is determined by i) vegetation color, ii) soil moisture, iii) maximum air temperature and iv) net outgoing longwave radiation. Global drivers are represented by tropical Pacific and Indian Ocean sea temperatures. Composite and regression analysis of drought reveals a mid-tropospheric anticyclone over Namibia induces equatorward flow and subsidence that drives away atmospheric moisture. This feature is associated with the Pacific El Niño, positive Indian Ocean Dipole and an accelerated westerly jet stream. Ocean warming east of Madagascar draws NW-cloud bands there. The advection of anticyclonic vorticity from the South Atlantic and standing atmospheric Rossby wave-trains are key features of South African drought. Dry spells in the summers of 2015, 2016 and 2019 were more intense than 1983 and 1992, as reflected by S-pan potential evaporation measurements >14 mm/day. Despite water deficits, maize yields and river discharge appear stable, due to the uptake of scientific advice and innovative engineering.

IMPACT OF ARCTIC SEA ICE REDUCTION ON ATMOSPHERIC CIRCULATION PATTERNS

This paper focuses on the effect of sea ice melting under the effect of the mechanism of decreasing albedo of dry and wet ice and snow on the structure of atmospheric circulation. In particular, the Impact on storm tracks in the Pacific and Atlantic Oceans is analyzed. Extreme weather events are usually associated with atmospheric blocking conditions. Blocking is such meteorological conditions in which a large anticyclonic atmospheric vortex is observed over an area for several days. The Molteni-Tibaldi blocking criterion and the magnitude of the local anticyclonic wave activity (LAWA) are used to estimate the number of blockings. Extreme values of LAWA may indicate the presence of atmospheric blockings. As a result, there is a weakening and eastward shift of Atlantic storm trajectories. There is almost no influence on the Pacific storm tracks.

The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations

Abstract. Warm Atlantic-origin waters are modified in the Lofoten Basin in the Nordic Seas on their way toward the Arctic. An energetic eddy field redistributes these waters in the basin. Retained for extended periods, the warm waters result in large surface heat losses to the atmosphere and have an impact on fisheries and regional climate. Here, we describe the eddy field in the Lofoten Basin by analyzing Lagrangian simulations forced by a high-resolution numerical model. We obtain trajectories of particles seeded at three levels – near the surface, at 200 m and at 500 m depth – using 2D and 3D velocity fields. About 200 000 particle trajectories are analyzed from each level and each simulation. Using multivariate wavelet ridge analysis, we identify coherent cyclonic and anticyclonic vortices in the trajectories and describe their characteristics. We then compare the evolution of water properties inside cyclones and anticyclones as well as in the ambient flow outside vortices. As measured from Lagrangian particles, anticyclones have longer lifetimes than cyclones (16–24 d compared to 13–19 d), a larger radius (20–22 km compared to 17–19 km) and a more circular shape (ellipse linearity of 0.45–0.50 compared to 0.51–0.57). The angular frequencies for cyclones and anticyclones have similar magnitudes (absolute values of about 0.05f). The anticyclones are characterized by warm temperature anomalies, whereas cyclones are colder than the background state. Along their path, water parcels in anticyclones cool at a rate of 0.02–0.04 ∘Cd-1, while those in cyclones warm at a rate of 0.01–0.02 ∘Cd-1. Water parcels experience a net downward motion in anticyclones and upward motion in cyclones, often found to be related to changes in temperature and density. The along-path changes in temperature, density and depth are smaller for particles in the ambient flow. An analysis of the net temperature and vorticity fluxes into the Lofoten Basin shows that while vortices contribute significantly to the heat and vorticity budgets, they only cover a small fraction of the domain area (about 6 %). The ambient flow, including filaments and other non-coherent variability undetected by the ridge analysis, hence plays a major role in closing the budgets of the basin.

Interaction of near-inertial waves with an anticyclonic vortex

AbstractAnticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initialwave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.

Evolution of convective characteristics during tropical cyclogenesis

AbstractFour idealized, high‐resolution (500 m horizontal grid spacing), numerical simulations are used to investigate the evolution of convective structures during tropical cyclogenesis. The simulations all begin with a weak initial axisymmetric cloud‐free vortex in a quiescent environment, but differ in the moisture level of the initial sounding and whether or not ice microphysical processes are considered. Irrespective of experimental setup, there is only a short period where shallow or congestus clouds dominate. The shallow cloud phase is slightly extended with the drier initial environmental sounding. The composite structure of the convective elements sampled changes markedly throughout the genesis period. For much of the genesis phase, vertical profiles of the mean convective cell show significant amounts of anticyclonic vorticity produced in cells in the inner core. Towards the end of the genesis phase, there is a large increase in the production of cyclonic vertical vorticity in inner‐core convection, and cyclonic vorticity becomes dominant at low‐mid levels. The evolution from roughly equal strength vertical profiles of cyclonic/anticyclonic vorticity at low‐mid levels to profiles where cyclonic vorticity dominates occurs at relatively low system wind speeds (Vmax less than 10 m·s−1). This finding indicates a change in the structure of vortical convection prior to rapid intensification. In outer‐core convection, there are roughly equal strength vertical vorticity dipoles produced throughout the genesis period.

Transient generation of spiral inertia-gravity waves from a geostrophic vortex

The emission of inertia-gravity waves (IGWs) from an exact geostrophic vortex in a rotating and stratified fluid is investigated by three-dimensional numerical modeling. An initially balanced geostrophic vortex inevitably generates IGWs with spiral patterns within a short transient time period through an instability mechanism. This result reinforces the nonexistence of exactly invariant slow manifolds. The direction of the rotation of spiral IGWs is clockwise for both anticyclonic and cyclonic geostrophic vortices, which is consistent with the theoretical prediction. Spiral patterning can be regarded as a universal feature of IGWs, which occurs in the transient generation process. In the vertical direction, the energy of IGWs is dominated by mode-1 in the generation and propagation processes, leading to weak dissipation and long-distance propagation. A comparison of barotropic and baroclinic vortices suggests that horizontal nonzero strain and vorticity are essential for the occurrence of this instability mechanism, while the presence of vortex baroclinicity increases the intensity of the IGWs. The amplitude of the IGWs increases linearly with the Rossby number in the range of 0.04–0.1. Additionally, the IGWs emitted from an anticyclonic vortex are stronger than those radiated from a cyclonic vortex. Anticyclonic and cyclonic geostrophic vortices transfer approximately 0.54% and 0.41% of their kinetic energy to IGWs in this transient generation process, respectively. This transient generation of IGWs can supply an energy pathway from mesoscale eddies to diapycnal mixing processes in the interior of the oceans.

Application of Symmetric Instability Parameterization in the Coastal and Regional Ocean Community Model (CROCO)

AbstractAs one kind of submesoscale instability, symmetric instability (SI) of the ocean surface mixed layer (SML) plays a significant role in modulating the SML energetics and material transport. The small spatial scales of SI, O(10 m–1 km), are not resolved by current climate ocean models and most regional models. This study describes comparisons in an idealized configuration of the SI parameterization scheme proposed by Bachman et al. (2017, https://doi.org/10.1016/j.ocemod.2016.12.003) (SI‐parameterized) versus the K‐Profile Parameterization scheme (SI‐neglected) as compared to a SI‐permitting model; all variants use the Coastal and Regional Ocean Community Model version of the Regional Ocean Modeling System and this study also serves to introduce the SI parameterization in that model. In both the SI‐parameterized and SI‐permitting models, the geostrophic shear production is enhanced and anticyclonic potential vorticity is reduced versus the SI‐neglected model. A comprehensive comparison of the energetics (geostrophic shear production, vertical buoyancy flux), mixed layer thickness, potential vorticity, and tracer redistribution indicate that all these variables in the SI‐parameterized case have structures closer to the SI‐permitting case in contrast to the SI‐neglected one, demonstrating that the SI scheme qualitatively improves representation of the impacts of SI. This work builds toward applying the SI scheme in a realistic regional or climate model.

The merger of two three-dimensional quasi-geostrophic baroclinic tripolar eddies

We investigate the strong interaction between two baroclinic tripolar eddies in a three-dimensional, rapidly-rotating, continuously stratified flow under the quasi-geostrophic approximation. Each tripolar eddy consists of an anticyclonic central vortex with two oblate cyclonic vortices located above and below the anticyclone. The interaction depends on the vertical and horizontal offsets between the two tripolar eddies. For small and low PV oblate cyclones, each tripolar eddy alone is only weakly unstable to a baroclinic mode. The instability puts the three vortices out of alignment. Most of the eddy however survives the instability. When two tripolar eddies interact, their constituent vortices may merge. Merger occurs when the eddies are close enough together, and shows similarities with the merger of monopolar vortices. Vertically separated eddies do not align vertically. This suggests the importance of an external flow for the alignment, observed in the oceans, to occur. We finally show that the interaction between two tripolar eddies with intense oblate cyclones is very different and show similarities with the dynamics of dipolar baroclinic eddies known as hetons.

A mechanism for formation of the western North Pacific monsoon trough: nonlinear upscale cascade

Linear and nonlinear barotropic vorticity model frameworks are constructed to understand the formation of the monsoon trough in boreal summer over the western North Pacific. The governing equation is written with respect to specified zonal background flows, and a wave perturbation is prescribed in the eastern boundary. Whereas a uniform background mean flow leads no scale contraction, a confluent background zonal flow causes the contraction of zonal wavelength. Under linear dynamics, the wave contraction leads to the development of smaller scale vorticity perturbations. As a result, there is no upscale cascade. Under nonlinear dynamics, cyclonic (anticyclonic) wave disturbances shift northward (southward) away from the central latitude due to the vorticity segregation process. The merging of small-scale cyclonic and anticyclonic perturbations finally leads to the generation of a pair of large-scale cyclonic and anti-cyclonic vorticity gyres, straddling across the central latitude. The large-scale cyclonic circulation due to nonlinear upscale cascade can be further strengthened through a positive convection-circulation feedback.

Effects of system rotation on diffusion of the disturbances in inhomogeneous strongly stratified flow

This study is an extension of our previous study [O. Iida, “Turbulent structure of stably stratified inhomogeneous flow,” Phys. Fluids 30, 045101 (2018)] where direct numerical simulations of a spectral method are performed for an inhomogeneous flow under stable density stratification by the inclusion of a fringe region where an artificial body force is imposed to make the flow locally disturbed, and thus, generated disturbances are horizontally diffused into the undisturbed laminar region. In this study, moreover, additional effects of system rotation on diffused disturbances are investigated in detail. As a result, we find that rotation makes the horizontally diffused disturbances anticyclonic vortices and that with a further increase in rotation, the horizontal diffusion is significantly attenuated, and their horizontal and vertical lengths decrease and increase, respectively. In addition, it is found that attenuating the energy cascade in the vertical direction simultaneously attenuates the horizontal enlargement of anticyclonic vortices.