Relative Vorticity Gradient Research Articles

Interannual Variations of the β Drift of Tropical Cyclones over the Western North Pacific

Abstract Tropical cyclone (TC) movement consists essentially of the large-scale environmental steering and the β drift, and the latter is also affected by large-scale environmental flows. However, few previous studies have been conducted to examine the interannual variations of the β drift of TCs in the western North Pacific (WNP) basin and the corresponding relationship with the large-scale environmental flow. Using the observed TC tracks and reanalysis dataset during 1965–2019, it is found that the β drift plays a more important role in the TC motion than environmental steering at the interannual time scale. The westward (northward) component of the β drift in the environment with anticyclonic relative vorticity is slower (faster) than that with cyclonic relative vorticity, while an equatorward (poleward) relative vorticity gradient decreases (increases) the westward component of the β drift, in agreement with previous numerical simulations. It is also found that the southwestward migration of the WNP subtropical high can decrease (increase) the westward (northward) component of the β drift. This study suggests that environmental influence on the β drift should be considered when understanding interannual variations of TC movement and tracks, which are important to TC landfall, emergency management, and hazard mitigation. Significance Statement The β drift is an important component of TC movement and is affected by large-scale environmental flows. However, few previous studies have been conducted to examine the interannual variations of the β drift of TCs in the western North Pacific basin and the corresponding relationship with the large-scale environmental flow. This study finds that the β drift plays a more important role in the TC motion than environmental steering at the interannual time scale, and the interannual variations of the β drift are also controlled by large-scale environmental flows. It is suggested that environmental influence on the β drift should be considered when understanding interannual variations of TC movement and tracks, which are important to TC landfall, emergency management, and hazard mitigation.

The role of Rossby waves in polar weather and climate

Abstract. Recent Arctic warming has fuelled interest in the weather and climate of the polar regions and how this interacts with lower latitudes. Several interesting theories of polar-midlatitude linkages involve Rossby wave propagation as a key process even though the meridional gradient in planetary vorticity, crucial for these waves, is weak at high latitudes. Here we review some basic theory and suggest that Rossby waves can indeed explain some features of polar variability, especially when relative vorticity gradients are present. We suggest that large-scale polar flow can be conceptualised as a mix of geostrophic turbulence and Rossby wave propagation, as in the midlatitudes, but with the balance tipped further in favour of turbulent flow. Hence, isolated vortices often dominate but some wavelike features remain. As an example, quasi-stationary or weakly westward-propagating subpolar anomalies emerge from statistical analysis of observed data, and these are consistent with some role for wave propagation. The noted persistence of polar cyclones and anticyclones is attributed in part to the weakened effects of wave dispersion, the mechanism responsible for the decay of midlatitude anomalies in downstream development. We also suggest that the vortex-dominated nature of polar dynamics encourages the emergence of annular mode structures in principal component analyses of extratropical circulation. Finally, we consider how Rossby waves may be triggered from high latitudes. The linear mechanisms known to balance localised heating at lower latitudes are shown to be less efficient in the polar regions. Instead, we suggest the direct response to sea ice loss often manifests as a heat low, with radiative cooling balancing the heating. If the relative vorticity gradient is favourable this does have the potential to trigger a Rossby wave response, although this will often be weak compared to waves forced from lower latitudes.

Impact of Global Warming on the Western North Pacific Circulation Anomaly during Developing El Niño

AbstractEl Niño stimulates an anomalous cyclone over the North Pacific during its developing phase. Using 30 CGCMs and 11 AGCMs from CMIP5, we find a weakly strengthened anomalous North Pacific cyclone (NPC) in a warmer climate in CGCMs, and intermodel uncertainty exists. A similar change of the anomalous NPC is found in AGCMs with increased mean state SST but with a stronger amplitude of enhancement. Based on a simple Gill model, the diabatic heating anomaly, mean state static stability, and meridional gradient of relative vorticity are identified to be responsible for the change of the anomalous NPC. Analyses of the CMIP5 models suggest that the change of the anomalous NPC is largely determined by the competition between the enhanced diabatic heating anomaly and the enhanced mean state static stability. The amplitude of enhancement of the anomalous NPC is strongly modulated by the change of precipitation anomaly over the equatorial central-eastern Pacific, which depends on the changes of mean state SST and the El Niño–related SST anomaly. Compared with a uniform warming, an El Niño–like mean state SST warming favors a much stronger enhancement of the anomalous NPC, by enhancing the mean state precipitation and latent heating anomaly associated with the precipitation anomaly over the equatorial Pacific. However, the air–sea coupling acts to weaken the SST anomaly associated with El Niño in the CGCMs, which further reduces the enhancement of the anomalous NPC.

The influence of wave trains in mid-high latitudes on persistent heavy rain during the first rainy season over South China

Based on daily precipitation data from the Chinese Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy, the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated. The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features, with a period of 8–24 days. The influence of wave trains in the mid-high latitudes to low-frequency persistent heavy rain event (PHR-LF event, the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed. During the first rainy season over South China, there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet. Analysis of wave activity flux indicates that the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east, and then propagates downward over South China. Accordingly, the disturbance of the relative vorticity of the cyclonic anomalies over eastern China is strengthened, which enhances the meridional gradient of relative vorticity. Owing to the transport of low-frequency relative vorticity and geostrophic vorticity by meridional wind, the ascending motion over South China intensifies and lasts for a long time, triggering a PHR-LF event. In addition, the tropical system is also a key factor to PHR-LF event. The QBWO of the convection over the South China Sea provide moisture for PHR-LF events, maintaining persistent rainfall and vertical ascending motion over South China.

Changes in Atmospheric Blocking Circulations Linked with Winter Arctic Warming: A New Perspective

AbstractWinter atmospheric blocking circulations such as Ural blocking (UB) have been recognized to play an important role in recent winter Eurasian cooling. Observational analyses performed here reveal that the winter warming in the Barents–Kara Seas (BKS) related to the recent decline of sea ice concentration (SIC) has been accompanied by a large increase in the mean duration of the UB events. A new energy dispersion index (EDI) is designed to help reveal the physics behind this association and show how the BKS warming can influence the mean duration of UB events. This EDI mainly reflects the role of the meridional potential vorticity (PV) gradient in the blocking persistence and it characterizes the changes in energy dispersion and nonlinearity strength of blocking. The meridional PV gradient combines the relative vorticity gradient (related to the nonuniform meridional shear of the mean zonal wind) and the mean zonal wind strength. It is revealed that the BKS warming leads to a significant lengthening of the UB duration because of weakened energy dispersion and intensified nonlinearity of the UB through reduced meridional PV gradient. Furthermore, the duration of the UB is found to depend more strongly on the meridional PV gradient than the mean westerly wind strength, although the meridional PV gradient includes the effect of mean westerly wind strength. Thus, the meridional PV gradient is a better indicator of the change in the blocking duration related to Arctic warming than the zonal wind strength index.

Sensitivity of Tropical Cyclone Track to the Vertical Structure of a Nearby Monsoon Gyre

Abstract The impact of different vertical structures of a nearby monsoon gyre (MG) on a tropical cyclone (TC) track is investigated using idealized numerical simulations. In the experiment with a relatively deeper MG, the TC experiences a sharp northward turn at a critical point when its zonal westward-moving speed slows down to zero. At the same time, the total vorticity tendency for the TC wavenumber-1 component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC. At this point, the TC motion is dominated by the beta effect, as in a no-mean-flow environment, and takes a sharp northward turn. In contrast, the TC does not exhibit a sharp northward turn with a shallower MG nearby. In the case with a deeper MG, a greater relative vorticity gradient of the MG promotes a quicker attraction between the TC and MG through the vorticity segregation process. In addition, a larger outer size of the TC also favors a faster westward propagation from its initial position, thus having more potential to collocate with the MG. Once the coalescence is in place, the Rossby wave energy dispersion associated with the TC and MG together is enhanced and rapidly strengthens the southwesterly flow on the eastern flank of both systems. The steering flow from both the beta gyre and the Rossby wave dispersion leads the TC to take a sharp northward track when the total vorticity tendency is at its minimum. This study indicates the importance of good representations of the TC structure and its nearby environmental flows in order to accurately predict TC motions.

Formation and variability of the Lofoten basin vortex in a high-resolution ocean model

The Lofoten Basin of the Norwegian Sea is characterized by a local maximum of eddy kinetic energy and it is an important transit region for the warm and saline Atlantic waters on their way towards the Arctic Ocean. Eddies are generated by the Norwegian Atlantic Current and propagate anticlockwise around the center of the basin. In situ and satellite observations have discovered a rather small (with a radius of a few tens of km), but strong quasi-permanent anticyclonic vortex that resides in the center of the Lofoten Basin near 3°E, 69.8°N. The objective of this paper is to understand how and why the vortex is formed and to investigate what processes support its stability and drive its variability. To achieve this objective, we have conducted three high-resolution numerical experiments with the mean horizontal grid spacing of 18km, 9km, and 4km. The Lofoten Vortex did not form in the 18-km experiment. The most realistic (compared to available observations) simulation of the vortex is provided by the 4-km experiment, which better reproduces eddy variability in the region. The experiments thus provide experimental evidence of the importance of eddies in the formation and stability of the vortex. We demonstrate how anticyclonic eddies, that are usually stronger and more numerous in the basin than cyclonic eddies, contribute to the intensification of the Lofoten Vortex. The Lofoten Vortex itself is not stationary and drifts cyclonically within the area bounded by approximately the 3250m isobath. The analysis of the barotropic vorticity budget in the 4-km experiment shows that the advection of the relative vorticity gradient by eddies is the main mechanism that drives the variability of the Lofoten Vortex. The direct impact of wind/buoyancy forcing is found to be small to negligible.

Eddy kinetic energy redistribution within idealized extratropical cyclones using a two‐layer quasi‐geostrophic model

Eddy kinetic energy (EKE) redistribution within midlatitude cyclones is investigated using a two‐layer quasi‐geostrophic model. The flow is composed of synoptic localized cyclones in both layers interacting with a baroclinic zonal basic flow. The effects of the planetary vorticity gradient β and the relative vorticity gradient of the westerly basic flow are to reduce the upstream ageostrophic geopotential fluxes in the lower layer and to intensify the downstream fluxes in the upper layer. Furthermore, they act to reduce the downward ageostrophic geopotential fluxes and to intensify the upward ageostrophic fluxes. When cyclones are embedded in a cyclonic shear, EKE rapidly accumulates on the southwestern flank of the lower‐layer cyclone before being cyclonically redistributed by the rearward cyclonically oriented ageostrophic fluxes and nonlinear advection. In contrast, when cyclones are embedded in an anticyclonic shear, the lower‐layer ageostrophic fluxes are mainly westward‐oriented and the pressure work combines perfectly with nonlinear advection to maintain the EKE increase at the same place upstream or downstream of the lower‐layer cyclone, depending on the value of β. Finally, a simulation showing the crossing of a westerly jet by cyclones condenses all the above effects. At the early stages, when cyclones lie on the anticyclonic side of the jet, a strong EKE increase occurs downstream of the lower‐layer cyclone. Just after the jet crossing, the EKE is rapidly rearward and cyclonically redistributed by the ageostrophic fluxes and nonlinear advection, leading to the formation of a lower‐layer jet to the south of the cyclone.

On the Northward Motion of Midlatitude Cyclones in a Barotropic Meandering Jet

Abstract The combined effects of the deformation (horizontal stretching and shearing) and nonlinearities on the beta drift of midlatitude cyclones are studied using a barotropic quasigeostrophic model on the beta plane. It is found that, without any background flow, a cyclonic vortex moves more rapidly northward when it is initially strongly stretched along a mostly north–south direction. This meridional stretching is more efficient at forming an anticyclone to the east of the cyclone through Rossby wave radiation. The cyclone–anticyclone couple then forms a nonlinear vortex dipole that propagates mostly northward. The case of a cyclone embedded in uniformly sheared zonal flows is then studied. A cyclone evolving in an anticyclonic shear is stretched more strongly, develops a stronger anticyclone, and moves faster northward than a cyclone embedded in a cyclonic shear, which remains almost isotropic. Similar results are found in the general case of uniformly sheared nonzonal flows. The evolution of cyclones is also investigated in the case of a more realistic meandering jet whose relative vorticity gradient creates an effective beta and whose deformation field is spatially varying. A statistical study reveals a strong correlation among the cyclone’s stretching, the anticyclone strength, and the velocity toward the jet center. These different observations agree with the more idealized cases. Finally, these results provide a rationale for the existence of preferential zones for the jet-crossing phase: that is, the phase when a cyclone crosses a jet from its anticyclonic to its cyclonic side.

Relationship between eddy‐driven jet latitude and width

The relationship between the latitude and the width of the eddy‐driven jet is examined. We find that there is strong correlation between jet latitude and jet width, with jets located towards the pole being broader. The broadening of the jet with increased latitude appears to be a consequence of increased barotropic instability. When the jet is located towards the pole, the reduced planetary vorticity gradient is more easily overwhelmed by the negative relative vorticity gradient on the flanks of the jet, and this allows a horizontal shear instability to occur. Enstrophy diagnostics show that when the condition of a negative vorticity gradient is met, the effects of barotropic instability are indeed more prevalent.

The Roles of Vortex Rossby Waves in Hurricane Secondary Eyewall Formation

Abstract A high-resolution, full-physics model initiated with an idealized tropical cyclone–like vortex is used to simulate and investigate the secondary eyewall formation. The beta skirt axisymmetrization (BSA) hypothesis previously proposed is examined and the roles of axisymmetrizing vortex Rossby waves (VRWs) in the secondary eyewall formation are further investigated. During the formation period, convection outside the inner-core region is organized into an outer spiral rainband. The PV dipoles that are persistently generated by convective updrafts through tilting effect move along the rainband and inward toward inner-core region and are finally axisymmetrized in the preexisting beta skirt region. The formation of the secondary eyewall is preceded by a rapid intensification period, during which vortical hot towers, discrete VRWs, and sheared VRWs dominate the inner-core asymmetric structures. Sheared VRWs are repeatedly emanated from the outer edge of the eyewall and become more concentric when propagating outward, leading to the formation of a weak but nonnegligible secondary circulation near the VRWs’ stagnant radius. The mean tangential flow is accelerated by the low-level convergence associated with the secondary circulation and also by the wave–mean flow interaction mechanism, both of which are elucidated by absolute angular momentum budget calculation. The mean radial gradient of relative vorticity is enhanced across the stagnant radius, causing the extension of beta skirt to outer radii in the lower-tropospheric levels. Results from this study suggest that the stagnant radius mechanism and the BSA mechanism may work cooperatively in the sense that the former helps to establish an extensive beta skirt and the latter takes charge from then on.

Observational relationship of climatologic beta drift with large‐scale environmental flows

While many numerical studies suggested that large‐scale environmental flows significantly affect beta drift, this influence was not verified observationally and included in assessing changes of tropical cyclone (TC) tracks under the background of global warming. This study investigates the influence of large‐scale environmental flow on climatologic beta drift in the northern West Pacific using the best track dataset from Joint Typhoon Warming Center (JTWC) and the NCEP monthly reanalysis over the period from 1965 to 2007. Consistent with most previous numerical studies, the meridional shear of zonal wind, vertical shear of zonal wind and meridional gradient of relative vorticity, which are mainly associated with the zonal environmental wind, play an important role in affecting beta drift. For the meridional shear of zonal flows, the northward beta drift in an anticyclonic shear is faster than that in a cyclonic shear. A poleward (equatorward) gradient of relative vorticity leads to faster (slower) westward beta drift than in a resting environment. TCs tend to move to the left of the vector of vertical wind shear.

A high‐resolution simulation of the ocean during the POMME experiment: Simulation results and comparison with observations

A modeling study of physical processes occurring in an area of the northeast Atlantic (21.33°–15.33°W, 38.00°–45.00°N) that was extensively sampled during the Programme Océan Multidisciplinaire Méso Echelle (POMME) (October 2000–September 2001) is carried out. The model is a mesoscale version of the ocean general circulation model OPA developed at the Laboratoire d'Océanographie Dynamique et de Climatologie in Paris. It is used in a three‐dimensional limited area domain with a high‐resolution grid (approximately 5 km horizontal spacing, 69 vertical levels) and realistic boundary conditions (initial state, air‐sea fluxes, open boundary fluxes, and bottom topography). The objectives of the study are to properly simulate the upper ocean dynamics, particularly mesoscale activity and mixed layer evolution, during a key period (restratification) of the POMME experiment (POMME 1 and POMME 2, from February to May 2001) and to compare model results with oceanographic observations collected during the experiment in order to establish confidence in the model. Some results provided by the high‐resolution simulation, in particular features related to mixed layer depth and vertical velocities, are also presented. There is no pronounced north‐south mixed layer depth gradient, but strong filament‐shaped structures associated with stirring at the periphery of eddies are present. Mixed layer restratification is simulated. It is associated with submesoscale mixed layer depth structures and intense vertical velocity filaments in the upper ocean correlated with the relative vorticity gradient field.

Development and application of a mesoscale climate model for the tropics: Influence of sea surface temperature anomalies on the West African monsoon

A mesoscale climate model (MCM) is developed from the Pennsylvania State University/National Center for Atmospheric Research mesoscale model 5 to simulate the West African summer monsoon. Results from the MCM are compared to observations, a reanalysis, and global climate model (GCM) output to show that the MCM reasonably simulates the West African monsoon climate and its variability and improves on many shortcomings of the GCM simulations. The MCM's ability to capture correctly processes that cause interannual variability, and thereby allow us to improve our physical understanding of that variability, is investigated by examining the influence of Gulf of Guinea sea surface temperature anomalies (SSTAs) on the West African monsoon. Similar to observations, precipitation decreases over the Sahel and increases along the Guinean coast in response to warm SSTAs in the gulf. The increase in rainfall along the Guinean coast is associated with an increase in lower tropospheric water vapor content due to enhanced evaporation over the warm SSTAs and northward moisture advection in the monsoon flow. This stronger precipitation on the coast occurs despite a decrease in the land/sea temperature gradient, which is the fundamental driver of the monsoon. The decrease in rainfall over the southern Sahel is associated with lower tropospheric subsidence replacing rising vertical motions as the monsoon circulation is shifted equatorward. Dynamically, the subsidence over the southern Sahel is associated with shrinking of both planetary and relative vorticity. The former is related to the equatorward shift of the monsoon, which results in an expansion of the equatorward flow from the Sahel to the northern Guinean coast. The latter occurs because the equatorward flow over the southern Sahel occurs lower in the troposphere (i.e., 850 mbar) than in the control (i.e., 700 mbar), where the meridional relative vorticity gradient is opposite to the gradient at 700 mbar.

歪んだ一般流の中のサイクロンのβ-Gyreの回転の方程式

An equation of the β-gyre rotation of a cyclone in a non-uniformly straining environmental flow is derived in a barotropic non-divergent system. The environmental flow, which is generally both zonally and meridionally varying, is assumed to be a quadratic function of coordinates. The total flow is assumed to consist of the environmental flow, a symmetric cyclonic circulation and a flow of β-gyres. The resulting equation says the followings. The β effect forces the β-gyre centres to lie perpendicular to the direction of β, i. e., of the gradient of the Coriolis parameter. The symmetric cyclonic circulation advects the β-gyres counter-clockwise. The deformational component of the environmental flow forces the β-gyres centres to lie along the dilatation axis of the defomation matrix. The rotational component of the environmental flow rotates the β-gyres with an angular velocity equal to half the environmental relative vorticity. The second derivative of the environmental flow affects the β-gyre rotation only through the relative vorticity gradient, and the other components of the second derivative have no influence on the rotation.

Propagation of a Tropical Cyclone in a Meridionally Varying Zonal Flow: An Energetics Analysis

Abstract Tropical cyclone propagation (the beta drift) is driven by a secondary circulation associated with axially asymmetric gyres (beta gyres) in the vicinity of the cyclone center. In the presence of the beta effect, the environmental flow may interact with the symmetric circulation and beta gyres of the cyclone, affecting the development of the gyres and thereby the cyclone propagation. An energetics analysis is carried out to elucidate the development mechanism of the beta gyres and to explain variations in propagation speed of a barotropic cyclone embedded in a meridionally varying zonal flow on a beta plane. Two types of zonal flows are considered: one with a constant meridional shear resembling those in the vicinity of a subtropical ridge or a monsoon trough, and the other with a constant relative vorticity gradient as in the vicinity of an easterly (westerly) jet. Zonal flow with a constant meridional shear changes the generation rate of the gyre kinetic energy through an exchange of energy dire...

Large-Scale Lateral Entrainment and Detrainment at the Edge of a Geostrophic Shear Layer

The evolution of large-amplitude disturbances at the outer edge of a quasi-geostrophic shear layer depends on the sign of the outward gradient of potential vorticity. Entrainment of ambient water can occur when the gradient of relative vorticity dominates in the potential vorticity, and detrainment from the current can occur when gradient of isopycnal thickness dominates. In the latter case long, thin filaments of finite area are “pinched off” into the surrounding water mass. This is verified using a quasi-geostrophic model having piecewise uniform potential vorticity. Contour dynamical calculations for many initial conditions allow us to define and tabulate an entrainment/detrainment velocity. This is used for an order of magnitude estimate of the flux of heat or salt on an isopycnal surface in a warm core ring.

Zonal mean properties of Jupiter's upper troposphere from voyager infrared observations

The highest spatial resolution Voyager IRIS spectra are used to produce zonal averages of the temperature at the 150- and 270-mb pressure levels, of the para-hydrogen fraction at 270 mb, of the ammonia abundance near the 680-mb level, and of two infrared cloud optical depths, one near 5 μm and one near 45 μm wavelength. There are two cloud components, one uniformly distributed and only apparent at 5 μm, and another that correlates strongly with the ammonia abundance and that is apparent at both 5 and 45 μm. From the ratio of optical depths at the two wavelengths, the particles in the variable cloud are between 3 and 10 μm in radius. This cloud is located near the ammonia condensation level. The other particles are either smaller or deeper. The cloud and ammonia distribution is consistent with concentration by upward vertical motion at the equatorward edges of prograde atmospheric jets. The temperature field is also consistent with such vertical motion, with radiative heating balancing adiabatic expansional cooling. The para-hydrogen distribution also appears consistent, but noise levels are high. The thermal wind shear indicates decay of the jets with height within the upper troposphere, with a vertical scale of two or three scale heights. The entire set of upper troposphere data is consistent with a simple axisymmetric dynamical model with Coriolis acceleration of the zonal wind balanced by a linear drag. The meridional residual mean circulation in the model, if interpreted also as a Lagrangian mean circulation, would explain nicely the distribution of ammonia and para-hydrogen. The circulation is a response to a deeper tropospheric flow of unknown origin. However, the horizontal scale of jets is on the order of the deformation radius based on a scale height at the base of the upper troposphere. It is conjectured that the physics of the flow may require this to be true, and may also require that the relative vorticity gradient be of the same order as the planetary vorticity gradient, thereby fixing both the dimensions and amplitudes of the jets.

On the preferred mode of Ekman-CISK

The relationship between the vertical partitioning of the diabatic heating and the preferred mode of motion is explored for an Ekman-CISK model. First an unrealistic, but heuristically useful, sinusoidal heating distribution in the vertical is used to establish the link between the growth rate of unstable modes, the Rossby radius of deformation and the diabatic heating profile. The results of the analysis, augmented with those obtained with a more realistic heating distribution, indicate that a rapidly growing, tropical cyclone scale, preferred mode will result from a diabatic heating profile that has a maximum at a low-level. The vertical structure of this mode possesses some realistic features. In particular it has a large vertical gradient of relative vorticity compared with modes associated with mid-tropospheric diabatic heating maximae. DOI: 10.1111/j.2153-3490.1979.tb00919.x