Potential Vorticity Research Articles

Shallow-water modelling of the atmospheric circulation regimes of brown dwarfs and their observable features

ABSTRACTObservations of time-varying thermal emission from brown dwarfs suggest that they have large-scale atmospheric circulation. The magnitude of this variability ranges from a few per cent to tens of per cent, implying a range of sizes of atmospheric perturbations. Periodograms of phase curves of the thermal emission reveal a range of peaks with different periods and widths, suggesting different atmospheric flow speeds and directions. This implies a variety of atmospheric circulations in the different brown dwarfs observed to date, but there is no general theoretical understanding of the circulation regimes these objects can support, and the resulting sizes and velocities of their atmospheric features. We therefore use an idealized 2D shallow-water model of a brown dwarf atmosphere to understand their potential large-scale circulation regimes. We non-dimensionalize the model to reduce the number of input parameters to two non-dimensional numbers: the thermal Rossby number and the non-dimensional radiative time-scale. This allows us to define a parameter space that bounds the entire range of brown dwarf behaviour possible in our model. We analyse the resulting height, velocity, and potential vorticity fields in this parameter space, and simulate observed phase curve and periodograms for comparison with real observations. We use our results to qualitatively define four circulation regimes, which we hope will be useful for interpreting observations and for guiding simulations with more detailed physical models.

Variation of Dynamical Parameters with Upper Tropospheric Potential Vorticity in Tropical Cyclone over the North Indian Ocean Using WRF Model

Meteorologists are experiencing many challenges in the reliable forecasting of the track and intensity of tropical cyclones (TC). Uses of the potential vorticity (PV) technique will enrich the current forecasting system. The use of PV analysis of TC intensification over the North Indian Ocean (NIO) is rare. In this study, the authors analyze the behaviour of upper-level PV with dynamic parameters of TCs over NIO. The authors used NCEP FNL reanalysis 1 × 1 degree data as input in WRF model version 4.0.3 with one-way nesting between the parent and child domains. The authors used a coupling of the Kain-Fritsch (new Eta) scheme and the WSM 6-class graupel scheme as cumulus and microphysics options to run the model. The authors found that at least one potential vorticity unit (PVU) (1 PVU = 10–6 m2 s –1KKg–1) upper PV is required to maintain the intensification of TC. Larger upper PV accelerates the fall of central pressure. The high value of upper PV yields the intensification of TC. The wind shear and upper PV exhibited almost identical temporal evolution. Upper PV cannot intensify the TCs at negative wind shear and shear above the threshold value of 12 ms–1. The upper PV and geopotential heights of 500 hPa change mutually in opposite trends. The upper PV calculated by the model is comparable to that of ECMWF results. Therefore, the findings of this study are admissible.

The role of boundary layer processes in summer-time Arctic cyclones

Abstract. Arctic cyclones are the most energetic weather systems in the Arctic, producing strong winds and precipitation that present major weather hazards. In summer, when the sea ice cover is reduced and more mobile, Arctic cyclones can have large impacts on ocean waves and sea ice. While the development of mid-latitude cyclones is known to be dependent on boundary layer (BL) turbulent fluxes, the dynamics of summer-time Arctic cyclones and their dependence on surface exchange processes have not been investigated. The purpose of this study is to characterise the BL processes acting in summer-time Arctic cyclones and understand their influence on cyclone evolution. The study focuses on two cyclone case studies, each characterised by a different structure during growth in the Arctic: (A) low-level-dominant vorticity (warm-core) structure and (B) upper-level-dominant vorticity (cold-core) structure, linked with a tropopause polar vortex. A potential vorticity (PV) framework is used to diagnose the BL processes in model runs from the ECMWF Integrated Forecasting System model. Both cyclones are associated with frictional Ekman pumping and downward sensible heat fluxes over sea ice. However, a third process, the frictional baroclinic generation of PV, acts differently in A and B due to differences in their low-level temperature structures. Positive PV is generated in Cyclone A near the bent-back warm front, like in typical mid-latitude cyclones. However, the same process produces negative PV tendencies in B, shown to be a consequence of the vertically aligned axisymmetric cold-core structure. This frictional process also acts to cool the lower troposphere, reducing the warm-core anomaly in A and amplifying the cold-core anomaly in B. Both cyclones attain a vertically aligned cold-core structure that persists for several days after maximum intensity, which is consistent with cooling from frictional Ekman pumping, frictional baroclinic PV generation, and downward sensible heat fluxes. This may help to explain the longevity of isolated cold-core Arctic cyclones with columnar vorticity structure.

Climatological features of future MCSs over the Canadian Prairies using convection-permitting climate models

This study explores the potential effects of global warming on Mesoscale Convective Systems (MCSs) initiation and duration in the Canadian Prairies. By utilizing high-resolution convection-permitting climate models (CPCMs), the study examines 15 parameters to identify the most significant factor for predicting future changes, using the Pseudo Global Warming (PGW) approach. The results suggest that the frequency of MCSs will significantly increase in the future, with varying initiation and duration patterns between daytime and nighttime. The study highlights Preciptable Water (PW), Isentropic Potential Vorticity (IPV), and Most Unstable Convective Potential Energy (MUCAPE) as the most significant parameters for long-lived nighttime MCSs, while Relative Humidity at 850 hPa (RH850), PW, and Storm Relative Helicity 0-1 km (SRH1km) are identified as the most significant parameters for long-lived daytime MCSs. The findings are supported by violin plots that show the differences in the distribution of the 15 parameters in the initiation regions of short- and long-lived MCSs. Additionally, this study partially confirms previous research findings, such as the Mountain-plains solenoids, which suggest that increased PW and RH850 in the future are in agreement with increased total precipitation in the northern region. The study emphasizes the importance of utilizing newly released forcing datasets to investigate the potential impact of global warming on MCSs initiation and duration in the Canadian Prairies.

A Comparative Analysis of the Synoptic Conditions of Two Heavy Rainfall Cases West of the Pearl River Estuary

This paper compares the synoptic-scale circulation and thermodynamic conditions of two representative heavy rainfall cases that were concentrated west of the Pearl River Estuary under different seasonal backgrounds. The first case occurred in the early rainy season, with strong precipitation lasting for several hours. The second case occurred in the late rainy season, with extreme flash downpour in just two hours. The analysis reveals that the main cause was different for the two rainfall cases. The main cause of the first rainfall case was converging polar cold air and marine warm air that formed a synoptic scale front that drove the strong ascent. The main cause of the second case was a cold backflow from the Taiwan Strait, resulting in a sharply contrasting equivalent potential temperature front and triggering very strong upward motion. The moist potential vorticity (MPV) showed a negative value before both rainstorms, indicative of the importance of convective instability. A sandwich-like configuration of the MPV led to the development of strong vertical motion for the second case.

Characterization of the 2012 and 2013 Metro Manila “Enhanced Habagat” Heavy Rainfall Events

The strong southwest monsoon episodes in August 2012 and 2013, locally referred to as the “Enhanced Habagat ” 2012 and 2013, respectively, resulted in rainfall that exceeded the monthly mean rainfall of the affected regions along western Luzon, including Metro Manila. The prolonged heavy rainfall events were the result of tropical cyclone enhancement of the monsoon winds. The synoptic environment in the two events was characterized by the deepening of the Asian monsoon trough depicted by the zonally-oriented and eastward-extended 1000 hPa isobar. The deep troughs were caused each year by a combination of tropical cyclones located to the northeast of the Philippines – Typhoon Haikui in 2012 and Severe Tropical Storm Trami in 2013, and remnant tropical cyclones in the northern South China Sea region that formed several days prior to the enhanced Habagat episodes. The monsoon trough induced a low-level westerly jet in the South China Sea toward the western Luzon region that transported a narrow stream of moisture-laden air mass to Metro Manila and surrounding areas. Consequently, heavy precipitation ensued. Analysis showed the remnant lows are as important as the enhancing tropical cyclones Haikui and Trami in inducing the westerly jets. Removal of the enhancing tropical cyclones in numerical model simulations still showed a relatively deep monsoon trough that led to heavy rainfall along western Luzon. In addition, the intrusion of low potential vorticity area to the eastern and northern flanks of the tropical cyclones facilitated the strengthening of the steering ridge that resulted in their westward and slow translational motion, making the events last for several days and exacerbating the impacts. Furthermore, the Madden-Julian Oscillation possibly serves as a precursor to heavy rainfall events. Compounded with a populous megacity, understanding the mechanisms that lead to these extreme hazards is vital to future forecasting and disaster risk management of similar events.

Aspects of potential vorticity circulation in the Northern Hemisphere: climatology and variation

This study revisits the concept of potential vorticity (PV) circulation (PVC) and presents new findings. Results suggest that PVC can cross the isentropic surface. The gross PV in the Northern Hemisphere (NH) depends solely on the total flux of PVC crossing the atmospheric upper boundary, bottom, and cross-section along the equator. In terms of climate, the cross-upper boundary PVC flux is critical for forming the positive basic state of the gross PV in the NH. In terms of variation, a cancelation intrinsically rooted in the PV dynamics between the cross-upper boundary PVC flux and cross-equator PVC flux means that the NH’s gross PV anomaly is largely determined by the cross-bottom PVC flux. Further analysis sheds light on a seminal atmospheric process in which anomalous PVC inflowing from the NH’s upper boundary outflows from the cross-section along the equator and vice versa. An analysis of the quasi-biennial oscillation verifies the process. All results imply that the PVC is a novel tool for examining the interaction between the upper and lower levels of the atmosphere and the interaction between hemispheres.

Theoretical analysis and numerical approximation for the stochastic thermal quasi-geostrophic model

This paper investigates the mathematical properties of a stochastic version of the balanced 2D thermal quasigeostrophic (TQG) model of potential vorticity dynamics. This stochastic TQG model is intended as a basis for parametrization of the dynamical creation of unresolved degrees of freedom in computational simulations of upper ocean dynamics when horizontal buoyancy gradients and bathymetry affect the dynamics, particularly at the submesoscale (250 m–10 km). Specifically, we have chosen the Stochastic Advection by Lie Transport (SALT) algorithm introduced in [D. D. Holm, Variational principles for stochastic fluid dynamics, Proc. Roy. Soc. A: Math. Phys. Eng. Sci. 471 (2015) 20140963, http://dx.doi.org/10.1098/rspa.2014.0963 ] and applied in [C. Cotter, D. Crisan, D. Holm, W. Pan and I. Shevchenko, Modelling uncertainty using stochastic transport noise in a 2-layer quasi-geostrophic model, Found. Data Sci. 2 (2020) 173, https://doi.org/10.3934/fods.2020010 ; Numerically modeling stochastic lie transport in fluid dynamics, SIAM Multiscale Model. Simul. 17 (2019) 192–232, https://doi.org/10.1137/18M1167929 ] as our modeling approach. The SALT approach preserves the Kelvin circulation theorem and an infinite family of integral conservation laws for TQG. The goal of the SALT algorithm is to quantify the uncertainty in the process of up-scaling, or coarse-graining of either observed or synthetic data at fine scales, for use in computational simulations at coarser scales. The present work provides a rigorous mathematical analysis of the solution properties of the thermal quasigeostrophic (TQG) equations with SALT [D. D. Holm and E. Luesink, Stochastic wave-current interaction in thermal shallow water dynamics, J. Nonlinear Sci. 31 (2021), https://doi.org/10.1007/s00332-021-09682-9 ; D. D. Holm, E. Luesink and W. Pan, Stochastic mesoscale circulation dynamics in the thermal ocean, Phys. Fluids 33 (2021) 046603, https://doi.org/10.1063/5.0040026 ].

Annular self-organization of the two-dimensional vorticity condensate

Self-organization of the vorticity condensate of forced two-dimensional turbulence is examined in analogy with the mixing of a background planetary vorticity gradient in geophysical flows. Starting from the theoretical vorticity profile of the condensate, different scenarios are illustrated by the construction of idealized angular momentum conserving rearrangements of vorticity into a staircase profile similar to the potential vorticity staircase of $\beta$ -plane turbulence. Two sets of numerical experiments are then presented that illustrate a similar self-organization of the background vorticity gradient in fully turbulent flows. In the first set of experiments, the flow is initialized with a laminar vortex dipole corresponding to the theoretical condensate vorticity profile. A fluctuation vorticity field is then induced by a stochastic forcing at smaller scales, which induces an azimuthally symmetric self-organization of the laminar flow into distinct annular bands. In the second set of experiments, the flow is initialized from rest with stochastic forcing generating a turbulent inverse energy cascade, out of which emerges the condensate in a self-consistent evolution as the turbulent energy accumulates at the domain scale. A self-organization of the condensate is again observed, giving a distinct annular structure on top of the theoretically predicted condensate profile. A major difference from the potential vorticity staircase of geophysical flows is the emergence in many cases of significantly non-monotonic radial vorticity profiles.

Filaments of uniform quasi-geostrophic potential vorticity in pure strain

Three-dimensional filaments of quasi-geostrophic potential vorticity are generic features of atmospheric and oceanic flows. They are often generated during the strong interactions between three-dimensional quasi-geostrophic vortices. They contribute to a direct cascade of enstrophy in spectral space. These filaments correspond to shear zones. Therefore they may be sensitive to shear instabilities akin to the Kelvin–Helmholtz instability of the classical two-dimensional vorticity strip. They are, however, often subjected to a straining flow induced by the surrounding vortices. This straining flow affects their robustness. This paper focuses on a simplified model of this situation. We consider the effect of a pure strain on a three-dimensional filament of uniform quasi-geostrophic potential vorticity. We first consider a quasi-static situation where the strain, assumed small, only affects the cross-sectional shape of the filament, but not the velocity field. We address the linear stability of the filament in that context and also show examples of the filament's nonlinear evolution. We then consider the linearised dynamics of the filament in pure strain. In particular we focus on the maximum perturbation amplification observed in the filament. We conclude that small to moderate strain rates are efficient at preventing a large perturbation growth. Nonlinear effects can nevertheless leads to the roll-up of weakly strained filaments.

The relationship between extra-tropical cyclone intensity and precipitation in idealised current and future climates

Abstract. Extra-tropical cyclones (ETCs) are the main cause of precipitation in the mid-latitudes, and there is substantial evidence that ETC-related precipitation will increase in the future. However, little is known about how this will impact on the dynamical strength of ETCs, and whether the impact will differ for different types of ETCs. We quantify the linear relationship between maximum vorticity and ETC-related precipitation in the current and idealised future climates and determine how this relationship depends on the structure and characteristics of the ETC. Three 10-year-long aqua-planet simulations are performed with a state-of-the-art global model, OpenIFS, that differ in their specified sea surface temperature (SST) distributions. A control simulation, a uniform warming simulation, and a polar amplification simulation are performed. ETCs are objectively identified using the feature-tracking software TRACK, and k-means clustering is applied to the ETC precipitation field to group the ETCs into clusters with similar precipitation structures. In all experiments, ETCs with stronger maximum vorticity are associated with more precipitation. For all cyclones considered together, we find that the slope of the linear relationship between maximum cyclone vorticity and ETC precipitation is larger in the uniform warming and polar amplification simulations than in the control simulation. We hypothesise that if an increase in precipitation in warmer climates were to feed back, via diabatic heating and potential vorticity anomalies, onto the dynamical intensity of the ETCs, precipitation and vorticity would increase at similar rates, and hence the slope of the linear regression line between precipitation and vorticity would remain similar. Our results indicate either that there is no feedback or that the increase in vorticity due to diabatic heating is masked by the decrease in the Eady growth rate which occurs in both the uniform warming and polar amplification simulations compared to the control. The k-means clustering identifies four distinct and physically realistic types of ETCs which are present in all experiments meaning that the average precipitation patterns associated with ETCs are unlikely to change in the future. The strongest dependency between ETC maximum vorticity and precipitation occurs for ETCs that have the most precipitation associated with the warm front. ETCs with the heaviest precipitation along the cold front, which are the most intense storms in terms of maximum vorticity, also exhibit a strong dependency between precipitation and maximum vorticity, but this dependency is weaker and has a smaller correlation coefficient than the warm-front ETCs. Not all ETC types exhibit a strong dependency between precipitation and maximum vorticity. ETCs located at high latitudes with weak precipitation show little dependency due to the lack of moisture, whereas ETCs with the precipitation located mainly in the centre of the ETCs have the weakest linear regression slope, which is likely due to the lack of upper-level forcing. These results stress that despite small changes in the strength of the cyclones, the precipitation increases are large, indicating potential future increases in flooding associated with cyclones.

Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre‐scale simulation

AbstractThe influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause‐intersecting isentropic surfaces. Lagrangian “PV gradient thinking” is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics‐processing‐unit‐enabled numerical weather prediction model Consortium for Small‐Scale Modelling featuring on‐line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change.

The dynamics of slow‐moving coherent cyclonic potential vorticity anomalies and their links to heavy rainfall over the eastern seaboard of Australia

Heavy rainfall occurs frequently on the subtropical eastern seaboard of Australia (ESB). Many rainfall events are associated with slow‐moving, upper‐level low‐pressure systems. Combined with moisture‐rich easterly winds associated with a surface anticyclone, these systems can produce heavy rainfall, leading to flooding events. Although the general meteorology of these events has been documented, much of the focus has been on surface processes, with limited attention paid previously to the dynamics aloft. Here, we investigate the upper‐level dynamics associated with heavy rainfall events over the ESB with the use of a coherent potential vorticity (PV) minimum climatology on the 330‐K isentropic level. Slow‐moving coherent cyclonic PV anomalies produce more rainfall than fast‐moving anomalies over the ESB. Rossby‐wave breaking is responsible for the development of a slow‐moving coherent cyclonic PV anomaly as well as the commonly observed surface patterns that are necessary for heavy rainfall over the ESB. Slow cyclonic coherent PV anomalies are either transported equatorwards into the Tropics, where they may influence tropical weather systems, or removed from the region by the restoration of the subtropical jet over continental Australia.

A Multifaceted Isoneutral Eddy Transport Diagnostic Framework and Its Application in the Southern Ocean

AbstractWe propose an isoneutral eddy transport diagnostic framework and apply it in the Southern Ocean to analyze eddy potential vorticity (PV) transport dynamics. We leverage a thickness‐weighted spatiotemporal scale separation to define the generalized eddy flux. A local sampling method is employed and validated to estimate the transport tensor. We introduce Leonard's decomposition in Large Eddy Simulations (LES) to split the process into the Reynolds and non‐Reynolds parts. We also use the stationary‐transient decomposition. The non‐Reynolds term and the stationary effect contribute significantly to the transport properties. The Reynolds and transient parts have distinctions, that show oceanic mesoscale dynamics are not synergistic in time and spatial scales. We investigate the spatial scale dependence of each part and find that the transient and stationary components of the Reynolds (non‐Reynolds) term have consistent (dissimilar) scale dependence. The scale dependence of the generalized eddy flux and the transport eigenvalues is controlled by the non‐Reynolds term. We further investigate the eigenvalues in terms of the potential enstrophy and anisotropy. The potential enstrophy in the Southern Ocean holistically has a forward cascade on the researched scales. The Reynolds and transient effects efficiently promote the forward enstrophy cascade. The non‐Reynolds and stationary effects stimulate drastically anisotropic eddy transport. The PV gradient barrier mechanism dominates the drastic anisotropic case. When the anisotropy is weak, a special anisotropic case with a locally balanced potential enstrophy budget overwhelms the case of real isotropy. We also introduce several LES concepts, hoping to enlighten any potential ocean modeling and parameterization.

О СВЯЗИ ПЕРИСТОЙ ОБЛАЧНОСТИ И ПОЛЕЙ ОТНОСИТЕЛЬНОЙ ВЛАЖНОСТИ В ВЕРХНЕЙ ТРОПОСФЕРЕ С СИММЕТРИЧНОЙ НЕУСТОЙЧИВОСТЬЮ

The relationship of cirrus clouds and areas of increased relative humidity in the upper troposphere with symmetric instability developing on the subtropical (anticyclonic) side of jet streams are explored. Theoretical arguments are supplemented by case studies related to the Northern and Southern hemispheres. Using satellite images of cirrus clouds and reanalysis data, the values of the vertical eddy viscosity, which compensates for the inviscid growth of disturbances in case of symmetric instability, are estimated. At the levels of 250 and 200 hPa, a significant correlation between high relative humidity and potential vorticity characteristic of symmetric instability is confirmed.

Three-Dimensional Fujiwhara Effect for Binary Tropical Cyclones in the Western North Pacific

Abstract Recent idealized simulations have shown that a system of binary tropical cyclones (TCs) induces vertical wind shear (VWS) in each TC, which can subsequently modify the tracks of these TCs through asymmetric diabatic heating. This study investigates these three-dimensional effects in the western North Pacific using the best track and ERA5 reanalysis data. The TC motion was found to deviate systematically from the steering flow. The direction of deviation is clockwise and repelling with respect to the midpoint of the binary TCs with a separation distance of more than 1000 km. The large-scale upper-level anticyclonic and lower-level cyclonic circulations serve as the VWS for each TC in a manner consistent with the idealized simulations. The VWS of a TC tends to be directed to the rear-left quadrant from the direction of the counterpart TC, where the maxima of rainfall and diabatic heating are observed. The potential vorticity budget analysis shows that the actual TC motion is modulated by the diabatic heating asymmetry that offsets the counterclockwise and approaching motion owing to horizontal advection when the separation distance of the binary TCs is 1000–2000 km. With a small separation distance (<1000 km), horizontal advection becomes significant, but the impact of diabatic heating asymmetry is not negligible. The abovementioned features are robust, while there are some dependencies on the TC intensities, size, circulation, duration, and geographical location. This research sheds light on the motion of binary TCs that has not been previously explained by a two-dimensional barotropic framework.

Incidence and Reflection of Offshore Subinertial and Barotropic Pressure Signals in Wide Shelf Seas

Abstract The response of a wide shelf to subinertial and barotropic offshore pressure signals from the shelf edge was investigated. By relaxing the semigeostrophic approximation, an elliptical wave structure equation was formulated and solved with the integral transform method. It was found that when the imposed offshore signal has an along-shelf length scale similar to the shelf width, it can efficiently break the potential vorticity barrier and propagate toward the coast, producing a significant coastal sea level setup. Thereafter, the pressure signal reflects from the coast or the sloping topography, producing a transient eddy and propagates to the downshelf. The intensities of the coastal setup and the eddy increase as the along-shelf scale of the subinertial signal decreases or when its time scale is close to the inertial period. For a signal with longer time scale, the eddy is insignificant. The nature of the shelf response is controlled by the shelf conductivity κ ≡ r/(fsB), in which r is the Rayleigh friction coefficient, f is the Coriolis parameter, s is the shelf slope, and B is the shelf width, respectively. For a given offshore signal, coastal setup increases with κ. For large κ, the eddy energy is concentrated at low modes, producing a large eddy, whereas a small κ produces a small eddy. The proposed theory can explain coastal sea level fluctuations under eddy impingement in the Mid-Atlantic Bight or other similar areas. Significance Statement Coastal sea level and shelf circulation are greatly affected by offshore pressure signals, e.g., mesoscale eddy impingements or boundary current fluctuations. It is often assumed that the along-shelf length scale of the forcing is much larger than the shelf width, i.e., the semigeostrophic approximation. Here in this study, we found this approximation significantly underestimates the shelf–ocean interaction. A general shelf wave equation was developed that relaxed the semigeostrophic approximation and was solved analytically with a novel mathematical method. The solution can characterize the shelf response to subinertial offshore forcing at arbitrary spatiotemporal scales. It was found that for a subinertial signal with scale close to or smaller than the shelf width, significant coastal sea level setup and transient eddy can be formed, which was consistent with realistic phenomena. The new theory could promote the understanding of coastal sea level variations and along-/cross-shelf transports at synoptic and intermediate scales.

Quasi-two-day wave amplification through interhemispheric coupling during the 2010 austral summer

A quasi-two-day wave (QTDW) event during January-February 2010 is investigated using the MERRA-2 reanalysis dataset. MERRA-2 data reveals the growth of the QTDW westward propagating wavenumber 3 preceded by a weak westward wavenumber 4. The diagnostic analysis suggests that the QTDW growth near tropical stratopause is mainly supported by barotropic (BT) instability in the presence of the summer easterly jet. BT instability during this event is suggested to be linked to the planetary wave breaking (PWB) in the winter hemisphere. The connecting link between the two appears to be the inertial instability (II) supported by the cross-equatorial transport of potential vorticity in association with PWB in the winter hemisphere which is consistent with the previous studies. Signatures of II are evident from the MERRA-2 temperature and wind anomalies in the vicinity of cross-equatorial transport of potential vorticity. One important observation brought out in this study is that the strength of meridional wind anomalies in the regions of the II appears to control the meridional curvature of the zonal mean zonal wind and, hence, BT instability of the jet. Overall, the temporal evolution of the QTDW at tropical stratopause during this event is consistent with the episodes of PWB, strength of meridional wind anomalies around the II and meridional curvature of zonal mean zonal wind. The latitude-altitude growth of the QTDW W3 in the zonal wind conforms to the location of critical layers and region of negative meridional gradient of potential vorticity.

The Different Characteristics of the Mass Transport between the Stratosphere and the Troposphere in Two Types of Cyclonic Rossby Wave-Breaking Events

Using the ERA5 reanalysis data and trajectory analysis provided by Hysplit4, a comparative analysis was conducted on the primary pathways of air particles and the dominant weather systems in two distinct cases of equatorward and poleward cyclonic Rossby wave-breaking (CWB) events. Subsequently, the characteristics of mass exchange between the stratosphere and troposphere in both CWBs were estimated and discussed. CWB events are frequently associated with the development of an upper front in subtropics and a ridge or blocking in mid-latitudes, leading to a tropopause anomaly characterized by a downward depression in the subtropics and an upward bulge in the mid-latitudes. High potential vorticity (PV) particles exhibit negligible vertical motion and are instead controlled by the circulation of the ridge or blocking, leading to a significant poleward transport. In contrast, low PV particles display noticeable vertical motion, with approximately one fourth of them ascending on the north side of the upper-level jet exit region. After CWB occurrence, approximately 25% of low PV particles moved southward and sank below 500 hPa with the downstream trough’s cold air. Most high PV particles remained in the stratosphere, and low PV particles predominantly remained in the troposphere. Only a small proportion (2% to 6%) of particles underwent stratosphere–troposphere exchange (STE). In equatorward CWB, STE manifested as transport from stratosphere to troposphere, occurring mainly in 24–48 h post breaking with a maximum mass transport of approximately 1.54 × 1013 kg. In poleward CWB, STE involved transport from troposphere to stratosphere, occurring mainly within 0–18 h post breaking with a maximum mass transport of approximately 1.48 × 1013 kg.

Seasonal Variability of the Deep Western Boundary Current in the Philippine Sea

The deep western boundary current (DWBC) in the Philippine Sea has been expected to play a crucial role in transporting lower circumpolar deep water and to contribute to regional and global climate regulation. Two-year-long near-bottom current measurements reveal a southward-flowing DWBC with a mean velocity of 5 cm/s and seasonal variations—weaker in summer and stronger in winter. Seasonal variability in the DWBC is hypothesized to be induced by changes in the North Equatorial Current bifurcation latitude (NECBL) and upper pycnocline depth through potential vorticity conservation. Data-assimilated reanalysis model (GLORYS12V1) outputs, which reproduce the seasonal variability of DWBC similarly to the observation, are used for further analysis. During the seasonal period, the NECBL displays significant coherence (>0.9) with the first-mode empirical orthogonal function principal component of the simulated along-slope DWBC. The upper pycnocline depth, varying seasonally within a range of approximately 27 m, induces seasonal variability in a deep anticyclonic eddy trapped by topography. In summer, the intensified deep anticyclonic eddy obstructs the adjacent southward-flowing DWBC, weakening its strength, whereas in winter, the southward flow of the DWBC is enhanced due to the weakening of the deep anticyclonic eddy.