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A simplified model of the Earth’s atmosphere is considered, which on the one hand exactly reproduces the real characteristics of the Earth, and on the other hand extremely simplifies the configuration of continents and oceans and completely ignores the topology of the real planet. The aim of this formulation is to analyze the generation of large-scale mid-latitude eddies and waves on a zonally homogeneous flat planet with energy balance and atmospheric properties similar to those of the Earth. It is shown that the preservation of a relatively small equatorial ocean on the background of a flat desert planet is sufficient to reproduce the general atmospheric circulation and the dynamics of mid-latitude wave and vortex structures that are realistic for the Earth. The considered idealized system correctly reproduces not only the spatial structure of the emerging baroclinic waves, but also their seasonal dynamics, including the variability of the spectral composition of the observed waves. The modes with wave numbers 3–8 contain most of the wave energy. The spectra for higher modes are characterized by a power law E m ∼ m γ with a slope γ ≈ - 3.5 for the winter season and γ ≈ - 3 for the summer season. During the baroclinic season, there is a change in the dominant wave number from 7 to 4. Another particular feature of baroclinic waves in a considered configuration is a significant temporal variation of the phase velocity with two distinct stages of monotonic increase and decrease. There is a time lag between beginning of the gradual increase of the phase velocity for different modes. The higher modes begin to accelerate earlier and as a result lower modes move slower than higher modes. The temporal evolution of the baroclinic wave intensity is well described by the Eady parameter. [ABSTRACT FROM AUTHOR]

The westerlies in the southern hemisphere have intensified and shifted southward since the middle of the twentieth century. Previous studies have indicated that the expected increase in isopycnal slopes and acceleration of the Antarctic Circumpolar Current (ACC) is considerably weakened by the strengthening of mesoscale eddies and that this "eddy saturation" occurs mainly downstream of the major bottom topographic features such as the Kerguelen Plateau. Such eddy "hotspots" are thus considered to regulate the ACC responses to changes in external forcing. To improve our understanding of the ACC response to intensified winds, a sensitivity study is conducted using an eddy-resolving quasi-global ocean general circulation model named "OFES." The reference run is driven by a climatological atmospheric forcing and the sensitivity run is driven by artificially intensified climatological westerlies. Our new finding is that the baroclinic energy pathway is enhanced over the Subantarctic Front (SAF) as well as over the hotspots identified by previous studies. A linear stability analysis indicates that the spin-up of the subtropical gyres north of the SAF and the enhanced Ekman upwelling south of the SAF by the intensified wind stress curl increase the vertical shear of zonal velocity along the SAF, enhancing baroclinic instability. We have also performed the same stability analysis comparing the 1985–2018 and 1955–1984 periods of a hindcast run of OFES, confirming the result from the climatological sensitivity study. These results suggest that the SAF is another eddy hotspot when the wind stress curl keeps increasing. [ABSTRACT FROM AUTHOR]

Under the influence of buoyancy and the earth rotation, river outflows leaving an estuary typically have a two‐part structure, including a recirculating bulge near the river mouth and a coastal current propagating downstream. A continuous river outflow causes the bulge to expand in size and to accumulate freshwater, with the bulge eventually becoming unstable due to baroclinic instability. We conducted a series of laboratory experiments on a rotating tank to simulate an idealized river plume under various Coriolis frequencies, density differences, and shelf slopes. The horizontal velocity structures of the river plume were qutantified using particle imaginary velocimetry (PIV). We designed a velocity‐based vortex identification and tracking algorithm to capture anticyclonic eddies (ACEs) at the bulge center, cyclonic eddies (CEs) on the plume front, and coastal cyclonic return flow (CCRF) at the corner between estuary and coastal wall. Our results suggest a linear relationship between bulge instability parameter and bulge wavenumber, which can be used to predict the bulge instability patterns that are classified according to vortex stretching, splitting, and squeezing. Finally, we estimated the eddy kinetic energy contained within ACEs, CEs, and CCRF to explore the cross‐scale energy transfer and dissipation. Our results show that the bugle is more unstable in gentle slope cases, and its instability decreases with the inflow Rossby number and increases with the Froude number. The generation of CEs on the plume front may extract the energy from the larger scale anticyclonic core, which plays an important role in mass transport and frontal mixing of river plumes. Plain Language Summary: When the river flows into the coastal ocean, it forms a river plume and serves as an important source of land‐borne materials. Due to the density differences between river and ocean water, the river plume stays on top of the ocean water and forms a circular bore that rotates counter‐clockwise in the northern hemisphere. As the riverine water continuously flows into the ocean, this circular bore grows in size with the accumulation of riverine water and finally becomes unstable. To investigate this phenomenon, we carried out a series of experiments on the rotating tank in the laboratory and changed three parameters: rotation periods, fluid density differences, and the angle of continental shelf slopes. Our experimental results showed that a gentler slope, shorter rotation period, and less fluid density difference led to a more unstable river plume with more small‐scale vortices at the edge of the plume. We categorized the river plumes into three types based on their stability status and proposed an empirical relationship between easy‐obtain plume parameters and their stability status. We believe that the relationship can be utilized to predict the river plume stability based on satellite images or field observations. Key Points: Small scale eddies induced by baroclinic instabilities in river plume are triggered by gentler slopes, shorter rotation periods and weaker fluid density differencesBaroclinic instability parameter is proposed to estimate bulge wavenumber and thereby to predict plume instabilityEKE is more distributed in submesoscale processes along the bulge front in an unstable river plume system [ABSTRACT FROM AUTHOR]

Broadened width of high chlorophyll concentration band with wavy structures, patches, and filaments are often observed along the western coastal next to the Pearl River Estuary over the northern South China Sea shelf during the transition period from winter to summer monsoon. Whereas, there is no such wide band in other seasons. By using a high‐resolution numerical model, we reveal that the complex structure and wider band of high coastal chlorophyll concentration results from the smaller scale eddies (about 20–50 km in diameter) associated with buoyant plume‐induced salinity front and density fronts, which are roughly along the 30 and 50 m isobaths, respectively. Two trains of eddies are formed along the fronts by the baroclinic instability triggered by frequently alternating winds over the fronts during the period of monsoon transition. The influences of these two trains of eddies are extended in the cross‐shelf direction by their interactions, and they can temporally enhance the cross‐shelf flow and material exchange. They serve as an efficient pathway to link the inner shelf toward the continental slope. Plain Language Summary: Cross‐shelf flow is a major agent of promoting connectivity in coastal oceans and plays a dominant role in the onshore/offshore delivery and exchange of nutrients, pollutants, biomass, and other materials. Coastal oceans are often influenced by buoyant water which forms fronts between nearshore and offshore. The front‐associated eddies and filaments can facilitate particles in overcoming topographic barriers and therefore could transport materials in the cross‐shelf direction. The shelf region in the northern South China Sea influenced by the discharge of Pearl River is an ideal testbed to illustrate and investigate how instabilities of coastal fronts help to enhance cross‐shelf exchange. Using high‐resolution simulations and Lagrangian drifter experiments, we reveal that the instability eddies generated at the fronts can significantly extend the cross‐shelf connectivity of the inner shelf to the slope, and serve as an effective path for exchange coastal water and materials in the cross‐shelf direction. Our findings advance the knowledge of how coastal fronts boost the cross‐shelf connectivity with implications for marine tracer distribution and ecological environments. Key Points: Eddies are generated along the edge of the river plume over the northern South China Sea shelf during the monsoon transition periodBaroclinic instability of the fronts triggered by the frequent change of monsoon wind plays an important role for eddy generationThe interaction of the eddies enhances the cross‐shelf exchange and serves as a pathway to link the coast toward the continental slope [ABSTRACT FROM AUTHOR]

Intraseasonal variabilities (ISVs) of the western boundary currents (WBCs) east of Luzon Island were explored using acoustic Doppler current profiler (ADCP) measurements from three moorings at 18°N during 2018–2020. Besides the traditionally known surface‐intensified ISV, subsurface‐intensified ISV with a typical period of approximately 60 days was also detected in the currents, and the strongest signal appeared between 400 and 800 m. Further analysis indicates that they are highly associated with subsurface eddies. Based on their lifespan, subsurface eddies are classified into two categories: short‐lived and medium‐to long‐lived eddies. The short‐lived eddies are primarily generated locally near the eastern coast of Luzon Island, whereas the medium‐to long‐lived eddies are mainly generated away from the western boundary, in the region west of 135°E. Additional energy diagnosis suggests that baroclinic instability induced by the velocity shear of the North Equatorial Current (NEC)/subtropical countercurrent (STCC) system dominates the generation of medium‐to long‐lived subsurface eddies in the interior ocean, while barotropic instability and baroclinic instability play a comparable role in the generation of short‐lived eddies near the eastern coast of Luzon Island. Plain Language Summary: The Kuroshio and Luzon Undercurrent (LUC) have a far‐reaching impact on regional climate change as a crucial part of the western boundary current (WBC) system. In situ measurements east of Luzon Island have revealed that the currents exhibit significant intraseasonal variabilities (ISVs). However, the vertical difference of the ISVs is unclear, and we still do not know if subsurface ISVs exist in this region. In this work, we found the currents east of Luzon Island are characterized by two types of ISVs with a period of approximately 60 days: surface‐intensified and subsurface‐intensified. The two types of ISVs are linked to the surface and subsurface mesoscale eddies, respectively. Previous studies have revealed that baroclinic instability associated with the vertical shear of velocity in the North Equatorial Current/subtropical countercurrent (NEC/STCC) system is the primary generation mechanism of surface eddies. This study suggests that baroclinic instability of the NEC/STCC system also dominates the generation of subsurface eddies in the interior ocean, while barotropic instability associated with the horizontal shear of currents near the Luzon coast is non‐negligible in the generation of coastal subsurface eddies. These results significantly deepen our understanding of the ISVs of the currents in this region. Key Points: Quasi‐60‐day intraseasonal variability in the currents east of Luzon Island exhibits surface‐ and subsurface‐intensified vertical featuresSubsurface‐intensified eddies are revealed appearing east of Luzon Island and modulating the western boundary currentsBaroclinic instability dominates the generation of subsurface eddies east of Luzon while coastal barotropic instability is non‐negligible [ABSTRACT FROM AUTHOR]

In order to quantify pelagic‐benthic coupling on high‐latitude shelves, it is imperative to identify the different physical mechanisms by which phytoplankton are exported to the sediments. In June–July 2023, a field program documented the evolution of an under‐ice phytoplankton bloom on the northeast Chukchi shelf. Here, we use in situ data from the cruise, a simple numerical model, historical water column data, and ocean reanalysis fields to characterize the physical setting and describe the dynamically driven vertical export of chlorophyll associated with the bloom. A water mass front separating cold, high‐nutrient winter water in the north and warmer summer waters to the south—roughly coincident with the ice edge—supported a baroclinic jet which is part of the Central Channel flow branch that veers eastward toward Barrow Canyon. A plume of high chlorophyll fluorescence extending from the near‐surface bloom in the winter water downwards along the front was measured throughout the cruise. Using a passive tracer to represent phytoplankton in the model, it was demonstrated that the plume is the result of subduction due to baroclinic instability of the frontal jet. This process, in concert with the gravitational sinking, pumps the chlorophyll downwards an order of magnitude faster than gravitational sinking alone. Particle tracking using the ocean reanalysis fields reveals that a substantial portion of the chlorophyll away from the front is advected off of the northeast Chukchi shelf before reaching the bottom. This highlights the importance of the frontal subduction process for delivering carbon to the sea floor. Plain Language Summary: The Chukchi Sea shelf north of Bering Strait is known to experience some of the largest phytoplankton blooms in the Arctic Ocean. In 2023, a field program was carried out to quantify aspects of the early summer bloom, with an emphasis on characterizing how the phytoplankton biomass from the bloom is exported to the sea floor. A large bloom was measured under the pack ice in very cold, high‐nutrient water, just north of warmer, ice‐free waters. The front separating the warm and cold waters supported a current flowing eastward, which is one of the main flow pathways on the Chukchi shelf. A plume of high chlorophyll fluorescence extending from the near‐surface bloom downwards along the front was measured throughout the cruise. We demonstrate that this vertical pumping was due to a dynamical process associated with the current which resulted in much faster downward export of phytoplankton than gravitational sinking alone. Tracking the fate of particles on the northeast Chukchi shelf using an ocean simulation revealed that much of the phytoplankton biomass away from the front is carried off the shelf before reaching the bottom. This highlights the importance of the frontal process for delivering chlorophyll to the sea floor. Key Points: An under‐ice phytoplankton bloom developed during June–July in the northeast Chukchi Sea within the Central Channel flow branchA plume of chlorophyll fluorescence extending downwards from the bloom along the current's water mass front was continually presentA simple numerical model demonstrates that the plume is the result of baroclinic instability of the frontal jet [ABSTRACT FROM AUTHOR]

The linear instability of baroclinic flows in two superimposed and coupled, immiscible fluids is studied theoretically. These flows are westerlies, and they are thermally or mechanically coupled. The fluids are stably stratified, internally and mutually (the upper fluid is lighter than the lower fluid). In each fluid, two-level surface quasi-geostrophy governs the evolution of the perturbed westerly flow. The perturbations are horizontal normal modes. Firstly, the models are not coupled and the flow instability in each fluid is validated separately against the results of the classical Eady model of baroclinic instability. Secondly, the two fluids are thermally and/or mechanically coupled. With thermal coupling, and for meridionally uniform perturbations, a new mode of instability appears for long waves. This pair of unstable modes converges towards the modes of the uncoupled fluids at medium wavelengths. For perturbations with a non trivial meridional structure, the thermal coupling essentially damps the instability. For an upper flow with a larger deformation radius than in the lower flow, the growth rates of the perturbation are therefore more strongly altered in the former than in the latter. With mechanical coupling, the instability is essentially damped at large to medium scales, while the short-wave cut-off is extended towards smaller waves. When the fluids are both thermally and mechanically coupled, these effects add up. This very idealised study is a first step towards studying more realistic cases. [ABSTRACT FROM AUTHOR]

We report on a novel fluid dynamics experiment configuration based on a modified version of the differentially heated rotating annulus, a widely used laboratory model of the large-scale mid-latitude atmospheric circulation. Through applying an azimuthally (zonally) inhomogeneous, dipole-like thermal boundary condition—imitating a permanent "day side" and "night side" in the rotating setting—we explore the character of the flow patterns emerging at different values of the zonal temperature contrast and rotation rate. This configuration may prove to be a useful minimal model of the large-scale atmospheric circulation of tidally locked exoplanets. [ABSTRACT FROM AUTHOR]

It is traditionally believed that in a liquid of uniform density, an axisymmetric barotropic flow with solid-body rotation is stable. Within the framework of a two-level quasi-geostrophic model, this work shows that this is not in the case of a baroclinic flow with solid-body rotation of the heton type. Such a flow has different directions of rotation at two levels. Due to the vertical velocity shift, this flow is always unstable. This paper develops a linear theory of the instability of such flows both in a model without friction and in a model with Ekman friction. It is shown that, for instability in a model with friction, the horizontal wave number of the disturbance should not exceed a certain critical value. It has been established that instability with respect to longwave disturbances in the model without friction is absolute in nature; i.e., it always exists. The development of instability may be associated with the formation of observed disturbances in the axial zone of intense atmospheric vortices. [ABSTRACT FROM AUTHOR]

This study explores the effect of submesoscale topography on baroclinic instability under the quadric shear basic zonal flow is discussed. On the beta plane approximation, the quasi-geostrophic model is used for numerical simulation, and the multiscale model to discuss and verify this problem. A multiscale framework is used to explain the interaction between mesoscale eddies, submesoscale variability, and topographic effects. The multiscale method makes it possible to express the dynamic characteristics of the governing equations with a set of closed systems containing only mesoscale variables. It is found that the interaction between submesoscale topography and baroclinic instability affects some characteristics of mesoscale variability, such as promoting the formation of mesoscale eddies. and the energy and momentum transport of eddies. [ABSTRACT FROM AUTHOR]

We note that the atmosphere has distinct tropical and extratropical regimes. The tropical regime is significantly dependent on the greenhouse effect and is characterized by temperatures that are largely horizontally homogenized. The extratropical regime is dominated by large scale unstable convective eddies that transport heat between the tropics and the poles (leaving the poles warmer than they otherwise would be) and serve to determine the temperature difference between the tropics and the poles. Changes in tropical temperature and in the tropics-to-pole temperature difference both contribute to changes in global mean temperature. It turns out that changes in global mean temperature associated with major climate change (i.e., the last glacial maximum and the warm period of the Eocene about 50 million years ago) were associated primarily with changes in the tropics-to-pole temperature differences. By contrast, changes in global mean temperature over the past 150 years or so are almost entirely associated with changes in tropical temperature. Thus, there is no intrinsic amplification associated with a change in the tropics-to-pole temperature difference. However, model simulations of climate behave differently from both observations and from each other. In particular, they all show more significant contributions for the tropics-to-pole temperature difference – sometimes much more significant. They also show excessive tropical warming. [ABSTRACT FROM AUTHOR]

Eddy activity is particularly prominent in the Southern Ocean due to the instabilities of the Antarctic Circumpolar Current, which plays a critical role in energy transport of the global ocean. In this study, a systematic energetics analysis framework is employed to investigate notable anomalies of an intensified Eddy Kinetic Energy (EKE) event observed in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017, utilizing a reanalysis product. The EKE anomalies, presenting across all depths, emerged in April, peaked during the austral winter, and persisted into the subsequent summer. Energetics analysis indicates that the pronounced EKE anomalies are primarily determined by baroclinic instability, with distinct governing mechanisms at the surface and in the internal ocean. The anomalous intrusion of warm Circumpolar Deep Water intensified the baroclinic energy conversion in the subsurface, contributing significantly to the EKE anomalies. Moreover, strong anomalous wind‐induced Ekman pumping served to amplify the lifting of isopycnals, which enhanced the baroclinic instability and subsequently intensified the EKE anomalies. This study sheds new light on underlying mechanisms governing local polar dynamics and provides insights into the intricate interaction between ocean dynamics and energy distribution in the Antarctic region. Plain Language Summary: The Indian sector of the Southern Ocean is known for its dynamic variability, often manifested as jets and eddies. Eddy Kinetic Energy (EKE) is widely used to measure the kinetic energy (KE) as the difference between the total KE and the KE of mean currents. This study found an anomalous event of significant increases in EKE in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017. We applied a systematic energetics analysis framework to a reanalysis product to investigate the processes responsible for the observed anomalous event. The results suggest that the main cause was the anomalous intrusion of warm water masses in the upper and deeper ocean layers, which led to the increases in density gradients and then intensified the energy conversion from available potential energy to EKE. Moreover, changes in wind patterns have an impact on the variations of EKE in the upper ocean. This study enhances the understanding of the energy conversions and underlying mechanisms for EKE in polar regimes. Key Points: The subpolar region to the southwest of the Kerguelen Plateau was characterized by anomalous strong Eddy Kinetic Energy (EKE) in 2017The strong regional anomalies in EKE can be primarily attributed to baroclinic instability, with inverse barotropic energy conversionBaroclinic instability is mainly caused by anomalous intrusion of Circumpolar Deep Water, particularly at depths between 500 and 2,000 m [ABSTRACT FROM AUTHOR]

The main goal of the present work is to compare and contrast the characteristics of distinct monsoon systems like the Indian monsoon system (IMS) and South American monsoon system (SAMS) for the period (1979–2022). In addition, we discuss in some detail the theoretical aspects of the two monsoon systems by examining the energetics and the applicability of "convective quasi-equilibrium, (CQE)." We have also analyzed the precipitation interannual variability of SAMS and IMS considering neutral, El Niño, and La Niña years. Then, a discussion of the applicability of CQE along with the recent changes in surface entropy is presented. In our analysis, we found that interannual variability in the case of SAMS is less than that of IMS, and rainfall of SAMS is not drastically affected by ENSO when compared to IMS. We observed that rainfall characteristics over IMS are more complex than SAMS. The annual cycle of the vertically integrated total kinetic energy (KE) over SAMS is maximum in Austral spring, while the precipitation is higher in Austral summer. This is in sharp contrast to the IMS, where the maximum KE and rainfall coincide, both occurring in July and August. Further analysis showed that the conversion from the mean available potential energy PZ to the eddy available potential energy PE and conversion from PE to KE are important over SAMS. This shows that in South America, baroclinic conversions associated with baroclinic instability are important in austral summer, while over IMS, baroclinic conversions are not important in boreal summer. Our results support CQE for the IMS, but in the case of El Niño, we found that CQE is invalid. For SAMS, the applicability of CQE is climatologically doubtful, but during El Niño, the applicability of CQE is robustly visible. [ABSTRACT FROM AUTHOR]

Extreme precipitation events (EPEs) are among the most pervasive weather hazards in the western Himalayan region (WHR), posing widespread damage to life, infrastructure, and agriculture. This study investigates the synoptic and large‐scale characteristics linked to winter precipitation extremes over the WHR. EPEs are identified as events surpassing the 95th percentile threshold. A composite analysis is employed using two reanalyses—the fifth‐generation European Centre for Medium‐Range Weather Forecasts Reanalysis (ERA5) and the Indian Monsoon Data Assimilation and Analysis (IMDAA)—to elucidate the synoptic conditions conducive to EPEs. Our findings suggest that EPEs in the WHR are linked to an intensified subtropical westerly jet, characteristically shifted to south than normal. Enhanced kinetic energy in the upper troposphere, attributed to increased baroclinic instability, reinforces moisture convergence and strengthens synoptic‐scale circulation, triggering deep convection and supporting EPEs. Notably, the interplay of pronounced Rossby waves sinking over the WHR and regional orography significantly modulates the intensity of western disturbances (WDs). Employing clustering analysis, we observed that the strongest EPEs are linked to anomalous vorticity in the upper to middle troposphere, together with deep convection via strengthened WDs, suggesting the potential role of large‐scale influences. Using Lagrangian method, we identify that the Arabian Sea is the primary moisture source for EPEs in the WHR. We further delved into the role of large‐scale connections and EPEs through quasi‐resonant amplification (QRA) analysis. The findings unveil distinct QRA fingerprints in meridional temperature gradients along with notably magnified, quasi‐stationary midlatitude planetary waves characterized by zonal wave numbers 6/7/8 (baroclinic waves) contributing to EPEs. Overall, our results highlight the underlying physical mechanisms for winter precipitation extremes, emphasizing QRA's role in amplifying planetary waves and promoting EPEs, underscoring the WHR's vulnerability to evolving climatic conditions. [ABSTRACT FROM AUTHOR]

Polar lows (PLs) are intense maritime mesoscale weather systems that develop during marine cold air outbreaks at high latitudes. The objective of this review is to describe the advances in polar low research since the last literature review--published 3 years ago--, indicate the knowledge gaps that remain, and suggest promising courses of research. Among the breakthroughs identified here are the first climatology of PLs obtained with a global atmospheric model, and increased evidence showing that baroclinic instability is the main mechanism leading to PL development. Despite these advances, many challenges persist such as the lack of conventional observations of PLs and the need to better understand coupled atmosphere-ocean processes involved in PL development. With the rapid advances in deep learning, this method has the potential to be used for PL forecasting. [ABSTRACT FROM AUTHOR]

Based on a submesoscale-resolving glider observation from April 25 to May 4, 2018, characteristics and underlying dynamics of submesoscale variability at the edge of an anticyclonic eddy shed from Kuroshio in the Northern South China Sea are explored in this study. Three underwater gliders traveled across the frontal zone and implemented ~ 300 dives, covering a horizontal distance of ~ 160 km and a vertical depth of ~ 500 m in 9 days. The character of k−2 slope for spectral potential energy and the strong lateral buoyancy gradient indicate frontogenesis-induced submesoscale motions on the eddy edge. Further analysis focusing on the potential vorticity and balanced Richardson number reveals the development of symmetric instability (SI), which is associated with the strong lateral gradient of buoyancy at the edge of the anticyclonic eddy in the late spring. [ABSTRACT FROM AUTHOR]

Observational analyses reveal that a dominant mode in the South Asian Monsoon region in boreal summer is a westward-propagating synoptic-scale disturbance with a typical wavelength of 4000 km that is coupled with moistening and precipitation processes. The disturbances exhibit an eastward tilt during their development before reaching their maximum activity center. A 2.5-layer model that extends a classic 2-level quasi-geostrophic model by including a prognostic lower-tropospheric moisture tendency equation and an interactive planetary boundary layer was constructed. The eigenvalue analysis of this model shows that the most unstable mode has a preferred zonal wavelength of 4000 km, a westward phase speed of 6 m s−1, an eastward tilt vertical structure, and a westward shift of maximum moisture/precipitation center relative to the lower-tropospheric vorticity center, all of which agree with the observations. Sensitivity experiments show that the moisture–vortex instability determines, to a large extent, the growth rate, while the baroclinic instability helps set up the preferred zonal scale. Ekman-pumping-induced vertical moisture advection prompts an in-phase component of perturbation moisture relative to the low-level cyclonic center, allowing the generation of available potential energy and perturbation growth, regardless of whether or not a low-level mean westerly is presented. In contrast to a previous study, the growth rate is reversely proportional to the convective adjustment time. The current work sheds light on understanding the moisture–vortex and the baroclinic instability in a monsoonal environment with a pronounced easterly vertical shear. [ABSTRACT FROM AUTHOR]

Watermass transformation in the Irminger Sea, a key region for the Atlantic Meridional Overturning Circulation, is influenced by atmospheric and oceanic variability. Strong wintertime atmospheric forcing in 2015 resulted in enhanced convection and the densification of the Irminger Sea. Deep convection persisted until 2018, even though winters following 2015 were mild. We show that this behavior can be attributed to an initially slow convergence of buoyancy, followed by more rapid convergence of buoyancy. This two‐stage recovery, in turn, is consistent with restratification driven by baroclinic instability of the Irminger Current (IC), that flows around the basin. The initial, slow restratification resulted from the weak horizontal density gradients created by the widespread 2015 atmospheric heat loss. Faster restratification occurred once the IC recovered. This mechanism explains the delayed recovery of the Irminger Sea following a single extreme winter and has implications for the ventilation and overturning that occurs in the basin. Plain Language Summary: The Irminger Sea, between Greenland and Iceland, is known to be an important driver of variability in the global ocean circulation that regulates global climate. During the 2015 winter, the Irminger Sea experienced widespread cooling and buoyancy loss down to 1,000 m, resulting in deeper wintertime mixing than had been observed in the region for many years. This low buoyancy state and deep wintertime mixing persisted from 2015 to 2018, despite a return to average atmospheric wintertime conditions. Here we study how processes that typically provide summertime buoyancy gain to balance wintertime buoyancy loss contribute to the multi‐year recovery of the region. We characterize the recovery of the interior of the Irminger Sea as having two stages: initially slow recovery, followed by more rapid recovery. Our findings are consistent with earlier studies stating that the main source of buoyancy to the interior is eddies shed from the current flowing around the Irminger Sea. This study shows that the deep mixing that is important for global circulation and climate is influenced by changes in the current around the basin. Key Points: Widespread buoyancy loss across the Irminger interior and Irminger Current (IC) delayed the recovery of the interior from strong cooling in 2015Baroclinic instabilities shed from the IC are the dominant source of buoyancy restratifying the sub‐surface Irminger interiorIt is important to consider changes in the IC when considering drivers of variability in convection, ventilation, and the Atlantic Meridional Overturning Circulation [ABSTRACT FROM AUTHOR]

Mesoscale eddies are a prominent oceanic phenomenon that plays an important role in oceanic mass transport and energy conversion. Characterizing by rotational speed, the eddy intensity is one of the most fundamental properties of an eddy. However, the seasonal spatiotemporal variation in eddy intensity has not been examined from a global ocean perspective. In this study, we unveil the seasonal spatiotemporal characteristics of eddy intensity in the global ocean by using the latest satellite-altimetry-derived eddy trajectory data set. The results suggest that the eddy intensity has a distinct seasonal variation, reaching a peak in spring while attaining a minimum in autumn in the Northern Hemisphere and the opposite in the Southern Hemisphere. The seasonal variation of eddy intensity is more intense in the tropical-subtropical transition zones within latitudinal bands between 15° and 30° in the western Pacific Ocean, the northwestern Atlantic Ocean, and the eastern Indian Ocean because baroclinic instability in these areas changes sharply. Further analysis found that the seasonal variation of baroclinic instability precedes the eddy intensity by a phase of 2–3 months due to the initial perturbations needing time to grow into mesoscale eddies. [ABSTRACT FROM AUTHOR]

The characteristics of cold core eddies and their effects on the hydrodynamics and biological production in the Andaman seas were studied using in-situ and satellite observations. The existence and spatial extent of the eddy are revealed by the precise structure and patterns of the temperature-salinity (T–S) profiles, nutrients, and chl a. A good illustration of this is the anomaly in sea surface height (SSHA). The Cyclonic Eddies (CE1 and CE2) are monitored using the Okubo-Weiss parameter (− 2 × 10–11/s2) calculated from the satellite SSHA and a geostrophic current centered at 8°N and 92°E and 13°N and 93°E, respectively. At the eastern flank of CE1, measurements are taken in-situ along 8°N and 92.5°–93.5°E. Vertical currents measured with Acoustic Doppler Current Profiles (ADCP) at 0.3 m/s show northward flow along the track. A significant northward flow (0.3 m/s) can be seen in the vertical currents recorded with ADCP, yet a weak southern flow can be seen over the western margin. In addition, the SSHA acquired from altimetry demonstrates the spatial extent, supporting the occurrence of cyclonic eddies. The vertical shear in the horizontal flow is the main contributor to baroclinic instability (Ri 0.0001) in the water column, according to an analysis of the factors that could result in the formation of an eddy. The T–S profiles show that the area contains Bay of Bengal (BoB) water, and there are semi-annual Rossby waves there as well. However, the wind stress curl did not provide a reliable indication of divergence in the region. The region's biological production (chl a) and nutrient distribution (NO2, NO3, PO4, and SiO4) were impacted by the eddy. CE2 is connected to convective mixing processes that take place along the northwest coast of the Andaman Islands as a result of the primarily cold, dry continental air flowing from the north east. [ABSTRACT FROM AUTHOR]

The classical problem of geophysical hydrodynamics is the problem of instability of a zonal geostrophic flow with a vertical velocity shear. At present, the instability with respect to symmetric perturbations that do not depend on the coordinate along the flow has been most thoroughly studied. For a symmetric instability to arise, the two-dimensional perturbation wave vector must lie inside a certain sector in the vertical plane of the wave numbers. In this paper, we study the instability with respect to unsymmetric perturbations oriented at an angle to the flow. Fundamentally new features of the temporal dynamics of the amplitudes of such perturbations are found. The main feature is associated with the existence of a stage of exponential explosive growth of finite duration. A kinematic interpretation of this stage is given that is related to the passage of the projection of the three-dimensional wave vector onto the plane transversely to the flow through the sector of symmetric instability. [ABSTRACT FROM AUTHOR]

Subtropical highs have a profound influence on the weather and climate of adjacent continents. In this study, we use reanalysis data to investigate the interannual variability and trends in winter subtropical highs from 1979 to 2021. We find dynamical relationships between subtropical high intensity, the Hadley and Ferrel Circulation intensity, and the Eady Growth Rate (EGR). A poleward shift of the maximum in EGR is associated with a strengthening of the descending branches of the Ferrel and Hadley Cells, with subtropical troposphere adiabatic warming and an increased intensity and poleward movement of the subtropical highs. Shifts in the poleward EGR are dominated by changes in vertical wind shear which, in turn, are in thermal wind balance with variations and trends in temperature. The mechanism for the intensification of the subtropical highs involves feedbacks from high‐frequency transient eddies. Strong North Pacific and South Pacific Subtropical highs are associated with La‐Niña conditions. We also show that the mechanisms for interannual variations are similar to those for trends in the highs. Plain Language Summary: Wintertime subtropical highs have large climatic impacts but the dynamical explanation for their interannual variability and trends is incomplete. Here we find that wintertime subtropical high intensity is related to overturning cell intensities and the Eady Growth Rate (EGR), which is a measurement of baroclinic instability, in both interannual variability and long‐term trends. The northward (southward) shift of the northern (southern) hemisphere EGR is associated with an increasing intensity of the Ferrel Cell and Hadley Cell. The enhancement of their descending branches strengthens the intensity of subtropical highs. The EGR change is dominated by the vertical wind shear change. The North Pacific Subtropical High and South Pacific Subtropical high are intensified by cold anomalies of sea surface temperature in the Central and Eastern Pacific but are also trending stronger and moving polewards. Key Points: Winter subtropical highs get stronger and shift polewards when the Eady Growth Rate (EGR) shifts polewards and the Hadley Cell strengthensThe EGR shift is dominated by the vertical wind shear, in thermal wind balance with the variations and trends of temperatureSimilar mechanisms are at play on interannual and longer time scales and the Pacific subtropical highs are strengthening over time [ABSTRACT FROM AUTHOR]

The stability of flow with a constant vertical shear is investigated as part of a two-level quasi-geostrophic model. Analytical expressions for the increment of perturbation growth in linear stability theory are obtained. The Galerkin method with three basic Fourier harmonics is used to describe the nonlinear dynamics of perturbations. A nonlinear system of ordinary differential equations is formulated for amplitudes of Fourier harmonics. It is shown that, in the absence of bottom friction, all solutions of the system describe a periodic mode of nonlinear oscillations or vascillations. The situation changes fundamentally in the model with bottom friction. In this case, for a wide range of parameter values, the system solutions exhibit complex chaotic behavior. Thus, chaos or turbulence emerges for large-scale motions. [ABSTRACT FROM AUTHOR]

Polar lows (PLs) are maritime mesoscale cyclones associated with severe weather. They develop during marine cold air outbreaks near coastlines and the sea ice edge. Unfortunately, our knowledge about the mechanisms leading to PL development is still incomplete. This study aims to provide a detailed analysis of the development mechanisms of a PL that formed over the Norwegian Sea on 25 March 2019 using the output of a simulation with the sixth version of the Canadian Regional Climate Model (CRCM6/GEM4), a convection-permitting model. First, the life cycle of the PL is described and the vertical wind shear environment is analysed. Then, the horizontal wind divergence and the baroclinic conversion term are computed, and a surface pressure tendency equation is developed. In addition, the roles of atmospheric static stability, latent heat release, and surface heat and moisture fluxes are explored. The results show that the PL developed in a forward-shear environment and that moist baroclinic instability played a major role in its genesis and intensification. Baroclinic instability was initially only present at low levels of the atmosphere, but later extended upward until it reached the mid-troposphere. Whereas the latent heat of condensation and the surface heat fluxes also contributed to the development of the PL, convective available potential energy and barotropic conversion do not seem to have played a major role in its intensification. In conclusion, this study shows that a convection-permitting model simulation is a powerful tool to study the details of the structure of PLs, as well as their development mechanisms. [ABSTRACT FROM AUTHOR]

A new shallow layer laboratory model of global atmospheric circulation is realised and studied by experiments and numerical simulations. A shallow rotating cylindrical fluid layer of 30 mm thickness and 690 mm diameter, with a localised heater at the bottom periphery and localised cooler in the central part of the upper boundary is considered. The rim heater imitates the equator heating and disc cooler – the North pole cooling. The flow transforms from the Hadley-like regime to the baroclinic wave regime through transitional states. The decrease in the thermal Rossby number for the fixed value of Taylor number results in the regularisation of the baroclinic waves. All wave regimes, even with regular wave structures, are characterised by strong non-periodic fluctuations. The observed baroclinic wave structures are a combination of temporarily evolving different baroclinic modes. The important outcome of the shallow layer model is a realisation of the Earth-like meridional three-cell structure. It is shown that the three-cell structure with analogs of polar, Ferrel and Hadley cells exist only in a limited range of parameters. A comparison of the results for the water and silicon oil demonstrated that the physical properties of the fluid can have a strong impact on the baroclinic wave structure. [ABSTRACT FROM AUTHOR]

Interannual eddy variability in the eastern Ryukyu Island chain (RIC) was analyzed based on eddy trajectories and assimilation data. Analysis of the eddy trajectory data suggested a predominance of eddies originating near the study area. The occurrence frequency of eddies has interannual variability, but it demonstrates a different tendency with eddy kinetic energy (EKE): few eddies appear when EKE is high; however, they are strong with fast rotation speeds and increase in size. The EKE variability was correlated with the baroclinic conversion rates, suggesting a vital role of baroclinic instability in the study area. Baroclinic instability was found to be affected by the North Equatorial Current (NEC), during which more NEC water entered the eastern RIC, intensifying the vertical velocity shear between the upper and lower oceans. Consequently, baroclinic instability in the study area was enhanced.

The large-scale flows of the oceans and the atmosphere are driven by a non-uniform surface heating over latitude, and rotation. For many years scientists try to understand these flows by doing laboratory experiments. In the present paper we discuss two rather new laboratory experiments designed to study certain aspects of the atmospheric circulation. One of the experiments, the differentially heated rotating annulus at the Brandenburg University of Technology (BTU) Cottbus, has a cooled inner cylinder and a heated outer wall. However, the structure of the atmospheric meridional circulation motivates a variation of this "classical" design. In the second experiment described, operational at the Institute of Continuous Media Mechanics (ICMM) in Perm, heating and cooling is performed at different vertical levels that resembles more the atmospheric situation. Recent results of both experiments are presented and discussed. Differences and consistencies are highlighted. Though many issues are still open we conclude that both setups have their merits. The variation with heating and cooling at different levels might be more suited to study processes in the transition zone between pure rotating convection and the zone of westerly winds. On the other hand, the simpler boundary conditions of the BTU experiment make this experiment easier to control. [ABSTRACT FROM AUTHOR]

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Within the framework of a discrete quasi-geostrophic model with two vertical levels, the problem of linear stability of the flow of a stratified rotating fluid with constant vertical and horizontal velocity shifts is solved. It is shown that taking into account the horizontal shear leads to a qualitative change in the dynamics of unstable wave disturbances. The main feature is related to the effect of temporary exponential growth of unstable perturbations, i.e., growth over a finite time period. This effect manifests itself in the alternation of stages of smooth oscillating behavior (in time) with stages of the exponential (explosive) growth of finite duration. A kinematic interpretation of the effect of temporal exponential growth is given, which is associated with the passage of a time-dependent perturbation wave vector through the region of exponential instability that exists in the absence of a horizontal shear. It is shown that mathematically this effect is described by solutions of a second-order differential equation containing turning points. Asymptotic solutions of the equation are given for weak horizontal shifts. [ABSTRACT FROM AUTHOR]

• Enhanced Kuroshio looping path across the Luzon Strait is observed during 1993–2020. • Eddy Kinetic Energy shows long-term increasing in the Northeastern South China Sea. • Energy from baroclinic instability is dominant for the EKE long-term increasing. The seasonal to interannual variability of eddy kinetic energy (EKE) in the Northeastern South China Sea (NE-SCS) has been widely studied and it is recognized that they are strongly related to the state of the Kuroshio pathway in the Luzon Strait. While, due to the lack of long-term observations and high-resolution simulations, the decadal change of EKE in NE-SCS remains unexplored. In this study, we show the EKE trend in the past ∼ 30 years in the NE-SCS by using satellite observation and global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation. It is found that due to the weakening of the Kuroshio in the Luzon Strait since 1990 s, the Kuroshio shows an enhanced looping path in the NE-SCS, inducing stronger EKE in this region. Further analysis confirms that the energy transfer by baroclinic instability is dominant for the increasing of EKE, when the Kuroshio intrudes into the NE-SCS and brings more potential energy inside the circulation. The Kuroshio state along the Luzon Strait is the key for modulating the EKE in the NE-SCS. Furthermore, the long-term weakening of Kuroshio current along the Luzon strait during 1993–2020 is determined by the decreasing of subtropical mode water, corresponding to the positive phase of the Atlantic Multidecadal Oscillation. This study provides insight into the interaction between marginal sea (i.e., the SCS) and the open ocean (i.e., the western Pacific Ocean), finally linking to the global climate change. [ABSTRACT FROM AUTHOR]

The quasi 5‐day wave (Q5DW) with zonal wavenumber 1 is a dominant planetary wave (PW) oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. In this paper, the Q5DW signal derived from 16 years (2007–2022) of Microwave Limb Sounder temperature observations is used to investigate the role of this PW mode on the onset of PMC seasons in the northern hemisphere (NH). PMC data from the Cloud Imaging and Particle Size (CIPS) instrument during this time indicates that NH PMC season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Except 2013 and 2022, the other four earlier onsets were also characterized by enhanced Q5DW activity. The wave amplification appears to be driven by baroclinic instability arising from the negative meridional gradient of potential vorticity in the high‐latitude summer mesosphere. CIPS data show that when the Q5DW was present at the beginning of the season, clouds formed preferentially in the cold troughs of the wave. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While the 11‐year solar cycle, inter‐ and intra‐hemispheric coupling due to gravity wave and PW activity have been shown to contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Plain Language Summary: The quasi 5‐day wave (Q5DW), a wave that takes ∼5 days to travel around a latitude circle, is a dominant planetary scale oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. We present the first study of the Q5DW impact on the northern hemisphere (NH) PMC season onset. The Q5DW activity is derived from 16 years (2007–2022) of satellite temperature observations. This activity is then compared to the onset of NH PMC seasons determined from the Cloud Imaging and Particle Size (CIPS) observations. The season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Earlier onsets in 2015, 2019, 2020, and 2021 were characterized by increased Q5DW power presumably driven by instability due to large vertical shear in the zonal winds. PMCs are found to form in the cold troughs of the Q5DW. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While several other factors contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Key Points: There is year‐to‐year variability in the quasi 5‐day wave (Q5DW) activity in the northern hemisphere summerObservations from 2007 to 2022 indicate that four of the six earliest polar mesospheric cloud (PMC) season onsets are characterized by higher Q5DW activityPMC first form in the longitude sector coincident with the cold trough of the Q5DW [ABSTRACT FROM AUTHOR]

Many factors are affecting the downstream development of baroclinic waves, among which zonal shear flow is one of the factors that need to be considered. In this paper, the influence of zonal shear flow and β on the downstream development of unstable chaotic baroclinic waves is studied from the two-layer model in a wide channel controlled by quasi-geostrophic potential vorticity equation. Through the obtained Lorentz equation, We concentrated on the influence of zonal shear flow (the second derivative of baseline zonal flow is not zero) on the downstream development of baroclinic waves. In the absence of zonal shear flow, chaotic behavior along feature points would occur, and the amplitude would change rapidly from one feature to another, that is, it would change very quickly in space. When zonal shear flow is introduced, the influence of zonal shear flow on the downstream development of unstable baroclinic waves is examined categorically. And from Lorentz's final equation, we're investigating a change in his solution. It is found that the zonal shear flow smoothes the solution of the equation and reduces the instability, and with the increase of zonal shear flow, the stability in space will increase gradually. The second derivative of the zonal shear flow (the quadrical shear flow) therefore has a major influence on the stability of space. [ABSTRACT FROM AUTHOR]

By using satellite altimeter observations and the eddy-permitting Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), the seasonal variation of eddy kinetic energy (EKE) along the East Australian Current (EAC) is investigated. Both observations and ECCO2 outputs indicate active intra-seasonal EKE along the EAC path. The ECCO2 result reveals that the intra-seasonal EKE is mainly concentrated in the upper 500 m layer, and shows a prominent seasonal cycle, strong in austral summer and weak in austral winter. Eddy energy budget diagnosis reveals that the evolution of EKE is controlled by barotropic instability of the mean EAC. The seasonal variation of baroclinic instability is opposite to the barotropic instability variation, but of a much smaller magnitude. Further analysis indicates that the seasonal cycle of mesoscale signals in this region is related to the transport variability of the EAC. [ABSTRACT FROM AUTHOR]

Observational data from the submesoscale hydrological polygon confirm the presence of summer runoff of dense Antarctic shelf water (ASW) on the continental slope in the Commonwealth Sea. It is provided by a huge amount of ASW accumulated in shelf depressions during winter. The ASW overflows the shelf edge in the form of discrete meanders, which are formed as a result of baroclinic instability of a deep ( 150–250 m) ASW front. The estimated instability scale along the dense water front km coincides with the observed spatial discreteness of ASW. At the same time, the distribution of near-bottom density currents affected by the bottom irregularities, is noted on the shelf. The Antarctic Slope Front (ASF) is formed on the slope due to the interaction of ASW with warm Circumpolar Deep Water (CDW). Due to the continuity of motion in the near-slope area, 3D cascading is accompanied by compensatory upwelling of CDW, which complements its large-scale advection. These processes form the intrusive structure of water on the edge of the shelf and on the shelf. Depending on the bathymetric characteristics of the slope in different parts of the polygon, the cascading of ASW has a different regime. [ABSTRACT FROM AUTHOR]

The eastern part of the Tsugaru Strait (located at ~ 41.5° N) is a remarkable region of the coastal flow system around Japan due to the Tsugaru Warm Current (TgWC), an intensive eastward current that flows over its complex topography and exhibits marked tidal variation. The TgWC has a jetlike structure and demonstrates drastic seasonal changes in its path and vertical current structure. Using a high-resolution dataset obtained from a high-frequency radar system (HFR) and a coastal data assimilation experiment (JCOPE-T DA), we focused on the frontal variation, which is shorter than the seasonal timescale, along the axis of the TgWC in relation to the meanders of the jet. We found variations in the axis with multiple timescales between 1 and 14 days, especially in summer and autumn. The ~ 14-day variation, a remarkable occurrence on the west side of the eastern Tsugaru Strait, was consistent with the tidal variation in the strait. The zonal scale of the axis meander estimated by the HFR matched a scale several times larger than the internal deformation radius, especially in summer and autumn. JCOPE-T DA outputs showed that the crest of such a meander at greater depths progresses in relation to that at the surface. The outputs also suggested instabilities, indicating potential and kinetic energy conversion from the mean to the eddy field. The propagation speed of the several-day variation in the axis estimated from the HFR data was consistent with that of the two-layer baroclinic instability. The several-day variation also showed relatively high coherence with wind variation from summer to autumn, during which time the seasonal wind over the strait switched from easterly to westerly. These variations with multiple timescales imply an effect on north–south water mass mixing in the eastern part of the strait, including rapid acidification in the strait and intensification of stratification. [ABSTRACT FROM AUTHOR]

Explosive cyclones (ECs) pose serious challenges for weather forecasting and significant threats to human life and property. In searching for the key points that make a cyclone go through explosive deepening off the East Asian coast, we present a comparative analysis of ECs and nonexplosive cyclones (or ordinary cyclones; OCs) using 10 years of ERA5 reanalysis data with high temporal and spatial resolutions. Their differences in synoptic backgrounds are shown, and mechanisms of the initial developments are compared quantitatively from the perspective of potential vorticity (PV). Among the identified 135 cyclones, 72 went through explosive growth and 37.5/36.1/20.8/5.6% of these ECs are ranked as weak/medium/strong/super ECs. ECs feature stronger low-level baroclinicity and higher PV than OCs. The decomposition of the local PV tendency shows the dominant role of the PV advection (with a correlation coefficient of 0.8). During the initial development, ECs have an average meridional temperature contrast 4 K larger than OCs within 20 latitudes at the low troposphere in the upstream, due to a stronger cold advection. The upstream colder air increases the horizontal temperature gradient and thus produces steeper isentropic surfaces inclining to the west. Since the PV intrusion is mainly along the isentropic surfaces, the increase in their slope significantly enhances the downward transport of PV from upper air. The importance of the horizontal gradient of potential temperature is further proved by numerical experiments with the Weather Research and Forecast (WRF) model on typical winter ECs. In sensitivity experiments, the low troposphere meridional temperature contrast decreasing by the average difference between ECs and OCs significantly decreases PV and stops the cyclones from explosive deepening. Despite the importance of diabatic processes in the deepening of mid-latitude cyclones emphasized by many studies, this study shows that the PV intrusion dominated by cold air mass is the key cause of winter explosive cyclogenesis in this region.

Baroclinic instability is one of the main mechanisms for the formation of synoptic scale systems. Previous studies examined the exponential growth of small perturbations for a stably stratified troposphere in the case of a constant meridional temperature gradient ignoring the stratosphere (Eady’s model). However, since stratospheric flow also affects to some extent the motions in the troposphere, in this work we investigate the effect of stratospheric wind shear on baroclinic instability using the tools of Generalized Stability Theory (GST). GST is a linear stability theory that addresses both the exponential growth of perturbations that is pertinent in the large time asymptotic limit and the transient growth of perturbations at finite time. The optimal initial perturbations leading to the largest growth over a specified time interval are calculated for three main cases of stratospheric shear: positive, zero and negative shear over the stratosphere. It is found that the inclusion of stratospheric shear in all three cases decreases perturbation growth and influences the scale of the structures that will dominate the flow. For optimizing times of the order of a week, the development of systems with larger spatial scale compared to the prediction of the Eady model is expected, while for optimizing times of the order of a day, smaller scale systems are found to develop.

Flow configurations that maximize the instantaneous rate of conversion from potential to kinetic energy are sought using a combination of analytical and numerical methods. A hydrostatic model is briefly investigated, but the presence of unrealistic ageostrophic flow configurations renders the results unrealistic. In the quasigeostrophic (QG) model, flow configurations that locally optimize the conversion rate are found, but it remains unclear if these flow configurations produce the global maximum conversion rate. The difficulty is associated with the fact that in the QG model, the vertical velocity is a quadratic function of the QG streamfunction, which renders the conversion rate a cubic function of the QG streamfunction. For these locally maximal conversion rates, the rate of conversion depends on the horizontal length scale of the flow: for scales larger than the deformation radius, the maximal rates are small and decrease as the horizontal scale increases; for scales smaller than the deformation radius, the maximal conversion rate rises until it becomes comparable to the maximal rate at which potential energy can be extracted from the mean flow. [ABSTRACT FROM AUTHOR]

In the real atmosphere, the development of large-scale motion is often related to the baroclinic properties of the atmosphere. So, it is necessary to discuss the stability condition of baroclinic flow. It is advantageous to use a layered model to discuss baroclinic instability, not only to apply the potential vortex equation directly, but also to deal with shear of basic flow. The stability and oscillatory shear ability of Rossby waves are studied based on the two-layer Phillips model in the β plane; then, we summarize the baroclinic instability of time-dependent zonal shear flows. The multiscale method is used to eliminate some terms of natural frequency oscillations of nonlinear operators in the third-order expansion, thus generating an equation about the amplitude of the lowest-order Rossby wave in the long-time variable. The large amplitude perturbation begins to decrease, which produces the desired behavior. After the amplitude decreases for some time, the amplitude of Rossby waves can still be found to oscillate periodically with the time variable. [ABSTRACT FROM AUTHOR]

Intra‐seasonal variations have been frequently observed in the North Equatorial Undercurrent (NEUC) jets, yet their dynamical nature remains elusive. Based on field observations and model results, we identify three flavors of the ISVs: Wave (accounting for 10%), wave‐vortex dipole (WVD) (20%) and vortex (70%). The wave flavor refers to a Rossby wave, the WVD flavor consists of a pair of counter‐rotating vortices, and the vortex flavor refers to a sub‐thermocline monopole eddy. To our knowledge, the WVD is identified for the first time and is found characterized by a unique dynamical feature: It manifests both as a second baroclinic mode‐like Rossby wave and as a pair of dipole‐like sub‐thermocline eddies. It is further identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave in an equilibrium being fueled by baroclinic conversion. These results indicate that mesoscale instability waves are an important component of subthermocline intra‐seasonal and mesoscale variations. Plain Language Summary: At the depths of 200–1,000 m of the Pacific Ocean between 10°N $10{}^{\circ}\mathrm{N}$ and 18°N $18{}^{\circ}\mathrm{N}$, the North Equatorial Undercurrent consisting of several jets flows eastward with mean velocity of ∼5 cm s−1. But it is not simply a steady current; instead, eastward and westward perturbations interlace, with an oscillation period of 70–120 days, which is called intra‐seasonal variations (ISVs). These variations are known to exist from previous field observations, but their dynamical nature remains elusive. Here we present that they are possibly associated with three distinct flavors of subsurface mesoscale activities with a length scale of O(100 km). They are called, "vortex wave", "wave‐vortex dipole" and "vortex". Among them, the wavevortex dipole flavor is an instability wave, dynamically intermediate between vortex wave and vortex, which might be an important component of oceanic intra‐seasonal and mesoscale variations. Key Points: Three flavors of the sub‐thermocline variability—wave, vortex (mesoscale eddy) and wave‐vortex dipole (WVD)—of the North Equatorial Undercurrent are identifiedThe WVD consists of a pair of antisymmetric vortices and is characterized by lens‐like temperature anomaliesThe WVD is identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave fueled by baroclinic conversion [ABSTRACT FROM AUTHOR]

As a key problem in meteorological research, the dynamical processes of warm-sector rainstorms have been of continuing interest to scholars. Using the functional analysis apparatus of multiscale window transform (MWT), and the MWT-based localized energetics analysis and theory of canonical transfer, we have analyzed the warm-sector torrential rain on May 7, 2018 in southern Fujian Province for an understanding of its underlying dynamical processes. First, the atmospheric fields from the ERA5 data are reconstructed onto three scale windows, namely, the background window, the synoptic scale window, and the rainstorm window. From the reconstructed fields the upper-layer and lower-layer jets are well represented on the background window, and the vertical motion is clearly seen on the rainstorm window. Previously it has been believed that the warm-sector rainstorms are characterized by weak baroclinicity, but here baroclinic instability matters as well as barotropic instability. It is found that the dynamical processes differ with height. In the lower layer, barotropic instability dominates, and the barotropic canonical transfer directs to the rainstorm window from both the background and synoptic scale windows. In the middle layer, the instability is mixed; apart from barotropic instability, the baroclinic canonical transfer also brings available potential energy from the background window to the rainstorm window, which is then converted into kinetic energy, maintaining the rainstorm-scale motion in the middle layer. The same as the lower layer, the upper layer sees only the canonical energy transfer from the background window to the rainstorm window. [ABSTRACT FROM AUTHOR]

The upwelling of cold water from the depths of the Southern Ocean to its surface closes the global overturning circulation and facilitates uptake of anthropogenic heat and carbon. Upwelling is often conceptualised in a zonally averaged framework as the result of isopycnal flattening via baroclinic eddies. However, upwelling is zonally non-uniform and occurs in discrete hotspots near topographic features. The mechanisms that facilitate topographically confined eddy upwelling remain poorly understood and thus limit the accuracy of parameterisations in coarse-resolution climate models.Using a high-resolution global ocean sea-ice model, we calculate spatial distributions of upwelling transport and energy conversions associated with barotropic and baroclinic instability, derived from a thickness-weighted energetics framework. We find that five major topographic hotspots of upwelling, covering less than 30% of the circumpolar longitude range, account for up to 76% of the southward eddy upwelling transport. The conversion of energy into eddies via baroclinic instability is highly spatially correlated with upwelling transport, unlike the barotropic energy conversion, which is also an order of magnitude smaller than the baroclinic conversion. This result suggests that eddy parameterisations that quantify baroclinic energy conversions could be used to improve the simulation of upwelling hotspots in climate models. We also find that eddy kinetic energy maxima are found on average 110 km downstream of upwelling hotspots in accordance with sparse observations. Our findings demonstrate the importance of localised mechanisms to Southern Ocean dynamics.

The Asian monsoon anticyclone (AMA) exhibits a trimodal distribution of sub‐vortices and the western Pacific is one of the preferred locations. Amplification of the western Pacific anticyclone (WPA) is often linked with eastward eddy shedding from the AMA, although the processes are not well understood. This study investigates the dynamics driving eastward eddy shedding associated with the emergence of the WPA in the upper troposphere and lower stratosphere on synoptic scales. Using reanalysis data during 1979–2019, our composite analysis reveals that amplified WPA events are tied to the upstream Silk Road (SR) wave‐train pattern over midlatitude Eurasia as identified in previous studies. The quasi‐stationary eastward propagating eddies result from baroclinic excitation along the westerly jet, as identified by coherent eddy heat fluxes and weakening of the low‐level temperature gradient. The upper‐level westerly jet is important in determining the longitudinal phase‐locking of wave trains, which are anchored and amplify near the jet exit. Occasionally enhanced convection near the Philippines also triggers anticyclonic eddies that propagate upward and northeastward via the Pacific‐Japan (PJ) pattern, forming the WPA in the upper troposphere. Correlation analysis suggests that the SR and PJ mechanisms are not physically correlated. Key Points: Eastward eddy shedding from the Asian monsoon anticyclone is often associated with the emergence of an isolated western Pacific (WP) anticycloneWP anticyclone is closely related to the Silk Road pattern. Eddies grow along the jet associated with baroclinic heat fluxesEastward eddy shedding occasionally occurs in association with the Pacific‐Japan pattern forced by strong convection near the Philippines [ABSTRACT FROM AUTHOR]

Coherent large‐scale vortices in the open ocean can retain their structure and properties for periods as long as several years. However, the patterns of potential vorticity in such vortices suggest that they are baroclinically unstable and therefore should rapidly disintegrate. This study proposes a plausible explanation of the longevity of large‐scale ocean rings based on bottom roughness, which restricts flow in the lower layer and thereby stabilizes the eddy. We perform a series of simulations in which topography is represented by the observationally derived Goff‐Jordan spectrum. We demonstrate that topography stabilizes coherent vortices and dramatically prolongs their lifespan. In contrast, the same vortices in the flat‐bottom model exhibit strong instability and fragmentation on the timescale of weeks. A critical depth variance exists that allows vortices to remain stable and circularly symmetric indefinitely. Our investigation underscores the essential role played by topography in the dynamics of large‐ and meso‐scale flows. Plain Language Summary: Swirls of circular currents tens to hundreds of kilometers in diameter known as ocean rings are critically important for transporting heat and nutrients throughout the ocean. Such structures usually reside in the upper ocean (top 1,000 m) and can last from months to several years. Ocean rings are often emitted by strong currents, such as the Gulf Stream, Agulhas, and Kuroshio. However, scientists are not entirely sure what allows rings to maintain their strength and persist for long periods. In particular, early theoretical models suggest that such large‐scale vortices are unstable and therefore should quickly disintegrate. In this study, we suggest that the resolution of the vortex longevity conundrum could lie in an unexpected direction—topography. The seafloor contains numerous underwater mountains, ridges, and valleys which, surprisingly, can dramatically affect rings that spin several kilometers above the bottom. Using numerical simulations with flat and realistically varying bottom, we demonstrate that eddies above rough topography persist much longer than their counterparts with identical parameters above a flat seafloor. We also show that there is a critical height of the bottom roughness which allows the surface‐intensified rings to remain stable and maintain their structure for years. Key Points: Coherent surface‐intensified vortices with scales greater than the radius of deformation are stabilized by rough bottom topographyThe lifespan of large vortex rings increases with the increasing amplitude of bottom topographyA critical threshold of the depth variance exists, above which the vortices are linearly stable [ABSTRACT FROM AUTHOR]

The seasonality of mesoscale eddy intensity in the southeastern tropical Indian Ocean (SETIO) is investigated using the latest eddy dataset and marine hydrological reanalysis data. The results show that the eddy intensity in an area to the southwest coast of the Java Island has prominent seasonality—eddies in this area are relatively weak during the first half of the year but tend to enhance in August and peak in October. Further analysis reveals that the strong eddies in October are actually developed from the ones mainly formed in July to September, and the barotropic instability and baroclinic instability are the key dynamics for eddy development, but each plays a different role at different development stages. The barotropic instability resulting from the horizontal shear of surface current plays an important role in the early stage of eddy development. However, in the late development stage, the baroclinic instability induced by the sloping pycnocline becomes the major energy contributor of eddy development.

This paper reports a complete diagnosis of energetics of the three representative structurally distinct unstable modes in the Charney problem with a generic stratosphere presented in Mak et al (2021). The diagnosis of the vertically-local-energetics of these modes (Charney+ mode, Green mode and Tropopause mode) helps delineate the physical nature of the impacts of a tropopause upon their vertical structures. The total-energetics diagnosis reveals that the computational noise is negligible in the significantly unstable modes, but that in a weakly unstable mode is measurable. Treating the top boundary as a sufficiently high free-surface adequately permits the existence of vertically propagating modes with negligibly small energy passing through it. The analysis of this generalized Charney problem could be relevant to the study of troposphere-stratosphere interaction capable of inducing anomalous tropospheric weather regimes.

Significant surface-intensified and subsurface-intensified intraseasonal variability (ISV) of the North Equatorial Current/Undercurrent with different periods are detected to coexist with mooring ADCP measurements at 13°N, 130°E. The ISV of the currents in the upper 200 m has a relatively shorter period of 45 days, while the period of the subsurface-intensified ISV between 400 and 800 m is around 85 days. By combining with sea surface height measurements from satellite altimeters and outputs from an eddy-resolving ocean general circulation model, the origin and dynamic mechanism of the two flavors of ISV are investigated. Eddy trajectory tracking and energy analysis indicate that both the surface-intensified and subsurface-intensified ISV are related to locally generated meso-scale eddies near the mooring sites. Stability analysis suggests the surface-intensified ISV is related to the baroclinic instability induced by the vertical velocity shear of the North Equatorial Current. While the generation of the subsurface-intensified ISV is more complex and is partly related to the enhanced barotropic instability induced by the intensified horizontal shear of the subsurface zonal background flow.