Relative Vorticity Research Articles

Reconstructing the Three-Dimensional Structure of Loop Current Rings from Satellite Altimetry and In Situ Data Using the Gravest Empirical Modes Method

The three-dimensional structure of Gulf of Mexico’s warm-core rings, detaching from the Loop Current, is investigated using satellite altimetry and a large set of ARGO float profiles. Reconstruction of the Loop Current rings (LCRs) vertical structure from sea surface height observations is made possible by the use of the gravest empirical modes method (GEM). The GEMs are transfer functions that associate a value of temperature and salinity for each variable pair {dynamic height; pressure}, and are computed by estimating an empirical relationship between dynamic height and the vertical thermohaline structure of the ocean. Between 1993 and 2021, 40 LCRs were detected in the altimetry and their three-dimensional thermohaline structure was reconstructed, as well as a number of dynamically relevant variables (geostrophic and cyclogeostrophic velocity, relative vorticity, potential vorticity, available potential energy and kinetic energy density, etc.). The structure of a typical LCR was computed by fitting an analytical stream function to the LCRs dynamic height signature and reconstructing its vertical structure with the GEM. The total heat and salt contents and energy of each LCR were computed and their cumulative effect on the Gulf of Mexico’s heat, salt and energy balance is discussed. We show that LCRs have a dramatic impact on these balances and estimate that residual surface heat fluxes of −13 W m−2 are necessary to compensate their heat input, while the fresh water outflow of the Mississippi river approximately compensates for their salt excess input. An average energy dissipation of O [10−10–10−9] W kg−1 would be necessary to balance their energy input.

Vertical structural variability of diurnal internal tides inside a mesoscale anticyclonic eddy based on single virtual-moored Slocum glider observations

The vertical structural variability of the diurnal internal tide (DIT) with a mode-1 wavelength of ~420 km inside a mesoscale baroclinic anticyclonic eddy was examined based on observations by a single virtual-moored (VM) Slocum glider. During the glider observational period from 10 to 19 September 2018, the eddy traveled northward at approximately 50 km, allowing the glider to scan a cross section of 50 km wide and 800 m deep inside the eddy. VM observations showed that DIT experienced a noticeable vertical structural variability near the eddy center. In a range of 30 km horizontally from the eddy center (inner center), DIT’s vertical displacements were significantly intensified in the 400–800-m depth below the eddy. In the range of 30–50 km from the eddy center (outer center), DIT was almost uniformly distributed from the surface to 800-m depth. Owing to the spatiotemporally restricted dataset by the glider, the significance of DIT’s modulation observed inside the eddy can be questionable. As a result of comparing DIT’s vertical structural variability in two domains in terms of available potential energy (APE) and horizontal kinetic energy (HKE) using CTDs inside the eddy and ADCPs outside the eddy, DIT’s vertical structure inside the eddy was significantly distinguished from that outside the eddy. The relative vorticity inside the eddy was estimated based on the satellite dataset; it was negatively great in the inner center (approximately 0.35 – 0.25f) and small in the outer center (approximately 0.2 – 0.1f). These observational behaviors indicate a close relationship between them; the vorticity-dependent modulation of the DIT seems to be observationally confirmed inside the eddy. Further, in order to examine the energy transferring behavior in low vertical modes, a wavenumber spectral analysis was performed on the DIT displacements and the lowest four wavenumbers, Kz (1) through Kz (4), showed a similar behavior to those observed in DIT’s vertical structural variability inside of the eddy; the relative power of the sum of Kz (2) ~ Kz (4) with respect to Kz (1) was strong in the inner center and weakened in the outer center. These results seem to support that the wave–eddy interaction is non-uniform inside the eddy and partially depends on the relative vorticity.

Extreme anomalies under El Niño and La Niña developing tendencies restrain subsequent severe typhoons in the western North Pacific

Forecasting typhoon features over a few upcoming months is necessary for planning disaster prevention and predicting the drought/flood situation. However, the current methods are more suitable for forecasting the interannual to interdecadal variation trends or demanding huge computing resources and are unsuitable for the operation of short-term climate forecasts. In order to devise other effective approaches, the correlation between the Oceanic Niño Index (ONI) in January and the frequency of severe typhoons (STYs) during 1959–2018 was analyzed for the two scenarios when the ONI trend during the previous year was higher and lower than zero. The reason for this classification is that the air‒sea conditions were different when the oceanic phenomena developed toward an El Niño or La Niña phase during the previous year, and the results of these trends affect the subsequent features of atmospheric dynamic factors favorable for TC development (relative vorticity and vertical velocity). The extreme anomalies under both the increasing and decreasing tendencies of ONI in January typically retrained the growth of STYs, as the subsequent decreasing ocean heat anomaly of the previous year was accompanied by a negative vorticity anomaly and downward vertical velocity anomaly in the lower atmosphere and vice versa. Results from the forecast test show that the accuracy of the new approach was acceptable and somewhat higher than that of the current methods. Thus, these results can assist in developing a practical approach for typhoon forecasts.

The Influence of Convectively Coupled Kelvin Waves on African Easterly Waves in a Wave-Following Framework

Abstract While considerable attention has been given to how convectively coupled Kelvin waves (CCKWs) influence the genesis of tropical cyclones (TCs) in the Atlantic Ocean, less attention has been given to their direct influence on African easterly waves (AEWs). This study builds a climatology of AEW and CCKW passages from 1981 to 2019 using an AEW-following framework. Vertical and horizontal composites of these passages are developed and divided into categories based on AEW position and CCKW strength. Many of the relationships that have previously been found for TC genesis also hold true for non-developing AEWs. This includes an increase in convective coverage surrounding the AEW center in phase with the convectively enhanced (“active”) CCKW crest, as well as a buildup of relative vorticity from the lower to upper troposphere following this active crest. Additionally, a new finding is that CCKWs induce specific humidity anomalies around AEWs that are qualitatively similar to those of relative vorticity. These modifications to specific humidity are more pronounced when AEWs are at lower latitudes and interacting with stronger CCKWs. While the influence of CCKWs on AEWs is mostly transient and short lived, CCKWs do modify the AEW propagation speed and westward-filtered relative vorticity, indicating that they may have some longer-term influences on the AEW life cycle. Overall, this analysis provides a more comprehensive view of the AEW–CCKW relationship than has previously been established, and supports assertions by previous studies that CCKW-associated convection, specific humidity, and vorticity may modify the favorability of AEWs to TC genesis over the Atlantic.

Enhanced Near-Inertial Waves and Turbulent Diapycnal Mixing Observed in a Cold- and Warm-Core Eddy in the Kuroshio Extension Region

Abstract Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models.

Recent decrease in western North Pacific tropical cyclone rapid intensification during June

AbstractThis study investigates the long‐term tendency in the ratio of rapid intensification (RI) cases to the total number of tropical cyclone cases over the western North Pacific (WNP) during June between 1982 and 2021. There is a significant decreasing trend in RI ratio during June, which is opposite to the increasing trend in RI ratio during July–November. From 1982–2005 to 2006–2021, the June RI ratio significantly decreases over the region of 5°–20°N and 120°–150°N, while showing weak changes over other parts of the WNP. Over the region with a significant RI ratio reduction, significantly reduced 850‐hPa relative vorticity and 200‐hPa divergence and significantly enhanced 850 to 200‐hPa vertical wind shear are the likely primary drivers. The RI‐suppressing effect of the above changes in dynamical variables exceeds the RI‐favoring effect of the warming ocean, for example, significantly increased sea surface temperature and tropical cyclone heat potential, over this same region. Moreover, the changes in dynamic conditions can be related to an anomalous low‐level anticyclonic circulation, anomalous upper‐level convergent flow and a greater enhancement of upper‐level easterlies than lower‐level easterlies. Our study highlights the importance of changes in dynamic variables (e.g., vertical wind shear) in modulating WNP RI activity.

Modulation of western North Pacific tropical cyclone formation by central Pacific El Niño–Southern Oscillation on decadal and interannual timescales

AbstractThis study investigates the impact of central Pacific (CP) El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) frequency over the western North Pacific (WNP) on decadal and interannual timescales. The CP ENSO‐TC frequency relationship is strong and significant only on decadal timescales, with enhanced TC formation over most of the WNP associated with warm CP ENSO phases. TC formation changes, driven by CP ENSO, exhibit a southeast–northwest dipole pattern on interannual timescales, similar to the typical pattern induced by eastern Pacific ENSO. Associated with warm CP ENSO phases on decadal timescales are increases in TC formation east of 125°E, while increases in TC formation east of 140°E are observed with warm CP ENSO phases on interannual timescales. These increases in TC activity are primarily caused by favourable dynamic conditions, including increased 850‐hPa relative vorticity and 200‐hPa divergence and decreased 850–200‐hPa vertical wind shear. The main TC formation difference on decadal and interannual timescales is concentrated over the longitudinal band of 125°–140°E, where reduced TC formation is observed on interannual timescales during CP El Niño years, predominately due to suppressed thermodynamic factors, for example, reduced maximum potential intensity and 700–500‐hPa relative humidity. We find insignificant (significant) changes in thermodynamic conditions between 125°E and 140°E due to weak (strong) descending motion in the western cell of the anomalous Walker circulation on decadal (interannual) timescales. This can be explained by different patterns of CP ENSO‐induced sea surface temperature anomalies on different timescales.

Characteristics of dynamical and thermo-dynamical variables during heavy rainfall events over the Indian region

There has been an increase in the frequency and intensity of heavy rainfall events (HREs) over the Indian region in recent years, and it is a huge challenge for the forecaster to accurately predict HREs with adequate lead time. In this study, four HREs, i.e., Kerala (KRA), Mumbai (MUM), Chennai (CHE), and Uttarakhand (UTK), occurred over different geographical regions of India considered. Total of five days (2 days before and after the event day-D0) of the HREs are extensively investigated. The ECMWF Fifth Generation Reanalysis (ERA5) and India Meteorological Department (IMD) data are used to examine rainfall and associated variables in these HREs. ERA5 is able to capture rainfall reasonably well in most of the HREs; however, there is a distinct underestimation found in UTK compared to IMD. Overall, the rainfall skills of ERA are mediocre for higher rainfall thresholds (>100 mm/hr). In general, higher convective available potential energy (CAPE) and lower convective inhibition (CIN) is distinctly noted for HREs. The strong monsoon flow facilitating intense moisture transport is the key factor for KRA and MUM events. Besides, unique convective signatures such as sudden and intense vertical velocity, flaring of relative vorticity, convergence, and moisture accumulation are seen for UTK, and thereafter, these variables rapidly weaken, indicating a short duration cloud burst type HRE. This is in clear contrast to other HRE events where convective conditioning and weakening of key variables occur in a gradual manner before and after D0, respectively. The ice hydrometeors are the major source of heavy rainfall in HREs except for UTK, where snow is the major contributor of rainfall in terms of rainwater and cloud liquid water. Further, it is revealed that an elevated moist static energy both at the lower and upper troposphere is found for UTK and MUM events compared to CHE and KRA. The intense lower troposphere moisture, low-level convergence, upper-level divergence, and intense relative vorticity, along with vertical motions, are key precursors for these HREs. Finally, a consolidated schematic flow of mechanisms responsible for HRE is proposed. The findings of this study are highly useful for improving early warning and disaster preparedness.

Impact of extratropical cyclone intensity and speed on the extreme wave trends in the Atlantic Ocean

This work analyses the extratropical cyclone-related extreme waves in the ocean surface and their trends in the North and South Atlantic Oceans. Atmospheric and ocean wave products are obtained from ERA5, from 1979 to 2020 with 1-hourly outputs, covering 42 years with the present climate changes evaluated by the difference between the two 21-years-time slices. The cyclones are tracked through the relative vorticity at 850 hPa and then associated with extreme wave events using an automated scheme that searches for an extreme wave region 1500 km from the centre of the cyclone, following criteria that exclude possible swell dominated events. The hot spot regions of cyclone-related waves occurrence found by the method agree with previous studies and relate to the cyclogenesis region, and storm track orientation. Most cyclones associated with extreme wave events are generated in the western boundary of the domains. The east-poleward side of the ocean basins presents the highest density of occurrences related to the higher density of cyclone track and the dominance of more mature stage cyclones while in the west side prevail systems on developing stages, with notable propagating fronts and consequently, lower wind persistence. The storm track variations alone cannot explain the observed changes in the wave occurrence during the period due to the lack of statistical confidence. However, the wave occurrence responds to changes in the cyclone intensity, modulated by cyclone displacement speed. Regions with an increase of extreme waves are related to the effect of more intense cyclones or cyclones with slower propagation, being the last associated with a longer interaction of winds with the ocean surface.

Impact of the mid-latitude zonal circulation on dynamic mechanism of anomalous precipitation over China in summer 2021

China was hit by extreme precipitation in July and August of 2021 along with significant monthly variation. The positive precipitation anomalies observed in July were mainly located in eastern China, but these anomalies shifted to the middle and lower reaches of the Yangtze River (MLYR) in August. In this study, we investigated the July and August precipitation anomaly dynamics based on the three-pattern decomposition of global atmospheric circulation (3P-DGAC) method. The results showed that the mid-latitude zonal circulation played a dominant role in July 2021, causing anomalous positive relative vorticity advection over the center of the positive precipitation anomalies, thus providing strong convective activity for the anomalous precipitation. Nevertheless, the August anomalies could be better explained from a climate perspective. The anomalous relative vorticity distribution patterns recorded in August were often accompanied by extreme precipitation in the MLYR, and the negative relative vorticity anomalies identified in Northeast Asia also played a crucial role. In conclusion, the mechanisms of precipitation anomalies in July and August 2021 were almost completely disparate. In this study, we emphasized the role of the mid-latitude zonal circulation in extreme precipitation events, and suggested that the impact of mid-latitude zonal circulation will be further accentuated under global warming.

Global smooth solutions to the two dimensional compressible Euler equations with the rotational Coriolis forcing

In this paper, we investigate the small initial value problem for the two dimensional compressible Euler equations with the rotational Coriolis forcing. Based on the effect of Klein–Gordon equations, the L∞ estimate, the ghost weight energy estimate and the bootstrap argument, global smooth solutions are proved under the assumption of small data and zero relative vorticity.

The structure of strong Indian monsoon low‐pressure systems in subseasonal‐to‐seasonal prediction models

AbstractThe structure of strong Indian monsoon low‐pressure systems (LPSs) up to forecast lead times of 15 days in 11 models of the Subseasonal‐to‐Seasonal (S2S) Prediction Project is analysed. Strong LPS (SLPS) tracks are obtained from a catalogue of LPSs tracked in all ensemble members of the S2S models during a common reforecast period of June–September 1999–2010. SLPSs, which have a minimum intensity equal to at least the upper‐quartile intensity of all LPSs, are then composited to generate horizontal and vertical structures of several dynamic and thermodynamic fields. The evolution of fields with forecast lead time and during LPS lifecycle is analysed. Furthermore, the simulation of the lower‐tropospheric monsoon circulation, precipitation biases, and the precipitation contribution of LPSs are analysed. All S2S models and the multimodel mean simulate the lower‐tropospheric monsoon circulation, but prominent dry biases are observed in the Australian Bureau of Meteorology and Environment and Climate Change Canada models. The precipitation contribution of LPSs to the summer mean precipitation is smaller in all S2S models than in tracks derived from ERA‐Interim reanalysis. The location and amplitude of the lower‐tropospheric cold core and the location of maximum precipitation are not well simulated by many models, particularly by the Hydrometeorological Centre of Russia model, in which the cold core is missing altogether. The structure of relative vorticity anomaly in all S2S models and the multimodel mean is shallower and weaker than in ERA‐Interim and MERRA‐2 reanalyses. Though the cold core intensifies through the LPS lifecycle in all models, the warm core features a midlife maximum, except in models such as Australian Bureau of Meteorology and China Meteorological Administration. These results demonstrate the potential for S2S models at simulating the structure of SLPSs, benefiting stakeholders that use S2S models for forecasting.

Jet Stream Changes over Southeast Australia during the Early Cool Season in Response to Accelerated Global Warming

In recent decades, southeast Australia has experienced both extreme drought and record-breaking rainfall, with devastating societal impacts. Variations in the Australian polar-front jet (PFJ) and the subtropical jet (STJ) determine, for example, the location and frequency of the cool season (April–September) weather systems influencing rainfall events and, consequently, water availability for the southern half of Australia. Changes in jet stream wind speeds also are important for aviation fuel and safety requirements. A split jet occurs when the single jet separates into the STJ and PFJ in the early cool season (April–May). This study focusses on split jet characteristics over Australian/New Zealand longitudes in recent decades. During the accelerated global warming from the mid-1990s, higher mean wind speeds were found in the PJF across the Australian region during June–September, compared to the STJ. In contrast, significant wind speed increases occur in the early cool season (April–May) at STJ latitudes, which straddle the East Coast of Australia and the adjacent Tasman Sea. These changes are linked to major changes in the mean atmospheric circulation, and they include relative vorticity and humidity, both being vital for the development of rain-bearing weather systems that affect the region.

Response of a Mesoscale Dipole Eddy to the Passage of a Tropical Cyclone: A Case Study Using Satellite Observations and Numerical Modeling

Mesoscale eddies occurring in the world’s oceans typically exist in pairs known as mesoscale dipole eddies or simply dipole eddies. Tropical cyclones (hereafter TCs) that move over the world’s oceans often encounter and interact with these dipole eddies. Through this interaction, TCs induce significant perturbations in the mesoscale eddies. However, the specific influences that the passage of a TC on a dipole eddy have not been addressed. In this paper, a case study of the dipole eddy’s response to the passage of a TC is conducted by using satellite observations and numerical simulation. The passage of a TC induces a long-duration response in the dipole eddy. First, the cyclonic ocean eddy component (COE) of the dipole is amplified, and the anticyclonic ocean eddy component (AOE) is weakened or even destroyed during the interaction. The amplification of the COE and weakening of the AOE primarily manifests as a change in their amplitudes and radii and as the adjustment of their vertical structure. The dipole eddy’s response to the interaction with a TC manifests as an upwelling anomaly and the injection of positive relative vorticity. Following the passage of the TC, the COE gradually stabilizes, and AOE slowly recovers after the disturbance energy from the interaction dissipates, which facilitates the reestablishment of the dipole eddy. The dipole reaches an equilibrium state through a quasi-geostrophic adjustment process. As a consequence, the overall effect of the interaction of the dipole with the TC leads to an asymmetric signature on the dipole eddy. The eddy–eddy interaction in a dipole may allow it to stabilize in a shorter time relative to that of a solitary eddy.

Eyewall asymmetries and their contributions to the intensification of an idealized tropical cyclone translating in uniform flow

Abstract Scale-dependent processes within the tropical cyclone (TC) eyewall and their contributions to intensification are examined in an idealized simulation of a TC translating in uniform environmental flow. The TC circulation is partitioned into axisymmetric, low-wavenumber (m = 1–3), and high-wavenumber (m > 3) categories, and scale-dependent contributions to the intensification process are quantified through the azimuthal-mean relative (vertical) vorticity and tangential momentum budgets. To further account for the interdependent relationship between the axisymmetric vortex structure and eyewall asymmetries, the analyses are subdivided into three periods—early, middle, and late—that represent the approximate quartiles of the full intensification period prior to the TC attaining its maximum intensity. The asymmetries become concentrated among lower azimuthal wavenumbers during the intensification process and are persistently distributed among a broader range of azimuthal scales at higher altitudes. The scale-dependent budgets demonstrate that the axisymmetric and asymmetric processes generally oppose each other during TC intensification. The axisymmetric processes are mostly characterized by a radial spin-up dipole pattern, with a tangential momentum spin-up tendency concentrated along the radius of maximum tangential winds (RMW) and a spin-down tendency concentrated radially inward of the RMW. The asymmetric processes are mostly characterized by an opposing spin-down dipole pattern that is slightly weaker in magnitude. The most salient exception occurs from high-wavenumber processes contributing to a relatively modest, net spin-up along the RMW between ~2–4 km altitude. Given that the maximum tangential winds persistently reside below 2-km altitude, eyewall asymmetries are primarily found to impede TC intensification.

A Case Study on the Dynamics of Phytoplankton Blooms Caused by Tropical Cyclones in the Southeastern Arabian Sea

A phytoplankton bloom was observed in the southeastern Arabian Sea after the passage of the anti-S-type track Tropical cyclone ARB 06 in 1998, which lasted for 22 days. The dynamic mechanism of the phytoplankton bloom was investigated with high-resolution satellite remote sensing data, reanalysis data and buoy data. The results show that the double thermocline structure hindered the uplift of deep nutrients to the surface before the tropical cyclone arrival. During and after the passage of the cyclone, the shallower thermocline disappeared, the deeper thermocline changed from 45.7 m to 76.5 m, the mixed layer deepened, and the upper oceanic stratification weakened. Additionally, a weak ocean eddy pair was enhanced gradually after the passage of cyclone. The relative vorticity of the ocean eddy pair was calculated. In the oscillation with a period of two days, the vorticity of the cyclonic eddy was stronger than that of the anticyclonic eddy, which enhanced a strong upwelling at a depth of more than 55 m for more than 10 days. The vertical current shear generated by the oscillation not only enhanced the vertical mixing, but also plays an important role in the distribution of phytoplankton blooms. The phytoplankton bloom was triggered by the nutrient below the thermocline into the sea surface, where a strong upwelling and weakened oceanic stratification occurred after the passage of cyclone. This study offers new insights on the mechanism of phytoplankton bloom induced by tropical cyclones and will be helpful for evaluating tropical cyclone-induced biological responses in future.

Upper‐Troposphere Saddle‐Like Response to Springtime Surface Sensible Heating Over the Tibetan Plateau: Combined Effect From Baroclinic and Barotropic Process

AbstractThe surface sensible heating over the Tibetan Plateau (TP) is a significant, elevated heating source in spring (March‐April‐May, MAM). It exerts considerable thermal effects on both regional and global climate. During MAM, the surface sensible heating over the TP triggers a giant saddle‐like circulation anomaly in the upper troposphere at 200 hPa. Although this structure was observed a long time ago and it contributes to the stationary wave, its formation mechanism remains unclear. Using the Linear Baroclinic Model, we reveal the evolution of this structure in detail. In the initial state, a typical baroclinic anticyclone with a vertical phase tilt forms around the heating source. When the wave propagates downstream due to strong relative vorticity advection and favorable lower level circulation, an anticyclone with a barotropic structure forms downstream. Our analysis indicates that both the barotropic process and baroclinic process contribute to the formation of the saddle‐like structure, and the latter is associated with the spring unique background circulation and vertical heating structure over the TP.

Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica.

A novel dynamical diagnosis of relative vorticity equation based on three-pattern decomposition of global atmospheric circulation: a case study of the western Pacific subtropical high in 2020

A novel dynamical diagnosis of relative vorticity equation based on three-pattern decomposition of global atmospheric circulation: a case study of the western Pacific subtropical high in 2020

Seasonal variability of the potential vorticity in the Lofoten vortex

The anticyclonic quasi-permanent Lofoten vortex is a strong dynamic formation in the center of the Lofoten Basin in the Norwegian Sea. We used the oceanic reanalysis data GLORYS12V1. We analyzed a seasonal distribution of the relative and potential vorticity, as well as the orbital velocities and the potential density in the Lofoten vortex. The main points of the potential vorticity calculating are considered. The values of the Lofoten vortex volume, the horizontal dimension (the diameter) as well as the vertical scale for the period from 2000 to 2019 are obtained, as well as the method for estimates of these characteristics is described in detail. It was shown that the intensification of the vortex occurs in the summer, and its relaxation -- in the winter.