Potential Vorticity Gradient Research Articles

Motivating Mechanisms of the South Asian Jet Wave Train by Disturbances over the North Atlantic in Winter

Abstract This study investigated the motivation of the South Asian jet wave train (SAJW) and its underlying mechanism, by distinguishing different evolutions of disturbance sources over the North Atlantic. The disturbance sources propagate along the SAJW toward the western North Pacific in the propagating cases, while they are constrained over the North Atlantic and Europe in the interrupted cases. The possible motivating mechanisms are disclosed in the perspectives of energy dispersion and background potential vorticity gradient (PVG) affecting energy attraction. In the propagating cases, wave energy propagates eastward over the North Atlantic and converges into the Mediterranean Sea and Middle East, leading to the amplification and development of the wave train. The enhancement of PVG over south Europe–Mediterranean Sea is a key factor attracting such eastward propagation, which is associated with the positive PV anomaly over north Europe driven by the diabatic heating of precipitation. In the interrupted cases, energy dispersion is more divergent over the North Atlantic because of the weaker PVG, resulting in the failure of the SAJW excitement. The impacts of the SAJW on the temperature and precipitation along its path are evident, with the modulated snow depth agreeing more with temperature rather than precipitation except over the Tibetan Plateau. Hence, understanding the motivating mechanism of the SAJW is essential for improving weather forecasts over subtropical Eurasia by using the disturbances over the North Atlantic.

Quasi-geostrophic monopoles in sheared zonal jets and multiple-jet flows

This work continues our earlier studies of the interaction between a monopolar vortex and a sheared zonal flow in the framework of a 1.5-layer quasi-geostrophic model, based on numerical experiments with singular vortices. Earlier examination of flows with shears of fixed sign showed that the interaction depends strongly on the latitudinal distribution of the gradient of background potential vorticity b(y) (y being the latitude). The latitude y0 at which b(y) changes sign turns out to be of particular importance. In the vicinity of y0, under certain conditions, there arises the zonal-strip region, which attracts (repels) prograde (retrograde) vortices. This effect is examined here for the zonal flows in the form of individual jets as well as for the systems of alternating zonal jets; in all these cases, the background-flow velocity shear and the parameter b(y) can change sign depending on y. It is shown that the vortex drifts to the nearest latitude y0 on the prograde side of the zonal flow, and the meridional speed of the trapped vortex almost vanishes, but its zonal speed is directed westward and approaches the Rossby-wave drift velocity.

Distinct Impacts of Topographic versus Planetary PV Gradients on Baroclinic Turbulence

Abstract The impacts of topographic versus planetary potential vorticity (PV) gradients on fully developed geostrophic turbulence are often treated as dynamically equivalent in theoretical understanding and parameterizations of ocean mesoscale turbulence. Using a suite of homogeneous, two-layer quasigeostrophic (QG) turbulence simulations, we identify similarities and distinctions of topographic versus planetary PV gradients in modulating turbulent eddy energy and fluxes. We show that, while an elevated background PV gradient can suppress geostrophic turbulence, a positive (negative) bottom slope with respect to the orientation of isopycnal slope barotropizes (debarotropizes) the turbulent energy at large scales, which contrasts with a dynamically inert planetary PV gradient in the modewise energy transfer. Then, in the presence of weak bottom drag, a positive slope energizes large-scale along-slope jets and limits small-scale barotropic eddies, both of which yield stronger eddy suppression effects than from a planetary PV gradient; by contrast, a negative slope hinders along-slope jet formation by enhancing the dual-energy cascade cycling, which alleviates the topographic suppression on eddies. In the presence of strong bottom drag, a positive slope elevates barotropic eddy energy, which further enhances the eddy-driven fluxes; by contrast, a negative slope confines turbulent energy to the baroclinic mode, which is strongly damped, causing further weakened turbulent energy and eddy fluxes. A flow regime captured by linear QG theories also emerges as the turbulent energy cascade is jointly suppressed by negative slopes and strong bottom drag. This study provides insights into parameterizing mesoscale eddy effects across sloping bathymetry in predictive ocean models.

Formation of Eastern Boundary Undercurrents via Mesoscale Eddy Rectification

Abstract Eastern boundary upwelling systems (EBUSs) host equatorward wind-driven near-surface currents overlying poleward subsurface undercurrents. Various previous theories for these undercurrents have emphasized the role of poleward alongshore pressure gradient forces (APFs). Energetic mesoscale variability may also serve to accelerate undercurrents via mesoscale stirring of the potential vorticity gradient imposed by the continental slope. However, it remains unclear whether this eddy rectification mechanism contributes substantially to driving poleward undercurrents in EBUS. This study isolates the influence of eddy rectification on undercurrents via a suite of idealized simulations forced either by alongshore winds, with or without an APF, or by randomly generated mesoscale eddies. It is found that the simulations develop undercurrents with strengths comparable to those found in nature in both wind-forced and randomly forced experiments. Analysis of the momentum budget reveals that the along-isobath undercurrent flow is accelerated by isopycnal advective eddy momentum fluxes and the APF and retarded by frictional drag. The undercurrent acceleration may manifest as eddy momentum fluxes or as topographic form stress depending on the coordinate system used to compute the momentum budget, which reconciles these findings with previous work that linked eddy acceleration of the undercurrent to topographic form stress. The leading-order momentum balance motivates a scaling for the strength of the undercurrent that explains most of the variance across the simulations. These findings indicate that eddy rectification is of comparable importance to the APF in driving poleward undercurrents in EBUSs and motivate further work to diagnose this effect in high-resolution models and observations and to parameterize it in coarse-resolution ocean/climate models.

Instability and Mesoscale Eddy Fluxes in an Idealized 3‐Layer Beaufort Gyre

AbstractWe study the impacts of a continental slope on instability and mesoscale eddy fluxes in idealized 3‐layer numerical model simulations. The simulations are inspired by and mimic the situation in the Arctic Ocean's Beaufort Gyre, where anti‐cyclonic winds drive anti‐cyclonic currents that are guided by the continental slope. The forcing and currents are retrograde with respect to topographic Rossby waves. The focus of the analysis is on eddy potential vorticity (PV) fluxes and eddy‐mean flow interactions under the Transformed Eulerian Mean framework. Eddy lateral vorticity fluxes dominate over the continental slope where eddy form stress, that is, vertical momentum flux, is suppressed due to the topographic PV gradient. The diagnosis also shows that while eddy momentum fluxes are up‐gradient over parts of the slope, the total quasi‐geostrophic PV flux is down‐gradient everywhere. We then calculate the linearly unstable modes of the time‐mean state and find that the most unstable mode contains several key features of the observed finite‐amplitude fluxes over the slope, including down‐gradient PV fluxes. When accounting for additional unstable modes, more qualitative features of the observed eddy fluxes in the numerical model are reproduced.

Rapid summer Russian Arctic sea-ice loss enhances the risk of recent Eastern Siberian wildfires

In recent decades boreal wildfires have occurred frequently over eastern Siberia, leading to increased emissions of carbon dioxide and pollutants. However, it is unclear what factors have contributed to recent increases in these wildfires. Here, using the data we show that background eastern Siberian Arctic warming (BAW) related to summer Russian Arctic sea-ice decline accounts for ~79% of the increase in summer vapor pressure deficit (VPD) that controls wildfires over eastern Siberia over 2004-2021 with the remaining ~21% related to internal atmospheric variability associated with changes in Siberian blocking events. We further demonstrate that Siberian blocking events are occurring at higher latitudes, are more persistent and have larger zonal scales and slower decay due to smaller meridional potential vorticity gradients caused by stronger BAW under lower sea-ice. These changes lead to more persistent, widespread and intense high-latitude warming and VPD, thus contributing to recent increases in eastern Siberian high-latitude wildfires.

The Role of Surface Potential Vorticity in the Vertical Structure of Mesoscale Eddies in Wind-Driven Ocean Circulations

Abstract The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extratropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasigeostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.

Suppression of Mesoscale Eddy Mixing by Topographic PV Gradients

Abstract Oceanic mesoscale eddy mixing plays a crucial role in Earth’s climate system by redistributing heat, salt, and carbon. For many ocean and climate models, mesoscale eddies still need to be parameterized. This is often done via an eddy diffusivity , which sets the strength of turbulent downgradient tracer fluxes. A well-known effect is the modulation of in the presence of background potential vorticity (PV) gradients, which suppresses cross-PV gradient mixing. Topographic slopes can induce such suppression through topographic PV gradients. However, this effect has received little attention, and topographic effects are often not included in parameterizations for . In this study, we show that it is possible to describe the effect of topography on analytically in a barotropic framework, using a simple stochastic representation of eddy–eddy interactions. We obtain an analytical expression for the depth-averaged as a function of the bottom slope, which we validate against diagnosed eddy diffusivities from a numerical model. The obtained analytical expression can be generalized to any constant barotropic PV gradient. Moreover, the expression is consistent with empirical parameterizations for eddy diffusivity over topography from previous studies and provides a physical rationalization for these parameterizations. The new expression helps to understand how eddy diffusivities vary across the ocean, and thus how mesoscale eddies impact ocean mixing processes. Significance Statement Large oceanic “whirls,” called eddies, can mix and transport ocean properties such as heat, salt, carbon, and nutrients. Mixing plays an important role for oceanic ecosystems and the climate system. In numerical simulations of Earth’s climate, eddy mixing is typically represented using a simplified expression. However, an effect that is often not included is that eddy mixing is weaker over a sloping seafloor. In most areas of the ocean the bottom slope is steep enough for this effect to be significant. In this study we derive an expression for eddy mixing that accounts for oceanic bottom slopes. The present effort provides a physical basis for eddy mixing over oceanic bottom slopes, justifying their use in climate models.

The response of atmospheric blocking and East Asian cold extremes to future Arctic Sea ice loss

The effect of Arctic warming on cold extremes over middle latitude remains a controversial and unresolved issue, particularly considering potential enhanced Arctic amplification in future scenarios. Here we examine the possible responses of winter cold extremes to the projected sea ice loss under 2 °C global warming scenario in four models, based on the atmosphere-only experiments of the Polar Amplification Model Inter-comparison Project (PAMIP). We find agreement with the increased responses of extreme cold days over East Asia (EA), characterized by longer duration or stronger intensity of temperature drop during cold events. Accordingly, most models exhibit a weak response of the Ural blocking in the corresponding composite circulation fields, whereas the westward movement of an anomalous anticyclonic system from eastern Siberia before the maturing of the Ural blocking is notable in four models. We suggest that this westward shifting pattern may prolong the duration of Ural blocking or modulate the strength of the Northeast Asian cold vortex, resulting in longer or stronger cold anomalies in EA. Accordingly, increased blocking frequency with a more pronounced westward response of shipment signals is also found over eastern Siberia, where a reduced meridional potential vorticity gradient (PVy) exists. The background of weak PVy can enhance the atmospheric nonlinearity, resulting in the anticyclonic anomalies less prone to dispersion and presenting a westward characteristic. Our results highlight the impact of the nonlinear response of circulation to Arctic sea ice loss. In contrast to more concerned Ural blocking influence, it is suggested that the downstream impact of eastern Siberia blocking on cold extremes in EA may be more pronounced in an enhanced Arctic amplification climate.

Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

AbstractMost of the ocean's kinetic energy is contained within the mesoscale eddy field. Models that do not resolve these eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large‐scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models rely on an assumption of a flat ocean floor even though observations and high‐resolution modeling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. We show that buoyancy transport coefficient diagnosed from idealized eddy‐resolving simulations is indeed reduced over both prograde and retrograde bottom slopes (topographic wave propagation along or against the mean flow, respectively) and that the reduction can be skilfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale. This modified “GM” parameterization enhances the strength of thermal wind currents over the slopes in coarse‐resolution, non‐eddying, simulations. We find that in realistic global coarse‐resolution simulations the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean‐flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean biases.

Summer Russian Heat Waves Linked to Arctic Sea Ice Anomalies in 2010 and 2016

Abstract This study investigates dominant features of the atmospheric circulation evolution associated with extreme heat waves (HWs) in Russia during the summers of 2010 and 2016, respectively, and their possible association with Arctic sea ice loss. Results show that a region of Russia (20°–70°E, 45°–65°N) experienced a lasting 44-day HW event from 4 July to 16 August 2010 and a 26-day HW event from 2 to 27 August 2016. The associated atmospheric circulation anomalies are characterized by the summer Arctic cold anomaly in the mid- to low troposphere and an anticyclonic circulation anomaly over the Ural Mountains. Simulation experiments forced by summer Arctic sea ice anomalies reproduce the major characteristics of observational associations. Observations and numerical simulations indicate that summer Arctic sea ice anomaly is conducive to the formation of the summer Arctic cold anomaly, which is often accompanied by the enhanced baroclinicity in most of the Arctic troposphere and increased and decreased meridional temperature gradient in the high and midlatitudes, respectively. Such a configuration strengthens westerly winds over most of the Arctic and weakens zonal westerly over the southern Ural Mountains. This anomalous zonal wind pattern establishes the background conditions for the sustained positive geopotential height anomaly in the mid- to low troposphere that dynamically facilitates the prevalence of Russian HW events. Moreover, when compared with 2016, the weaker meridional potential vorticity gradient anomaly in the summer of 2010 prolonged the persistence of Ural blocking, which may lead to longer HW events in Russia.

Moist bias in the Pacific upper troposphere and lower stratosphere (UTLS) in climate models affects regional circulation patterns

Abstract. Water vapour in the upper troposphere and lower stratosphere (UTLS) is a key radiative agent and a crucial factor in the Earth's climate system. Here, we investigate a common regional moist bias in the Pacific UTLS during Northern Hemisphere summer in state-of-the-art climate models. We demonstrate, through a combination of climate model experiments and satellite observations, that the Pacific moist bias amplifies local long-wave cooling, which ultimately impacts regional circulation systems in the UTLS. Related impacts involve a strengthening of isentropic potential vorticity gradients, strengthened westerlies in the Pacific westerly duct region, and a zonally displaced anticyclonic monsoon circulation. Furthermore, we show that the regional Pacific moist bias can be significantly reduced by applying a Lagrangian, less-diffusive transport scheme and that such a model improvement could be important for improving the simulation of regional circulation systems, in particular in the Asian monsoon and Pacific region.

Effects of Stratospheric Warming on Ural blocking Events in Winter

AbstractUtilizing the Open Integrated Forecasting System, the responses of Ural blocking (UB) to different stratospheric warming scenarios are investigated. Numerical results show that stratospheric warming with moderate strength in minor patterns prolongs the UB duration and enhances its intensity, while strong stratospheric warming in minor patterns tends to shorten its duration and weaken its intensity, even leading to the collapse of the UB events. Further diagnosis reveals that the planetary wave activity flux propagates downward from the stratosphere to the troposphere after stratospheric warming. Moreover, the convergence of planetary wave activity flux is a key factor for UB enhancement and maintenance. In addition, the weakened meridional temperature gradients, decelerated zonal westerly winds, and a reduced meridional potential vorticity gradient (PVy) result in UB enhancement in response to stratospheric warming with moderate strength. As stratospheric warming strengthens, planetary wave activity flux diverges, westerly winds in the tropospheric mid‐latitudes accelerate and the PVy in the Ural sector enlarges, which further weakens UB. Regarding the stratospheric perturbations in major patterns, they have similar influences on UB events, that is, UB enhances with moderate stratospheric warming and weakens with strong warming. However, the strengthened warming would trigger UB re‐enhancement, which is closely associated with anomalous activities of tropospheric synoptic‐scale waves induced by stratospheric perturbations. These results reveal UB events respond differently to stratospheric warming with various intensities and patterns in the short term, which makes a contribution to understanding stratosphere‐troposphere coupling.

Cold Air Outbreaks in Winter over the Continental United States and Its Possible Linkage with Arctic Sea Ice Loss

The mechanism for the paradox of global warming and successive cold winters in mid-latitudes remains controversial. In this study, the connection between Arctic sea ice (ASI) loss and frequent cold air outbreaks in eastern Continental United States (CONUS) is explored. Two distinct periods of high and low ASI (hereafter high- and low-ice phases) are identified for comparative study. It is demonstrated that cold air outbreaks occur more frequently during the low-ice phase compared to that during the high-ice phase. The polar vortex is weakened and shifted southward during the low-ice phase. Correspondingly, the spatial pattern of 500 hPa geopotential height (GPH), which represents the mid-tropospheric circulation, shows a clear negative Arctic Oscillation-like pattern in the low-ice phase. Specifically, positive GPH anomalies in the Arctic region with two centers, respectively located over Greenland and the Barents Sea, significantly weaken the low-pressure system centered around the Baffin Island, and enhance Ural blocking in the low-ice phase. Meanwhile, the high ridge extending from Alaska to the west coast of North America further intensifies, while the low trough over eastern CONUS deepens. As a result, the atmospheric circulation in North America becomes more conductive to frigid Arctic air outbreaks. It is concluded that the ASI loss contributes to more cold air outbreaks in winter in eastern CONUS through the polar vortex weakening with southward displacement of the polar vortex edge, which lead to the weakening of the meridional potential vorticity gradient between the Arctic and mid-latitude and thus are conducive to the strengthening and long-term maintenance of the blocking high.

Structure and Characteristics of Synoptic-Scale Waves in the Northern Eurasian Storm Track during Summer

Abstract This study examined the dominant structure and characteristics of synoptic-scale (2–8-day periods) waves over northern Eurasia during 40 summer seasons (June–August, 1979–2018). The synoptic-scale wave patterns are isolated using an extended empirical orthogonal function (EEOF) analysis on the 300-hPa geopotential height anomalies, and a composite based on atmospheric circulation fields and gridded precipitation product. The wave patterns are classified into two types from two pairs of EEOF modes. These two different wave types are defined as the polar frontal (PF) mode and Arctic frontal (AF) mode, respectively. The PF-mode waves are initiated in the North Atlantic sector to the west of the British Isles. They propagate eastward across Siberia into the North Pacific, and produce precipitation mainly over the Eurasian polar frontal zone. The AF-mode wave train arcs along the climatological Arctic frontal zone (AFZ). The AF-mode waves originate near the Scandinavian Peninsula. Their eastward passage brings precipitation along the AFZ. The development of the synoptic-scale waves is reflected by unique background conditions over northern Eurasia. The lower-tropospheric baroclinicity in southern Siberia and central Asia favored the baroclinic growth of the PF-mode waves. The AF-mode waves are trapped in the well-organized baroclinic zone along the north coast of the Eurasian continent. The baroclinic zone is coupled with a band of large meridional gradient of potential vorticity in the upper troposphere, suggesting that this band acts as a waveguide for the AF-mode waves. Significance Statement This study examines the synoptic-scale waves in the 2–8-day range of time scales over northern Eurasia during summer. The synoptic-scale waves are categorized into two distinct types at different latitude bands by the EEOF analysis on the 300-hPa z anomalies. They are defined as polar frontal (PF) mode and Arctic frontal (AF) mode. Then the EEOF-based composite analysis is conducted to detect the large-scale circulation anomalies associated with the propagation of different types of synoptic-scale waves. The structure and characteristics are examined. The roles of the mean background conditions in the development and propagation of the respective types are discussed. The behavior of these wave disturbances as rain-producing weather systems is also examined.

Different influences of La Niña types on the winter sub-seasonal Eurasian cold anomalies linked to Ural blocking

The effects of different types of La Niña on winter sub-seasonal cold anomalies or cold extremes over Eurasia and Siberia are examined through exploring changes in Ural blocking (UB). It is found that the Central Pacific (CP) La Niña can significantly influence surface air temperature over Eurasia mainly by regulating the movement, persistence, zonal scale and location of the cyclonic anomaly associated with the UB. This influence is much greater than that resulting from an Eastern Pacific (EP) La Niña. A long-lived and quasi-stationary cyclonic anomaly with a large zonal scale of UB is induced to the southeast of the Ural Mountains for CP-type La Niña conditions, which leads to a strong warm Arctic-cold Eurasia pattern with an intense sub-seasonal cold anomaly or cold event in the broad region of Eurasia due to strongly reduced downward infrared radiation over Eurasia. In contrast, for EP-type La Niña winter the cyclonic anomaly, while still present, is much weaker, short-lived and eastward propagating, resulting in only a very weak cold anomaly or cold event over a small region of Eurasia. The reduced (enhanced) eastward movement and increased (decreased) persistence of the cyclonic anomaly with large (small) zonal scale of UB are shown to be associated with reduced (enhanced) meridional potential vorticity gradient in the midlatitudes (30o-50oN) to the east of 60°E during the CP-type (EP-type) La Niña winter.

The Predictability of the 2021 SSW Event Controlled by the Zonal‐Mean State in the Upper Troposphere and Lower Stratosphere

AbstractSudden stratospheric warming (SSW) describes a disruption of the stratospheric polar vortex in the winter hemisphere. It affects not only the stratospheric circulation but also the surface climate for up to 2 months, serving as an important source of subseasonal‐to‐seasonal (S2S) predictability in midlatitudes. This study evaluates the predictability of the 2021 SSW and investigates the crucial factors that determine its predictability in the ECMWF and JMA S2S real‐time forecasts. In both models, only a subset of the ensemble members predicted the SSW at the lead time of about 2 weeks before the onset. By comparing the 10 ensembles with successful SSW predictions and those with failed predictions, we found that the ensembles predicting the SSW have relatively stronger wave fluxes from the upper troposphere to the stratosphere than the others. Stronger wave fluxes, particularly those of zonal wavenumber one, are not the result of the tropospheric precursors such as the Ural blocking and Aleutian cyclones but they result from the modulation of the wave propagation by the background state. In particular, the ensembles with failed SSW predictions tend to have a negative potential vorticity gradient in the upper troposphere and lower stratosphere, which limits the upward wave propagation into the stratosphere and provides an unfavorable condition for the SSW. This result suggests that not only the wave sources in the troposphere but also the background state in the upper troposphere and lower stratosphere can modulate the predictability of SSW in S2S prediction models.

The Indian Easterly Jet During the Pre‐Monsoon Season in India

AbstractWe identify for the first time an Indian Easterly Jet (IEJ) in the mid‐troposphere during the pre‐monsoon using reanalysis data. The IEJ is weaker and smaller than the African Easterly Jet over West Africa, with a climatological location of 10°N, 60–90°E, 700 hPa, and strength 6–7 m s−1 during March–May. The IEJ is a thermal wind associated with low‐level meridional gradients in temperature (positive) and moisture (negative), resulting from equatorward moist convection and poleward dry convection. The IEJ is associated with a negative meridional potential vorticity gradient, therefore satisfying the Charney‐Stern necessary condition for instability. However, no wave activity is detected, suggesting that the potential for combined barotropic‐baroclinic instability is not often realized. IEJ strong (weak) years feature increased (reduced) near‐surface temperatures and drier (wetter) conditions over India. This study provides an introduction to the IEJ's role in pre‐monsoon dynamics, and a platform for further research.

Stirring across the Antarctic Circumpolar Current's southern boundary at the prime meridian, Weddell Sea

Abstract. At the southern boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the southern boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the southern boundary in the Weddell Sea sector at the prime meridian is surveyed for the first time in high resolution over 2 months during an austral summer with underwater gliders occupying a transect across the front on five occasions. The five transects show that the frontal structure (i.e. hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, transient (a few weeks) variability of the southern boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the southern boundary’s frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the ACC and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the southern boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using satellite altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasise that locally and rapidly changing barrier properties of the southern boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the southern boundary is located near the Antarctic shelf break (e.g. in the West Antarctic sector).

Loss of autumn Kara-East Siberian Sea ice intensifies winter Ural blocking and cold anomalies in high latitudes of Eurasia

Sea ice in the Kara-East Siberian Sea (KESS) declined to the lowest record in the autumn of 2020 and has been suggested to affect the Eurasian climate in the following winter. By analyzing reanalysis data from 1979 to 2021 and performing model experiments, this study demonstrates that the loss of autumn KESS sea ice acts to intensify the Eurasian cold spells in winter. Low autumn KESS sea ice attenuates the stratospheric polar vortex through driving upward planetary wave activities. The weakening of polar vortex persists to winter and induces anomalies in tropospheric zonal wind and meridional gradient of potential vorticity through downward wave propagation. These intraseasonal to cross-seasonal processes lead to enhancement and prolonged duration of Ural blocking that aggravates Eurasian cold spells. A positive feedback likely exists between the weak stratospheric polar vortex induced by autumn sea ice loss and the wintertime Ural blocking via vertical wave propagation. The results establish a link between autumn KESS sea ice and Eurasian winter climate, with implications for seasonal prediction of Eurasian cold events.