Vortex Response Research Articles

Structural fluctuations of the Arctic Oscillation tied to the Atlantic Multidecadal Oscillation

The Arctic Oscillation (AO) has been observed to undergo distinct decadal structural fluctuations that significantly influence regional weather and climate. Understanding the drivers and mechanisms behind the AO’s spatial nonstationarity is critical for improving climate predictions related to the AO. We present evidence that the Atlantic Multidecadal Oscillation (AMO) plays a pivotal role in modulating AO’s Pacific center in recent decades. The poleward amplified cooling associated with negative AMO enhances the north-south temperature gradient and results the strengthened westerly winds and stratospheric polar vortex (SPV) responses, which reflects more planetary waves from the North Pacific to the North Atlantic. This enhances the atmospheric coupling between these regions and leads to a more pronounced Pacific center within the AO pattern. Numerical simulations from ECHAM5 and 35 CMIP6 models further corroborate the essential role of the AMO. These findings advance our understanding of the mechanisms driving the variability of the AO pattern.

Influence of quasi-biennial oscillations (QBO) on the stratospheric polar vortex in the Antarctic

For many years, the dominant opinion in the literature has been that the stratospheric polar vortex is weaker in the easterly phase of the QBO than in the westerly phase, which is known as the Holton–Tan effect. While for the Northern Hemisphere vortex this is true during winter, for the Southern Hemisphere vortex the dependence on the QBO is observed only in spring. This feature is usually explained by the greater intensity of the Southern Hemisphere vortex compared to the Northern Hemisphere vortex, and the QBO can modulate its strength only during the vortex breaking season in October-November. Usually, the vortex response to the QBO is determined based on the equatorial wind direction at a certain vertical level, and the conclusions depend strongly on the level at which the QBO phase is determined. However, it has long been shown that using the equatorial wind sign at one or even a combination of several levels is not quite correct because it does not take into account the time of descent of the QBO wind relative to the seasons of the year, and it is not known at what altitudes the QBO wind has the strongest influence on the extratropical stratosphere. Due to the variable period of the QBO cycles, the phase relationship between the seasonal cycle and the QBO cycle is constantly changing, resulting in many variants of the vertical structure of the wind QBO during the Antarctic winter vortex and ozone hole. However, the seasonal regularities of QBO used in this work lead to a strictly limited number of possible variants of coincidence of the phases of the QBO cycles with the seasons of the year, which allows us to reveal typical features of interannual variations of the polar vortex and ozone hole in the Antarctic that are due to the QBO. The analysis of observational data indicates unexpected peculiarities of the QBO modulation of the stratospheric polar vortex in the Antarctic. The QBO effect in the vortex intensity is observed not only in spring during the weakening phase of the winter vortex, but also during the vortex maximum in June–August. At the same time, changes in the wind speed of the vortex during its maximum in winter are opposite to those in the spring during the ozone hole period. If the winter vortex is more intense (weak), then during the ozone hole period the vortex is weaker (more intense) than the average level.

Broadband high-efficiency acoustic vortices via a topology-optimized space-coiling-cavity metasurface

Acoustic vortices carrying orbital angular momentum are significant in wave-matter interactions, allowing for versatile devices with promising applications. However, current metasurface-based vortex generators have limited bandwidth, operating within a narrow frequency range. Here, we propose a broadband acoustic metasurface with four space-coiling-cavity units through the bottom-up topology optimization. These units have constant phase differences with high-efficiency transmission (>0.8) over a desired frequency range [3.0, 4.0] kHz, indicating the unique wideband vortex response of their assembled metasurfaces. Our simulations and experiments further demonstrate this excellent performance. This breakthrough in broadband vortices allows for the development of devices like acoustic tweezers, antennas, and spanners.

High-frequency vortex dynamics and losses in the mixed state of nanostructured superconductors with columnar defects

We suggest a theoretical model for the microwave response of Abrikosov vortices in nanostructured superconductors with columnar defects, which serve as effective pinning sites for vortices that are considered elastic strings. For this case, we suppose that microwave losses related to the vortex oscillations in a microwave field arise caused by viscous oscillations of vortex kinks, which connect different parts of the vortex line pinned on adjacent columnar defects and vortex segments pinned on these defects. We consider the case of inclined magnetic field H, which creates vortex staircases consisting of pinned vortex segments and vortex kinks inside the superconductor. In this case, rf losses due to viscous kinks oscillations under the microwave current action arise even at T = 0 if the inclination angle of the magnetic field θΗ, concerning the columnar pinning site axis (z axis), exceeds some locking angle value θL.

The Influence of Large-Scale Radiation Anomalies on Tropical Cyclone Frequency

Abstract The response of tropical cyclone (TC) frequency to sea surface warming is uncertain in climate models. We hypothesize that one source of uncertainty is the anomalies of large-scale atmospheric radiation in response to climate change, and whose influence on TC frequency is investigated. Given two atmospheric models with opposite TC frequency responses to uniform sea surface warming, we interchange their atmospheric radiation anomalies in experiments with prescribed radiative heating rates. The largest model discrepancy occurs in the western North Pacific, where the TC frequency tends to increase with anomalous large-scale ascent caused by prescribed positive radiation anomalies, while the TC frequency tends to decrease with anomalous large-scale descent caused by prescribed negative radiation anomalies. The model spread in TC frequency response is approximated by the model spread in the frequency response of pre-TC vortices (seeds), which is explained by changes in the large-scale circulation using a downscaling formula known as the seed propensity index. We further generalize the index to predict the influence of large-scale radiation anomalies on TC seed frequency. The results show that model spread in TC and seed frequency response can be reduced when constraining the large-scale radiation anomalies. Significance Statement It is difficult to predict whether tropical cyclones will occur more or less frequently in the future and by how much. We show that tropical cyclone frequency is strongly influenced by the global pattern of heating and cooling due to radiation, a process that has been neglected in existing theories. Our theory improves understanding of how tropical cyclones respond to climate change, explaining why one model may predict a frequency increase while a different but equally realistic model may predict a frequency decrease. One reason for the difficulty in predicting tropical cyclone frequency is found to be the difficulty in predicting how global cloud distribution will change in the future.

Southern Hemisphere Response to the Quasi-Biennial Oscillation in the CMIP5/6 Models

Abstract Using 25 quasi-biennial oscillation (QBO)-resolving models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6), this study systematically explores the Southern Hemisphere (SH) extratropical response to the QBO. The observed maximum stratospheric polar vortex–weakening response to the 20-hPa easterly QBO appears in the austral spring, which is reproduced qualitatively by seven models. The Eliassen–Palm (EP) flux convergence associated with enhanced upward propagation of waves from the SH midlatitude troposphere is also reasonably simulated following easterly QBO in these seven models. The Brewer–Dobson (BD) circulation response, especially its deep branch, has a large bias among models. The anomalous upwelling of the deep branch of the BD circulation and the cold anomalies are confined to midlatitudes above 20 hPa in reanalyses, while this deep branch is simulated to expand to the Antarctic stratosphere with unrealistically cold anomalies for some models. Models with a decent stratospheric pathway tend to simulate the observed negative Southern Annular Mode (SAM)-like response in the troposphere. Further, the largest enhanced convection center is observed over the tropical Indo-Pacific Ocean and simulated by 12 models, most of which also simulate a more reasonable height anomaly pattern in the troposphere. The near-surface response to the QBO also resembles the SAM pattern with warm anomalies over eastern Antarctic, which are simulated by only five models. Seven models simulate the observed wet signals in the midlatitude southern Indo-Pacific Ocean, where an anomalous low develops. Overall, models with a better reproduction of the SH stratospheric polar vortex response exhibit a more reasonable near-surface response, though for all models the response is too weak. Significance Statement The equatorial stratospheric quasi-biennial oscillation (QBO) has been widely reported to influence the Northern Hemisphere. This paper focuses on the possible impact of the QBO on the Southern Hemisphere (SH) using 25 models. The maximum QBO signal in the Antarctic stratosphere forms in austral spring when the climatological vortex intensity has weakened relative to austral winter. The timing and magnitude of the QBO signal is simulated with different degrees of success by the 25 models, and those with a reasonable polar vortex response tend to better simulate the near-surface response. The QBO has another pathway for the extratropical circulation change via modifying tropical convection, and models also simulate a large spread in the tropical convection change associated with the QBO.

Rheological transient effects on steady-state contraction flows

It may be assumed that the steady-state kinematics of viscoelastic contraction flows depends on the time-independent rheological properties only. This idea is supported by the large number of references explaining steady simulation results by considering only steady-state material functions. Even with numerical simulations, it would be difficult to prove such a statement wrong. However, using the Bautista-Manero-Puig class of models allows to obtain the same steady rheological response but with different transient evolution. Here, we considered two fluids, one displaying a monotonic trend towards the steady-state and the other with at least one visible overshoot in the material functions. Our results show that for the transient evolution with the overshoot fluid, a significant increase in the steady pressure drop is gathered. In addition, vortex response is quite different for the two fluids. This research gives evidence that the transient evolution in rheometrical functions has great impact on steady-state flow behavior.

Visualization and Electrical Response of Electroconvective Vortices on the Surface of Homo/Heterogeneous Ion Exchange Membranes

Visualization and Electrical Response of Electroconvective Vortices on the Surface of Homo/Heterogeneous Ion Exchange Membranes

Experiment on jet–vortex interaction for variable mutual spacing

The interaction of a line vortex with a collinearly aligned jet is a prototypical configuration for various important applications in aeronautics. The purpose of this study is to analyze the impact of the jet flow on the kinematics and dynamics of a trailing vortex. A particular emphasis is on the effect of a variable relative jet–vortex spacing. To this end, we realized four different jet–vortex configurations in a wind tunnel experiment at a chord-based Reynolds number of 1.7×105 using high-speed stereo particle image velocimetry measurements in five transversal planes located between 2 and 26 chords behind the wing. Stochastic analyses reveal that the jet generally contributes an external excitation to the vortex as a function of the mutual spacing. Compared with the configuration without jets, the vortex amplification increases upon reducing the jet–vortex spacing. Most notably, for all but the closest spacing, the vortex response is qualitatively the same, changing mainly in magnitude. For the closest spacing, however, the dynamics is considerably different, which we suspect to be a consequence of jet entrainment. Proper orthogonal decomposition reveals, for the first time, that the collinearly aligned jet tends to excite a progressively broader range of vortex modes as the jet–vortex spacing is reduced. A close examination of the vortex mean flow seems to preclude linear vortex instabilities, while the vortex characteristics hint toward some form of receptivity mechanism to disturbances being located in the free stream. Our analyses are useful to validate simulation tools on configurations combining simultaneous lift and thrust effects.

Non‐Linear Response of the Extratropics to Tropical Climate Variability

AbstractThere are established El Niño‐Southern Oscillation (ENSO) and Quasi‐biennial Oscillation (QBO) teleconnections to the extratropics that alter the strength of the polar vortex. Here, we demonstrate non‐linearity in the stratospheric polar vortex response to ENSO and QBO combined, in climate model simulations and observational data. We show that the combined effect of El Niño and QBO easterly phase (QBO‐E) on the polar vortex is greater than the linear combination of the individual responses to El Niño and QBO‐E separately. Observations support the model results, despite a relatively small sample of observed cases. We propose that non‐linearity in the teleconnection to ENSO and QBO is caused by the combined effects of an increase in wave driving from El Niño and more poleward propagation of wave activity during QBO‐E. Our results have implications for extreme winters when both El Niño and QBO‐E are active.

The Effects of Numerical Dissipation on Hurricane Rapid Intensification With Observational Heating

AbstractThe computational fluid dynamics of hurricane rapid intensification (RI) is examined through idealized simulations using two codes: a community‐based, finite‐difference/split‐explicit model (WRF) and a spectral‐element/semi‐implicit model (NUMA). The focus of the analysis is on the effects of implicit numerical dissipation (IND) in the energetics of the vortex response to heating, which embodies the fundamental dynamics in the hurricane RI process. The heating considered here is derived from observations: four‐dimensional, fully nonlinear, latent heating/cooling rates calculated from airborne Doppler radar measurements collected in a hurricane undergoing RI. The results continue to show significant IND in WRF relative to NUMA with a reduction in various intensity metrics: (a) time‐integrated, mean kinetic energy values in WRF are ∼20% lower than NUMA and (b) peak, localized wind speeds in WRF are ∼12 m/s lower than NUMA. Values of the eddy diffusivity in WRF need to be reduced by ∼50% from those in NUMA to produce a similar intensity time series. Kinetic energy budgets demonstrate that the pressure contribution is the main factor in the model differences with WRF producing smaller energy input to the vortex by ∼23%, on average. The low‐order spatial discretization of the pressure gradient in WRF is implicated in the IND. In addition, the eddy transport term is found to have a largely positive impact on the vortex intensification with a mean contribution of ∼20%. Overall, these results have important implications for the research and operational forecasting communities that use WRF and WRF‐like numerical models.

Uncertainty in the Winter Tropospheric Response to Arctic Sea Ice Loss: The Role of Stratospheric Polar Vortex Internal Variability

Abstract Arctic sea ice has declined rapidly over the past four decades and climate models project a seasonally ice-free Arctic Ocean by the middle of this century, with attendant consequences for regional climate. However, modeling studies lack consensus on how the large-scale atmospheric circulation will respond to Arctic sea ice loss. In this study, the authors conduct a series of 200-member ensemble experiments with the Community Atmosphere Model version 6 (CAM6) to isolate the atmospheric response to past and future sea ice loss following the Polar Amplification Model Intercomparison Project (PAMIP) protocol. They find that the stratospheric polar vortex response is small compared to internal variability, which in turn influences the signal-to-noise ratio of the wintertime tropospheric circulation response to ice loss. In particular, a strong (weak) stratospheric polar vortex induces a positive (negative) tropospheric northern annular mode (and North Atlantic Oscillation), obscuring the forced component of the tropospheric response, even in 100-member averages. Stratospheric internal variability is closely tied to upward wave propagation from the troposphere and can be explained by linear wave interference between the anomalous and climatological planetary waves. Implications for the detection of recent observed trends and model realism are also presented. These results highlight the inherent uncertainty of the large-scale tropospheric circulation response to Arctic sea ice loss arising from stratospheric internal variability.

Interactions between the stratospheric polar vortex and Atlantic circulation on seasonal to multi-decadal timescales

Abstract. Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSWs), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation, and cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000 year preindustrial control simulation of the UK Earth System Model to study such interactions, using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary, with spectral power in vortex variability at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response through a pathway involving the equatorial Pacific and quasi-biennial oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use regression analysis to estimate the potential contribution of the 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus.

The Arctic Polar Vortex Response to Volcanic Forcing of Different Strengths

AbstractTropical volcanic eruptions injecting sulfur into the stratosphere are assumed to not only scatter radiation and cool Earth's surface but also alter atmospheric circulation and in particular to strengthen the stratospheric polar vortex in boreal winter. The exact impact is difficult to estimate because of the small number of well observed eruptions and the high internal variability of the vortex. We use large (100‐member) ensembles of simulations with an Earth system model for idealized volcanic aerosol distributions resulting from sulfur injections between 2.5 and 20 Tg. We suggest the existence of a threshold somewhere between 2.5 and 5 Tg(S) below which the vortex does not show a detectable response to the injection. This nonlinearity is introduced partly through the infrared aerosol optical density which increases much stronger than linear with increasing particle size occurring for increasing injection amount. Additionally, the dynamical mechanism causing the vortex strengthening seems not to set in for small aerosol loading. Furthermore, we add to the recent discussion concerning a possible downward propagation of the circulation response leading to a winter warming in Northern Eurasia. At latitudes northward of about 50°N, our simulations do show such an average warming pattern that is statistically significant for injections of 10 Tg(S) or more.

Sensitivity of QBO teleconnection to model circulation biases

AbstractThe Quasi‐biennial oscillation (QBO) in the equatorial stratospheric winds is known to modulate the extratropical atmospheric circulation so that an easterly QBO phase is associated with a weaker Arctic stratospheric vortex and a negative North Atlantic Oscillation in winter. The link can potentially be used to improve seasonal forecasts and is usually captured by climate models but with a smaller magnitude than that in observations. Here, the sensitivity of the QBO teleconnection to biases in model circulation is explored in an atmosphere‐only model ECHAM6 with an internally generated QBO, by contrasting simulations by an original, biased model, and a model version in which biases are artificially reduced. We find that the strength of the winter mean Arctic stratospheric vortex response is better reproduced by the bias‐corrected model. On the other hand, the response is delayed by 1 month and there is no improvement in the tropospheric response. Stronger response of the polar vortex in the bias‐corrected model is likely associated with larger magnitude of QBO winds and better‐represented stratospheric mean climate and planetary wave fluxes, but the exact mechanism remains unclear. The results of this study have implications for seasonal forecast model development.

Roughness-induced secondary flows in stably stratified turbulent boundary layers

Spanwise heterogeneous roughness, more specifically, spanwise-adjacent strips of relatively high and low roughness, is known to cause large-scale secondary flows in the vertical domain in neutral turbulent boundary layers. In this work, we study the response of secondary vortices to thermal stratification with focus on the stably stratified case. We systematically vary the Monin–Obukhov (MO) length scale from L/h = 0.3, which corresponds to relatively strong stratification, to ∞, which corresponds to the neutral condition. Here, L is the MO length and h is an outer length scale. The results show that stable stratification suppresses the vertical motions and reduces the vertical size and strength of the first off-wall secondary vortex. Moreover, the reduced vertical extent of the first off-wall vortex and the shear near its top together give rise to a second vortex, and the second to a third vortex. This leads to a stack of secondary vortices in a stably stratified boundary layer flow with decreasing strength when moving away from the wall.

How Does the Quasi-Biennial Oscillation Affect the Boreal Winter Tropospheric Circulation in CMIP5/6 Models?

AbstractUsing 17 CMIP5 and CMIP6 models with a spontaneously generated quasi-biennial oscillation (QBO)-like phenomenon, this study explores and evaluates three dynamical pathways for impacts of the QBO on the troposphere: 1) the Holtan–Tan (HT) effect on the stratospheric polar vortex and the northern annular mode (NAM), 2) the subtropical zonal wind downward arching over the Pacific, and 3) changes in local convection over the Maritime Continent and Indo-Pacific Ocean. More than half of the models can reproduce at least one of the three pathways, but few models can reproduce all of the three routes. First, seven models are able to simulate a weakened polar vortex during easterly QBO (EQBO) winters, in agreement with the HT effect in the reanalysis. However, the weakened polar vortex response during EQBO winters is underestimated or not present at all in other models, and hence the chain for QBO, vortex, and tropospheric NAM/AO is not simulated. For the second pathway associated with the downward arching of the QBO winds, 10 models simulate an inconsistent extratropical easterly anomaly center over 20°–40°N in the Pacific sector during EQBO, and hence the negative relative vorticity anomalies poleward of the easterly center is not present in those models, leading to no consensus on the height response over the North Pacific between those models and the reanalysis. However, the other seven models do capture this effect. The third pathway is only observed in the Indo-Pacific Ocean, where the strong climatological deep convection and the warm pool are situated. Seven models can simulate the convection anomalies associated with the QBO over the Maritime Continent, which is likely caused by the near-tropopause low buoyancy frequency anomalies. No robust relationship between the QBO and El Niño–Southern Oscillation (ENSO) events can be established using the JRA55 reanalysis, and 10 models consistently confirm little modulation of the ocean basinwide Walker circulation and ENSO events by the QBO.

Non‐Additivity of the Midlatitude Circulation Response to Regional Arctic Temperature Anomalies: The Role of the Stratosphere

AbstractPrevious studies have documented the impact of the Arctic sea ice loss and associated warming on the midlatitude weather and climate, especially the influence of sea ice retreat over the Barents‐Kara Sea on the North Atlantic and Europe regions. However, less attention has been given to other geographical locations over the Arctic, and to the linear additivity of the circulation response to regional Arctic sea ice loss and temperature anomalies. Using a simplified dry dynamical core model, we demonstrate that responses to regional Arctic temperature anomalies over the Barents‐Kara Sea, Baffin Bay‐Davis Strait‐Labrador Sea, and East Siberia‐Chukchi Sea, separately, cause similar equatorward shift of the tropospheric jet, but different stratospheric polar vortex responses. Furthermore, responses to regional Arctic temperature anomalies are not linearly additive, and the residual resembles a positive Northern Annular Mode‐like structure. Additional targeted experiments highlight the stratospheric influence in the non‐additivity of the midlatitude tropospheric response.

Influence of Enhanced Planetary Wave Activity on the Polar Vortex Enhancement Related to Energetic Electron Precipitation

AbstractThe Northern polar vortex experiences considerable interannual variability, which is also reflected to tropospheric weather. Recent research has established a link between polar vortex variations and energetic electron precipitation (EEP) from the near‐Earth space into the polar atmosphere, which is mediated by EEP‐induced chemical changes causing ozone loss in the mesosphere and stratosphere. The most dramatic changes in the polar vortex are due to sudden stratospheric warmings (SSWs). Enhanced planetary wave convergence and meridional circulation may cause an SSW, a temporary breakdown of the polar vortex. Here we study the relation of SSWs to the atmospheric response to EEP in 1957–2017 using combined ERA‐40 and ERA‐Interim reanalysis data and geomagnetic activity as a proxy of EEP. We find that the EEP‐related enhancement of the polar vortex and other associated dynamical responses are seen only during winters when an SSW occurs and that the EEP‐related changes are observed systematically slightly before the SSW onset.We show that during these times, the planetary wave activity into the stratosphere is systematically increased, thus favoring enhanced wave‐mean‐flow interaction, which can dynamically amplify the initial polar vortex enhancement caused by ozone loss. These results highlight the importance of considering planetary wave activity as a necessary condition for observing the effects of EEP on the polar vortex.

Boreal Winter Surface Air Temperature Responses to Large Tropical Volcanic Eruptions in CMIP5 Models

AbstractWe analyzed global surface air temperature (SAT) responses to five major tropical volcanic eruptions from 1870 to 2005 using outputs from 97 historical and 58 Atmospheric Model Intercomparison Project (AMIP) runs that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). In observations, there was a 3-yr global cooling trend after the eruption due to reduced shortwave radiation, and a 0.1-K average global-mean SAT recovery, consisting of El Niño–like tropical warming and Eurasian warming, occurred in the first posteruption boreal winter. This global cooling pause was simulated by the multimodel ensemble (MME) mean of the AMIP runs, but not the MME of the historical runs due to the absence of El Niño–like warming. In the historical runs, simulation of El Niño–like warming was influenced by the initial ocean condition (IOC). An El Niño–like response was simulated when the IOC was not in an El Niño state, but the warming was much weaker compared to observations. The Eurasian warming response, despite being reproduced by the MME mean of both AMIP and historical runs, was not as strong as in observations. This is because the simulated positive polar vortex response, an important stratospheric forcing for Eurasian warming, was very weak, which suggests that the CMIP5 models, and even the Climate Forecast System model, underestimate volcanic effects on the stratosphere. Most of the coupled models failed to replicate both the El Niño and the enhanced polar vortex responses, indicating an urgent need for improving air–sea interaction and stratospheric processes in these models.