Jet Variability Research Articles

A Tale of Two Novembers: Confounding Influences on La Niña’s Relationship with Rainfall in Australia

Abstract Despite common background La Niña conditions, Australia was very dry in November 2020 and wet in November 2021. This paper aims to provide an explanation for this difference. Large-scale drivers of Australian rainfall, including El Niño–Southern Oscillation, Indian Ocean dipole, Southern Annular Mode, and Madden–Julian oscillation, were examined but did not provide obvious clues for the differences. We found that the absence (in 2020) or presence (in 2021) of an enhanced thermal wind and subtropical jet over the Australian continent contributed to the rainfall anomalies. In general, La Niña sets up warm sea surface temperatures around northern Australia, which enhances the meridional temperature gradient over the continent and hence thermal wind and subtropical jet. In November 2021, these warm sea surface temperatures, coupled with a persistent midlatitude trough, which advected cold air over the Australian continent, led to an enhanced meridional temperature gradient and subtropical jet over Australia. The enhanced jet provided favorable conditions for the development of rain-bearing weather systems across Australia. In 2020, the continent was warm, displacing the latitude of maximum meridional temperature gradient south of the continent, resulting in fewer instances of the subtropical jet over Australia, and little development of weather systems over the continent. We highlight that although La Niña tilts the odds to wetter conditions for Australia, in any given month, variability in temperatures over the continent can contribute to subtropical jet variability and resulting rainfall in ways which confound the normal expectation from La Niña. Significance Statement Forecasts of El Niño–Southern Oscillation are eagerly awaited, as the state of this climate driver has profound impacts on the likelihood of rainfall in regions around the world. While El Niño and La Niña do change rainfall likelihoods, the actual outcomes of these events are sometimes counter to expectation. This work explores one of the confounding factors to those expectations in the Australian context—the role of the meridional temperature gradient over the continent in modifying the storm track over Australia, which can disrupt the expected El Niño and La Niña teleconnections. We present case studies for two La Niña springs, highlighting that the Australian continent can help shape its own weather toward wetter or drier outcomes.

Recent two decades witness an uptick in monsoon depressions over the northern Arabian Sea

The analysis reveals that the occurrence of summer Monsoon Depressions (MDs) over the North Arabian Sea is doubling during 2001–2022 compared to the 1981–2000 period. This increase stems from changes in the region’s dynamic and thermodynamic conditions. The heightened genesis potential parameter with sea surface temperature and moisture flux transport and its convergence over the North Arabian Sea inducing MDs formation, contrasting to the Bay of Bengal. The dynamic processes involved in its formation, a combination of barotropic and dynamical instability, are leading to increased rainfall over northwestern India. Strong East Asian jet variability, with an anomalous anticyclone in the north and weak cyclonic anomalies in the south, induces prevailing easterly wind anomalies along the monsoon trough. This leads to a poleward shift (~1.13°) in the low-level jet, significantly altering dynamic and thermodynamic parameters in the northern Arabian Sea region leading to a notable increase in MDs.

Influence of ENSO and Volcanic Eruptions on Himalayan Jet Latitude

AbstractThe position of the subtropical jet over the Himalayas (Himalayan jet) affects extreme precipitation and heat over Central and South Asia. We examine the influence of two major natural factors‐the El Niño/Southern Oscillation (ENSO) and explosive volcanic eruptions—on Himalayan jet interannual variability during the past millennium using simulations from the Community Earth System Model. We find that both El Niño events and eruptions shift the Himalayan jet equatorward by up to 3°. If an El Niño occurs following an eruption, this enhances the equatorward Himalayan jet shift, while La Niña tends to favor poleward jet migration. Subtropical cooling during El Niño or following eruptions is the primary cause of equatorward Himalayan jet shifts, while poleward shifts are associated with subtropical warming. Consistent across the CMIP6 models over the historical period, our results suggest that both ENSO and eruptions are the key drivers of interannual Himalayan jet variability.

Probabilistic Causal Network Modeling of Southern Hemisphere Jet Subseasonal to Seasonal Predictability

Abstract Skillful prediction of the Southern Hemisphere (SH) eddy-driven jet is crucial for representation of mid-to-high-latitude SH climate variability. In the austral spring-to-summer months, the jet and the stratospheric polar vortex variabilities are strongly coupled. Since the vortex is more predictable and influenced by long-lead drivers 1 month or more ahead, the stratosphere is considered a promising pathway for improving forecasts in the region on subseasonal to seasonal (S2S) time scales. However, a quantification of this predictability has been lacking, as most modeling studies address only one of the several interacting drivers at a time, while statistical analyses quantify association but not skill. This methodological gap is addressed through a knowledge-driven probabilistic causal network approach, quantified with seasonal ensemble hindcast data. The approach enables to quantify the jet’s long-range predictability arising from known late-winter drivers, namely, El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), upward wave activity flux, and polar night jet oscillation, mediated by the vortex variability in spring. Network-based predictions confirm the vortex as determinant for skillful jet predictions, both for the jet’s poleward shift in late spring and its equatorward shift in early summer. ENSO, IOD, late-winter wave activity flux, and polar night jet oscillation only provide moderate prediction skill to the vortex. This points to early spring submonthly variability as important for determining the vortex state leading up to its breakdown, creating a predictability bottleneck for the jet. The method developed here offers a new avenue to quantify the predictability provided by multiple, interacting drivers on S2S time scales. Significance Statement Predictions of the Southern Hemisphere midlatitude jet stream are crucial for skillful forecasts of the austral mid-to-high latitudes. Several oceanic and atmospheric phenomena could, if better represented in models, improve spring-to-summer jet predictions on subseasonal to seasonal time scales. However, the combined potential skill arising from the inclusion of such phenomena has not been quantified. This study does so by using a probabilistic causal network model, representing the connections between those drivers and the jet with conditional probabilities, trained on large sets of model data. The stratospheric polar vortex is confirmed as crucial predictor of jet variability but is itself hard to predict a month in advance due to submonthly variability, creating a predictability bottleneck for the jet.

Diabatic Eddy Forcing Increases Persistence and Opposes Propagation of the Southern Annular Mode in MERRA-2

Abstract As a dominant mode of jet variability on subseasonal time scales, the Southern Annular Mode (SAM) provides a window into how the atmosphere can produce internal oscillations on longer-than-synoptic time scales. While SAM’s existence can be explained by dry, purely barotropic theories, the time scale for its persistence and propagation is set by a lagged interaction between barotropic and baroclinic mechanisms, making the exact physical mechanisms challenging to identify and to simulate, even in latest generation models. By partitioning the eddy momentum flux convergence in MERRA-2 using an eddy–mean flow interaction framework, we demonstrate that diabatic processes (condensation and radiative heating) are the main contributors to SAM’s persistence in its stationary regime, as well as the key for preventing propagation in this regime. In SAM’s propagating regime, baroclinic and diabatic feedbacks also dominate the eddy–jet feedback. However, propagation is initiated by barotropic shifts in upper-level wave breaking and then sustained by a baroclinic response, leading to a roughly 60-day oscillation period. This barotropic propagation mechanism has been identified in dry, idealized models, but here we show evidence of this mechanism for the first time in reanalysis. The diabatic feedbacks on SAM are consistent with modulation of the storm-track latitude by SAM, altering the emission temperature and cloud cover over individual waves. Therefore, future attempts to improve the SAM time scale in models should focus on the storm-track location, as well as the roles of the cloud and moisture parameterizations. Significance Statement As they circumnavigate the planet, the tropospheric jet streams slowly drift north and south over about 30 days, longer than the normal limit of weather prediction. Understanding the source of this “memory” could improve our knowledge of how the atmosphere organizes itself and our ability to make long-term forecasts. Current theories have identified several possible internal atmospheric interactions responsible for this memory. Yet most of the theories for understanding the jets’ behavior assume that this behavior is only weakly influenced by atmospheric water vapor. We show that this assumption is not enough to understand jet persistence. Instead, clouds and precipitation are more important contributors in reanalysis data than internal “dry” mechanisms to this memory of the Southern Hemisphere jet.

11-year solar cycle influences on the late-wintertime South Asian jet variability

The South Asian jet leading wave train (SAJLWT) is a zonally elongated and equivalently barotropic stationary Rossby wave train along the wintertime South Asian subtropical jet, which stands out as the leading empirical orthogonal function mode of monthly meridional winds at the upper troposphere over southern Eurasia. The SAJLWT is closely related to weather and climate extremes over southern Eurasia, but up to now, the mechanisms of SAJLWT variability are still not fully understood. The present study reveals a significant 11-year solar cycle (SC) influence on the SAJLWT variability in late winter (January–March). The in-phase correlation between the SC and the SAJLWT is not only statistically significant but also consistent over time. Associated with the solar-induced SAJLWT anomalies, significant surface cooling exists over northeastern Africa and Middle East, while significant positive precipitation anomalies appear over southern China. Two routes of mechanisms are presented to clarify the SC–SAJLWT linkage. One is due to the solar-induced northwest–southeast tilted North Atlantic Oscillation (TNAO) pattern, which may reflect a “top–down” solar forcing and the ocean–atmosphere couplings in the North Atlantic Ocean. This TNAO pattern would trigger southeastward wave activity fluxes toward the SAJ, thus significantly impacting the upstream portions of the SAJLWT pattern. The other mechanism involves weakened convection over the Maritime Continent (MC) region, reflecting solar-induced weakening of Pacific Walker Circulation via “bottom–up” processes. The MC convection anomaly would also excite a SAJLWT-like circulation pattern. Our analysis highlights that the solar “top–down” and “bottom–up” mechanisms may act in concert to exert a significant impact on the SAJLWT and indicates that the SC forcing is an appreciable source of decadal predictability in southern Eurasia.

3C 120 Disk/Corona vs. Jet Variability in X-rays

The 3C120 (Mrk 1506, UGC 03087, Mrk 9014) is a type 1 Seyfert (Sy1)/broad-line radio galaxy (BLRG) with intriguing variable jet activity featuring “dip” and “outburst” phases. Significant X-ray observational datasets have been collected for 3C120 by INTEGRAL, XMM-Newton, SWIFT, Suzaku, and other X-ray observational facilities. The overall X-ray spectrum of 3C 120 is too soft for typical radio-loud AGN, likely due to both variable spectral shape and jet contamination. Separating the “jet base” and nuclear (disc/corona) counterparts in the X-ray spectrum of 3C 120 can provide us with the possibility to investigate its variability in a more detailed way. Our objectives are to estimate separately the time variations of the accretion disc/corona and SSC/IC jet emission counterparts in the 3C 120 X-ray spectra and to analyze the physical state of the nucleus during different phases. Here, we attempt to use the connections between the synchrotron radio- and X-ray SSC/IC jet spectra and their photon indices and the dependence between the nuclear continuum and Fe-K iron luminescent line emission near 6.4 keV to separate the nuclear and jet base contributions to the total X-ray continuum. Using the X-ray observational dataset of 3C 120, we obtained separated fluxes that were interpreted as originating from the nucleus (disc/corona) and non-thermal SSC/IC jet base contributions. After this component separation, we identified the accretion disc/corona and jet states during different phases and compared them with the “jet/disk cycle” (Lohfink) and “magnetic plasmoid reconnection” (Shukla/Manheim) models.

The SOUL view of IRAS 20126+4104

Context. We exploit the increased sensitivity of the recently installed adaptive optics SOUL at the LBT to obtain new high-spatial-resolution near-infrared images of the massive young stellar object IRAS20126+4104 and its outflow. Aims. We aim to derive the jet proper motions and kinematics, as well as to study its photometric variability by combining the novel performances of SOUL together with previous near-infrared images. Methods. We used both broad-band (Ks, K′) and narrow-band (Brγ, H2) observations from a number of near-infrared cameras (UKIRT/UFTI, SUBARU/CIAO, TNG/NICS, LBT/PISCES, and LBT/LUCI1) to derive maps of the continuum and the H2 emission in the 2.12 µm line. Three sets of images, obtained with adaptive optics (AO) systems (CIAO, in 2003; FLAO, in 2012; SOUL, in 2020), allowed us to derive the proper motions of a large number of H2 knots along the jet. Photometry from all images was used to study the jet variability. Results. We derived knot proper motions in the range of 1.7–20.3 mas yr−1 (i.e. 13–158 km s−1 at 1.64 kpc), implying an average outflow tangential velocity of ~80 km s−1. The derived knot dynamical age spans a ~200–4000 yr interval. A ring-like H2 feature near the protostar location exhibits peculiar kinematics and may represent the outcome of a wide-angle wind impinging on the outflow cavity. Both H2 geometry and velocities agree with those inferred from proper motions of the H2O masers, located at a smaller distance from the protostar. Although the total H2 line emission from the knots does not exhibit time variations at a ⪞0.3 mag level, we have found a clear continuum flux variation (radiation scattered by the dust in the cavity opened by the jet) which is anti-correlated between the blue-shifted and red-shifted lobes and may be periodic (with a period of ~12–18 yr). We suggest that the continuum variability might be related to inner-disc oscillations which have also caused the jet precession. Conclusions. Our analysis shows that multi-epoch high-spatial-resolution imaging in the near-infrared is a powerful tool to unveil the physical properties of highly embedded massive protostars.

A Self-Organizing Maps Analysis of Wintertime North Pacific Jet Stream Variability

Abstract Previous research regarding the intraseasonal variability of the wintertime Pacific jet has employed empirical orthogonal function (EOF)/principal component (PC) analysis to characterize two leading modes of variability: a zonal extension or retraction and a ∼20° meridional shift of the jet exit region. These leading modes are intimately tied to the large-scale structure, sensible weather phenomena, and forecast skill in and around the vast North Pacific basin. However, variability within the wintertime Pacific jet and the relative importance of tropical and extratropical processes in driving such variability, is poorly understood. Here, a self-organizing maps (SOM) analysis is applied to 73 Northern Hemisphere cold seasons of 250-hPa zonal winds from the NCEP–NCAR reanalysis data to identify 12 characteristic physical jet states, some of which resemble the leading EOF Pacific jet patterns and combinations of them. Examination of teleconnection patterns such as El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) provide insight into the varying nature of the 12 SOM nodes at inter- and intraseasonal time scales. These relationships suggest that the hitherto more common EOF/PC analysis of jet variability obscures important subtleties of jet structure, revealed by the SOM analysis, which bear on the underlying physical processes associated with Pacific jet variability as well as the nature of its downstream impacts.

Gaussian Process Modeling Blazar Multiwavelength Variability: Indirectly Resolving Jet Structure

Blazar jet structure can be indirectly resolved by analyzing the multiwavelength variability. In this work, we analyze the long-term variability of blazars in radio, optical, and X-ray energies with the Gaussian process (GP) method. The multiwavelength variability can be successfully characterized by the damped-random walk model. The nonthermal optical characteristic timescales of 38 blazars are statistically consistent with the γ-ray characteristic timescales of 22 blazars. For three individual sources (3C 273, PKS 1510-089, and BL Lac), the nonthermal optical, X-ray, and γ-ray characteristic timescales are also consistent within the measured 95% errors, but the radio timescale of 3C 273 is too large to be constrained by the decade-long light curve. The synchrotron and inverse-Compton emissions have the same power spectral density, suggesting that the long-term jet variability is irrelevant to the emission mechanism. In the plot of the rest-frame timescale versus black hole mass, the optical-γ-ray timescales of the jet variability occupy almost the same space with the timescales of accretion disk emission from normal quasars, which may imply that the long-term variabilities of the jet and accretion disk are driven by the same physical process. It is suggested that the nonthermal optical-X-ray and γ-ray emissions are produced in the same region, while the radio core, which can be resolved by very long baseline interferometry, locates at a far more distant region from the black hole. Our study suggests a new methodology for comparing thermal and nonthermal emissions, which is achieved by using the standard GP method.

A Simple Mechanistic Model of Wave–Mean Flow Feedbacks, Poleward Jet Shifts, and the Annular Mode

Abstract Changes in the latitude of the zonal-mean midlatitude jet play an important role for both natural variability and the response of the atmospheric circulation to greenhouse gases and other external forcing. Nevertheless, the jet response to external forcing exhibits perplexing and nonintuitive behavior. For example, external forcing that acts to strengthen the jet will also shift the jet poleward. In addition, for internal jet variability, zonal wind anomalies slowly propagate poleward over most latitudes; however, this propagation stalls somewhat at latitudes on the flanks of the mean jet. At these latitudes zonal wind anomalies are more stationary, and therefore, anomaly persistence is maximized. These same persistent latitudes are collocated with the zonal wind anomalies associated with the annular mode. Feedbacks between the zonal-mean zonal wind and the eddy momentum fluxes are responsible for the above behaviors. Here a simple mechanistic model of the effect of the zonal-mean zonal wind on the eddy momentum fluxes is developed. The model reproduces the wave–mean flow feedbacks that maintain the annular mode, cause stronger jets to shift poleward (and vice versa), and cause the poleward propagation of zonal wind anomalies. In the model, the effect of the mean flow on the eddy momentum fluxes is determined solely by the critical level and the reflecting level. The model is used to distill the essential dynamics of annular variability and change such as why stronger jets shift poleward, why high-frequency eddies are responsible for the positive feedback and why the intricate structure of propagating versus stationary zonal wind anomalies exists.

Interannual variability of the double easterly jets over the tropical western Pacific and their effects on tropical cyclone genesis

AbstractDifferent from other upper‐tropospheric jet stream that owns the single jet axis, the tropical easterly jet (TEJ) generally splits into two branches over the tropical western Pacific where the TEJ inflows from June to September, with the north branch locating near 20°N and the south branch appearing near the equator. However, how these double easterly jets change from year to year and whether they have effects on the tropical cyclones (TCs) over the tropical western Pacific are still unknown. Observational evidence has demonstrated that the double easterly jets in the TEJ's inflow region exhibit an out‐of‐phase relationship in the interannual time scale and is significantly related to the TCs genesis. Corresponding to a stronger south branch and a weaker north branch, the accelerated (decelerated) easterly wind for the south (north) branch would induce a northerly (southerly) ageostrophic wind, thus triggering an upper‐tropospheric divergence between the double easterly jets. This divergence can further lead to anomalous ascending flows over the western North Pacific, which turns out to be the primary cause that increases the TCs number over there. The results highlight that the out‐of‐phase variation of the double easterly jets could dynamically modulate the occurrence of the TCs over the western North Pacific by inducing the secondary vertical circulation.

Mitigating Climate Biases in the Midlatitude North Atlantic by Increasing Model Resolution: SST Gradients and Their Relation to Blocking and the Jet

Abstract Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multimodel ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multimodel ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the storm track. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

On the Relative Importance of Stratospheric and Tropospheric Drivers for the North Atlantic Jet Response to Sudden Stratospheric Warming Events

Abstract Roughly two-thirds of the observed sudden stratospheric warming (SSW) events are followed by an equatorward shift of the tropospheric jet in the North Atlantic, while the other events generally show a poleward shift. It is however not resolved which drivers lead to the large inter-event variability in the surface impact. Using an intermediate complexity atmospheric model, we analyze the contribution of different factors to the downward response: polar cap geopotential height anomalies in the lower stratosphere, downstream influence from the northeastern Pacific, and local tropospheric conditions in the North Atlantic at the time of the initial response. As in reanalysis, an equatorward shift of the North Atlantic jet is found to occur for two-thirds of SSWs in the model. We find that around 40% of the variance of the tropospheric jet response after SSW events can be explained by the lower stratosphere geopotential height anomalies, while around 25% can be explained by zonal wind anomalies over the northeastern Pacific region. Local Atlantic conditions at the time of the SSW onset are also found to contribute to the surface response. To isolate the role of the stratosphere from tropospheric variability, we use model experiments where the zonal mean stratospheric winds are nudged toward climatology. When stratospheric variability is suppressed, the Pacific influence is found to be weaker. These findings shed light on the contribution of the stratosphere to the diverse downward impacts of SSW events, and may help to improve the predictability of tropospheric jet variability in the North Atlantic.

The flickering radio jet from the quiescent black hole X-ray binary A0620-00

ABSTRACT Weakly accreting black hole X-ray binaries launch compact radio jets that persist even in the quiescent spectral state, at X-ray luminosities ≲ 10−5 of the Eddington luminosity. However, radio continuum emission has been detected from only a few of these quiescent systems, and little is known about their radio variability. Jet variability can lead to misclassification of accreting compact objects in quiescence, and affects the detectability of black hole X-ray binaries in next-generation radio surveys. Here we present the results of a radio monitoring campaign of A0620 − 00, one of the best-studied and least-luminous known quiescent black hole X-ray binaries. We observed A0620 − 00 at 9.8 GHz using the Karl G Jansky Very Large Array on 31 epochs from 2017 to 2020, detecting the source $\sim 75{{\ \rm per\, cent}}$ of the time. We see significant variability over all time-scales sampled, and the observed flux densities follow a lognormal distribution with μ = 12.5 μJy and σ = 0.22 dex. In no epoch was A0620 − 00 as bright as in 2005 (51 ± 7 μJy), implying either that this original detection was obtained during an unusually bright flare, or that the system is fading in the radio over time. We present tentative evidence that the quiescent radio emission from A0620 − 00 is less variable than that of V404 Cyg, the only other black hole binary with comparable data. Given that V404 Cyg has a jet radio luminosity ∼20 times higher than A0620 − 00, this comparison could suggest that less luminous jets are less variable in quiescence.

Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models

Abstract Two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations, in the scope of the current Coupled Model Intercomparison Project (CMIP6), are compared with their fully coupled counterparts. The atmospheric models simulate less barotropic atmospheric circulation variability over the North Atlantic and more barotropic atmospheric circulation variability over the North Pacific when compared with reanalysis variability, at intraseasonal and interannual scales. The coupled climate simulations have smaller global barotropic variability than the corresponding AMIP simulations. The smaller variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the North Atlantic. The results suggest that the reduction of the biases, in the North Pacific barotropic atmospheric variability, of coupled climate simulations is achieved through compensating biases in the mean sea surface temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic.

On the perspectives for separating the radio flux variations of the nucleus and the jet of active galactic based on data from monitoring programs and VLBI observations

The subject and purpose of the work: the possibility of separating the spectral flux densities of the active core and the jet of sources (3C 120, 3C 273, 3C 345, 3C 454.3, 3C 446, Bl Lac, CTA 102, OJ 287, OT 081) is considered. The data of long-term monitoring of changes in the integral flux of extragalactic radio sources performed on the 26-meter telescope of the University of Michigan UMRAO (1969–2012) at frequencies of 14.5, 8, 4.8 GHz are used. Also, data on the core flux and images of active galactic nuclei at a frequency of 15–15.4 GHz VLBA monitoring MOJAVE are used. The subject of this work is to separate the jet flow from the integral flux from the source, as well as to compile the catalog of the quasi-periods of variability of nuclei and jets separately. Methods and methodology: Fourier-periodogram analysis is used to distinguish long-term and short-period variability of nuclei and jets of extragalactic sources. Autocorrelation calculations were carried out in order to confirm the periods of variability. Results: conclusions were made about the applicability of the described method for separating the jet flux, and a catalog of long-term and short-period variability of radio emission fluxes from nuclei and jets for the investigated radio sources was compiled.

Variability in fluvially‐dominated, fine‐grained, shallow‐water deltas

AbstractFluvially‐dominated, fine‐grained, shallow‐water deltas are more variable than is generally recognized. Studies of the Mississippi River imply rather persistent river‐mouth jet dynamics, resulting in either progressive channel bifurcation around middle‐ground mouth bars on lobate deltas, or steady channel progradation to form elongate deltas akin to the modern Mississippi Delta. By contrast, satellite imagery and historical maps of less well‐known fluvially‐dominated deltas show diverse river‐mouth deposits, with plan‐form shapes ranging from river‐mouth fans, through splay, triangular, frond and tongue‐shaped mouth bars, to elongate channels with prominent subaqueous levées. Critically, such deposits may vary along individual channels and across or between individual lobes suggesting that jet variability can be normal, and that a spectrum of jets forms a suite of stable river‐mouth deposits. Channel elongation can also be common. In addition to high‐inertia jets and delta‐head or backwater‐mediated avulsion, two further mechanisms are recognized to form elongate channels. Following channel splitting, dominant splits can shoulder aside mouth bars, so that subordinate splits wither and dominant splits extend the parent channel. Alternatively, river‐mouth fans, subordinate distributary networks and lobes may be abandoned, so that a dominant parent distributary is rejuvenated by receiving an increase in discharge, allowing progradation to continue. Individual real‐world deltas are expected to be characterized by a range of river‐mouth deposits, both laterally and over time. However, such variable river‐mouth deposits, and different modes of channel elongation, are not generally considered in models of fluvially‐dominated deltas – omissions that may have significant impact for land remediation projects on modern deltas, and descriptions of the subsurface constructed to aid resource extraction.

Relationship between southern hemispheric jet variability and forced response: the role of the stratosphere

Abstract. Climate models show a wide range of southern hemispheric jet responses to greenhouse gas forcing. One approach to constrain the future jet response is by utilising the fluctuation–dissipation theorem (FDT) which links the forced response to internal variability timescales, with the Southern Annular Mode (SAM) the most dominant mode of variability of the southern hemispheric jet. We show that interannual stratospheric variability approximately doubles the SAM timescale during austral summer in both re-analysis data and models from the Coupled Model Intercomparison Project, Phases 5 (CMIP5) and 6 (CMIP6). Using a simple barotropic model, we demonstrate how the enhanced SAM timescale subsequently leads to an overestimate of the forced jet response based on the FDT, and we introduce a method to correct for the stratospheric influence. This result helps to resolve a previously identified discrepancy between the seasonality of jet response and the internal variability timescale. However, even after accounting for this influence, the SAM timescale cannot explain inter-model differences in the forced jet shift across CMIP models during austral summer.

A Jet-Centered Framework for Investigating High Plains Winter Storm Severity

Abstract This study utilizes a winter severity index (WSI) to characterize the impacts of High (Great) Plains winter storms during the 2006/07–2018/19 winter seasons across Nebraska and the Colorado Front Range. Winter storms are specifically defined based on the severity of their meteorological impacts and are required to influence a majority of Department of Transportation (DOT) districts within both states. Following their identification, winter storms are examined using a jet-centered framework based on the two leading modes of North Pacific jet (NPJ) and North Atlantic jet (NAJ) variability. The analysis reveals that a retracted or equatorward-shifted NPJ establishes a highly amplified flow pattern conducive to cyclogenesis over the central United States, while a poleward- or equatorward-shifted NAJ favors the development of a strongly baroclinic environment across the study region that serves as a focal region for cyclogenesis and precipitation. Composite analyses of winter storms that rank in the top 25% and bottom 25% in terms of their aggregate WSI are also performed to identify characteristics of the synoptic-scale evolution that discriminate between “high impact” and “low impact” events, respectively. High-impact events are found to feature a more amplified upper-tropospheric flow pattern over the eastern North Pacific and western United States relative to low-impact events, which subsequently favors stronger cyclogenesis over the southern plains. The integration of jet regimes with winter storm severity metrics as part of this study offers the potential to enhance impact-based decision support services and provide the weather enterprise, and its stakeholders, with critical life-saving information.