Zonal Wind Shear Research Articles

Climatological Investigation of Ionospheric Es Layer Based on Occultation Data

Sporadic E (Es) layers are irregular structures that occur at the E-layer height of the ionosphere, significantly affecting the reliability and accuracy of wireless communications, navigation, and satellite remote sensing. This study utilized the S4max data collected from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) occultation observations from 2007 to 2016 to identify the Es layer and investigate its climatological variations. The Horizontal Wind Field model (HWM14), in conjunction with the International Geomagnetic Reference Field model (IGRF13), is used to calculate vertical ion convergence (VIC) and analyze its correlation to the Es layers. The results of this study showed that the occurrence of Es has apparent hemispheric asymmetry. In the mid- and low latitudes, Es layer activity is more intense in the summer hemispheres, with center peak altitudes of around 105 km. The summer hemisphere exhibits a semi-diurnal periodic pattern, whereas the winter hemisphere shows a weakened diurnal variation. Simulation studies indicate that VIC induced by neutral wind shear contributes to the asymmetry in Es layer activities observed between the Northern and Southern hemispheres, and the zonal wind shear plays a more critical role than the meridional wind one.

Impact of shifting patterns of the South Asian High on interannual variability of Indian summer monsoon rainfall in homogeneous regions

AbstractThe movement and intensity indices of the South Asian High (SAH), an upper‐level anticyclonic‐circulation over the Tibetan Plateau, play a crucial role in Indian summer monsoon rainfall (ISMR) during June–September. The present study aims to document (i) the association between SAH and ISMR in the periods of June–July–August (JJA) and July–August–September (JAS), and (ii) the ISMR changes at different spatial and temporal scales. The Bayesian change‐point detection method identifies 1981 as change point and the dataset has been split into two groups (past climate:1940–1980 and current climate:1981–2020) to analyze temporal variations in ISMR. Results indicate that the northwest–southeast index (INW–SE), north–south index (INS) and intensity index (IINT) of the SAH are strongly correlated (˜0.67, ˜0.60, and ˜0.51, respectively) with ISMR whereas the east–west index (IEW) is negatively correlated (˜−0.52) during the JAS season. This relation was stronger in the past climate than the current climate, except for the IINT index during the JAS season. The INW–SE and INS indices are closely associated with all‐India, northwest India (NWI), and central India (CI) rainfall, whereas IINT is associated with south peninsular India (SPI) during the JAS season. The increased rainfall over NWI and SPI in the current climate is strongly associated with positive INS and IINT indices, respectively, during the JAS season. The northeast India (NEI) rainfall did not show any association with SAH indices; however, the IEW is strongly associated with increased NEI rainfall during El Niño years. A significant positive (negative) relation between meridional (zonal) wind shear and SAH indices except the IEW index is observed. Further, the positive (negative) SAH indices favor more (moderate) ISMR due to strong positive (negative) moisture anomalies. The study demonstrates the movement and intensity of the SAH in the dynamical shifts of seasonal rainfall patterns across the Indian homogeneous regions.

Effect of Vertical Shear in the Zonal Wind on Equatorial Electrojet Sidebands: An Observational Perspective Using Swarm and ICON Data

AbstractThe wind dynamo in the ionosphere leads to differential motion of ions and electrons, which in turn sets up electric fields and currents. Observations show that daytime lower thermospheric horizontal winds have large vertical gradients. Numerical modeling conducted approximately 50 years ago demonstrated that the zonal wind shears in the ∼130–180 km altitude range can generate off‐equatorial relative minima (dips) in the daytime height‐integrated eastward current density, appearing as westward sidebands north and south of the equatorial electrojet (EEJ). This study observationally confirms this connection for the first time by combining Ionospheric CONnection explorer zonal wind profiles and Swarm latitudinal zonal currents. We demonstrate observationally that the magnitude of the EEJ sideband current is proportional to the strength of westward turning winds with altitude in the Pedersen conductivity dominated region. Additional numerical experiments explain the importance of wind shear in different altitude regions in generating the sideband current. This study contributes to the better understanding of the neutral wind effect on the local current generation.

Suppressive MJO in April 2014 Downgraded the 2014/15 El Niño

Abstract The sudden halting of the extreme 2014/15 El Niño expected by many was attributed to the absence of westerly wind bursts (WWBs) in late spring and early summer 2014 in previous works, yet the cause of the lack of WWBs was overlooked. Using the ERA5 reanalysis and IBTrACS dataset, as well as a set of coupled model experiments, we showed that the absence of WWBs in May efficiently downgraded the intensity of the 2014/15 El Niño from a moderate to a weak event and was closely associated with a strong suppressive MJO originating from the central tropical Indian Ocean in mid-April 2014. The suppressive MJO underwent two pathways once passing through the Maritime Continent in early May. Along the eastward pathway, the strong suppressive MJO prevailed over the western–central equatorial Pacific, directly prohibiting the occurrence of WWBs at the equator via inducing equatorial easterly anomaly. Along the northeastward pathway, the downward motions with relative dry air and strong vertical zonal wind shear associated with the suppressive MJO suppressed the activity of the tropical cyclones in the northwestern tropical Pacific, another source of WWBs. Our results indicate that the contributions of MJO to the development of El Niño from both the direct and indirect ways should be taken into account for improving El Niño prediction.

Statistics of Traveling Ionospheric Disturbances at High Latitudes Using a Rapid‐Run Ionosonde

AbstractThe potential of deep learning for the investigation of medium scale traveling ionospheric disturbances (MSTIDs) has been exploited through the Sodankylä rapid‐run ionosonde in this statistical study. The complementing observations of the Sodankylä ionosonde with those of the Sodankylä meteor radar reveals the diurnal and seasonal occurrence rate of high‐latitude MSTIDs in the recent low solar activity period, 2018–2020. In our results, the daytime, nighttime and dusk MSTIDs are predominantly identified during winter, summer, and equinoctial months, respectively. The winter daytime higher (lower) occurrence rate is well correlated with the lower (higher) altitude of the height of the F2‐layer peak (hmF2), and the low occurrence rate of the summer daytime is well correlated with the mesosphere‐lower‐thermosphere wind shear and higher gradient of temperature. Relatively high occurrence rate (>0.4) of summer nighttime MSTIDs has a general—but not one‐to‐one agreement—with post‐noon to evening IU (eastward auroral current index) inferred ionospheric conductivity. Rather, we see a one‐to‐one relationship between the summer nighttime MSTIDs and zonal wind shear suggesting that the wind shear‐induced electrodynamic processes could play significant roles for higher occurrence rate of MSTIDs. Furthermore, significant MSTIDs with ∼0.4 occurrence rate are so far revealed during spring and autumn transition periods. The enhanced nighttime MSTID amplitudes during the equinox are observed to be well correlated with IL index (westward auroral current indicator) suggesting that the particle precipitation during substorms could be the primary cause.

Impact of Extratropical Circulation in East Asia on Western North Pacific Tropical Cyclone Frequency and Intensity

Abstract This study examines the East Asia and western North Pacific (WNP) monsoon circulation patterns for strong and weak WNP tropical cyclone (TC) numbers in summer. It suggested that years with more intense TCs are coupled with the tropical monsoon circulations, including the northward cross-equatorial airflow and the extending tropical monsoon trough toward the central-eastern WNP. However, a higher frequency of weak TCs can be largely attributed to the mutual interactions among the tropical monsoon trough west of 140°E, the westward South Asia high, and the high pressure anomaly in Northeast Asia (NEA). Then the potential influence of the NEA extratropical system is focused on. The resultant local negative potential vorticity (PV) anomaly is carried southeastward by the prevailing flow. It stimulates a descending flow around 30°N, which favors the westward retreat of the South Asian high and the decreased zonal vertical wind shear around 20°N. The associated lower-level outflow converges in the tropical WNP and reinforces the ascending motion around 10°–20°N. Meanwhile, the warm air column in NEA also contributes to anomalous easterlies in a band around 30°N, intensifying the lower-level cyclonic vorticity in the northwestern WNP. Consequently, the ascending motion, cyclonic vorticity, and the weakened zonal vertical wind shear in northwestern WNP promote the WTC formation. A set of physically based empirical models is developed using various physically based predictors to reconstruct the number of intense and weak TCs. Cross-validated hindcasts suggest that the NEA extratropical circulation can serve as an additional source of predictability for the weak TC variability. Significance Statement Tropical cyclones (TCs) are a highly destructive type of natural disaster that have garnered widespread attention. By comparison with intense TCs (ITCs), weak TCs (WTCs) are much more numerous and often form closer to the coastal regions of East Asia, whose mechanism has not been fully understood. In this study, we suggest that more ITCs are controlled by tropical monsoon circulations, while the WTC variability is closely coupled with both tropical and extratropical monsoon systems. In addition to the tropical monsoon trough west of 140°E and the westward South Asian high, the Northeast Asian circulation can regulate the WTC number by changing the lower-level vorticity, vertical motion, and vertical wind shear in the WTC genesis-prone region, which can be applied to improve the seasonal prediction skill of WTCs.

Influences of Different Factors on Gravity Wave Activity in the Lower Stratosphere of the Indian Region

The gravity wave (GW) potential energy (Ep) in the lower stratosphere (LS) of the altitude range between 20 and 30 km over the Indian region (60°E–100°E, 0°–30°N) is retrieved using the dry temperature profiles from the Constellation Observing System for Meteorology Ionosphere and Climate-2 (COSMIC-2) radio occultation (RO) mission from December 2019 to November 2021. Through correlation analysis and dominance analysis (DA) methods, the impacts of multiple influencing factors on the local LS GW activity are quantified and compared. The results demonstrate that in the central and northern part of Indian region, the three factors, including the convective activity (using outgoing long-wave radiation as the proxy) mainly caused by the Indian summer monsoon, the mean zonal wind speed between 15 and 17 km, the height range where the maximum tropical easterly jet (TEJ) wind speed appears, and the mean zonal wind speed between 20 and 30 km, have the greatest impacts on the LS GW activity. In the southern part of the Indian Peninsula and over the Indian Ocean, the mean zonal wind shear between 20 and 30 km plays a dominant role in the LS GW activity, which is due to the fact that the GW energy can be attenuated by large background wind shears. It can be concluded that the LS GW activity in the Indian region is mainly influenced by the Indian summer monsoon, the TEJ, and the wind activity in the LS, while over different local areas, differences exist in which factors are the dominant ones.

On the Linkage Between Daytime E Region Field‐Aligned Irregularities and Sporadic E Layer at Low Latitude

AbstractWe investigate the generation of low latitude daytime E region field‐aligned irregularities (FAIs) and focus on whether the FAIs are exclusively linked with sporadic E (Es) and whether the FAIs are driven by wind‐driven gradient‐drift instability (GDI). For this investigation, we use observations of FAIs made by the 30 MHz Gadanki Ionospheric Radar Interferometer (GIRI) and Es made by the DPS‐4D digital ionosonde, both collocated at Gadanki. We have also examined simultaneous GIRI and Ionospheric Connection Explorer wind observations to substantiate our results. Results show that daytime E region FAIs are closely linked with Es activity and FAIs are not formed when Es is absent. FAIs are formed when blanketing frequency of Es (fbEs) exceeds frequency of the E layer. Both signal‐to‐noise ratio (SNR) and spectral width of the FAIs echoes display close relationship with top frequency of Es (ftEs). Further, SNR and spectral width of the FAIs echoes display a near‐linear relationship with minimum deviation when Es activity is moderate and show dispersion from near‐linear relationship when Es activity is strong. Zonal drift of the FAIs are predominantly eastward displaying both positive and negative vertical shear. Results also show that Es activity in terms of fbEs and ftEs is found to be closely related to the vertical shear in the zonal drift of the FAIs. Considering the height region of FAIs being highly collisional, we argue that zonal drifts are dominated by zonal wind and vertical shear in the zonal neutral wind is responsible for the formation/maintenance of Es layer and the eastward neutral wind is primarily responsible for the generation of the FAIs through the GDI.

Interhemispheric Temperature Gradient and Equatorial Pacific SSTs Drive Sahel Monsoon Uncertainties under Global Warming

Abstract The Sahel is one of the most vulnerable regions to climate change. Robust estimation of future changes in the Sahel monsoon is therefore essential for effective climate change adaptation. Unfortunately, state-of-the-art climate models show large uncertainties in their projections of Sahel rainfall. In this study, we use 32 models from CMIP6 to identify the sources of this large intermodel spread of Sahel rainfall. By using maximum covariance analysis, we first highlight two new key drivers of this spread during boreal summer: the interhemispheric temperature gradient and equatorial Pacific sea surface temperature (SST) changes. This contrasts with previous studies, which have focused mainly on the Northern Hemisphere rather than the global scale, and in which the Pacific Ocean has been neglected in favor of the Atlantic. Next, we unravel the physical mechanisms behind these statistical relationships. First, the modulation of the interhemispheric temperature gradient across the models leads to varying latitudinal positions of the intertropical convergence zone and, consequently, varying Sahel rainfall intensity. Second, models that exhibit less warming than the multimodel mean in the equatorial Pacific, thereby projecting a less “El Niño–like” mean state, simulate enhanced precipitation over the central Sahel in the future through modulations of the Walker circulation, the tropical easterly jet, the meridional tropospheric temperature gradient, and hence regional zonal wind shear. Finally, we show that these two indices collectively explain 62% of Sahel rainfall change uncertainty: 40% due to the interhemispheric temperature gradient and 22% through equatorial Pacific SST.

Comparison between Developing and Nondeveloping Disturbances for Tropical Cyclogenesis in Different Large-Scale Flow Patterns over the Western North Pacific

Abstract This study classifies 407 developing disturbances (DEV) and 2309 nondeveloping disturbances (NONDEV) over the western North Pacific into five large-scale circulation patterns, namely the pre-existing cyclone (PC), easterly wave (EW), zonal wind convergence (CON), zonal wind shear line (SL), and mixed zonal wind convergence and shear line (CON-SL) patterns. The SL pattern has the highest TC yield percentage, followed by the CON-SL, while the EW is the least favorable pattern. The composite analysis shows that upper-level divergence, midlevel relative humidity, and surface heat flux (SHF) growth are crucial to the disturbance development in all the five patterns. Besides, large lower-level barotropic kinetic energy conversion and a well-developed primary circulation are good indicators for disturbance development in the PC, EW, and CON rather than in the SL and CON-SL patterns. Furthermore, for the PC, EW and CON patterns, the DEV features strong and rapidly growing SHF and mesoscale convective systems (MCS) closer to the disturbance center, which allows deep-layer warming and moistening, and drives a deep secondary circulation. Interestingly, due to an environment with high lower-level vorticity, the SL and CON-SL patterns typically foster a relatively mature primary circulation with strong SHF and MCS concentrated close to the center, especially for the NONDEV at the pre-genesis stage. However, a drier mid-to-upper-level environment for the NONDEV inhibits deep convection, which may explain its shallow secondary circulation and therefore poor potential to develop further.

Prediction of South China Sea summer monsoon onset in the CAMS‐CSM climate forecast system

AbstractBased on a set of hindcast experiments from 2011 to 2020, the prediction skills of the Chinese Academy of Meteorological Science Climate System Model (CAMS‐CSM) climate forecast system (CFS) on the abrupt changes and characteristic processes associated with the South China Sea summer monsoon (SCSSM) establishment are evaluated. We studied predictions for three different lead times, that is, LT1‐30, LT31‐60 and LT61‐90. The CAMS‐CSM CFS captures the climatological SCSSM onset date at all lead times, while the prediction skill of the SCSSM onset index decreases with the increasing lead times. The features of abrupt changes and characteristic processes related to the SCSSM establishment can be generally reproduced in the predictions, but different biases exist at three lead times. Besides, the CAMS‐CSM CFS provides higher skill on the transition of the zonal wind shear (ZWS) than that of meridional temperature difference (MTG). The transition date of MTG can only be accurately predicted at LT1‐30, indicating the limited ability of predicting temperature. Further results confirm that the CFS cannot well capture the eastward withdrawal of the western North Pacific subtropical high on late onset years, thereby leading a poorer prediction skill than early onset years. In addition, skillful prediction on the eastward extension of westerlies from the equatorial Indian Ocean to SCS also contributes to increase predictability of SCSSM onset.

The Pacific Meridional Mode Influences ENSO by Inducing Springtime Near-Equatorial TCs in the Western North Pacific

Abstract Previous research has shown that tropical cyclones in the near-equatorial western North Pacific (NE-WNP TCs) in boreal spring are associated with the onset of El Niño–Southern Oscillation (ENSO). However, the cause of these TCs is unclear. Here, we find that the interannual activity of the springtime NE-WNP TCs is insensitive to the simultaneous ENSO intensity but is closely related to the springtime Pacific meridional mode (PMM). The lead–lag correlation between PMM and NE-WNP TCs is high from January to April, and the evolution of the SST anomaly associated with the NE-WNP TCs strongly manifests the seasonal footprint of the PMM. Analysis using AGCM experiments confirms that the positive PMM tends to reduce the vertical shear of zonal wind and sea level pressure and to increase vertical velocity in the middle atmosphere and surface humidity in the WNP. All of these conditions are favorable for NE-WNP TC genesis. We conducted CGCM experiments to validate that the NE-WNP TCs significantly influence ENSO development. A statistical regression model with respect to the impact of PMM on NE-WNP TCs and further on ENSO intensity was also conducted. The results imply that modulating the springtime NE-WNP TCs is another effective way in which the PMM influences ENSO. Significance Statement Some studies have found that springtime near-equatorial western North Pacific (NE-WNP) tropical cyclones (TCs) heavily impact ENSO development. Using observations and AGCM experiments, we showed that the genesis of these TCs is closely associated with the Pacific meridional mode (PMM), presumably because the PMM provides favorable conditions for TC genesis. We further showed that including the PMM-induced NE-WNP TCs in a CGCM leads to an El Niño–like response. The linkage between the PMM and NE-WNP TCs not only supplements the existing theories about tropical–subtropical interactions but also provides a new way to improve ENSO simulation and prediction.

The Role of Subtropical Rossby Waves in Amplifying the Divergent Circulation of the Madden–Julian Oscillation

Abstract The composite structure of the Madden–Julian oscillation (MJO) has long been known to feature pronounced Rossby gyres in the subtropical upper troposphere, whose existence can be interpreted as the forced response to convective heating anomalies in the presence of a subtropical westerly jet. The question of interest here is whether these forced gyre circulations have any subsequent effects on divergence patterns in the tropics and the Kelvin-mode component of the MJO. A nonlinear spherical shallow water model is used to investigate how the introduction of different background jet profiles affects the model’s steady-state response to an imposed MJO-like stationary thermal forcing. Results show that a stronger jet leads to a stronger Kelvin-mode response in the tropics up to a critical jet speed, along with stronger divergence anomalies in the vicinity of the forcing. To understand this behavior, additional calculations are performed in which a localized vorticity forcing is imposed in the extratropics, without any thermal forcing in the tropics. The response is once again seen to include pronounced equatorial Kelvin waves, provided the jet is of sufficient amplitude. A detailed analysis of the vorticity budget reveals that the zonal-mean zonal wind shear plays a key role in amplifying the Kelvin-mode divergent winds near the equator, with the effects of nonlinearities being of negligible importance. These results help to explain why the MJO tends to be strongest during boreal winter when the Indo-Pacific jet is typically at its strongest. Significance Statement The MJO is a planetary-scale convectively coupled equatorial disturbance that serves as a primary source of atmospheric predictability on intraseasonal time scales (30–90 days). Due to its dominance and spontaneous recurrence, the MJO has a significant global impact, influencing hurricanes in the tropics, storm tracks, and atmosphere blocking events in the midlatitudes, and even weather systems near the poles. Despite steady improvements in subseasonal-to-seasonal (S2S) forecast models, the MJO prediction skill has still not reached its maximum potential. The root of this challenge is partly due to our lack of understanding of how the MJO interacts with the background mean flow. In this work, we use a simple one-layer atmospheric model with idealized heating and vorticity sources to understand the impact of the subtropical jet on the MJO amplitude and its horizontal structure.

Influence of monsoon trough length on interannual variability of Northwest Pacific multiple tropical cyclone events

Previous studies have established a link between the intensity of the monsoon trough (MT) and tropical cyclone (TC) activity. This study investigates the impact of the length of the MT on the interannual variability of multiple TC events (MTCEs) over the western North Pacific (WNP) during the typhoon season (June–October) during 1980–2020. Results show a significant positive correlation between the length of the MT and the occurrence frequency of MTCEs. A longer MT is favorable for the occurrence of more MTCEs. During the long MT years, the MT extends eastwards, resulting in strong low-level relative vorticity, mid-level upward motion, and sufficient moisture over the central-eastern WNP, which are conductive to the occurrence of MTCEs. A diagnosis of genesis potential index confirms the crucial role of positive synoptic-scale disturbances. Barotropic energy conversion analysis further shows that the MT-induced mean flow change can modulate barotropic energy conversion from the mean flow to the synoptic eddies, which is a critical energy source for the development of tropical depressions. In this process, changes in meridional shear of the large-scale zonal winds play a dominant role, followed by their meridional convergence. These changes lead to the increasing synoptic-scale disturbances over the central-eastern WNP during the long MT years, resulting in more MTCEs.

Opposite Changes in Monsoon Precipitation and Low Pressure System Frequency in Response to Orographic Forcing

Abstract Monsoon low pressure systems (LPS) are synoptic-scale disturbances that form along the quasi-stationary trough of the larger-scale South Asian summer monsoon, producing a large fraction of total monsoon precipitation. Here, we use an Earth system model to investigate the influence of Tibetan and Himalayan orography (THO) on mean monsoon strength and LPS activity. The influence of THO height on LPS activity has not been investigated before. The model simulates decreased mean monsoon circulation and precipitation when THO is removed, but the number of LPS and the meridional extent of LPS activity increase; this is an unexpected, important finding given that LPS are principal rain-bearing systems of the monsoon. The decreased mean monsoon circulation and precipitation on removal of THO are attributed to enhanced mixing of dry extratropical air into the humid monsoon domain, as demonstrated by prior studies. The increase in LPS frequency and the meridional extent of LPS activity is attributed to the increase in magnitude and meridional extent of the low-level meridional shear of zonal wind, which previous work showed amplifies LPS through barotropic growth. Specifically, as monsoon precipitation decreases, the sensible heat-driven low-level trough intensifies and shifts equatorward; this strengthens the shear zone in which LPS grow. Conversely, increasing THO height decreases the magnitude and meridional extent of cyclonic shear over India, decreasing LPS frequency and the spatial extent of LPS activity while increasing total monsoon precipitation. These results demonstrate that LPS activity and total monsoon rainfall can undergo large, opposing changes in response to imposed forcings. Significance Statement Monsoon low pressure systems (LPS) are propagating atmospheric vortices that deliver a large fraction of seasonal-mean precipitation to agricultural regions of India. The influence of Tibetan and Himalayan orography (THO) on the South Asian monsoon is an active area of research, but its effect on LPS has remained unexplored. Here, a global climate model is used to simulate LPS characteristics for different heights of THO. Flattening the THO reduces total summer monsoon precipitation but increases the number of LPS and the spatial extent of LPS activity. We attribute these LPS changes to a decrease in the magnitude and meridional extent of the hydrodynamically unstable low-level cyclonic shear zone over South Asia that occurs when orography is flattened.

Classification of large-scale environments that drive the formation of mesoscale convective systems over southern West Africa

Abstract. Mesoscale convective systems (MCSs) are frequently observed over southern West Africa (SWA) throughout most of the year. These MCS events are the dominant rain-bearing systems, contributing over 50 % of annual rainfall over SWA. However, it has not yet been identified what variations in typical large-scale environments of the seasonal cycle of the West African monsoon may favour MCS occurrence in this region. Here, nine distinct synoptic states are identified and are further associated with being a synoptic-circulation type of either a dry, transition, or monsoon season using self-organizing maps (SOMs) with inputs from reanalysis data. We identified a pronounced annual cycle of MCS numbers with frequency peaks in April and October that can be associated with the start of rainfall during the major rainy season and the maximum rainfall for the minor rainy season across SWA, respectively. Comparing daily MCS frequencies, MCSs are most likely to develop during transition conditions featuring a northward-displaced moisture anomaly (2.8 MCSs per day), which can be linked to strengthened low-level westerlies. Considering that these transition conditions occur predominantly during the pre- and post-monsoon period, these patterns may in some cases be representative of monsoon onset conditions or a delayed monsoon retreat. On the other hand, under monsoon conditions, we observe weakened low-level south-westerlies during MCS days, which reduce moisture content over the Sahel but introduce more moisture over the coast. Finally, we find a majority of MCS-day synoptic states exhibiting positive zonal wind shear anomalies. Seasons with the strongest zonal wind shear anomalies are associated with the strongest low-level temperature anomalies to the north of SWA, highlighting that a warmer Sahel can promote MCS-favourable conditions in SWA. Overall, the SOM-identified synoptic states converge towards high-moisture and high-shear conditions on MCS days in SWA, where the frequency at which these conditions occur depends on the synoptic state.

Dynamics of Mid‐Latitude Sporadic‐E and Its Impact on HF Propagation in the North American Sector

AbstractSporadic‐E (Es) are thin layers of enhanced ionization observed in the E‐region, typically between 95 and 120 km altitude. Es plays an important role in controlling the dynamics of the upper atmosphere and it is necessary to understand the geophysical factors influencing Es from both the scientific and operational perspectives. While the wind‐shear theory is widely accepted as an important mechanism responsible for the generation of Es, there are still gaps in the current state of our knowledge. For example, we are yet to determine precisely how changes in the dynamics of horizontal winds impact the formation, altitude, and destruction of Es layers. In this study, we report results from a coordinated experimental campaign between the Millstone Hill Incoherent Scatter Radar, the SuperDARN radar at Blackstone, and the Millstone Hill Digisonde to monitor the dynamics of mid‐latitude Es layers. We report observations during a 15‐hr window between 13 UT on 3 June 2022 and 4 UT on 4 June 2022, which was marked by the presence of a strong Es layer. We find that the height of the Es layer is collocated with strong vertical shears in atmospheric tides and that the zonal wind shears play a more important role than meridional wind shears in generating Es, especially at lower altitudes. Finally, we show that in the presence of Es, SuperDARN ground backscatter moves to closer ranges, and the height and critical frequency of the Es layer have a significant impact on the location and intensity of HF ground scatter.

Saturn's Atmosphere in Northern Summer Revealed by JWST/MIRI

AbstractSaturn's northern summertime hemisphere was mapped by JWST/Mid‐Infrared Instrument (4.9–27.9 µm) in November 2022, tracing the seasonal evolution of temperatures, aerosols, and chemical species in the 5 years since the end of the Cassini mission. The spectral region between reflected sunlight and thermal emission (5.1–6.8 µm) is mapped for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a system of two aerosol layers (an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars). Ammonia displays substantial equatorial enrichment, suggesting similar dynamical processes to those found in Jupiter's equatorial zone. Saturn's North Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds at p < 10 mbar, and exhibits localized enhancements in several hydrocarbons. The strongest latitudinal temperature gradients are co‐located with the peaks of the zonal winds, implying wind decay with altitude. Reflectivity contrasts at 5–6 µm compare favorably with albedo contrasts observed by Hubble, and several discrete vortices are observed. A warm equatorial stratospheric band in 2022 is not consistent with a 15‐year repeatability for the equatorial oscillation. A stacked system of windshear zones dominates Saturn's equatorial stratosphere, and implies a westward equatorial jet near 1–5 mbar at this epoch. Lower stratospheric temperatures, and local minima in the distributions of several hydrocarbons, imply low‐latitude upwelling and a reversal of Saturn's interhemispheric circulation since equinox. Latitudinal distributions of stratospheric ethylene, benzene, methyl, and carbon dioxide are presented for the first time, and we report the first detection of propane bands in the 8–11 µm region.

The quasi-biennial oscillation (QBO) and global-scale tropical waves in Aeolus wind observations, radiosonde data, and reanalyses

Abstract. The quasi-biennial oscillation (QBO) of the stratospheric tropical winds influences the global circulation over a wide range of latitudes and altitudes. Although it has strong effects on surface weather and climate, climate models have great difficulties in simulating a realistic QBO, especially in the lower stratosphere. Therefore, global wind observations in the tropical upper troposphere and lower stratosphere (UTLS) are of particular interest for investigating the QBO and the tropical waves that contribute significantly to its driving. In our work, we focus on the years 2018–2022 and investigate the QBO and different tropical wave modes in the UTLS region using global wind observations made by the Aeolus satellite instrument and three meteorological reanalyses: the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-5), the Japanese 55-year Reanalysis (JRA-55) of the Japan Meteorological Agency (JMA), and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). Further, we compare these data with observations of selected radiosonde stations. By comparison with Aeolus observations, we find that, on zonal average, the QBO in the lower stratosphere is well represented in all three reanalyses, with ERA-5 performing best. Averaged over the years 2018–2022, agreement between Aeolus and the reanalyses is better than 1 to 2 m s−1, with somewhat larger differences during some periods. Differently from zonal averages, radiosonde stations provide only local observations and are therefore biased by global-scale tropical waves, which limits their use as a QBO standard. While reanalyses perform well on zonal average, there can be considerable local biases between reanalyses and radiosondes. We also find that, in the tropical UTLS, zonal wind variances of stationary waves and the most prominent global-scale traveling equatorial wave modes, such as Kelvin waves, Rossby-gravity waves, and equatorial Rossby waves, are in good agreement between Aeolus and all three reanalyses (in most cases better than 20 % of the peak values in the UTLS). On zonal average, this supports the use of reanalyses as a reference for comparison with free-running climate models, while locally, certain biases exist, particularly in the QBO wind shear zones and around the 2019–2020 QBO disruption.

Equatorial Waves and Superrotation in the Stratosphere of a Titan General Circulation Model

We investigate the characteristics of equatorial waves associated with the maintenance of superrotation in the stratosphere of a Titan general circulation model. A variety of equatorial waves are present in the model atmosphere, including equatorial Kelvin waves, equatorial Rossby waves, and mixed Rossby–gravity waves. In the upper stratosphere, acceleration of superrotation is strongest around solstice and is due to interaction between equatorial Kelvin waves and Rossby-type waves in winter hemisphere midlatitudes. The existence of this “Rossby–Kelvin”-type wave appears to depend on strong meridional shear of the background zonal wind that occurs in the upper stratosphere at times away from the equinoxes. In the lower stratosphere, acceleration of superrotation occurs throughout the year and is partially induced by equatorial Rossby waves, which we speculate are generated by quasigeostrophic barotropic instability. Acceleration of superrotation is generally due to waves with phase speeds close to the zonal velocity of the mean flow. Consequently, they have short vertical wavelengths that are close to the model’s vertical grid scale and therefore likely to be not properly represented. We suggest that this may be a common issue among Titan general circulation models that should be addressed by future model development.