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Observational data inherently contain noise which manifests as uncertainties in the measured parameters and creates positive biases or noise floors in second‐order products like variances, fluxes, and spectra. Historical methods estimate and subsequently subtract noise floors, but struggle with accuracy. Gardner and Chu (2020, doi.org/10.1364/AO.400375) proposed an interleaved data processing method, which inherently eliminates biases from variances and fluxes, and suggested that the method could also eliminate noise floors of power spectra. We investigate the interleaved method for spectral analysis of atmospheric waves through theoretical studies, forward modeling, and demonstration with lidar data. Our work shows that calculating the cross‐power spectral density (CPSD) from two interleaved subsamples does reduce the spectral noise floor significantly. However, only the Co‐PSD (the real part of CPSD) eliminates the noise floor completely, while taking the absolute magnitude of CPSD adds a reduced noise floor back to the spectrum when the sample number is finite. This reduced noise floor can be further minimized through averaging over more observations, completely different from traditional spectrum calculations whose noise floor cannot be reduced by incorporating more samples. We demonstrate the first application of the interleaved method to spectral data, successfully eliminating the noise floor using the Co‐PSD in a forward model and in lidar observations of the vertical wavenumber of gravity waves at McMurdo, Antarctica. This high accuracy is gained by sacrificing precision due to photon‐count splitting, requiring additional observations to counter this effect. We provide quantitative assessment of accuracy and precision as well as application recommendations. Plain Language Summary: Atmospheric waves serve a vital role in global energy and momentum transportation between the lower and upper atmosphere, driving major atmospheric circulations. These waves exist across many scales, from large planetary waves to medium‐ and small‐scale gravity waves (GW). GW are a key factor driving many atmospheric phenomena, but due to their relatively smaller scales, they are difficult to study. The spectra of GW are important to understanding wave dynamics and informing the development of atmospheric models, as these spectra contain critical information about how wave‐transported energy is distributed amongst different temporal and spatial frequencies. A major tool in improving our knowledge and modeling of GW is their direct observations. Although being powerful wave observation tools, lidar and radar data contain noise in their measurements which manifests as noise floors, obscuring derived wave power spectra. These floors cannot be removed by averaging more samples, as is done for other parameters, making it difficult to accurately interpret the spectra. Pre‐existing techniques can remove this floor, but they struggle with accuracy, especially in high‐noise conditions. This study introduces and demonstrates the use of an interleaved method of spectral processing, which eliminates the noise floors altogether, enabling high‐accuracy calculation of wave spectra. Key Points: We develop an interleaved data processing technique to compute cross‐power spectral density (CPSD) for deriving unbiased wave spectraAccuracy and precision analyses of coincident‐power spectral density (Co‐PSD) and CPSD magnitude show Co‐PSD eliminating noise floor entirelyThe spectral interleaved method is shown to eliminate noise floor, demonstrated using a forward model and Antarctic lidar observations [ABSTRACT FROM AUTHOR]

The Tropical Cloud Oscillation (TCO) in the Martian atmosphere is a shift of clouds in the northern spring and summer tropical cloud belt between the eastern and western hemispheres on an intra‐seasonal timescale of about 10–40 sols. The TCO is a significant intraseasonal variation and may strongly affect the Martian general circulation, water cycle, and dust cycle. We examine TCOs using multiple data sets with a focus on the clouds observed in Mars Daily Global Maps during Mars Year (MY) 29–35. One or more TCO cycles are observed in each MY and the phenomenon is most prominent during Ls = 135°–185°. Space‐time spectral analysis shows a variety of waves which appear to follow the theoretical dispersion relationships of equatorial waves, such as Kelvin waves, Rossby waves, and Mixed Rossby Gravity waves. The TCO appears to be controlled by zonal wavenumber one traveling waves with Kelvin and Rossby wave characteristics and exhibits a fine‐scale latitudinal structure that requires modeling with sufficient resolution. Issues with current data assimilation products for use in studies of Martian equatorial waves due to this fine‐scale structure are discussed. Plain Language Summary: During martian northern spring and summer, a relatively thick water ice cloud belt is observed in the tropics (approximately 25°S–25°N), which modifies large‐scale circulation, global water transport and the atmospheric dust cycle. We report on a Tropical Cloud Oscillation (TCO) that is a periodic shift in the thickness of this cloud belt between the eastern and western hemispheres on timescales (about 10–40 Mars days) longer than that of weather but shorter than that of seasons. Since the TCO involves changes in clouds and implies changes in circulation, it may also modulate the dust and water cycles. We examine TCOs using image data for Mars Year 29–35 (late 2007 through early 2021). One or more TCO cycles are observed in each Mars Year and the phenomenon is most prominent in late northern summer. A wide variety of atmospheric waves are found during TCOs; however, the most important are equatorial waves traveling eastward and westward with one peak and one trough around the planet. These waves have narrow latitudinal length scales and thus require high resolution numerical models to simulate and properly represent in data assimilation products. Key Points: The Martian tropical cloud belt exhibits eastern—western zonal oscillations that are most prominent in late northern summerTropical Cloud Oscillations are controlled by large‐scale low frequency equatorial Kelvin and Rossby wavesEquatorial wave dynamics may have an important effect on the Martian water and dust cycles [ABSTRACT FROM AUTHOR]

High-amplitude quasi-stationary atmospheric Rossby waves with zonal wave numbers 6–8 associated with the phenomenon of quasi-resonant amplification (QRA) have been linked to persistent summer extreme weather events in the Northern Hemisphere. QRA is not well-resolved in current generation climate models, therefore, necessitating an alternative approach to assessing their behavior. Using a previously-developed fingerprint-based semi-empirical approach, we project future occurrence of QRA events based on a QRA index derived from the zonally averaged surface temperature field, comparing results from CMIP 5 and 6 (Coupled Model Intercomparison Project). There is a general agreement among models, with most simulations projecting substantial increase in QRA index. Larger increases are found among CMIP6-SSP5-8.5 (42 models, 46 realizations), with 85% of models displaying a positive trend, as compared with 60% of CMIP5-RCP8.5 (33 models, 75 realizations), with a reduced spread among CMIP6-SSP5-8.5 models. CMIP6-SSP3-7.0 (23 models, 26 realizations) simulations display qualitatively similar behavior to CMIP6-SSP5-8.5, indicating a substantial increase in QRA events under business-as-usual emissions scenarios, and the results hold regardless of the increase in climate sensitivity in CMIP6. Projected aerosol reductions in CMIP6-SSP3-7.0-lowNTCF (5 models, 16 realizations) lead to halting effect in QRA index and Arctic Amplification during the 1st half of the twenty-first century. Our analysis suggests that anthropogenic warming will likely lead to an even more substantial increase in QRA events (and associated summer weather extremes) than indicated by past analyses. [ABSTRACT FROM AUTHOR]

High humidity combined with high temperature decreases the human body's capability to dissipate heat, causing serious health issues. While increased attention has been paid to changes in humid‐heat extremes under the current climate and in the future, the causes of near‐surface humid‐heat extremes over the Yangtze River Delta region are still unclear. Here we investigate the relationships between quasi‐stationary waves (QSWs), which are atmospheric Rossby waves with phase speed close to zero, and humid‐heat extremes over the Yangtze River Delta region during the summer from 1959 to 2021. Additionally, the potential physical processes that link them are also explored. We find that the QSWs with wavenumber 1–8 present different vertical structures: Wave 1–2 are baroclinic, while Wave 3–8 are barotropic. Wave 1, 3, 5, 6, 7 and 8 show phase‐locking behaviour. When the amplitudes of Wave 1, 3, 5 and 6 are higher, the frequency of humid‐heat extremes near the surface at specific locations tends to be higher than the norm. The high‐amplitude waves in different wavenumbers modulate near‐surface temperature and/or humidity in various ways, ultimately resulting in anomalously intensive humid‐heat extremes. [ABSTRACT FROM AUTHOR]

The Quasi‐Biennial Oscillation and the Semiannual Oscillation have been identified to be the leading modes of variability in the tropical middle atmosphere. With reanalysis data and independent rocket soundings from a low latitude site, we report the existence of yet another variability in the tropical lower mesosphere which is primarily evident as easterly bursts in zonal winds during the months of May‐July. It occurs with a variable interval of 2–5 yrs in the late 20th century and 7–9 yrs in the early 21st century. These Quasi‐Periodic Easterly Bursts are found to have remote influences on the Antarctic polar vortex as well as residual circulation in the lower mesosphere. We identify a potential causative mechanism for the easterly bursts that involve enhanced cross equatorial advection of momentum as well as gravity wave drag. A close association with Quasi Biennial Oscillation winds is observed, however, cause of the observed periodicity remains elusive. Plain Language Summary: Circulation in the tropical middle atmosphere is distinctively different from that of the higher latitudes in that there exists long‐period oscillations chiefly driven by a broad spectrum of atmospheric waves. These oscillations that dominate the wind variability here, include the Quasi‐Biennial Oscillation (QBO) and the Semiannual Oscillation (SAO) with approximate periods of 28 and 6 months, respectively. With the help of reanalysis data sets and independent rocket soundings from a low latitude site, we identify the existence of a hitherto unrecognized variability in the tropical middle atmosphere. The newly identified variability seems to have association with the Southern Hemispheric polar vortex as well as the transport circulation in the lower mesosphere. Further, an attempt is made to identify the causative mechanism using the framework of interaction of large‐scale planetary waves with the mean flow. Key Points: A new pattern of variability in the tropical middle atmosphere is identifiedThe observed variability is associated with stronger polar vortex and an enhanced residual circulation in the Southern Hemispheric mesosphereA close connection with the presence of an extended layer of westerlies in the Quasi Biennial Oscillation regime is observed [ABSTRACT FROM AUTHOR]

Recent Greenland ice-sheet melt constitutes a considerable contribution to global sea-level rise. Observations indicate an approximate zero mass balance of the ice sheet until the late 1990s, after which a strong increase in melting occurred. This cannot be attributed linearly to gradually-increasing global warming. Instead the abrupt shift has been linked to atmospheric circulation changes, although causality is not fully understood. Here we show that changes of atmospheric waves over Greenland have significantly contributed to the shift into a strong melting state. This is evident after having applied a newly-developed methodology effectively decomposing atmospheric flow patterns into parts associated with waves of different scales such as Rossby waves and smaller perturbations. The onset of a westerly-flow reduction, consistent with anthropogenic Arctic warming, affected transports by atmospheric waves and led to a decrease in precipitation and an increase in surface warming, contributing to ice-sheet mass loss, in particular over the southwestern regions. As such, the Greenland ice-sheet melt is an example of a climate response non-linearly coupled to global warming. [ABSTRACT FROM AUTHOR]

Gravity wave (GW) momentum flux spectra help to uncover the mechanisms through which GWs influence momentum transfer in the atmosphere and provide crucial insights into accurately characterizing atmospheric wave processes. This study examines the momentum flux spectra of GWs in the troposphere (2–14 km) and stratosphere (18–28 km) over Koror Island (7.2°N, 134.3°W) using radiosonde data from 2013–2018. Utilizing hodograph analysis and spectral methods, the characteristics of momentum flux spectra are discussed. Given that the zonal momentum flux spectra of low-level atmospheric GWs generally follow a Gaussian distribution, Gaussian fitting was applied to the spectral structures. This fitting further explores the seasonal variations of the zonal momentum flux spectra and the average spectral parameters for each month. Additionally, the GW energy is analyzed using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) satellite data and compared with the results of the momentum flux spectra from radiosonde data, revealing the close negative correlation between wave energy and wave momentum for stratospheric GW changing with time. The findings indicate that the Gaussian peak shifts more eastward in both the troposphere and stratosphere, primarily due to the absorption of eastward-propagating GWs by the winter tropospheric westerly jet and critical layer filtering. The full width at half maximum (FWHM) in the stratosphere is larger than in the troposphere, especially in June and July, as the spectrum broadens due to propagation effects, filtering, and interactions among waves. The central phase speed in the stratosphere exceeds that in the troposphere, reflecting the influences of Doppler effects and background wind absorption. The momentum flux in the stratosphere is lower than in the troposphere, which is attributed to jet absorption, partial reflection, or the dissipation of GWs. [ABSTRACT FROM AUTHOR]

Parameterizations for internal gravity waves in atmospheric models are traditionally subject to a number of simplifications. Most notably, they rely on both neglecting wave propagation and advection in the horizontal direction (single-column assumption) and an instantaneous balance in the vertical direction (steady-state assumption). While these simplifications are well justified to cover some essential dynamic effects and keep the computational effort small, it has been shown that both mechanisms are potentially significant. In particular, the recently introduced Multiscale Gravity Wave Model (MS-GWaM) successfully applied ray-tracing methods in a novel type of transient but columnar internal gravity wave parameterization (MS-GWaM-1D). We extend this concept to a three-dimensional version of the parameterization (MS-GWaM-3D) to simulate subgrid-scale nonorographic internal gravity waves. The resulting global wave model—implemented into the weather forecast and climate code Icosahedral Nonhydrostatic (ICON)—contains three-dimensional transient propagation with accurate flux calculations, a latitude-dependent background source, and convectively generated waves. MS-GWaM-3D helps reproduce expected temperature and wind patterns in the mesopause region in the climatological zonal mean state and thus proves a viable internal gravity wave (IGW) parameterization. Analyzing the global wave action budget, we find that horizontal wave propagation is as important as vertical wave propagation. The corresponding wave refraction includes previously missing but well-known effects such as wave refraction into the polar jet streams. On a global scale, three-dimensional wave refraction leads to a horizontal flow-dependent redistribution of waves such that the structures of the zonal mean wave drag and consequently the zonal mean winds are modified. [ABSTRACT FROM AUTHOR]

Interannual sea surface temperature (SST) variations in the subtropical‐midlatitude Southern Hemisphere are often associated with a circumpolar wavenumber‐4 (W4) pattern. This study is the first attempt to successfully simulate the SST‐W4 pattern using a state‐of‐the‐art coupled model called SINTEX‐F2 and clarify the underlying physical processes. It is found that the SST variability in the southwestern subtropical Pacific (SWSP) plays a key role in triggering atmospheric variability and generating the SST‐W4 pattern during austral summer (December‐February). In contrast, the tropical SST variability has a very limited effect. The anomalous convection and associated divergence over the SWSP induce atmospheric Rossby waves confined in the westerly jet. Then, the synoptic disturbances circumnavigate the subtropical Southern Hemisphere, establishing an atmospheric W4 pattern. The atmospheric W4 pattern has an equivalent barotropic structure in the troposphere, and it interacts with the upper ocean, causing variations in mixed layer depth due to latent heat flux (LHF) anomalies. As incoming climatological solar radiation goes into a thinner (thicker) mixed layer, the shallower (deeper) mixed layer promotes surface warming (cooling). This leads to positive (negative) SST anomalies, developing the SST‐W4 pattern during austral summer. Subsequently, anomalous entrainment due to the temperature difference between the mixed layer and the water below the mixed layer, anomalous LHF, and disappearance of the overlying atmospheric W4 pattern cause the decay of the SST‐W4 pattern during austral autumn. These results indicate that accurate simulation of the atmospheric forcing and the associated atmosphere‐ocean interaction is essential to capture the SST‐W4 pattern in coupled models. Plain Language Summary: In the subtropical‐midlatitude Southern Hemisphere, we often observe year‐to‐year fluctuations in sea surface temperature (SST) linked to a specific pattern known as wavenumber‐4 (W4). This study represents the first successful attempt to simulate this temperature pattern using a climate emulator called SINTEX‐F2, allowing us to uncover its physical processes. Our research reveals that SST variations in the southwestern subtropical Pacific (SWSP) play a pivotal role in generating the W4 pattern in the atmosphere, subsequently influencing SST during austral summer. Interestingly, this pattern is almost independent of tropical SST variability. The process starts with heating in the SWSP, causing atmospheric disturbances. This leads to an undulation in mid‐latitude atmospheric flow, evolving into a well‐established global Rossby wave with four positive (negative) loading centers, forming a W4 pattern. This atmospheric wave interacts with the ocean's surface, leading to heat exchange between the atmosphere and the upper ocean. In turn, it influences the depth of the mixed layer in the upper ocean, which receives solar energy. When solar energy penetrates into a shallower (deeper) mixed layer, it warms (cools) the mixed layer effectively, resulting in higher (lower) SSTs. Afterwards, the energy exchange between the mixed layer and the deep ocean contributes to the decay of the SST pattern. Key Points: First attempt to successfully simulate the wavenumber‐4 (W4) pattern of Southern Ocean sea surface temperature (SST) using a coupled model, uncovering the underlying physical processesSouthwestern subtropical Pacific SST plays a crucial role in generating SST W4 pattern via circumpolar atmospheric variabilityThe ocean mixed layer and upper ocean processes are found to be important for the growth and decay of the SST pattern [ABSTRACT FROM AUTHOR]

The Venus atmosphere Superrotation (SR) is successfully simulated with the high‐resolution (1.25° × 1.25° in longitude and latitude) runs of the Venus Planetary Climate Model (PCM). The results show a clear spectrum and structure of atmospheric waves, primarily with periods of 5.65 and 8.5 days. The simulation reproduces long‐term quasi‐periodic oscillation of the zonal wind and primary planetary‐scale wave seen in observations. These oscillations occur with a period of 163–222 days, although their existence is still debated in observations. The Rossby waves show similarity in wave characteristics and angular momentum (AM) transport due to Rossby‐Kelvin instability by comparing the 5.65‐day wave in Venus PCM with the 5.8‐day wave simulated by AFES‐Venus, another Venus General Circulation Model. Similarities are also evident between the 8.5‐day wave in Venus PCM and the 7‐day wave obtained in AFES‐Venus. The long‐term variations in the AM budget indicate that the 5.65‐day wave is the dominant factor of the oscillation on the SR, and the 8.5‐day wave plays a secondary role. When the 5.65‐day wave grows, its AM and heat transport are enhanced and accelerate (decelerate) the lower‐cloud equatorial jet (cloud‐top mid‐latitude jets). Meanwhile, the 8.5‐day wave weakens, reducing its deceleration effect on the lower‐cloud equator. This further suppresses the meridional gradient of the background wind and weakens instability, leading to the decay of the 5.65‐day wave. And vice versa when the 5.65‐day wave decays. Plain Language Summary: On Venus, large‐scale waves in the atmosphere play a crucial role in maintaining its fast westward‐moving atmosphere, known as superrotation (SR). Previous observations suggest that there might be long‐term variations in the SR. We simulated the Venusian SR using a high‐resolution atmospheric computer model to understand the mechanism behind this phenomenon. The simulation reproduced the variations in SR and the main large‐scale waves observed in reality. Among these waves, the one with a 5.65‐day period is the strongest wave in the upper clouds of Venus, in addition to the atmospheric thermal tides. In the simulations, the SR always varies with the changes in the 5.65‐day wave. This result indicates that the 5.65‐day wave significantly influences SR, possibly explaining the long‐term variations observed in SR. Key Points: Rossby‐Kelvin instability wave mode and their angular momentum (AM) transport to super‐rotation are robust in at least 2 Venus General Circulation ModelsLong‐term quasi‐periodic (163–222 days) variations in the superrotation and dominant wave simulated by Venus Planetary Climate Model are close to observationsDynamically conserved AM from circulation, 5.65‐day, and 8.5‐day waves drives quasi‐periodic variations of the local jets [ABSTRACT FROM AUTHOR]

A record-setting extreme heatwave occurred over western North America (WNA) in the summer of 2021, which was associated with an extreme atmospheric Rossby wave ridge (ARR) over WNA and a minimum record event of the preceding winter Barents Sea ice concentration. We identify the remote effect and investigate how the Barents Sea ice loss (BSIL) in the preceding winter relates to the intensity of the ARR and extreme heatwaves in the following June and July (JJ) by analyzing the reanalysis data. Our results suggest that the atmospheric wave activity flux associated with the BSIL transferred wave energy towards the circumglobal teleconnection (CGT)-like wave trains, increasing the wave amplitude and enhancing the ARR over WNA. The anomalous sea ice melting in JJ, originating from the BSIL in the preceding winter through ocean-air-sea ice positive feedback processes, also induces a northward shift of jet stream by meridional temperature gradient change. The weakening of the local jet from the original jet axis area drives anticyclonic anomalies which increases atmospheric subsidence and shortwave radiation. The energy is transferred from the weakened jet to synoptic-scale disturbances via wave-current interaction. The northward jet as a waveguide causes the CGT to shift northward and receive signals from the higher latitude. Both the CGT-like wave trains and the shifted jet streams synergistically contribute to the increase of ARR intensity and the frequency of extreme heatwaves. Overall, the BSIL will serve as an early predictor for properly forecasting the extreme heatwaves over WNA six months in advance, which potentially improves the prediction skills. [ABSTRACT FROM AUTHOR]

Measurements of Very Low Frequency (VLF) signals from navy transmitters carry direct information about the D‐region ionosphere and have been widely utilized for detecting the electron density at D‐region altitudes, but not frequently for the atmospheric waves therein. Atmospheric waves have been extensively studied using the total ionospheric electron content, but if and how they are correlated with the D‐region ionosphere and VLF measurements still remains poorly investigated. In this study, we have conducted a comprehensive analysis using 7‐year measurements (2016–2022) of VLF signals from the JJI, NWC, and VTX transmitters as being recorded in Suizhou, China. These three transmitter‐receiver paths are representative and the corresponding observations constitute a valuable data set to investigate the periodicities of VLF data. Different from previous studies, we have utilized the ensemble empirical mode decomposition and Lomb‐Scargle methods to determine the periodicities of these data. By contrasting these paths, prominent periodicities ranging from 2 to 730 days have been found, with clear diurnal variation and suggestive latitudinal/longitudinal dependence. Moreover, we have found that the mesospheric temperature is closely related with the annual oscillation of VLF measurements, while this oscillation has a low correlation with solar Lyman‐α fluxes or geomagnetic activity. The oscillations with relatively shorter periods are likely atmospheric waves such as gravity waves, planetary waves, or harmonics of these waves. Our results suggest that, in addition to the electron density, the subionospheric VLF technique can be potentially utilized to remotely sense atmospheric waves that propagate up to or through the D‐region ionosphere. Plain Language Summary: Radio waves in the very low frequency range (VLF, 3–30 kHz) can propagate long distances within the Earth's atmosphere between the surface and the D‐region ionosphere, a region at altitudes of 60–100 km that is influenced by both solar fluxes that propagate through the upper atmosphere to the D‐region ionosphere and atmospheric waves from the lower atmosphere. Measurements of VLF waves from navy transmitters have been widely utilized to detect the electron density of D‐region ionosphere, but not the atmospheric waves therein. Toward this goal, we have investigated the VLF measurements recorded by our detector in Suizhou, China. These signals originate from the JJI transmitter in Japan, the NWC transmitter in Australia, and the VTX transmitter in India. The results show that these VLF signals have clear periodic oscillations ranging from 2 to 730 days, exhibiting notable variation between daytime and nighttime conditions, and suggestive dependence on the latitudes/longitudes the VLF propagation path crosses. Moreover, we have analyzed the main reason for the annual oscillation of VLF measurements and found that the annual oscillation is mainly driven by the mesospheric temperature. Our work demonstrates the potential usage of the VLF technique in remotely sensing the atmospheric waves of D‐region ionosphere. Key Points: We have quantified the periodicities of 7‐year VLF measurements using the empirical mode decomposition and Lomb‐Scargle methodsPeriods ranging from 2 to 730 days have been found with clear diurnal dependence and latitudinal/longitudinal dependenceThe annual oscillation is closely correlated with mesospheric temperature, while low correlation with Lyman‐α fluxes or geomagnetic activity [ABSTRACT FROM AUTHOR]

The purpose of this first article is to provide a physical basis for atmospheric Rossby waves at the tropopause to clarify their properties and improve our knowledge of their role in the genesis of extreme precipitation and heat waves. By analogy with the oceanic Rossby waves, the role played by the pycnocline in ocean Rossby waves is replaced here by the interface between the polar jet and the ascending air column at the meeting of the polar and Ferrel cell circulation or between the subtropical jet and the descending air column at the meeting of the Ferrel and Hadley cell circulation. In both cases, the Rossby waves are suitable for being resonantly forced in harmonic modes by tuning their natural period to the forcing period. Here, the forcing period is one year as a result of the variation in insolation due to solar declination. A search for cause-and-effect relationships is performed from the joint representation of the amplitude and phase of (1) the velocity of the cold or warm modulated airflows at 250 mb resulting from Rossby waves, (2) the geopotential height at 500 mb, and (3) the precipitation rate or ground air temperature. This is for the dominant harmonic mode whose period can be 1/16, 1/32, or 1/64 year, which reflects the intra-seasonal variations in the rising and falling air columns at the meeting of the polar, Ferrel, and Hadley cell circulation. Harmonics determine the duration of blocking. Two case studies referring to extreme cold and heat waves are presented. Dual cyclone–anticyclone systems seem to favor extreme events. They are formed by two joint vortices of opposite signs reversing over a period, concomitantly with the involved modulated airflows at the tropopause. A second article will be oriented toward (1) the examination of different case studies in order to ascertain the common characteristics of Rossby wave patterns leading to extreme events and (2) a map of the globe revealing future trends in the occurrence of extreme events. [ABSTRACT FROM AUTHOR]

The Radon and Hilbert transform and their applications to convectively coupled waves (CCWs) are reviewed. The Hilbert Transform is used to compute the wave envelope, whereas the Radon transform is used to estimate the phase and group velocities of CCWs. Together, they provide an objective method to understand CCW propagation. Results reveal phase speeds and group velocities for fast waves (mixed Rossby‐gravity, westward and eastward inertio‐gravity, and Kelvin) that are consistent with previous studies and with Matsuno's equatorial wave dispersion curves. However, slowly‐propagating tropical depression‐like systems and equatorial Rossby waves exhibit wave envelopes that propagate faster than the individual wave crests, which is not predicted by dry shallow water theory. [ABSTRACT FROM AUTHOR]

Biweekly sea surface temperature (SST) variability significantly contributes to over 50% of the intraseasonal variability in the eastern equatorial Pacific (EEP) and Atlantic (EEA). Our study investigates this biweekly variability, employing a blend of in–situ and reanalysis data sets. The research identifies biweekly signals in SST, meridional wind, and ocean currents, notably in September–November in EEP and June–August in EEA. Biweekly southerly (northerly) winds drive instantaneous northward (southward) ocean currents in EEP, but with a 1–2‐day phase delay in EEA. Consequently, these currents lead to SST anomalies with a 3–4‐day lag in both EEP and EEA due to the presence of the cold tongue. The study reveals the origin of biweekly wind fluctuations in the western Pacific for EEP and the subpolar Pacific for EEA, connected by atmospheric Rossby waves validated through a linearized non‐divergent barotropic model. This research affirms the influence of subtropical and subpolar atmospheric forcing on equatorial SST. Plain Language Summary: Our research focuses on understanding the regular changes in sea surface temperature (SST) occurring every 2 weeks, which significantly contribute to the seasonal variations in the eastern equatorial Pacific (EEP) and Atlantic (EEA). By analyzing a mix of direct and reconstructed data, we uncover the distinct patterns of these biweekly changes in SST, winds, and ocean currents. Our investigation shows that the movements of the ocean currents are closely linked to the shifts in wind direction, affecting the temperature of the ocean waters. Notably, we observe a delay in the relationship between wind and SST in both EEP and EEA. Through our analysis, we establish that the origins of these biweekly wind patterns can be traced to specific regions in the Pacific. Moreover, we identify the role of atmospheric Rossby waves in connecting these wind patterns to their source regions, which helps us understand how changes in atmospheric conditions in different parts of the ocean can impact the equatorial SST. Key Points: Over 50% of intraseasonal SST variability in the eastern equatorial Pacific and Atlantic is attributed to biweekly fluctuationsThe atmospheric winds play a crucial role in driving the biweekly SST variabilityBiweekly winds in the eastern equatorial Pacific and Atlantic stem from the subtropical and subpolar regions, respectively [ABSTRACT FROM AUTHOR]

This study investigates the day-to-day variability of equatorial plasma bubbles (EPBs) over the Atlantic–American region and their connections to atmospheric planetary waves during the sudden stratospheric warming (SSW) event of 2021. The investigation is conducted on the basis of the GOLD (Global Observations of the Limb and Disk) observations, the ICON (Ionospheric Connection Explorer) neutral wind dataset, ionosonde measurements, and simulations from the WACCM-X (Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension). We found that the intensity of EPBs was notably reduced by 35% during the SSW compared with the non-SSW period. Furthermore, GOLD observations and ionosonde data show that significant quasi-6-day oscillation (Q6DO) was observed in both the intensity of EPBs and the localized growth rate of Rayleigh–Taylor (R-T) instability during the 2021 SSW event. The analysis of WACCM-X simulations and ICON neutral winds reveals that the Q6DO pattern coincided with an amplification of the quasi-6-day wave (Q6DW) in WACCM-X simulations and noticeable ∼6-day periodicity in ICON zonal winds. The combination of these multi-instrument observations and numerical simulations demonstrates that certain planetary waves like the Q6DW can significantly influence the day-to-day variability of EPBs, especially during the SSW period, through modulating the strength of prereversal enhancement and the growth rate of R-T instability via the wind-driven dynamo. These findings provide novel insights into the connection between atmospheric planetary waves and ionospheric EPBs. [ABSTRACT FROM AUTHOR]

Atmospheric waveguides have been linked to amplified atmospheric Rossby waves and therefore to extreme weather events in the mid-latitudes. Waveguides have often been calculated on zonal-mean data, and/or on timescales of a month or longer. Here, I develop an objective algorithm to detect barotropic waveguides, and create a dataset of time- and spatially-varying waveguides in both summer and winter for both the Northern and Southern Hemisphere (NH/SH), including a metric of waveguide depth. In this dataset, waveguides for waves of zonal wavenumber 5 exist in the extra-tropics on more than 40 % of days across many longitudes, with the frequency of occurrence reducing for higher zonal wavenumbers. Waveguides tend to be more frequent, and deeper, in summer than in winter, and more frequent in the NH than the SH. Composites of days with high spatial mean waveguide depth over particular regions show a double jet structure associated with strong waveguide occurrence, consistent with previous research. Significant positive correlations exist between waveguide depth and the presence/strength of quasi-stationary waves. In the SH these correlations are strong across much of the mid-latitudes in both seasons, whilst in the NH significant correlations are found only over the Atlantic, Europe and Asia during NH summer, with the strongest correlations over the Atlantic and western Europe, a region notable for its strong positive trend in extreme heat temperature events in recent decades. [ABSTRACT FROM AUTHOR]

This study unravels the characteristics, mechanisms, and predictability of four consecutive record‐breaking heatwaves hitting North China in June and July 2023. The first three heatwaves primarily influenced the northern part of North China and were accompanied by consistent anticyclonic anomalies in the upper troposphere. The anomalous anticyclone was caused by the British–Baikal corridor teleconnection along the polar front jet, particularly during the second heatwave. In contrast, the fourth heatwave was induced by a distinct low‐pressure system, attributed to the Silk Road pattern along the subtropical jet. The presence of this low‐pressure system and its interaction with atmospheric rivers and local topography led to the foehn wind, further contributing to the rise in surface temperatures. Sub‐seasonal to seasonal models can effectively predict the occurrence of all heatwaves 2–5 days in advance despite underestimating the intensity. However, models exhibit limitations in providing reliable predictions when the lead time exceeds 2 weeks. Plain Language Summary: In the summer of 2023, North China experienced four consecutive extreme high‐temperature events, which are called heatwaves. This study investigates the main factors that cause the four events and shows how well the operational numerical models can predict the heatwaves. The first three heatwaves shared similar circulation, with high‐pressure systems controlling North China. This local circulation anomaly was related to an upstream quasi‐stationary wave train along the polar front jet. The fourth heatwave was associated with a low‐pressure system over North China and an upstream quasi‐stationary wave train along the subtropical jet. Moreover, predictions from operational models demonstrate their capability to forecast the occurrence of high temperatures 2–5 days ahead, with an underestimation in the intensity. However, models are not reliable when it comes to predicting heatwaves 2 weeks in advance. Key Points: North China witnessed record‐breaking heatwaves in June and July 2023, consisting of four sequential synoptic‐scale eventsThe first three and the last heatwaves are controlled by distinct local circulations induced by atmospheric wave trains across EurasiaS2S models capture the heatwave occurrences 2–5 days in advance, but predictive skill notably diminishes beyond 2 weeks [ABSTRACT FROM AUTHOR]

Observations of column water vapor in the tropics show significant variations in space and time, indicating that it is strongly influenced by the passage of weather systems. It is hypothesized that many of the influencing systems are moisture modes, systems whose thermodynamics are governed by moisture. On the basis of four objective criteria, results suggest that all oceanic convectively coupled tropical depression (TD)-like waves and equatorial Rossby waves are moisture modes. These modes occur where the horizontal column moisture gradient is steep and not where the column water vapor content is high. Despite geographical basic-state differences, the moisture modes are driven by the same mechanisms across all basins. The moist static energy (MSE) anomalies propagate westward by horizontal moisture advection by the trade winds. Their growth is determined by the advection of background moisture by the anomalous meridional winds and anomalous radiative heating. Horizontal maps of column moisture and 850-hPa streamfunction show that convection is partially collocated with the low-level circulation in nearly all the waves. Both this structure and the process of growth indicate that the moisture modes grow from moisture–vortex instability. Last, space–time spectral analysis reveals that column moisture and low-level meridional winds are coherent and exhibit a phasing that is consistent with a poleward latent energy transport. Collectively, these results indicate that moisture modes are ubiquitous across the tropics. That they occur in regions of steep horizontal moisture gradients and grow from moisture–vortex instability suggests that these gradients are inherently unstable and are subject to continuous stirring. Significance Statement: Over the tropics, column water vapor has been found to be highly correlated with precipitation, especially in slowly evolving systems. These observations and theory support the hypothesis that moisture modes exist, a type of precipitating weather system that does not exist in dry theory. In this study, we found that all oceanic tropical depression (TD)-like waves and equatorial Rossby waves are moisture modes. These systems exist in regions where moisture varies greatly in space, and they grow by transporting air from the humid areas of the tropics toward their low pressure center. These results indicate that the climatological-mean distribution of moisture in the tropics is unstable and is subject to stirring by moisture modes. [ABSTRACT FROM AUTHOR]

In this paper, we attempt to compare the Antarctic SSW of 2019 with the Arctic SSW of 2013. Despite being a minor SSW, the rare Antarctic event of September 2019 caused a significant enhancement of stratospheric temperature at high latitudes with more temperature deviation than the Arctic major SSW of 2013. On the basis of the duration and extent of stratospheric zonal mean zonal wind, the 2019 SSW is comparable to major SSWs. An exceptionally strong wave 1 planetary wave was observed during the 2019 SSW. We have investigated the response of the ionospheric total electron content (TEC) during both the SSWs over two low mid-latitude locations Okinawa and Darwin, which form magnetic conjugate pair. The extent of maximum ionospheric deviations during the 2019 SSW is almost comparable to the 2013 SSW in their respective hemispheres. We have observed mostly night time enhancement in TEC during the Arctic SSW, while daytime enhancement is noted during the Antarctic SSW. Hemispheric asymmetry is observed during both the SSWs. Minor geomagnetic storm has been noticed coinciding with each SSW during different phases of the warming. The SSW, solar flux, and storm contribution on the TEC have been calculated during both the SSWs, which shows clear contribution of SSW on the total TEC despite the presence of storm and during moderately high solar activity. We have also investigated the foF2 variations from ionosonde and IRI-16 at the same two locations. The peak electron density from ionosonde responds to the warming in a manner similar to TEC. The foF2 of IRI-16 exhibits overestimation during daytime and underestimation during nighttime on quiet days during low solar activity (2019), while during moderate solar activity (2013), it overestimates during morning hours. However, it can not predict the enhancements of foF2 during SSWs. [ABSTRACT FROM AUTHOR]

Rossby Wave Packets (RWPs) are atmospheric perturbations located at upper levels in mid‐latitudes which, in certain cases, terminate in Rossby Wave Breaking (RWB) events. When sufficiently persistent and spatially extended, these RWB events are synoptically identical to atmospheric blockings, which are linked to heatwaves and droughts. Thus, studying RWB events after RWPs propagation and their link with blocking is key to enhance extreme weather events detection 10–30 days in advance. Hence, here we assess (a) the occurrence of RWB events after the propagation of transient RWPs, (b) whether long‐lived RWPs (RWPs with a lifespan above 8 days, or LLRWPs) are linked to large‐scale RWB events that could form a blocking event, and (c) the proportion of blocking situations that occur near RWB events. To do so, we applied a tracking algorithm to detect transient RWPs in the southern hemisphere during summertime between 1979 and 2021, developed a wave breaking algorithm to identify RWB events, and searched for blocking events with different intensities. Results show that LLRWPs and the other RWPs displayed large‐scale RWB events around 40% of the time, and most RWB events in both distributions last around 1–2 days, which is not long enough to identify them as blocking situations. Nearly 17% of blockings have a RWB event nearby, but barely 5% of blockings are linked to RWPs, suggesting that transient RWPs are not strongly linked to blocking development. Lastly, large‐scale RWB events associated with RWPs that lasted less than 8 days are influenced by El Niño‐Southern Oscillation. Plain Language Summary: When an atmospheric wave breaks in the upper levels of the atmosphere, it modifies the wind flow in the upper atmosphere and can drastically change local weather conditions. If wave breaking events are sufficiently big and stable, they can produce atmospheric blocking events, which are linked to heatwaves and drought development. Here, we assess whether a link between wave breaking events caused by long‐lived traveling atmospheric waves (waves that last more than 8 days in the atmosphere) and blocking event development exists during southern hemisphere summer. Independently of the duration of the wave packets, near 4 out of 10 times they cause very extensive wave breaking events, but do not last long enough time in the atmosphere to be considered an atmospheric blocking. Oppositely, nearly 20% of blocking events manifest nearby a wave breaking event independently of the strength of the block, but these wave breaking events do not seem to be linked to traveling wave packets. Therefore, this study suggests that traveling atmospheric waves are not directly related to the development of atmospheric blockings. Also, the occurrence of extensive wave breaking events caused by traveling atmospheric waves that last less than 8 days is influenced by El Niño‐Southern Oscillation. Key Points: Large‐scale wave breaking events associated to transient long‐lived Rossby waves occur more often in the middle‐eastern Pacific basinTransient Rossby Wave Packets do not cause large‐scale wave breaking events that last enough to develop into an atmospheric blockNear 17% of blocking events appear preceded by a wave breaking event but most of them are not linked to transient Rossby Wave Packets [ABSTRACT FROM AUTHOR]

Since December 2015, Ultraviolet Imager (UVI) onboard Akatsuki has been observing Venus clouds at the wavelengths of 283 and 365 nm. Horizontal winds near the cloud top derived from the UVI images over ∼7 earth years are analyzed to elucidate spatial and temporal variability of the superrotation and planetary‐scale waves. Zonal winds averaged over the analysis period are asymmetric with respect to the equator, being faster in the southern hemisphere. This asymmetry varied temporarily and was occasionally reverted. Comparison of the winds from the two wavelengths suggests that it is uncertain whether the asymmetry is in the wind distribution or in the sensing altitude for winds. Mean zonal winds representing the superrotation exhibited broad low‐frequency variability with spectra resembling the red noise spectra. This is indicative of the presence of internal variability rather than responses to periodical external forcing. Planetary‐scale waves with zonal‐wavenumber 1 at periods around 4 and 5 days, which have been interpreted as equatorial Kelvin and Rossby waves, respectively, are quantified. While the ∼5‐day waves have nearly constant frequencies, the ∼4‐day waves have variable phase speeds that follow the superrotation speed. This result indicates that the ∼5‐day waves are likely to extend over a large depth below the cloud top and that the ∼4‐day waves are likely to be confined near the cloud top. Their possible generation mechanism through the coupling of Kelvin and Rossby waves is discussed. This study further reports wind variability of 10 to 15‐day periodicity, thermal‐tide structure, and comparison with minor species observations. Plain Language Summary: Akatsuki is the only currently operational artificial satellite of Venus. It has been conducting imaging observations at two ultraviolet wavelengths since December 2015. By measuring the movement of cloud features in the images, we obtain horizontal winds near the cloud‐top altitude where the famous superrotation is maximized. By using wind data over ∼7 earth years, we show that the superrotation speed varies over time, showing a broad spectrum. This is indicative of internally created variability rather than periodic variability created by external forcing. We also reveal that mean westward winds that characterize the superrotation have some differences between the northern and southern hemispheres and that these differences vary over time. However, the data do not allow us to conclude whether the asymmetry is in the winds or in the sensing altitude. Venus has long been known for having two kinds of planetary‐scale waves with periods around 4 and 5 days, but their vertical distributions are unknown. We indicate that the 5‐day waves have a deeper vertical distribution than the 4‐day waves from the differences in the long‐term changes in their frequencies. We further characterize the thermal tides excited by solar heating and suggest wind variability at periods of 10–15 days. Key Points: Venus' superrotation as measured near the cloud top exhibits rich low frequency variations indicating internal variabilityPlanetary‐scale ∼4‐ and ∼5‐day waves are suggested to reside in different altitude rangesTemporally varying hemispheric asymmetry is found in zonal winds, which might partly be due to the sensing altitude asymmetry [ABSTRACT FROM AUTHOR]

MATS (Mesospheric Airglow/Aerosol, Tomography & Spectroscopy) is a Swedish satellite designed to investigate atmospheric dynamics in the Mesosphere and Lower Thermosphere (MLT). From observing structures in noctilucent clouds over polar regions, and oxygen A-band emissions globally, MATS will supply the research community with properties of the MLT atmospheric wave field. Individual A-band images taken by the MATS main instrument, a 6-channel limb imager, are through tomography and spectroscopy turned into three-dimensional temperature fields in which the wave structures are embedded. To identify wave properties, in particular the gravity wave momentum flux, from the temperature field, smaller-scale perturbations, associated with the targeted waves, must be separated from large-scale background variations by a method of scale separation. This paper investigates the possibilities of employing a simple method based on smoothing polynomials to separate the smaller and larger scales. By using synthetic tomography data based on the HIAMCM (High Altitude Mechanistic Circulation Model) we demonstrate that smoothing polynomials can be applied to MLT temperatures to obtain fields corresponding to a global scale separation at zonal wavenumber 18. The simplicity of the method makes it a promising candidate for studying wave dynamics in the MATS temperature fields. [ABSTRACT FROM AUTHOR]

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

During 2013–16 and 2018–22, marine heatwaves (MHWs) occurred in the North Pacific, exhibiting similar extensive coverage, lengthy duration, and significant intensity but with different warming centers. The warming center of the 2013–16 event was in the Gulf of Alaska (GOA), while the 2018–22 event had warming centers in both the GOA and the coast of Japan (COJ). Our observational analysis indicates that these two events can be considered as two MHW variants induced by a basinwide MHW conditioning mode in the North Pacific. Both variants were driven thermodynamically by atmospheric wave trains propagating from the tropical Pacific to the North Pacific, within the conditioning mode. The origin and propagating path of these wave trains play a crucial role in determining the specific type of MHW variant. When a stronger wave train originates from the tropical central (western) Pacific, it leads to the GOA (COJ) variant. The cross-basin nature of the wave trains enables the two MHW variants to be accompanied by a tripolar pattern of sea surface temperature anomalies in the North Atlantic but with opposite phases. The association of these two MHW variants with the Atlantic Ocean also manifests in the decadal variations of their occurrence. Both variants tend to occur more frequently during the positive phase of the Atlantic multidecadal oscillation but less so during the negative phase. This study underscores the importance of cross-basin associations between the North Pacific and North Atlantic in shaping the dynamics of North Pacific MHWs. [ABSTRACT FROM AUTHOR]

The variations of the ionospheric current in the lower ionosphere and the plasma density of the upper ionosphere have been studied based on the monitoring of the horizontal component of the geomagnetic field induction and height-frequency sounding of the critical frequency of the ionosphere F layer at the Moscow Observatory. It is shown that, in the spectra of time variations of the geomagnetic field and the critical frequency of the F2 layer in the range of planetary waves in winter, there are both harmonics associated with solar activity and harmonics corresponding to quasi-5-, 10-, and 16-day planetary waves. The involvement of geomagnetic field registration data for a 20-year time interval (from 2001 to 2020) have made it possible to identify more subtle effects, namely, the harmonics associated with the modulation effect of longer period variations and tidal effects are identified. [ABSTRACT FROM AUTHOR]

Traveling planetary waves surrounding sudden stratospheric warming events can result from direct propagation from below or in situ generation. They can have significant impacts on the circulation in the mesosphere and lower thermosphere. Our study runs a series of ensembles initialized from the Whole Atmosphere Community Climate Model, version 4, nudged up to 50 km by 6-hourly Modern-Era Retrospective Analysis for Research and Application, version 2, reanalysis to compile a library of sudden stratospheric warming events. To our knowledge, we present the first composite or ensemble study that attempts to link direct propagation and in situ generation by evaluating the wave geometries associated with the overreflection perspective, a framework used to describe how planetary waves interact with critical and turning levels. The present study looks at the evolution of these interactions through the onset of sudden stratospheric warmings with an elevated stratopause (ES-SSWs). Robust and unique features of ES-SSWs are determined by employing an ensemble study that compares ES-SSWs with normal winters. Our study evaluates the production and impacts of westward-propagating, quasi-stationary, and eastward-propagating planetary waves surrounding ES-SSWs. Our results show that eastward-propagating planetary waves are generated within the westward stratospheric wind layer after ES-SSW onset which aids in restoring the eastward stratospheric wind. The interaction of quasi-stationary and westward-propagating waves with the westward stratospheric wind is explored from an overreflection perspective and reaffirms that westward-propagating planetary waves are produced from instabilities at the top of the westward stratospheric wind reversal. [ABSTRACT FROM AUTHOR]

Many previous studies have documented the relationship between the North Pacific Oscillation (NPO) and the weather/climate of upstream and downstream regions. However, the stability of NPO precursor signals of East Asian summer precipitation, which is important for climate prediction, has received little attention. This study identified temporal variations in the connection between the May NPO and subsequent midsummer precipitation over Northeast China (NEC) during 1961–2020. During 1986–2010, the correlation between the May NPO and midsummer precipitation over NEC was found significantly positive, whereas the relation was found statistically insignificant during 1961–1985/2011–2020. Further results indicated that the NPO stimulated a Rossby wave source over the North Pacific that was stronger during 1986–2010 than during 1961–1985/2011–2020. The Rossby wave source anomalies shifted to North America and to the North Atlantic during the subsequent June and midsummer, respectively. Consequently, a teleconnection wave train, which originated over the North Pacific, propagated eastward to North America, across the North Atlantic, and on to East Asia on the subseasonal time scale, influencing midsummer precipitation over NEC through modulation of the atmospheric circulation (e.g., horizontal wind, moisture transport, and vertical movement). Moreover, the intensified Rossby wave anomalies associated with the May NPO were attributed to the strengthened NPO, which could be related to the enhanced standard deviation of SLP over the North Pacific. Additionally, the strengthened out‐of‐phase relation between the NPO and the North Atlantic Oscillation, resulting from changes in the background circulation, partially contributed to intensification of the Rossby wave train. Plain Language Summary: The North Pacific Oscillation (NPO) is an important intrinsic atmospheric variability which is characterized by a north–south seesaw in sea level pressure over the North Pacific Ocean. The NPO has substantial influences on the weather/climate conditions over North America and East Asia. However, the relation between NPO and East Asian climate is not stationary. This study identified a temporal variation in the connection between the May NPO and the midsummer precipitation over Northeast China (NEC) during 1961–2020, which shows a strong (weak) correlation during 1986–2010 (1961–1985 and 2011–2020). This is attributed to the different atmospheric circulation patterns caused by the NPO during the three periods. The NPO can simulate stronger perturbation to the atmosphere during 1986–2010 than during 1961–1985 and 2011–2020. Consequently, the perturbed atmospheric waves can propagate eastward to North America, across the North Atlantic, and toward East Asia during 1986–2010, leading to anomalous atmospheric circulation in East Asia and influencing the midsummer precipitation over NEC. The strengthening of the NPO and the enhanced relation between the NPO and North Atlantic Oscillation during 1986–2010 can explain the above changes. This study may deepen our understanding of the influence of NPO on the East Asian climate and may be meaningful for climate prediction. Key Points: The effect of May North Pacific Oscillation (NPO) on summer rainfall at Northeast China (NEC) is not stable, with a strong (weak) relation for 1986–2010 (for 1961–1985 and 2011–2020)This is attributed to a stronger Rossby wave source stimulated by NPO during 1986–2010 than during 1961–1985 and 2011–2020The strengthened Rossby wave related with NPO is attributed to the intensification of NPO and to the enhanced relation between NPO and North Atlantic Oscillation [ABSTRACT FROM AUTHOR]

Interconnections between ocean basins are recognized as an important driver of climate variability. Recent modeling evidence suggests that the North Atlantic climate can respond to persistent warming of the tropical Indian Ocean sea surface temperature (SST) relative to the rest of the tropics (rTIO). Here, we use observational data to demonstrate that multi-decadal changes in pantropical ocean temperature gradients lead to variations of an SST-based proxy of the Atlantic Meridional Overturning Circulation (AMOC). The largest contribution to this temperature gradient-AMOC connection comes from gradients between the Indian and Atlantic Oceans. The rTIO index yields the strongest connection of this tropical temperature gradient to the AMOC. Focusing on the internally generated signal in three observational products reveals that an SST-based AMOC proxy index has closely followed low-frequency changes of rTIO temperature with about 26-year lag since 1870. Analyzing the pre-industrial control simulations of 44 CMIP6 climate models shows that the AMOC proxy index lags simulated mid-latitude AMOC variations by 4 ± 4 years. These model simulations reveal the mechanism connecting AMOC variations to pantropical ocean temperature gradients at a 27 ± 2 years lag, matching the observed time lag in 28 out of the 44 analyzed models. rTIO temperature changes affect the North Atlantic climate through atmospheric planetary waves, impacting temperature and salinity in the subpolar North Atlantic, which modifies deep convection and ultimately the AMOC. Through this mechanism, observed internal rTIO variations can serve as a multi-decadal precursor of AMOC changes with important implications for AMOC dynamics and predictability. [ABSTRACT FROM AUTHOR]

The effect of a subgrid-scale cloud field on the propagation of long atmospheric waves is investigated using a new scale-consistent formulation based on the asymptotic theory of homogenisation. A key aim is to quantify potential model errors in wave propagation speeds, introduced by using averaged fields in place of the fully resolved circulation, in the setting of a simple stratified Boussinesq midlatitude β-channel model. The effect of the cloud field, represented here by a random array of strongly nonlinear axisymmetric circulations, is found to appear in the large-scale governing equations through new terms which redistribute the large-scale buoyancy and horizontal momentum fields in the vertical. These new terms, which have the form of nonlocal integral operators, are linear in the cloud number density and are fully determined by the solution of a linear elliptic equation known as a cell problem. The cell problem in turn depends on the details of the nonlinear cloud circulations. The integral operators are calculated explicitly for example cloud fields and then dispersion relations are compared for different waves in the presence of clouds at realistic densities. The main finding is that baroclinic Rossby waves are significantly slowed and damped by the clouds, whilst inertia-gravity waves are affected almost exclusively by damping, most strongly at the lowest frequencies. In contrast, all waves with a barotropic structure are found to be almost unaffected by the presence of clouds, even at the highest realistic cloud densities. An important consequence of this study is a new approach to the closure of subgridscale cloud fields in the parameterisation of convection in large-scale atmospheric models. [ABSTRACT FROM AUTHOR]

A previous study has shown that a large portion of subseasonal-to-seasonal European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts for 2-m temperature exhibit properties of univariate bimodality, in some locations occurring in over 30% of forecasts. This study introduces a novel methodology to identify "bimodal events," meteorological events that trigger the development of spatially and temporally correlated bimodality in forecasts. Understanding such events not only provides insight into the dynamics of the meteorological phenomena causing bimodal events, but also indicates when Gaussian interpretations of forecasts are detrimental. The methodology that is developed allows one to systematically characterize the spatial and temporal scales of the derived bimodal events, and thus uncover the flow states that lead to them. Three distinct regions that exhibit high occurrence rates of bimodality are studied: one in South America, one in the Southern Ocean, and one in the North Atlantic. It is found that bimodal events in each region appear to be triggered by synoptic processes interacting with geographically specific processes: in South America, bimodality is often related to Andes blocking events; in the Southern Ocean, bimodality is often related to an atmospheric Rossby wave interacting with sea ice; and in the North Atlantic, bimodality is often connected to the displacement of a persistent subtropical high. This common pattern of large-scale circulation anomalies interacting with local boundary conditions suggests that any deeper dynamical understanding of these events should incorporate such interactions. Significance Statement: Repeatedly running weather forecasts with slightly different initial conditions provides some information on the confidence of a forecast. Occasionally, these sets of forecasts spread into two distinct groups or modes, making the "typical" interpretation of confidence inappropriate. What leads to such a behavior has yet to be fully understood. This study contributes to our understanding of this process by presenting a methodology that identifies coherent bimodal events in forecasts of near-surface air temperature. Applying this methodology to a database of such forecasts reveals several key dynamical features that can lead to bimodal events. Exploring and understanding these features is crucial for saving forecasters' resources, creating more skillful forecasts for the public, and improving our understanding of the weather. [ABSTRACT FROM AUTHOR]

Multiple subvortices corresponding to suction vortices in observations are obtained within a simulated tornado for the EF4 tornado case of Funing, China, on 23 June 2016. Within the simulation, the tornado evolves from a one-cell structure with vorticity maximum at its center to a two-cell structure with a ring of vorticity maximum. Five well-defined subvortices develop along the ring. The radial profile of tangential wind across the vorticity ring satisfies the necessary condition of barotropic instability associated with phase-locked, counterpropagating vortex Rossby waves (VRWs) along the ring edges. The phased-locked waves revolve around the parent vortex at a speed less than the maximum azimuthal-mean tangential velocity, agreeing with theoretically predicted VRW phase speed. The radii within which the wave activities are confined are also correctly predicted by the VRWtheory where radial group velocity approaches zero. Several other characteristics related to the simulated subvortices agree with VRW theories also. The most unstable azimuthal wavenumber depends on the width and the relative magnitude of vorticity of the vortex ring. Their values estimated from the simulation prior to subvortex formation correctly predict wavenumber 5 as the most unstable. The largest contribution to wave kinetic energy is diagnosed to be from the radial shear of azimuthal wind term, consistent with barotropic instability. Vorticity diagnostics show that vertical vorticity stretching is the primary vorticity source for the intensification and maintenance of the simulated subvortices. [ABSTRACT FROM AUTHOR]

The summer North American dipole (NAD) is a pattern of climate variability linked to variations in boreal forest seed production and migration of seed-eating birds. This is a modeling investigation of two teleconnections identified as drivers of the NAD in prior observational work: 1) tropically sourced atmospheric Rossby waves associated with anomalies in the phase distribution of the Madden–Julian oscillation (MJO) (i.e., phases 1 and 6 are anomalously prominent), and 2) a pan-Pacific atmospheric Rossby wave linked to East Asian monsoonal (EAM) convection. Sea surface temperature (SST) boundary forcing experiments were conducted with the Community Earth System Model 2 (CESM2) to trigger convection patterns that align with those observed during EAM and nonuniform phase distributions of MJO. For the EAM case, an El Niño–like SST dipole pattern combined with cool southern Japan SST forcing produced a convection and jet stream shift anomaly over East Asia and the northern Pacific with a positive NAD pattern downstream over North America, similar to the observed pattern when precipitation over East Asia (PEA) is relatively high. A companion experiment with only ENSO-like SST forcing also produced the NAD but featured a different structure over the Eurasian continent with a response resembling the summer east Atlantic (SEA) pattern over eastern North America and the eastern Atlantic. Simulation results suggest that the southern Japan SST forcing region has a secondary importance in triggering the NAD, producing only a somewhat NAD-like pattern by itself and only slightly improving the NAD produced by ENSO-like forcing. Simulations using SST forcing to induce seasonal convection anomalies with spatial patterns similar to anomalously frequent occurrence of MJO phase 1 (phase 6) produced circulation response patterns resembling the positive NAD (negative NAD). [ABSTRACT FROM AUTHOR]

The premonsoon circulation over South Asia in May shows remarkable interannual variations, and it can modulate the onset of the Asian summer monsoon. The present study derives two dominant stratosphere–troposphere modes over the North Atlantic in April via applying principal component (PC) analysis to regional geopotential height during 1979–2015. Both of the modes reflect the North Atlantic Oscillation (NAO)-like circulation at the midtroposphere, but the circulation patterns and planetary wave activity between the two modes are quite different. The first mode represents a stratosphere–troposphere-coupled mode. Further analysis indicates that during the years with a positive phase of the first mode, the low-level anomalous southerlies over the northern Barents Sea (BS) lead to a reduction in local sea ice, which could persist into May through the ice-albedo feedback. The BS sea ice anomalies in May could generate a southeastward propagating Rossby wave train, producing an anomalous anticyclonic circulation to the west of Qinghai–Tibet Plateau. This anomalous anticyclone induces increases in local air temperature and the meridional temperature gradient, resulting in a strengthened premonsoon circulation. Given that prediction of the premonsoon circulation over South Asia remains a challenging issue, the stratosphere–troposphere-coupling modes in April provide another potential predictability source. On the other hand, ENSO has a significant effect on the South Asian premonsoon circulations in May, and can also lead to the NAO-like circulation by influencing atmospheric Rossby waves. Such an NAO-like circulation is closely related to the second mode. [ABSTRACT FROM AUTHOR]

Using observations from the Global‐scale Observations of the Limb and Disk (GOLD) mission, we investigate post‐sunset ionospheric responses to the September 2019 Antarctic sudden stratospheric warming –first ever from a synoptic perspective. Observations reveal a prevalent quasi‐6‐day periodicity in the equatorial ionization anomaly region over South America and the Atlantic, coincident with enhanced quasi‐6‐day wave (Q6DW) activity in the mesosphere (Liu et al., 2021, https://doi.org/10.1029/2020JA028909). The atmosphere‐ionosphere coupling via large‐scale waves is rarely studied over the ocean due to the lack of observations. More importantly, further analyses suggest that multiple pathways are involved in transmitting the quasi‐6‐day periodicity from the middle atmosphere into the post‐sunset F‐region ionosphere, including modulation of F‐region field aligned winds and pre‐reversal enhancements by the tides and or Q6DW. A remarkable depletion in electron density, attributable to the overall change in thermosphere composition driven by the dissipative tides and or Q6DWs, is also seen during the period of enhanced Q6DW activity. Plain Language Summary: The equatorial ionization anomaly (EIA) owes an appreciable amount of variability to the lower/middle atmosphere wave forcing of various scales. The 2019 September Antarctic sudden stratospheric warming (SSW)—rarely occurs in the southern hemisphere—provides a great opportunity to explore the effects atmospheric waves have on the EIA, as a bunch of waves gets significantly reinforced following the polar vortex disruption. Combining the observations made by the NASA GOLD and TIMED missions reveals a coincident quasi‐6‐day modulation of the post‐sunset EIA crests and mesospheric temperatures during the course of this SSW event, demonstrating a solid case that the ionosphere couples with the atmosphere via planetary‐scale waves. Key Points: A prominent quasi‐6‐day periodicity in the post‐sunset equatorial ionization anomaly (EIA) is observed during the 2019 Antarctic sudden stratospheric warming, coincident with mesospheric quasi‐6‐day waves (Q6DWs)The EIA displays an overall depletion over South America and the Atlantic when the Q6DWs are quite activeMultiple mechanisms play roles in coupling of the Q6DW with the post‐sunset EIA [ABSTRACT FROM AUTHOR]

This study develops a new Bubble Index to quantify the intensity of 2‐D postsunset equatorial plasma bubbles (EPBs) in the American/Atlantic sector, using Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. A climatology and day‐to‐day variability analysis of EPBs is conducted based on the newly‐derived Bubble Index with the following results: (a) EPBs show considerable seasonal and solar activity dependence, with stronger (weaker) intensity around December (June) solstice and high (low) solar activity years. (b) EPBs exhibit opposite geomagnetic activity dependencies during different storm phases: EPBs are intensified concurrently with an increasing Kp, but are suppressed with high Kp occurring 3–6 hr earlier. (c) For the first time, we found that EPBs' day‐to‐day variation exhibited quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of Ionospheric Connection Explorer (ICON) winds and ionosonde data suggests that this multi‐day periodicity was related to the planetary wave modulation through the wind‐driven dynamo. Plain Language Summary: Equatorial plasma bubbles (EPBs) are plasma density depletions that frequently occur in the nighttime equatorial and low latitude ionosphere, which are well‐known for imposing significant space weather effects on navigation and communication systems. This study develops a novel Bubble Index to quantify EPBs' activity, using the Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. Based on this new Bubble Index, a climatology study and day‐to‐day variability analysis of GOLD‐captured EPBs were conducted. The EPBs' seasonal variation and solar and geomagnetic activity dependence were revealed and examined. Moreover, for the first time, we found the day‐to‐day variation of EPBs' intensity showed quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of neutral wind and ionosonde measurements suggests that this multi‐day periodicity was likely caused by planetary wave modulation via the wind‐driven dynamo. These new results shed new light on the day‐to‐day variability of the ionosphere and its possible connection with atmospheric waves. Key Points: A new Bubble Index is developed to quantify 2‐D EPBs' intensity over the American/Atlantic sector using GOLD nighttime disk image dataA first‐time statistical analysis of GOLD data shows that equatorial plasma bubbles (EPBs) have opposite geomagnetic activity dependencies during different storm phasesEPBs' intensity exhibits ∼3‐ and ∼6‐day periodicity, coinciding with planetary waves feature observed in atmospheric winds and hmF2 [ABSTRACT FROM AUTHOR]

Using the general circulation model of the middle and upper atmosphere (MUAM), a number of numerical scenarios were implemented to study the impact of individual planetary waves (PWs) on the global atmospheric circulation, including zonal wind, temperature, and residual meridional circulation (RMC). The calculations were performed for the winter conditions of the Northern Hemisphere (January–February). We show the contribution to the formation of the dynamic and temperature regimes of the MUAM made by equatorial Kelvin waves propagating to the east, as well as atmospheric normal modes (NMs) with periods from 4 to 16 d. In particular, it is demonstrated that the impact of a 5 d PW and an ultra-fast Kelvin wave (UFKW) can change the speed of circulation flows by up to 6 % in the areas of their amplitude maxima. At the same time, this effect can be significantly enhanced in certain periods of time. The presented research results are important for a deeper understanding of the mechanisms of large-scale atmospheric interactions. Despite the obviousness and simplicity of the problem, such work has not been carried out yet. [ABSTRACT FROM AUTHOR]

In 2019, the southern region of Eastern Siberia (located between 45° N and 60° N) experienced heavy floods, while the northern region (between 60° N and 75° N) saw intense forest fires that lasted for almost the entire summer, from 25 June to 12 August. To investigate the causes of these natural disasters, we analyzed the large-scale features of atmospheric circulation, specifically the Rossby wave breaking and atmospheric blocking events. In the summer of 2019, two types of Rossby wave breaking were observed: a cyclonic type, with a wave breaking over Siberia from the east (110° E–115° E), and an anticyclonic type, with a wave breaking over Siberia from the west (75° E–90° E). The sequence of the Rossby wave breaking and extreme weather events in summer, 2019 are as follows: 24–26 June (cyclonic type, extreme precipitation, flood), 28–29 June and 1–2 July (anticyclonic type, forest fires), 14–17 July (both types of breaking, forest fires), 25–28 July (cyclonic type, extreme precipitation, flood), 2 and 7 August (anticyclonic type, forest fires). Rossby wave breaking occurred three times, resulting in the formation and maintenance of atmospheric blocking over Eastern Siberia: 26 June–3 July, 12–21 July and 4–10 August. In general, the scenario of the summer events was as follows: cyclonic Rossby wave breaking over the southern part of Eastern Siberia (45° N–60° N) caused extreme precipitation (floods) and led to low gradients of potential vorticity and potential temperature in the west and east of Lake Baikal. The increased wave activity flux from the Europe–North Atlantic sector caused the anticyclonic-type Rossby wave breaking to occur west of the area of a low potential vorticity gradient and north of 60° N. This, in turn, contributed to the maintenance of blocking anticyclones in the north of Eastern Siberia, which led to the intensification and expansion of the area of forest fires. These events were preceded by an increase in the amplitude of the quasi-stationary wave structure over the North Atlantic and Europe during the first half of June. [ABSTRACT FROM AUTHOR]

The ERA5 reanalysis with hourly time steps and ∼30 km horizontal resolution resolves a substantially larger fraction of the gravity wave spectrum than its predecessors. Based on a representation of the two-sided zonal wavenumber–frequency spectrum, we show evidence of gravity wave signatures in a suite of atmospheric fields. Cross-spectrum analysis reveals (i) a substantial upward flux of geopotential for both eastward- and westward-propagating waves, (ii) an upward flux of westerly momentum in eastward-propagating waves and easterly momentum in westward-propagating waves, and (iii) anticyclonic rotation of the wind vector with time—all characteristics of vertically propagating gravity and inertio-gravity waves. Two-sided meridional wavenumber–frequency spectra, which are computed along individual meridians and then zonally averaged, exhibit characteristics similar to the spectra computed on latitude circles, indicating that these waves propagate in all directions. The three-dimensional structure of these waves is also documented in composites of the temperature field relative to grid-resolved, wave-induced downwelling events at individual reference grid points along the equator. It is shown that the waves radiate outward and upward relative to the respective reference grid points, and their amplitude decreases rapidly with time. Within the broad continuum of gravity wave phase speeds there are preferred values around ±49 and ±23 m s−1, the former associated with the first baroclinic mode in which the vertical velocity perturbations are of the same sign throughout the depth of the troposphere, and the latter with the second mode in which they are of opposing polarity in the lower and upper troposphere. [ABSTRACT FROM AUTHOR]

A high surface air temperature (SAT) record over Northwestern India was reported in April 2022. This study examines the contribution of interannual variability on Indian SAT and possible reasons for the extreme SAT during April 2022. Result shows that the interannual variability of SAT is captured by the first two leading modes using the empirical orthogonal function (EOF) analysis. Both of them show highest values over the Northwestern India and are simultaneous energetic during 2022. The EOF1 is related to the Indian Ocean Basin Mode (IOBM) and an anomalous anti-cyclone in the troposphere, while the Rossby wave train from the North Atlantic to the north of India controls the EOF2. The active IOBM and strong Rossby wave source account for the extreme SAT over northwestern India during April 2022. In addition, the Indian regional mean SAT during April is well represented by the indices of IOBM and tropospheric anomalous anti-cyclone, which can help to improve the prediction of SAT over India. [ABSTRACT FROM AUTHOR]

Worldwide tsunamis driven by atmospheric waves—or planetary meteotsunami waves—are extremely rare events. They mostly occur during supervolcano explosions or asteroid impacts capable to generate atmospheric acoustic-gravity waves including the Lamb waves that can circle the globe multiple times. Recently, such ocean waves have been globally recorded after the Hunga Tonga–Hunga Ha'apai volcano eruption on 15 January 2022, but did not pose any serious danger to the coastal communities. However, this study highlights that the mostly ignored destructive potential of planetary meteotsunami waves can be compared to the well-studied tsunami hazards. In practice, several process-oriented numerical experiments are designed to force a global ocean model with the realistic atmospheric response to the Hunga Tonga–Hunga Ha'apai event rescaled in speed and amplitude. These simulations demonstrate that the meteotsunami surges can be higher than 1 m (and up to 10 m) along more than 7% of the world coastlines. Planetary meteotsunami waves thus have the potential to cause serious coastal damages and even human casualties during volcanic explosions or asteroid impacts either releasing intense acoustic energy or producing internal atmospheric gravity waves triggering the deep-ocean Proudman resonance at a speed of ∼212 m s−1. Based on records of catastrophic events in Earth's history, both scenarios are found to be realistic, and consequently, the global meteotsunami hazards should now be properly assessed to prepare for the next big volcanic eruption or asteroid impact even occurring inland. [ABSTRACT FROM AUTHOR]

The poleward heat flux by atmospheric waves plays a pivotal role in maintaining the meridional temperature gradient. A recent study found that in the Northern Hemisphere the heat flux by transient eddies has been weakening, and the study attributed this weakening to the smaller equator‐to‐pole temperature gradient caused by Arctic warming. During the period of 1979–2019 examined here, for the annual mean, both the synoptic‐scale eddy heat flux and the temperature gradient had indeed declined. However, from October to April, the synoptic‐scale eddy flux trend is more closely tied to the planetary‐scale eddy heat flux trend, than to the temperature gradient trend. From June to August, the synoptic‐scale eddy flux decline can be attributed to a warming of the high‐latitude land areas. Therefore, a more comprehensive interpretation of the synoptic‐scale eddy heat flux trend needs to include the dynamics of the planetary‐scale waves and summer land warming. Plain Language Summary: The poleward heat transport by atmospheric circulation is an important process of maintaining the meridional temperature gradient. However, a recent study showed that the heat transport by atmospheric waves in the Northern Hemisphere has been decreasing over the past few decades. As a cause of this weakening trend, the decrease of the equator‐to‐pole temperature gradient due to Arctic warming has been suggested. In this study, we examine the monthly trends of the equator‐to‐pole temperature gradient and poleward heat transport by atmospheric waves that are decomposed into synoptic‐scale waves and large‐scale waves. We find that during the winter half‐year, the trend of heat transport by synoptic‐scale waves is more closely tied to that by large‐scale waves, than to the temperature gradient trend. From June to August, it is shown that the poleward heat transport by synoptic‐scale waves decreases after a warming of the high‐latitude land areas. Therefore, these findings underscore the importance of the planetary‐scale waves and summer land warming for understanding future changes in atmospheric heat transport. Key Points: Synoptic‐scale eddy heat flux trend is strongly linked to planetary‐scale eddy heat flux trend during boreal winter and springHigh‐latitude land warming leads to the decline of synoptic‐scale eddy heat flux during boreal summerShortwave radiation plays the major role in warming high‐latitude land areas which leads to the decline in synoptic‐scale eddy heat flux [ABSTRACT FROM AUTHOR]

The Geodynamic Laboratory in Książ includes investigations of various kinds of geodynamic signals. Among others, we registered harmonic signals of the range 10−3–10−4 Hz. These signals had been found in the measurement series of the long water-tube (WT) tiltmeters. The discovered signals consist of two classes of harmonics associated with various kinds of phenomena. The first class of these signals belongs to viscoelastic vibrations of the Earth's solid body, while the second class is produced possibly by the extremely long atmospheric infrasound waves. The signals of the vibrations of the Earth had been well recognized by the characteristic frequencies of the Earth's free vibrations' resonance, which occur mainly after strong earthquakes. The atmospheric pressure microvibrations affected the water level in the hydrodynamic systems of the WTs as a result of an inverse barometric effect. We observed that signals from both classes blend in the harmonics of similar frequencies and jointly affect the hydrodynamic systems of the WTs. We found that the amplitude of the second-class signals strongly depends on the location of water-tube gauges inside the underground, while the amplitudes of the first-class signals are similar for all the gauges. These observations clearly indicate the atmospheric origin of the second class of registered signals. [ABSTRACT FROM AUTHOR]

Negative storm surge (NSS) events frequently affect the Río de la Plata Estuary (RdP), limiting the access to major ports and waterways and emptying the freshwater intakes of one of the most socially developed basins of the Southwestern Atlantic Continental Shelf (SWACS). It is important to re-examine this issue, as the available studies are outdated, there are some contradictions in the literature and a regional framework of the adjacent shelf dynamics is not yet available. In this work (i) nonlinear statistical analyses were applied to review and update the climatological description of NSS in the RdP; and, then, (i i) an ocean numerical model was employed to describe and understand the surge dynamics framed into the larger scale dynamics of the atmospherically forced SWACS. An analysis of 87 years of hourly residual sea level observations (RSL) reveals that the number of yearly NSS events time series presents (i) a statistically significant multidecadal non-linear trend or envelope and (i i) two significant pseudo-cycles centred at 3.5 and 11.5 years that explain a large fraction of the interannual variability embedded in the multidecadal modulation. Numerical simulations results show that the surges (both negative and positive) that are generated in the SWACS and reach the RdP result from the passage of atmospheric planetary waves over Patagonia which produce winds with a significant alongshore (N-S) component. These winds force an Ekman transport either offshore or onshore at the coast, generating a negative or positive sea level anomaly, which will be larger as the atmospheric system is slower and more persistent. When synoptic atmospheric systems migrate away from the coast on their northeastward propagation, the forced sea level anomalies propagate along the coast to the north as free Kelvin waves, eventually reaching the estuary; thus the forcing for surges at the RdP can be remote. In this sense, despite the large gulfs and bays, the response of the Patagonian coast is similar to that of a linear coastline oriented in a north–south direction. In addition to remotely forced surges, locally forced events occur in the RdP as a result of the estuarine geometry and its resonant response to winds with a component longitudinal to the estuary axis (NW-SE), which could be forced by other atmospheric configurations besides Rossby waves, such as cyclogenesis of the Argentine Littoral. • The number of negative storm surges is modulated by climate variability. • The SWACS's wind response can be described as a combination of two orthogonal modes. • Atmospheric Rossby waves generate surges in the Southern SWACS by Ekman transport. • The forced surges propagate northward as Kelvin waves and eventually reach the RdP. • The strongest surge events occur in the RdP by resonance. [ABSTRACT FROM AUTHOR]

Using Perseverance (Mars 2020) and Mars Science Laboratory (MSL) measurements, we obtain planetary waves with a period of 1.5–30 sols in the 1.5 m temperature, winds (Mars 2020), surface pressure, and relative humidity. Planetary waves emerge and propagate latitudinally across all variables. Short‐period waves peak at periods of ∼2, 3, and 4.5 sols, confirming previous detections of waves, and wave amplitudes agree at Mars 2020 and MSL for all variables. The simultaneous detection of waves within Gale and Jezero craters in multiple variables indicates that planetary‐scale dynamics influences many facets of local weather throughout the year. Multiple waves are correlated or anti‐correlated between MSL and Mars 2020, suggesting waves originate in both hemispheres. Further, waves at one site do not always lead the other, suggesting a combination of baroclinic and barotropic processes as wave sources, determined from the phasing of the temperature and winds in Jezero compared to the Ensemble Mars Atmospheric Reanalysis System. Plain Language Summary: Atmospheric waves on Mars can initiate large dust storms, so understanding their properties is critical for the safety of missions. The surface pressure, temperature, relative humidity, and winds of Gale and Jezero craters are analyzed by Mars Science Laboratory (MSL) and Mars 2020 to quantify the signal of planetary waves over the first few months of the Mars 2020 mission. Mars 2020 and MSL detect waves in all variables with frequencies similar to observations from orbit and elsewhere on the surface. Comparing waves from Mars 2020, MSL, and orbit will provide needed context for these waves to enable predictions of dust storms. Key Points: Both rovers observe 1.5–30 sol waves in multiple variables during northern spring with amplitudes agreeing with a reanalysisMars Science Laboratory (MSL) and Mars 2020 observations alternatively correlate and anti‐correlate, pointing to planetary waves originating in different hemispheresNorth‐south propagating waves vacillate between first occurring at MSL or Mars 2020, suggesting waves grow from multiple types of instabilities [ABSTRACT FROM AUTHOR]

Ridging South Atlantic Anticyclones contribute an important amount of precipitation over South Africa. Here, we use a global coupled climate model and the ERA5 reanalysis to separate for the first time ridging highs (RHs) based on whether they occur together with Rossby wave breaking (RWB) or not. We show that the former type of RHs are associated with more precipitation than the latter type. The mean sea level pressure anomalies caused by the two types of RHs are characterized by distinct patterns, leading to differences in the flow of moisture‐laden air onto land. We additionally find that RWB mediates the effect of climate change on RHs during the twenty‐first century. Consequently, RHs occurring without RWB exhibit little change, while those occurring with RWB contribute more precipitation over the southern and less precipitation over the northeastern South Africa in the future. Plain Language Summary: The high pressure system located above the South Atlantic Ocean occasionally extends eastward over South Africa, leading to winds that blow onshore and carry moisture from the warm waters of the Southwest Indian Ocean to the coast. These events, termed ridging highs (RHs), bring an important contribution to precipitation over the southern and eastern parts of South Africa. Their occurrence is related to the propagation and breaking of atmospheric waves at the boundary between the troposphere and the stratosphere. This study categorizes RHs based on the behavior of atmospheric waves above and shows that events that are accompanied by wave breaking result in more precipitation over South Africa. In addition, model simulations are used to investigate the impact of climate change during the twenty‐first century on RHs and the associated precipitation. Although the model predicts that in total South Africa will experience drier conditions in the future, RHs contribute to this drying trend only in the northeastern part of the country. In the southern part of South Africa, the model simulates that RHs will bring more precipitation in the future. Key Points: Ridging South Atlantic Anticyclones are accompanied by Rossby wave breaking (RWB) aloft in 44% of the casesRidging highs that are accompanied by RWB lead to more precipitation over South Africa than those that are notRidging highs bring more precipitation over the southern and less precipitation over the northeastern part of South Africa in the future [ABSTRACT FROM AUTHOR]

Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this "remote" forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the "local" pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical "cloud plumes" through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale. Significance Statement: This study seeks to understand how circulations in the midlatitudes excite the weather systems in the tropics. Results show that the mechanisms, as well as the types of tropical weather systems excited, are strongly dependent on the mean large-scale wind structure. In particular, when the large-scale wind blows from east to west, a special type of eastward-moving tropical weather system, the Kelvin wave, is excited owing to its resonance with remote eastward-moving weather systems in the extratropics. On the contrary, when the average wind blows from west to east, midlatitude systems are observed to intrude into the lower latitudes and directly force tropical convection, the cloud plumes, while maintaining their extratropical nature. These results speak to how the midlatitudes can excite distinct types of tropical weather systems under different climatological wind regimes. Understanding these tropical weather systems and their interactions with the midlatitudes may ultimately help to improve predictions of weather beyond 2 weeks. [ABSTRACT FROM AUTHOR]

High-resolution numerical simulations of non-stationary, nonlinear acoustic–gravity waves (AGWs) propagating upwards from surface wave sources are performed for different temporal intervals relative to activation and deactivation times of the wave forcing. After activating surface wave sources, amplitudes of AGW spectral components reach a quasi-stationary state. Then the surface wave forcing is deactivated in the numerical model, and amplitudes of vertically traveling AGW modes quickly decrease at all altitudes due to discontinuations of the upward propagation of wave energy from the wave sources. However, later the standard deviation of residual and secondary wave perturbations experiences a slower quasi-exponential decrease. High-resolution simulations allowed, for the first time, for the estimation of the decay times of this wave noise produced by slow residual, quasi-standing and secondary AGW spectral components, which vary between 20 and 100 h depending on altitude and the rate of wave source activation and deactivation. The standard deviations of the wave noise are larger for the case of sharp activation and deactivation of the wave forcing compared to the steep processes. These results show that transient wave sources may create long-lived wave perturbations, which can form a background level of wave noise in the atmosphere. This should be taken into account in parameterizations of atmospheric AGW impacts. [ABSTRACT FROM AUTHOR]

The Ural blocking (UB) high is a weather system closely related to the cold air process during winter, which could trigger extreme cold events in East Asia. By retrieving five single blocking indexes, including accumulation frequency, central latitude, blocking intensity, mean duration and north rim, it is found that the UB in winter occurs more frequently, grows stronger, lasts longer and is located more northward after 2002, compared with 1985–2001. In order to describe the UB comprehensively, a new comprehensive blocking index (CBI) is developed based on the above five blocking indexes. The CBI can also reflect the interdecadal change of UB synthetically. Analysis on the corresponding atmospheric circulation shows that the relationship between the UB and atmospheric circulation, such as the polar vortex and jet, is closer in 2002–2018 than in 1985–2001. Compared with the atmospheric circulation in 1985–2001, the most prominent feature in 2002–2018 is that the intensity of the polar vortex is weaker at 100 hPa, and that the subtropical jet moves northward. Meanwhile, the East Asian trough downstream of the Urals deepens at 500 hPa and the Siberian high strengthens, indicating that the East Asia winter monsoon is stronger during 2002–2018. Further analysis on atmospheric waves and baroclinicity demonstrates that the meridional circulation of planetary waves strengthens, especially the 2-waves, which may increase the frequency of the UB and shift its location northward after 2002. Additionally, the baroclinicity ( − ∂ T / ∂ y ) in the mid-high latitudes is weakened during winter since 2002, which is also beneficial for the establishment of meridional circulation, causing a stronger intensity and longer duration of the UB. [ABSTRACT FROM AUTHOR]

The intraseasonal (IS) variability of the sea surface temperature (SST) in the Southwestern Atlantic Continental Shelf (SWACS, 45–33° S—70–50° W), and its relationship with that in the atmosphere, was studied for the austral warm season. SST satellite data (11-km resolution NOAA CoastWatch Program) and data of different atmospheric variables (Reanalysis1 NCEP/NCAR and ERA-Interim) were used. Data were filtered using a 10–90 day filter to isolate the IS variability. A Principal Component analysis was applied then to the filtered SST anomalies (SSTA) and the activity of the leading modes was described through the corresponding temporal series. The first three modes are significant. EOF1 (25.7% of variance) exhibits SSTA of opposite sign to the north/south of 42° S. EOF2 (9.0%) and EOF3 (5.1%) are related with centers of SSTA of opposite sign located off the Uruguayan coast and in the middle shelf. Composites of SSTA and of key atmospheric variables were made considering the days in which the main modes were active. They show that the SSTA described by the three modes are associated with distinctive regional sea level pressure anomalies that, in turn, seem to be related to atmospheric Rossby wave trains extending from the Australia area towards South America. The corresponding atmospheric wave sources vary depending on the mode. These results show, therefore, that the SSTA in the SWACS exhibit significant IS variability that is, in part, locally and remotely influenced by atmospheric anomalies oscillating on similar timescales. These ocean–atmosphere teleconnections could help to improve ocean predictability at those timescales in the future. [ABSTRACT FROM AUTHOR]