Effects Of Gravity Waves Research Articles

Surface Gravity Wave Effect on Hurricane Energetics

Theoretical researches have established that the energy dynamics of a mature tropical cyclone may be idealized to be very similar to a theoretical Carnot heat engine. Assuming the dissipative heating of the atmospheric boundary layer and the net production of mechanical energy in the cyclone dominate the energy budget of the storm, the potential maximum wind speed of the cyclone can be approximated as a function of the air–sea temperature difference (Ts − T0) and specific enthalpy (k0*−k) difference: |Vmax|2≈CkCDTs−T0T0(k0*−k). Although this theory gives a straighforward estimate of maximum tropical cyclone intensity, studies found that few real storms achieve this theoretical maximum estimated using climatological atmospheric conditions and sea surface temperatures. The discrepancies were attributed to a lack of knowledge of the values of the drag coefficient (CD) and surface exchange coefficient for enthalpy (Ck), and on insufficient upper ocean thermal measurements under hurricanes. Recent observational and numerical studies have unearthed another possible factor for these discrepancies by showing that the energy flux into surface gravity waves under extreme wind conditions can be an order of magnitude greater than formerly believed, and thus may play an important role in the energy budget of tropical cyclones. In this study, numerical experiments are performed to investigate the effect of surface gravity waves under a range of idealized tropical cyclone winds. The wave fields are simulated using the WAVEWATCH III model. Our results demonstrate that by considering the energy flux to surface gravity waves, the potential maximum wind speed can be reduced by up to 12% and this ratio varies with the storm size, intensity, and translation speed.

Experimental Investigation of the Effects of Intermediate Gravity Waves on the Water Evaporation Rate

Abstract A perfectly still water surface is an ideal hypothesis that is rarely encountered in real-world applications. It is impossible to find an open surface of water without any disturbance or waves, as well as an occupied closed water surface, such as a swimming pool. This experiment was done to predict the evaporation rate from the wavy water surface under the different convection regimes (free, forced, and mixed) at turbulent airflow conditions over a wide range of the ratio (Gr/Re2). The evaporation rate from the wavy water surface is strongly affected by combinations between wave steepness and main airflow velocity above the wavy water surface. Experimental results show that no pattern can be followed for which combinations of evaporation rate will increase. Thus, only two facts can be noticed: the evaporation rate is larger than that measured under the same airflow velocity conditions with no waves existing on evaporated water surface because the airflow is smooth and attached along the still water surface and when increasing the wave steepness (H/L,H/T), airflow will separate at the lee side of the wave crest near the bottom of the wave trough. Thus, the vortex will be generated in the airflow separation region. These vortexes are unstable and cause an increase in turbulence, reducing the water surface's resistance to vertical transport water vapor and increasing the evaporation rate. Also, experimental results show that the evaporation rates are somewhat less than that measured under the same airflow velocity with smaller wave steepness due to the air trapped region observed at the leeside of the wave crest near the bottom of the wave trough. Also, the result shows the evaporation rate increases with increased airflow velocity under the same convection regime. The current study implemented the particle image velocimetry (PIV) technique to analyze the airflow structure above the evaporated wavy water surface.

Effect of incoming gravity waves on the energy extraction efficiency of flapping wing hydroelectric generators

The energy extraction performance of flapping wing hydroelectric generators is subject to the effects of the free surface and incoming gravity waves. The present work addresses this issue by conducting transient numerical simulations of a two-dimensional flapping wing system. The effects of the wave wing phase difference, encounter frequency, and wing submergence depth on the energy extraction performance are investigated. The results indicate that the phase difference is the primary factor affecting the energy extraction efficiency. The incoming gravity wave has the most positive effect on the energy extraction efficiency when it's in the range of 0°–90°. In addition, the energy extraction efficiency increases with the increase in the encounter frequency. Moreover, the periodicity is best when the oscillation frequency of the wing is equal to the encounter frequency. Finally, the influence of the submerged depth on the energy extraction efficiency is also discussed.

Joint observation of the concentric gravity wave event on the Tibetan Plateau

A concentric gravity wave event was captured by a photographer in Nagarzê County (90.28°N, 28.33°E) between 02:00 and 04:00 (local time) on May 11, 2019. This concentric gravity wave event was also observed by the Suomi National Polar-orbiting Partnership satellite and the all-sky airglow imager at Yangbajing station (90.5°E, 30.1°N). The temporal and spatial information on gravity waves from the photographs provided a rare opportunity to study the propagation of gravity waves over the Tibetan Plateau. According to wind and temperature data from the MERRA-2 reanalysis (Modern-Era Retrospective analysis for Research and Applications, Version 2) and empirical models (NRLMSISE-00 [Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere] and HWM [horizontal wind model]), we inversely derived the propagation trajectory from the observed wave pattern to the source region by using the ray-tracing method. The source of the concentric gravity wave was identified as deep convection in Bangladesh (90.6°E, 25.0°N). The maximum background wind speed in the propagation direction (31.05 m/s) was less than the phase speed of 53 m/s, which is consistent with the wind-filtering theory.

Gravity Wave Focusing on the Antarctic Polar Vortex Using Gaussian Beam Approximation in Horizontally Nonuniform Flows

AbstractRay path theory is an asymptotic approximation to the wave equations. It represents efficiently gravity wave propagation in nonuniform background flows so that it is useful to develop schemes of gravity wave effects in general circulation models. One of the main limitations of ray path theory to be applied in realistic flows is in caustics where rays intersect and the ray solution has a singularity. Gaussian beam approximation is a higher-order asymptotic ray path approximation that considers neighboring rays to the central one, and thus, it is free of the singularities produced by caustics. A previous implementation of the Gaussian beam approximation assumes a horizontally uniform flow. In this work, we extend the Gaussian beam approximation to include horizontally nonuniform flows. Under these conditions, the wave packet can undergo horizontal wave refraction producing changes in the horizontal wavenumber, which affects the ray path as well as the ray tube cross-sectional area and so the wave amplitude via wave action conservation. As an evaluation of the Gaussian beam approximation in horizontally nonuniform flows, a series of proof-of-concept experiments is conducted comparing the approximation with the linear wave solution given by the WRF Model. A very good agreement in the wave field is found. An evaluation is conducted with conditions that mimic the Antarctic polar vortex and the orography of the southern flank of South America. The Gaussian beam approximation nicely reproduces the expected asymmetry of the wave field. A much stronger disturbance propagates toward higher latitudes (polar vortex) compared to lower latitudes.

Surface Gravity Wave Effects on Submesoscale Currents in the Open Ocean

AbstractA set of realistic coastal simulations in California allows for the exploration of surface gravity wave effects on currents (WEC) in an active submesoscale current regime. We use a new method that takes into account the full surface gravity wave spectrum and produces larger Stokes drift than the monochromatic peak-wave approximation. We investigate two high-wave events lasting several days—one from a remotely generated swell and another associated with local wind-generated waves—and perform a systematic comparison between solutions with and without WEC at two submesoscale-resolving horizontal grid resolutions (dx = 270 and 100 m). WEC results in the enhancement of open-ocean surface density and velocity gradients when the averaged significant wave height Hs is relatively large (>4.2 m). For smaller waves, WEC is a minor effect overall. For the remote swell (strong waves and weak winds), WEC maintains submesoscale structures and accentuates the cyclonic vorticity and horizontal convergence skewness of submesoscale fronts and filaments. The vertical enstrophy ζ2 budget in cyclonic regions (ζ/f > 2) reveals enhanced vertical shear and enstrophy production via vortex tilting and stretching. Wind-forced waves also enhance surface gradients, up to the point where they generate a small-submesoscale roll-cell pattern with high vorticity and divergence that extends vertically through the entire mixed layer. The emergence of these roll cells results in a buoyancy gradient sink near the surface that causes a modest reduction in the typically large submesoscale density gradients.

Effects of nonlinear internal gravity waves on normal-incident reflection measurements of marine sediments

Sonar data acquired by sub-bottom profilers and multi-beam echosounder systems are widely used to estimate geoacoustic properties of marine sediments. However, the uncertainty of the seabed property estimates caused by water column variability may limit the application. In this study, we focus on the acoustic focusing and defocusing effects of nonlinear internal waves on normal-incident reflection measurements in the South China Sea. The field experiment was conducted using a transceiver mooring consisted of a sound source and a four channel vertical hydrophone array. The variation of acoustic signal reflections from sub-bottom layers during three days at the same site is observed. The effects of the nonlinear internal waves passing through the transceiver mooring are identified. The impact on the sub-bottom property estimates is investigated. [Work supported by the Office of Naval Research.]

Orographically induced spontaneous imbalance within the jet causing a large-scale gravity wave event

Abstract. To better understand the impact of gravity waves (GWs) on the middle atmosphere in the current and future climate, it is essential to understand their excitation mechanisms and to quantify their basic properties. Here a new process for GW excitation by orography–jet interaction is discussed. In a case study, we identify the source of a GW observed over Greenland on 10 March 2016 during the POLSTRACC (POLar STRAtosphere in a Changing Climate) aircraft campaign. Measurements were taken with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument deployed on the High Altitude Long Range (HALO) German research aircraft. The measured infrared limb radiances are converted into a 3D observational temperature field through the use of inverse modelling and limited-angle tomography. We observe GWs along a transect through Greenland where the GW packet covers ≈ 1/3 of the Greenland mainland. GLORIA observations indicate GWs between 10 and 13 km of altitude with a horizontal wavelength of 330 km, a vertical wavelength of 2 km and a large temperature amplitude of 4.5 K. Slanted phase fronts indicate intrinsic propagation against the wind, while the ground-based propagation is with the wind. The GWs are arrested below a critical layer above the tropospheric jet. Compared to its intrinsic horizontal group velocity (25–72 m s−1) the GW packet has a slow vertical group velocity of 0.05–0.2 m s−1. This causes the GW packet to propagate long distances while spreading over a large area and remaining constrained to a narrow vertical layer. A plausible source is not only orography, but also out-of-balance winds in a jet exit region and wind shear. To identify the GW source, 3D GLORIA observations are combined with a gravity wave ray tracer, ERA5 reanalysis and high-resolution numerical experiments. In a numerical experiment with a smoothed orography, GW activity is quite weak, indicating that the GWs in the realistic orography experiment are due to orography. However, analysis shows that these GWs are not mountain waves. A favourable area for spontaneous GW emission is identified in the jet by the cross-stream ageostrophic wind, which indicates when the flow is out of geostrophic balance. Backwards ray-tracing experiments trace into the jet and regions where the Coriolis and the pressure gradient forces are out of balance. The difference between the full and a smooth-orography experiment is investigated to reveal the missing connection between orography and the out-of-balance jet. We find that this is flow over a broad area of elevated terrain which causes compression of air above Greenland. The orography modifies the wind flow over large horizontal and vertical scales, resulting in out-of-balance geostrophic components. The out-of-balance jet then excites GWs in order to bring the flow back into balance. This is the first observational evidence of GW generation by such an orography–jet mechanism.

Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere

The rapid displacement of the ocean floor during large ocean earthquakes or volcanic eruptions causes the propagation of tsunami waves on the surface of the ocean, and consequently internal gravity waves (IGWs) in the atmosphere. IGWs pierce through the troposphere and into the ionospheric layer. In addition to transferring energy to the ionosphere, they cause significant variations in ionospheric parameters, so they have considerable effects on the propagation of radio waves through this dispersive medium. In this study, double-frequency measurements of the Global Positioning System (GPS) and ionosonde data were used to determine the ionospheric disturbances and irregularities in response to the tsunami induced by the 2011 Tohoku earthquake. The critical frequency of the F2 layer (foF2) data obtained from the ionosonde data also showed clear disturbances that were consistent with the GPS observations. IGWs and tsunami waves have similar propagation properties, and IGWs were detected about 25 min faster than tsunami waves in GPS ground stations at the United States west coast, located about 7900 km away from the tsunami’s epicenter. As IGWs have a high vertical propagation velocity, and propagate obliquely into the atmosphere, IGWs can also be used for tsunami early warning. To further investigate the spatial variation in ionospheric electron density (IED), ionospheric profiles from FORMOSAT-3/COSMIC (F3/C) satellites were investigated for both reference and observation periods. During the tsunami, the reduction in IED started from 200 km and continued up to 272 km altitude. The minimum observed reduction was 2.68 × 105 el/cm3, which has happened at 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 105 el/cm3 at 355 km altitude.

Case Studies on Concentric Gravity Waves Source Using Lightning Flash Rate, Brightness Temperature and Backward Ray Tracing at São Martinho da Serra (29.44°S, 53.82°W)

AbstractWe relate the spatial and temporal distribution of lightning flash rates and cloud top brightness temperature (CTBT) to concentric atmospheric gravity wave (CGW) events observed at the Southern Space Observatory (SSO) in São Martinho da Serra (29.44°S, 53.82°W, 488.7 m) in southern Brazil. The selected identified cases from 2017 to 2018 were observed by a hydroxyl (OH) all‐sky imager. Backward ray tracing shows that the time of gravity wave excitation agrees with the highest values of lightning flash rates (indicating lightning jump) as well as the coldest brightness temperatures that indicate the time of convective overshoot. Radiosonde measurements show high convective available potential energy (CAPE), associated with a maximum updraft velocity just prior to the wave events. We find that these possible source locations correspond to the positions and times that convective plumes overshot the tropopause (seen in GOES‐16 CTBT images). We also show that higher spatial lightning density (i.e., number of lightning flashes at a given longitude and latitude) agree with the overshoot locations from the GOES satellite. We also find that the overshoot times from the GOES‐16 satellite agree with the times lightning jumps were observed in the lightning flash rate. Finally, we find that the periodicities in the lightning flash rate agree with the periods of the observed CGWs, which further strengthens the result that the CGWs were excited by the deep convective systems determined from backward ray tracing.

Infrasound Generation by Meteorological Fronts and Its Propagation in the Atmosphere

AbstractInfrasound parameters (amplitudes, coherences, grazing angles, azimuths, and horizontal phase speeds) derived during the passage of warm and cold fronts through the networks of microbarometers in the cities of Dubna and Moscow are presented. The significant differences observed in the temporal variations of the parameters of infrasound from warm and cold fronts are discussed. Such differences must be taken into account when detecting infrasound precursors of atmospheric storms. A possible mechanism for the generation of infrasound by the turbulent airstream flowing around the geometric irregularities of the meteorological front is proposed. The observed effect of internal gravity waves on the parameters of infrasound and its frequency spectrum is explained.

A Compact Rayleigh Autonomous Lidar (CORAL) for the middle atmosphere

Abstract. The Compact Rayleigh Autonomous Lidar (CORAL) is the first fully autonomous middle atmosphere lidar system to provide density and temperature profiles from 15 to approximately 90 km altitude. From October 2019 to October 2020, CORAL acquired temperature profiles on 243 out of the 365 nights (66 %) above Río Grande, southern Argentina, a cadence which is 3–8 times larger as compared to conventional human-operated lidars. The result is an unprecedented data set with measurements on 2 out of 3 nights on average and high temporal (20 min) and vertical (900 m) resolution. The first studies using CORAL data have shown, for example, the evolution of a strong atmospheric gravity wave event and its impact on the stratospheric circulation. We describe the instrument and its novel software which enables automatic and unattended observations over periods of more than a year. A frequency-doubled diode-pumped pulsed Nd:YAG laser is used as the light source, and backscattered photons are detected using three elastic channels (532 nm wavelength) and one Raman channel (608 nm wavelength). Automatic tracking of the laser beam is realized by the implementation of the conical scan (conscan) method. The CORAL software detects blue sky conditions and makes the decision to start the instrument based on local meteorological measurements, detection of stars in all-sky images, and analysis of European Center for Medium-range Weather Forecasts Integrated Forecasting System data. After the instrument is up and running, the strength of the lidar return signal is used as additional information to assess sky conditions. Safety features in the software allow for the operation of the lidar even in marginal weather, which is a prerequisite to achieving the very high observation cadence.

Variability of Gravity Wave Effects on the Zonal Mean Circulation and Migrating Terdiurnal Tide as Studied With the Middle and Upper Atmosphere Model (MUAM2019) Using a Nonlinear Gravity Wave Scheme

Implementing a nonlinear gravity wave (GW) parameterization into a mechanistic middle and upper atmosphere model, which extends to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating small-scale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GW spectrum on the climatology and tidal patterns of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a relatively small impact on the zonal mean climatology. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere climatology. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

Gravity Wave Breaking and Vortex Ring Formation Observed by PMC Turbo

AbstractPolar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9‐day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed. We identify vortex rings with diameters of 2–5 km and horizontal spacing comparable to their size. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s−1 and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s−2 and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05–0.4 W kg−1, which is consistent with other reported in situ measurements at the Arctic summer mesopause.

Application of the sampling moiré method to shallow open-channel flows with circular roughness elements

This research presents an application of the sampling moiré method, a novel technique originally developed for measuring slight deformation of a static object such as a plate, to fluctuating free water surface in rough open channel flows. The method was used to capture the two-dimensional water surface deformation at a high sampling rate by dyeing the color of water white. The target flow was a very shallow subcritical flow with a depth of 1.5 times the height of the roughness. As a result, a series of instantaneous water surface profiles with non-uniform rough planes were successfully measured at a sampling rate of 100 Hz.It has become clear that the effect of dispersive gravity waves traveling on the water surface is greater than that of non-dispersive waves driven by the turbulence, and that this feature becomes more pronounced as the Froude number increases in such shallow flow conditions. Furthermore, it was experimentally and theoretically verified that the intensity of water surface fluctuation increases with the square of the Froude number. The decomposition of surface fluctuations due to gravity waves and subsurface turbulence using the wavenumber frequency spectrum revealed that the effects of subsurface turbulence are about 20%. This is much less than the contribution from gravity waves in very shallow flows as in this experiment.

Mesospheric Gravity Wave Momentum Flux Associated With a Large Thunderstorm Complex

AbstractWe report on estimates of the vertical flux of horizontal momentum associated with an extensive and bright mesospheric gravity wave event that occurred over the El Leoncito Observatory, Argentina (31.8°S, 69.3°W), during the night of 17–18 March 2016. Using a combination of ground‐based and space‐based data sources, we propose that the event was generated by a large and distant thunderstorm complex located to the southeast of the observation site. The nightglow, including the wave structures, was easily visible to naked eye observers, a phenomenon known as a Bright Night. Such “extreme events” illuminate the important role of upward coupling in space weather research.

Response of Ozone to a Gravity Wave Process in the UTLS Region Over the Tibetan Plateau

In this paper, ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the Weather Research and Forecasting (WRF) mesoscale model are used to compare and simulate a topographic gravity wave event that occurred over the Tibetan Plateau between 18:00 on 3 May 2014 and 00:00 on 4 May 2014. The stratosphere–troposphere exchange (STE) process and its detailed characteristics during the gravity wave event are presented. The response of the upper troposphere and lower stratosphere (UTLS) ozone to the gravity wave is discovered from the perspective of the gravity wave dynamic process. The results show that the gravity wave structure between 90° E and 100° E had a westward tilt with height. From 19:00 to 21:00 the gravity waves decayed and from 22:00 to 23:00 their shape gradually blurred. Meanwhile, there was a response of the ozone mixing ratio in the UTLS over the Tibetan Plateau to the gravity waves breaking. After 21:00, this wave process caused the STE, as air from the stratosphere with a greater ozone concentration was injected into the upper troposphere, leading to rapidly increased ozone concentration there.

Characterization of gravity waves in the lower ionosphere using very low frequency observations at Comandante Ferraz Brazilian Antarctic Station

Abstract. The goal of this work is to investigate the gravity wave (GW) characteristics in the low ionosphere using very low frequency (VLF) radio signals. The spatial modulations produced by the GWs affect the conditions of the electron density at reflection height of the VLF signals, which produce fluctuations of the electrical conductivity in the D region that can be detected as variations in the amplitude and phase of VLF narrowband signals. The analysis considered the VLF signal transmitted from the US Cutler, Maine (NAA) station that was received at Comandante Ferraz Brazilian Antarctic Station (EACF, 62.1∘ S, 58.4∘ W), with its great circle path crossing the Drake Passage longitudinally. The wave periods of the GWs detected in the low ionosphere are obtained using the wavelet analysis applied to the VLF amplitude. Here the VLF technique was used as a new aspect for monitoring GW activity. It was validated comparing the wave period and duration properties of one GW event observed simultaneously with a co-located airglow all-sky imager both operating at EACF. The statistical analysis of the seasonal variation of the wave periods detected using VLF technique for 2007 showed that the GW events occurred all observed days, with the waves with a period between 5 and 10 min dominating during night hours from May to September, while during daytime hours the waves with a period between 0 and 5 min are predominant the whole year and dominate all days from November to April. These results show that VLF technique is a powerful tool to obtain the wave period and duration of GW events in the low ionosphere, with the advantage of being independent of sky conditions, and it can be used during the whole day and year-round.

The effect of internal gravity waves on cloud evolution in sub-stellar atmospheres

Context. Sub-stellar objects exhibit photometric variability, which is believed to be caused by a number of processes, such as magnetically-driven spots or inhomogeneous cloud coverage. Recent sub-stellar models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution. Aims. The aim of this paper is to investigate the effect of internal gravity waves on dust nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information. Methods. For a simplified atmosphere in two dimensions, we numerically solved the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an internal gravity wave. Furthermore, we derived an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density. Results. Numerical simulations show that the density, pressure, and temperature variations caused by gravity waves lead to an increase of the dust nucleation rate by up to a factor 20, and an increase of the dust mantle growth rate by up to a factor 1.6, compared to their equilibrium values. Through an exploration of the wider sub-stellar parameter space, we show that in absolute terms, the increase in dust nucleation due to internal gravity waves is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase, however, is greater in warm (L dwarf) and TiO2-poor atmospheres due to conditions that are less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, similar to the cloud structures observed on Earth. Conclusions. We show that internal gravity waves propagating in the atmosphere of sub-stellar objects can produce banded clouds structures similar to that observed on Earth. We propose a method with which potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects.

Impact of local gravity wave forcing in the lower stratosphere on the polar vortex stability: effect of longitudinal displacement

Abstract. The effects of gravity wave (GW) breaking hotspots in the lower stratosphere, especially the role of their longitudinal distribution, are evaluated through a sensitivity study by using a simplified middle atmosphere circulation model. For the position of the local GW hotspot, we first selected a fixed latitude range between 37.5 and 62.5∘ N and a longitude range from 112.5 to 168.75∘ E, as well as an altitude range between 18 and 30 km. This confined GW hotspot was then shifted in longitude by 45∘ steps, so that we created eight artificial GW hotspots in total. Strongly dependent on the location of the respective GW hotspot with regard to the phase of the stationary planetary wave of wavenumber 1 (SPW 1) generated in the model, the local GW forcing may interfere constructively or destructively with the modeled SPW 1. GW hotspots, which are located in North America near the Rocky Mountains, lead to an increase in the SPW 1 amplitude and EP flux, while hotspots located near the Caucasus, the Himalayas or the Scandinavian region lead to a decrease in these parameters. Thus, the polar vortex is less (Caucasus and Himalayan hotspots) or more weakened (Rocky Mountains hotspot) by the prevailing SPW activity. Because the local GW forcing generally suppresses wave propagation at midlatitudes, the SPWs 1 propagate into the polar region, where the refractive index turned to positive values for the majority of the artificial GW hotspots. An additional source of SPW 1 may be local instabilities indicated by the reversal in the meridional potential vorticity gradient in the polar region in connection with a positive EP divergence. In most cases, the SPWs 1 are breaking in the polar region and maintain the deceleration and, thus, the weakening of the polar vortex. While the SPWs 1 that form when the GW hotspots are located above North America propagate through the polar region into the middle atmosphere, the SPWs 1 in the remaining GW hotspot simulations were not able to propagate further upwards because of a negative refractive index above the positive refractive index anomaly in the polar region. GW hotspots, which are located near the Himalayas, influence the mesosphere–lower thermosphere region because of possible local instabilities in the lower mesosphere generating additional SPWs 1, which propagate upwards into the mesosphere.