Planetary-scale Waves Research Articles

Atmospheric Waves Driving Variability and Cloud Modulation on a Planetary-mass Object

Planetary-mass objects and brown dwarfs at the transition (T eff ∼ 1300 K) from relatively red L dwarfs to bluer mid-T dwarfs show enhanced spectrophotometric variability. Multiepoch observations support atmospheric planetary-scale (Kelvin or Rossby) waves as the primary source of this variability; however, large spots associated with the precipitation of silicate and metal clouds have also been theorized and suggested by Doppler imaging. We applied both wave and spotted models to fit near-infrared (NIR), multiband (Y/J/H/K) photometry of SIMP J013656.5+093347 (hereafter SIMP0136) collected at the Canada–France–Hawaii Telescope using the Wide-field InfraRed Camera. SIMP0136 is a planetary-mass object (12.7 ± 1.0 M J) at the L/T transition (T2 ± 0.5) known to exhibit light-curve evolution over multiple rotational periods. We measure the maximum peak-to-peak variability of 6.17% ± 0.46%, 6.45% ± 0.33%, 6.51% ± 0.42%, and 4.33% ± 0.38% in the Y, J, H, and K bands, respectively, and find evidence that wave models are preferred for all four NIR bands. Furthermore, we determine that the spot size necessary to reproduce the observed variations is larger than the Rossby deformation radius and Rhines scale, which is unphysical. Through the correlation between light curves produced by the waves and associated color variability, we find evidence of planetary-scale, wave-induced cloud modulation and breakup, similar to Jupiter’s atmosphere and supported by general circulation models. We also detect a 93.°8 ± 7.°4 (12.7σ) phase shift between the H − K and J − H color time series, providing evidence for complex vertical cloud structure in SIMP0136's atmosphere.

Latitude-dependent Atmospheric Waves and Long-period Modulations in Luhman 16 B from the Longest Light Curve of an Extrasolar World

In this work, we present the longest photometric monitoring of up to 1200 hr of the strongly variable brown dwarf binaries Luhman 16 AB and provide evidence of ±5% variability on a timescale of several to hundreds of hours for this object. We show that short-period rotational modulation around 5 hr (k = 1 wavenumber) and 2.5 hr (k = 2 wavenumber) dominate the variability under 10 hr, where the planetary-scale wave model composed of k = 1 and k = 2 waves provides good fits to both the periodograms and light curve. In particular, models consisting of three to four sine waves could explain the variability of the light-curve durations up to 100 hr. We show that the relative range of the k = 2 periods is narrower compared to the k = 1 periods. Using simple models of zonal banding in solar system giants, we suggest that the difference in period range arises from the difference in wind-speed distribution at low and mid-to-high latitudes in the atmosphere. Last, we show that Luhman 16 AB also exhibits long-period ±5% variability, with periods ranging from 15 hr up to 100 hr over the longest monitoring of this object. Our results for the k = 1 and k = 2 waves and long-period evolution are consistent with previous 3D atmosphere simulations, demonstrating that both latitude-dependent waves and slow-varying atmospheric features are potentially present in Luhman 16 AB atmospheres and are a significant contribution to the light-curve modulation over hundreds of rotations.

Analysis of Planetary Scale Waves Using Idealized Sudden Stratospheric Warming Simulations in Different Dynamical Cores

AbstractPlanetary waves from the troposphere are known to play an important role in sudden stratospheric warming (SSW). To evaluate the representation of large‐scale waves in idealized SSW simulations, three dynamical cores of the National Center for Atmospheric Research Community Atmosphere Model were tested in this study: the Eulerian (EUL) spectral‐transform, finite‐volume (FV), and spectral element (SE) models. Notable differences were observed among the dynamical cores, with FV being unable to generate major SSWs and simulating minor SSWs less effectively than the other models. Wave decomposition analysis revealed distinct planetary wave propagation patterns during the development of each event. Zonal Wave Number 2 wave activities primarily determined SSW types, and ZWN1 led SSWs to recovery during the ensuing periods. However, FV failed to adequately propagate large‐scale waves in minor SSW events, preventing the reversal of stratospheric zonal‐mean zonal winds. Interestingly, FV showed decreased upward Eliassen–Palm flux compared to that of other dynamical cores, even during climatology. Additional tests were performed to examine the reasons for FV's atypical results, but the dynamic impacts on planetary waves remain poorly understood.

The response of the North Pacific jet and stratosphere-to-troposphere transport of ozone over western North America to RCP8.5 climate forcing

Abstract. Stratosphere-to-troposphere transport (STT) is an important source of ozone for the troposphere, particularly over western North America. STT in this region is predominantly controlled by a combination of the variability and location of the Pacific jet stream and the amount of ozone in the lower stratosphere, two factors which are likely to change if greenhouse gas concentrations continue to increase. Here we use Whole Atmosphere Community Climate Model experiments with a tracer of stratospheric ozone (O3S) to study how end-of-the-century Representative Concentration Pathway (RCP) 8.5 sea surface temperatures (SSTs) and greenhouse gases (GHGs), in isolation and in combination, influence STT of ozone over western North America relative to a preindustrial control background state. We find that O3S increases by up to 37 % during late winter at 700 hPa over western North America in response to RCP8.5 forcing, with the increases tapering off somewhat during spring and summer. When this response to RCP8.5 greenhouse gas forcing is decomposed into the contributions made by future SSTs alone versus future GHGs alone, the latter are found to be primarily responsible for these O3S changes. Both the future SSTs alone and the future GHGs alone accelerate the Brewer–Dobson circulation, which modifies extratropical lower-stratospheric ozone mixing ratios. While the future GHGs alone promote a more zonally symmetric lower-stratospheric ozone change due to enhanced ozone production and some transport, the future SSTs alone increase lower-stratospheric ozone predominantly over the North Pacific via transport associated with a stationary planetary-scale wave. Ozone accumulates in the trough of this anomalous wave and is reduced over the wave's ridges, illustrating that the composition of the lower-stratospheric ozone reservoir in the future is dependent on the phase and position of the stationary planetary-scale wave response to future SSTs alone, in addition to the poleward mass transport provided by the accelerated Brewer–Dobson circulation. Further, the future SSTs alone account for most changes to the large-scale circulation in the troposphere and stratosphere compared to the effect of future GHGs alone. These changes include modifying the position and speed of the future North Pacific jet, lifting the tropopause, accelerating both the Brewer–Dobson circulation's shallow and deep branches, and enhancing two-way isentropic mixing in the stratosphere.

Planetary-scale MLT waves diagnosed through multi-station methods: a review

Most experimental investigations on planetary-scale waves in the mesosphere and lower thermosphere (MLT) region are based on single-station or -satellite spectral analysis methods, which suffer from intrinsic spectral aliasing/ambiguity. To overcome the aliasing, the author has developed and utilized dual- and multi-station spectral methods in a series of recent works. These methods were implemented on meteor radar observations and surface magnetometer observations. In the implements, a variety of waves were discovered or investigated in terms of seasonal variations and responses to sudden stratospheric warming events, such as lunar and solar tides (migrating and non-migrating), Rossby wave normal modes, ultra-fast Kelvin waves, and secondary waves of wave–wave nonlinear interactions between the previous waves. The current paper illustrates these methods using synthetic data, comparatively reviews the methods and results in plain language, and proposes a new representation, termed the adjusted Feynman diagram, to summarize the nonlinear interactions and explain their implications.Graphical

Traveling Planetary-scale Waves Cause Cloud Variability on Tidally Locked Aquaplanets

Cloud cover at the planetary limb of water-rich Earth-like planets is likely to weaken chemical signatures in transmission spectra, impeding attempts to characterize these atmospheres. However, based on observations of Earth and Solar System worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. We identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud–radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber 1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres, and this results in advection of clouds to or away from the planet’s eastern terminator. We study this oscillation in simulations of Proxima Centauri b, TRAPPIST-1e, and rapidly rotating versions of these worlds located at the inner edge of their stars’ habitable zones. We simulate time series of the transit depths of the 1.4 μm water feature and 2.7 μm carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of our simulations have variability significant enough to be detectable with current methods.

Asymptotic Solutions for Equatorial Waves on Venus

The atmosphere of Venus exhibits equatorial planetary-scale waves that are suspected to play an important role in its complex atmospheric circulation. Due to its particularly long sidereal day (243 terrestrial days against 24 h for the Earth), the Venusian waves must be described with the momentum equations for a cyclostrophic regime, but efforts to derive analytical wave solutions have been scarce. Following a classic approach for the terrestrial quasi-geostrophic regime, I present analytical solutions for equatorial waves in the atmosphere of Venus, assuming a single layer of a homogeneous incompressible fluid with a free surface and focusing on two asymptotic cases described by the ratio of their non-dimensional frequency and zonal wavenumber. One of the dispersion relations that has been obtained describes waves on a small spatial scale propagating upstream relative to the zonal flow, which is associated with a Rossby-type wave called “centrifugal”. The solutions for the other asymptotic case were interpreted as inertio-surface waves, which describe planetary-scale waves that can propagate “upstream” and “downstream” relative to the zonal winds and have null group velocity. These new wave solutions stress relevant differences between waves in geostrophic and cyclostrophic regimes and may be applicable to Saturn’s moon, Titan, and Venus-like exoplanets.

Compensation between Resolved Wave Forcing and Parameterized Orographic Gravity Wave Drag in the Northern Hemisphere Winter Stratosphere Revealed in NCEP CFS Reanalysis Data

Abstract Compensation between the resolved wave (RW) forcing and the parameterized orographic gravity wave drag (OGWD) accompanying barotropic/baroclinic (BT/BC) instability in the realistic atmosphere is investigated using Climate Forecast System Reanalysis data in the Northern Hemisphere winter stratosphere. When sufficiently narrow and/or strong negative OGWD drives instability, RWs are generated in situ, providing positive Eliassen–Palm flux divergence that compensates for the parameterized OGWD enhancement; this is consistent with the findings of previous studies based on the idealized general circulation models. However, dependence of the compensation rate on RW forcing differs from the nearly complete compensation in the previous studies, implying that an additional mechanism operates for the compensation: the refractive-index modification by BT/BC instability. The negative meridional gradient of the quasigeostrophic potential vorticity leads to the negative refractive index squared for RWs with phase speeds less than the zonal-mean zonal wind. This prevents RWs from entering the destabilized areas, resulting in the divergence of Eliassen–Palm fluxes that cancels out the parameterized OGWD perturbation. Although both mechanisms act simultaneously, the refractive-index modification plays an important role in the compensation processes in the stratosphere where RWs are dominated by the planetary-scale waves.

Stratospheric Modulation of Tropical Upper-Tropospheric Warming-Induced Circulation Changes in an Idealized General Circulation Model

Abstract The stratospheric polar vortex is dynamically coupled to the tropospheric circulation. Therefore, a better mechanistic understanding of this coupled system is important to interpret past and future circulation changes correctly. Previously, idealized simulations with a dry dynamical-core general circulation model and imposed tropical upper-tropospheric warming (TUTW) have shown that a critical warming level exists at which the polar vortex transitions from a weak and variable to a strong and stable regime. Here, we investigate the dynamical mechanism responsible for this regime transition and its influence on the troposphere by performing similar idealized experiments with (REF) and without a polar vortex (NPV). According to the critical-layer control mechanism, the strengthened upper flank of the subtropical jet in response to TUTW leads to an accelerated wave-driven residual circulation in both experiments. For the REF experiment, the stronger residual circulation is associated with changes in the lower-stratospheric thermal structure that are consistent with an equatorward shift of the polar vortex. At a certain threshold of TUTW in the REF experiment, the tropospheric jet and the stratospheric polar vortex form a confined waveguide for planetary-scale waves that presumably favors downward wave coupling events. Consistently, the polar vortex strengthens in combination with an enhanced poleward shift of the tropospheric jet compared to the NPV experiment. Overall, these idealized experiments suggest that a polar vortex strengthening can be caused by greenhouse gas–induced warmings via modifications of the waveguide. This mechanism might also be relevant to understand the polar vortex changes in more complex models.

Short-period planetary-scale waves in a Venus general circulation model: Rotational and divergent component structures and energy conversions

To elucidate the generation and coupling of the short-period waves in the Venus atmosphere, we investigated the three-dimensional wave structure in an AORI (Atmosphere and Ocean Research Institute, University of Tokyo, Japan) Venus general circulation model. The most predominant waves are 7.5-day waves with zonal wavenumber 1 and are slower than observed planetary-scale 5.5-day waves around the cloud bottom (∼50 km), associated with a slower zonal flow in the model. The 7.5-day waves comprise three types: Type I, a Rossby wave in the upper cloud layer; Type II, a Rossby wave around the polar tropopause (∼50 km); and Type III, an equatorial Kelvin-like wave around and below the cloud bottom.Around the cloud top (65–70 km), the Rossby wave (Type I) has a rotational eddy structure and is produced by potential-energy conversion from zonal mean to eddy, suggesting baroclinic instability in the cloud layer. The geopotential of the Rossby wave in the upper cloud layer vertically connects to that of the equatorial Kelvin-like wave (Type III) across the critical line at ∼55 km and 30° latitudes. At the cloud bottom (∼50 km), the Kelvin-like wave and Rossby wave (Type II) are separated at a critical latitude around the cloud bottom, but are horizontally jointed with the stream function and velocity potential at the critical latitude. These two waves are major equatorward momentum transporters and are generated by horizontal shear (or barotropic) and baroclinic instabilities associated with energy conversion at the critical line. For high-latitude Rossby waves around the polar tropopause (Type II), the positive kinetic-energy conversion by vertical momentum transport is predominant, tending to relax the vertical shear of the zonal flow, leading in turn to relaxation of the horizontal temperature gradient under the cyclostrophic thermal wind. In contrast, the negative potential-energy conversion tends to produce the horizontal temperature gradient in the lower atmosphere where the solar insolation is very weak.The equatorial jet core rotating with a 7.5-day period is composed of both divergent and rotational wind components of the equatorial Kelvin-like wave at the cloud bottom. Above the critical level of the Kelvin-like wave, the equatorial Rossby wave (downward and equatorward flank of the Type I wave) appears at around 54 km and is produced by the barotropic energy conversion.

Global shockwaves of the Hunga Tonga-Hunga Ha’apai volcano eruption measured at ground stations

The eruption of the Tonga volcano created globally propagating spherical shockwaves in the atmosphere. Analyses are done to data from two southern U.S. stations of the author sampling at 3-21s intervals and 189 weather stations at 1-5min intervals. The shockwaves arrived from two routes in the atmosphere: the shortest spherical arc and the longer spherical arc through the antipole. In most stations, signals up to the 6th path of shockwaves were recorded as the waves traveled around the globe multiple times. The speed of shockwaves is estimated to be 309.5± 2.9m/s, consistent with the speed of sound at the top of the troposphere where a waveguide exists. Discussion is made on the post-shockwave ringing of 4-8min as higher amplitude oscillations above the level of pre-shockwaves background noise. A theoretical wave dispersion is derived which verifies that the spherical shockwave's phase speed is the same as the speed of sound.

Meridional-energy-transport extremes and the general circulation of Northern Hemisphere mid-latitudes: dominant weather regimes and preferred zonal wavenumbers

Abstract. The extratropical meridional energy transport in the atmosphere is fundamentally intermittent in nature, having extremes large enough to affect the net seasonal transport. Here, we investigate how these extreme transports are associated with the dynamics of the atmosphere at multiple spatial scales, from planetary to synoptic. We use the ERA5 reanalysis data to perform a wavenumber decomposition of meridional energy transport in the Northern Hemisphere mid-latitudes during winter and summer. We then relate extreme transport events to atmospheric circulation anomalies and dominant weather regimes, identified by clustering 500 hPa geopotential height fields. In general, planetary-scale waves determine the strength and meridional position of the synoptic-scale baroclinic activity with their phase and amplitude, but important differences emerge between seasons. During winter, large wavenumbers (k = 2–3) are key drivers of the meridional-energy-transport extremes, and planetary- and synoptic-scale transport extremes virtually never co-occur. In summer, extremes are associated with higher wavenumbers (k = 4–6), identified as synoptic-scale motions. We link these waves and the transport extremes to recent results on exceptionally strong and persistent co-occurring summertime heat waves across the Northern Hemisphere mid-latitudes. We show that the weather regime structures associated with these heat wave events are typical for extremely large poleward-energy-transport events.

Southern Hemisphere Stratospheric Warmings and Coupling to the Mesosphere‐Lower Thermosphere

AbstractTwenty six years of medium frequency (MF) radar wind measurements made from 1994 to 2019 at Davis Station (68.6°S, 77.9°E) are used to study the mean response of the mesosphere‐lower thermosphere to stratospheric warmings in the Southern Hemisphere. Warming events were detected using Modern‐Era Retrospective Analysis for Research and Applications (MERRA2) data with a systematic search for reductions in the zonal‐mean circulation at 60°S and corresponding increases in polar temperatures. Some 37 events were identified, including the 2002 major warming and the large event of 2019, with an average of 1–2 warmings per year. At the 10 hPa level, the polar cap temperature increases ranged from 3 to 28 K, with a mean value of 12 K, while the zonal wind speed reductions varied between −6 and −43 ms−1, with a mean value of −15 ms−1. Peak values occurred near 40 km. Warmings occurred mainly between August and October, with a small peak in occurrence in April/May. The MF radar data showed an average reduction in the mesospheric eastward winds of about 5–7 ms−1 at heights near 75 km that occurred 3–4 days prior to the changes in the stratosphere. Warming events were driven by episodic intensifications in planetary wave amplitudes, with quasi‐stationary planetary scale waves (PW) 1 being especially important. PW Eliassen‐Palm flux divergences show a systematic behavior with time and height that is consistent with a poleward residual circulation and downwelling over the pole prior to the warming events and an equatorward flow and upwelling after the peak of the events.

Moisture Modes of Tropical Intraseasonal Oscillations—High Order and Anti‐Symmetric Solutions

AbstractThe Madden Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation are fundamental climate modes in the tropical atmosphere on the intraseasonal time scales. A recent study developed a linear moisture mode theory for unified treatment of these intraseasonal oscillations under different meridional moisture gradients. Using scaled Hermite polynomials as the basis for the meridional structure, it is shown that the system has infinite number of normal modes under the same parameter values, as n → ∞. The v = 0 and n = 1 analyzed in the previous study are two special cases. These moisture modes are non‐orthogonal. The idealized MJO solutions derived using the parameters in the boreal winter conditions bear similar meridional structure for planetary scale waves in the Earth‐like atmosphere, unlike the higher order solutions (which are often considered unrealistic) in the classical tropical waves theory. The solutions include both symmetric and anti‐symmetric modes. The symmetric modes are analogous to the Kelvin‐Rossby wave paradigm of the MJO. The anti‐symmetric modes features cross‐equatorial flow near the equator and subtropical gyres. Both growth rates and frequencies of the scaled higher order n modes display small increments compared to lower order modes. In the asymptotic limit n → ∞, a growth rate higher than other modes with finite n is achieved.

Joint evolution of equatorial oscillation and interhemispheric circulation in Saturn’s stratosphere

Planetary stratospheres are characterized by a subtle interplay between dynamics, radiation and chemistry. Observations of Saturn’s stratosphere have revealed a semi-annual equatorial oscillation of temperature and hinted at an interhemispheric circulation of hydrocarbon species. Both the forcing mechanisms of the former and the existence of the latter have remained debated. Here we use a new troposphere-to-stratosphere Saturn global climate model to argue that those two open questions are intimately connected. Our Saturn climate model reproduces a stratospheric oscillation exhibiting the observed semi-annual period, amplitude and downward propagation. In the same Saturn simulation, a prominent stratospheric summer-to-winter hemispheric circulation develops at the solstices, controlled by both the seasonal radiative gradients and Rossby-wave pumping in the winter-subsiding branch, analogous to Earth’s Brewer–Dobson circulation. Furthermore, we show that Saturn’s equatorial oscillation is driven by the seasonal variability of both the resolved planetary-scale wave activity and the interhemispheric circulation, akin to Earth’s Semi-Annual oscillation. A high-resolution three-dimensional global climate model of Saturn captures the small-scale dynamics of its stratosphere. It is able to reproduce the observed semi-annual equatorial oscillation and finds evidence of an interhemispheric meridional circulation that can explain the periodicity of its equatorial oscillation and the seasonal behaviour of hydrocarbon abundances.

Control of the oscillations of the martian Northern Hemisphere polar vortex by the Hadley cell and topographic forcing

The oscillations of the Martian Northern Hemisphere polar vortex are examined by using the Kida vortex model to interpret the dynamics of the vortex in reanalysis data. The vortex nutating around a fixed angle with little variability in its aspect ratio is shown to be controlled by the Hadley cell and wavenumber-2 planetary-scale waves that are characterised with a stationary component that is shown to be linked directly to Martian topography. The wave forcing and the Hadley cell structure are modelled as a strain and a rotational flow in Kida’s model. It is shown that the strain and rotational flow are strongly negatively correlated, acting against vortex elongation in Kida’s model. The result suggests that periods of high topographic forcing events are accompanied with a weakening in the Hadley cell winds and thus provides an explanation why vortex splits are rare on Mars.

Under the surface: Pressure-induced planetary-scale waves, volcanic lightning, and gaseous clouds caused by the submarine eruption of Hunga Tonga-Hunga Ha'apai volcano

We present a narrative of the eruptive events culminating in the cataclysmic January 15, 2022 eruption of Hunga Tonga-Hunga Ha'apai Volcano by synthesizing diverse preliminary seismic, volcanological, sound wave, and lightning data available within the first few weeks after the eruption occurred. The first hour of eruptive activity produced fast-propagating tsunami waves, long-period seismic waves, loud audible sound waves, infrasonic waves, exceptionally intense volcanic lightning and an unsteady volcanic plume that transiently reached—at 58 ​km—the Earth's mesosphere. Energetic seismic signals were recorded worldwide and the globally stacked seismogram showed episodic seismic events within the most intense periods of phreatoplinian activity, and they correlated well with the infrasound pressure waveform recorded in Fiji. Gravity wave signals were strong enough to be observed over the entire planet in just the first few hours, with some circling the Earth multiple times subsequently. These large-amplitude, long-wavelength atmospheric disturbances come from the Earth's atmosphere being forced by the magmatic mixture of tephra, melt and gasses emitted by the unsteady but quasi-continuous eruption from 0402±1–1800 UTC on January 15, 2022. Atmospheric forcing lasted much longer than rupturing from large earthquakes recorded on modern instruments, producing a type of shock wave that originated from the interaction between compressed air and ambient (wavy) sea surface. This scenario differs from conventional ideas of earthquake slip, landslides, or caldera collapse-generated tsunami waves because of the enormous (∼1000x) volumetric change due to the supercritical nature of volatiles associated with the hot, volatile-rich phreatoplinian plume. The time series of plume altitude can be translated to volumetric discharge and mass flow rate. For an eruption duration of ∼12 ​h, the eruptive volume and mass are estimated at 1.9 ​km3 and ∼2 900 ​Tg, respectively, corresponding to a VEI of 5–6 for this event. The high frequency and intensity of lightning was enhanced by the production of fine ash due to magma—seawater interaction with concomitant high charge per unit mass and the high pre-eruptive concentration of dissolved volatiles. Analysis of lightning flash frequencies provides a rapid metric for plume activity and eruption magnitude. Many aspects of this eruption await further investigation by multidisciplinary teams. It represents a unique opportunity for fundamental research regarding the complex, non-linear behavior of high energetic volcanic eruptions and attendant phenomena, with critical implications for hazard mitigation, volcano forecasting, and first-response efforts in future disasters.

Intensified Impact of Winter Arctic Oscillation on Simultaneous Precipitation Over the Mid–High Latitudes of Asia Since the Early 2000s

This study reveals an intensified impact of winter (November–February mean) Arctic Oscillation (AO) on simultaneous precipitation over the mid–high latitudes of Asia (MHA) since the early 2000s. The unstable relationship may be related to the changes in the tropospheric AO mode and the subtropical jet. Further analyses suggest that their changes may be attributable to the interdecadal changes in the stratospheric polar vortex. During 2002–2017, the anomalously weak stratospheric polar vortex is accompanied by intensified upward-propagating tropospheric planetary-scale waves anomalies. Subsequently, the stratospheric geopotential height anomalies over the North Atlantic high-latitudes propagate downward strongly, causing the changes in the tropospheric AO mode, that is, the positive height anomalies over the North Atlantic high-latitudes are stronger and extend southward, corresponding to the stronger and eastward extension of negative height anomalies over the North Atlantic mid-latitudes. Thus, the Rossby wave source anomalies over Baffin Bay and the Black Sea are strong, and correspondingly so too are their subsequently excited the Rossby waves anomalies. Meanwhile, the planetary-scale waves anomalies propagate weakly along the low-latitude waveguide, causing the intensified and southward shift of the subtropical jet. Therefore, the strong Rossby waves anomalies propagate eastward to the MHA. By contrast, during 1979–1999, the strong stratospheric polar vortex anomaly is accompanied by weak upward-propagating planetary-scale waves anomalies, resulting in weaker height anomalies over the North Atlantic mid–high latitudes. Consequently, the anomalous Rossby waves are weak. In addition, the subtropical jet weakens and shifts northward, which causes the Rossby waves anomalies to dominate over the North Atlantic, and thereby the impact of winter AO on simultaneous precipitation over the MHA is weak.

The Role of Planetary-Scale Eddies on the Recent Isentropic Slope Trend during Boreal Winter

AbstractAccording to baroclinic adjustment theory, the isentropic slope maintains its marginal state for baroclinic instability. However, the recent trend of Arctic warming raises the possibility that there could have been a systematic change in the extratropical isentropic slope. In this study, global reanalysis data are used to investigate this possibility. The result shows that tropospheric isentropes north of 50°N have been flattening significantly during winter for the recent 25 years. This trend pattern fluctuates at intraseasonal time scales. An examination of the temporal evolution indicates that it is the planetary-scale (zonal wavenumbers-1–3) eddy heat fluxes, not the synoptic-scale eddy heat fluxes, that flatten the isentropes; synoptic-scale eddy heat fluxes instead respond to the subsequent changes in isentropic slope. This extratropical planetary-scale wave growth is preceded by an enhanced zonal asymmetry of tropical heating and poleward wave activity vectors. A numerical model is used to test if the observed latent heating can generate the observed isentropic slope anomalies. The result shows that the tropical heating indeed contributes to the isentropic slope trend. The agreement between the model solution and the observation improves substantially if extratropical latent heating is also included in the forcing. The model temperature response shows a pattern resembling the warm-Arctic–cold-continent pattern. From these results, it is concluded that the recent flattening trend of isentropic slope north of 50°N is mostly caused by planetary-scale eddy activities generated from latent heating, and that this change is accompanied by a warm-Arctic–cold-continent pattern that permeates the entire troposphere.

Climatology of Three-Dimensional Eliassen–Palm Wave Activity Fluxes in the Northern Hemisphere Stratosphere from 1981 to 2020

Based on the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis data from 1981 to 2020, the climatological features of the vertical components of three-dimensional Eliassen–Palm (EP) wave activity fluxes (WAF) were investigated. The parameter is related to eddy heat flux and is a key indicator of the upward and downward propagation of quasi-stationary planetary-scale waves. Northern Hemisphere data from a 30 km height (or about a 10-hPa level) were used for the analysis. We evaluated the extreme values (daily maxima and minima) of the vertical WAFs, the probability of their recurrences, and their interannual and daily variability observed over the last four decades. The correlation between the upward EP WAF maxima and the 10-hPa stratosphere temperature anomalies were examined. The results show that very close relationships exist between these two parameters with a short time lag, but the initial state of the stratosphere is a key factor in determining the strength of these relationships. Moreover, trends over the last 40 years were evaluated. In this research, we did not find any significant changes in the extreme values of the vertical WAFs. Finally, the dominant spatial patterns of upward and downward extreme WAFs were evaluated. The results show that there are three main regions in the stratosphere where extremely intensive upward and downward WAFs can be observed.