Zonal Wave Research Articles

ИССЛЕДОВАНИЕ ИЗМЕНЕНИЙ ДИНАМИКИ СТРАТОСФЕРЫ СЕВЕРНОГО ПОЛУШАРИЯ В XXI ВЕКЕ ПО РАСЧЕТАМ КЛИМАТИЧЕСКОЙ МОДЕЛИ ИВМ РАН

Simulations of 5th version of INM RAS (Institute of Numerical Mathematics of the Russian Academy of Science) climate model performed in the framework of CMIP6 project for the future climate under ssp2–4.5 (moderate) and ssp5–8.5 (business as usual or hard) scenarios of green house gases (GHG) increase are employed to analyze temperature, zonal mean wind, stratospheric polar vortex, planetary wave activity, meridional circulation, sudden stratospheric warming (SSW) events, and stratospheric circulation spring break-up date changes during boreal winters from 2015 to 2100. Comparison of averages over two periods of 2080–2100 and 2015–2035 revealed that temperature will decrease from 1° in the lower stratosphere to 4° in the upper stratosphere under moderate scenario and up to 11° under hard scenario. Cooling of stratosphere will be accompanied by strengthening of zonal circulation and planetary wave activity propagation in the middle – upper stratosphere that in turn leads to increase (stronger under hard scenario) of planetary wave with zonal wave number 1 amplitude (wavenumber 1). 13 major sudden stratospheric warming events and 16 very cold stratospheric winter seasons were revealed under hard scenario. Under both scenarios early spring break-up dates will be accompanied by stronger wavenumber 1 in comparison with winter seasons with later spring break-up dates. Strengthening of zonal mean meridional circulation is expected in the late XXI century

Transient energy growth in the ageostrophic Eady model

The problem of optimal initial disturbances in thermal wind shear is revisited and extended to include non-hydrostatic effects. This systematic study compares transient and modal growth rates of submesoscale instabilities over a large range of zonal and meridional wave numbers, aspect ratios, and different Richardson number regimes. Selection criteria were derived to remove spurious and unresolved instability modes that arise from the eigenvalue problem and we generalize the study of the hydrostatic Eady problem by Heifetz and Farrell (2003, 2007, 2008) to thin fronts, characterized by large aspect ratios. Such fronts are commonly found at the early stages of frontogenesis, for example, in the ocean mesoscale eddies and near the eye wall of hurricanes. In particular, we show that transient energy growth rates are up to two orders of magnitude larger than modal counterparts for a wide range of Richardson number and that the effects of transient energy gain become even greater when non-hydrostatic effects become important and/or for large Richardson numbers. This study also compares the dominant energy pathways contributing to the energy growth at short and long times. For symmetric modes, we recover the inertia-gravity instability described in Xu et al. (2007). These mechanisms are shown to be the most powerful mediator of vertical transport when compared with the fastest growing baroclinic and symmetric modes. These results highlight the importance of transient processes in the ocean and the atmosphere.

Understanding of European Cold Extremes, Sudden Stratospheric Warming, and Siberian Snow Accumulation in the Winter of 2017/18

AbstractIt is unclear whether the Eurasian snow plays a role in the tropospheric driving of sudden stratospheric warming (SSW). The major SSW event of February 2018 is analyzed using reanalysis datasets. Characterized by predominant planetary waves of zonal wave 2, the SSW developed into a vortex split via wave–mean flow interaction. In the following two weeks, the downward migration of zonal-mean zonal wind anomalies was accompanied by a significant transition to the negative phase of the North Atlantic Oscillation, leading to extensive cold extremes across Europe. Here, we demonstrate that anomalous Siberian snow accumulation could have played an important role in the 2018 SSW occurrence. In the 2017/18 winter, snow depths over Siberia were much higher than normal. A lead–lag correlation analysis shows that the positive fluctuating snow depth anomalies, leading to intensified “cold domes” over eastern Siberia (i.e., in a region where the climatological upward planetary waves maximize), precede enhanced wave-2 pulses of meridional heat fluxes (100 hPa) by 7–8 days. The snow–SSW linkage over 2003–19 is further investigated, and some common traits among three split events are found. These include a time lag of about one week between the maximum anomalies of snow depth and wave-2 pulses (100 hPa), high sea level pressure favored by anomalous snowpack, and a ridge anchoring over Siberia as precursor of the splits. The role of tropospheric ridges over Alaska and the Urals in the wave-2 enhancement and the role of Arctic sea ice loss in Siberian snow accumulation are also discussed.

STATISTICALLY SIGNIFICANT ESTIMATES OF INFLUENCE OF SOLAR ACTIVITY ON PLANETARY WAVES IN THE MIDDLE ATMOSPHERE OF THE NORTHERN HEMISPHERE AS DERIVED FROM MUAM MODEL DATA

Numerical simulation has been used to examine the effect of changes in solar activity (SA) in the thermosphere on amplitudes of long-period planetary waves (PW) for the winter period in the Northern Hemisphere. The model of the middle and upper atmosphere (MUAM) is used. It allows simulations of general atmospheric circulation at altitudes 0–300 km. In order to reproduce SA changes, different values of the solar radio flux at a wavelength of 10.7 cm at an altitude of more than 100 km are set in the MUAM radiation block. To take into account the effect of charged particles in the ionosphere on the neutral gas dynamics, ionospheric conductivities for different SA levels are included in MUAM. To improve the statistical reliability of the results, two ensembles of model simulations consisting of 16 runs corresponding to the minimum and maximum SA have been obtained. The statistical confidence of average differences in PW amplitudes between high and low SA has been calculated. The results are shown to be reliable in almost the entire altitude range 0–300 km. Results of the simulations have shown for the first time that statistically significant differences in amplitudes of long-period PWs can reach 10–15 % in the middle atmosphere of the Northern Hemisphere, depending on the zonal wave number. At the same time, reflection of PWs at altitudes of lower thermosphere has a significant effect on the PW structure in the middle atmosphere.

СТАТИСТИЧЕСКИ ЗНАЧИМЫЕ ОЦЕНКИ ВЛИЯНИЯ СОЛНЕЧНОЙ АКТИВНОСТИ НА ПЛАНЕТАРНЫЕ ВОЛНЫ В СРЕДНЕЙ АТМОСФЕРЕ СЕВЕРНОГО ПОЛУШАРИЯ ПО ДАННЫМ МОДЕЛИ МСВА

Numerical simulation has been used to examine the effect of changes in solar activity (SA) in the thermosphere on amplitudes of long-period planetary waves (PW) for the winter period in the Northern Hemisphere. The model of the middle and upper atmosphere (MUAM) is used. It allows simulations of general atmospheric circulation at altitudes 0–300 km. In order to reproduce SA changes, different values of the solar radio flux at a wavelength of 10.7 cm at an altitude of more than 100 km are set in the MUAM radiation block. To take into account the effect of charged particles in the ionosphere on the neutral gas dynamics, ionospheric conductivities for different SA levels are included in MUAM. To improve the statistical reliability of the results, two ensembles of model simulations consisting of 16 runs corresponding to the minimum and maximum SA have been obtained. The statistical confidence of average differences in PW amplitudes between high and low SA has been calculated. The results are shown to be reliable in almost the entire altitude range 0–300 km. Results of the simulations have shown for the first time that statistically significant differences in amplitudes of long-period PWs can reach 10–15 % in the middle atmosphere of the Northern Hemisphere, depending on the zonal wave number. At the same time, reflection of PWs at altitudes of lower thermosphere has a significant effect on the PW structure in the middle atmosphere.

Characteristics of internal variability on summer rainfall in Northeast Asia in a changing climate

Using a 35-ensemble member simulated with the community earth system model-large ensemble (CESM-LE) experiment, we examined the characteristics of internal variability (IV) on the Northeast Asian summer rainfall anomaly (NASRA) in a changing climate. The NASRA, which is defined as the rainfall anomaly averaged across Northeast Asia (30°–40° N, 115°–145° E), consists of the tropics-related rainfall anomaly and the extratropics-related rainfall anomaly. In this study, the IV was estimated by the spread of rainfall simulated in each ensemble member from their ensemble mean. The IV associated with the tropics-related rainfall variability did not change significantly from the present climate to a future climate. In contrast, the IV associated with the NASRA gradually increased from the mid-twenty first century and this was mostly due to an increase of IV associated with the extratropics-related rainfall variability. This implies that the predictability of NASRA may decrease in a future changing climate because of the extratropics-related rainfall variability associated with zonal atmospheric wave train patterns including Eurasian-like pattern, Silk Road pattern or Circumglobal Teleconnection pattern. We also discussed the characteristic change of IV associated with the tropics-related and the extratropics-related rainfall variability in a future warmer climate.

An Updated Analysis of Northern Hemisphere Submonthly Retrograde Waves

Abstract This study performs an updated analysis of Northern Hemisphere retrograde disturbances that were first identified by classical observational studies as one of the dominating coherent structures in the higher latitudes on the submonthly time scale. Analyzing 8–30-day bandpass-filtered data based on reanalysis, a set of criteria on the phase and amplitude of zonal wave-1 Fourier coefficients of geopotential height anomalies at 250 mb (1 mb = 1 hPa) and 60°N are used to identify strong retrograde-wave events in the spirit of Madden and Speth. The new catalog of retrograde-wave events from 1979 to 2017 is used to extract basic statistics and structures of retrograde waves across all major events. The results broadly agree with those reported in the classical observational studies, reaffirming the robustness of the phenomenon. The new catalog can be used to aid further studies on the mechanisms and predictability of retrograde waves. As an example, an analysis of isentropic potential vorticity over the Pacific sector for selected retrograde-wave events reveals the common occurrence of an extrusion of low-PV air into the higher latitudes, followed by a westward shift of the low-PV patch and vortex shedding. Future directions of research surrounding the retrograde-wave phenomenon are discussed.

Subauroral and Polar Traveling Ionospheric Disturbances During the 7–9 September 2017 Storms

AbstractThis study provides new scenarios for storm time traveling ionospheric disturbance excitation and subsequent propagation at subauroral and polar latitudes. We used ground‐based total electron content observations from Global Navigation Satellite System receivers combined with wide field, subauroral ionospheric plasma parameters measured with the Millstone Hill Incoherent Scatter Radar during strong September 2017 geospace storms. Observations provide the first evidence of significant influences on traveling ionospheric disturbance (TID) propagation and excitation caused by the presence of large subauroral polarization stream flow channels. Simultaneous large‐ and medium‐scale TIDs evolved during the event in a broad subauroral and midlatitude area near dusk. Similar concurrent TIDs occurred near dawn sectors as well during a period of sustained southward Bz. Medium‐scale TIDs at subauroral and midlatitudes had wave fronts aligned northwest‐southeast near dusk, and northeast‐southwest near dawn. These wave fronts were highly correlated with the direction of storm time large zonal plasma drift enhancements at these latitudes. At high latitudes, unexpected, predominant, and persistent storm time TIDs were identified with 2000+ km zonal wave fronts and 15% total electron content perturbation amplitudes, moving in transpolar propagation pathways from the dayside into the nightside. This propagation direction in the polar region was opposite to the normal assumption that TIDs originated in the nightside auroral region. Results suggest that significant dayside sources, such as cusp regions, can be efficient in generating transpolar TIDs during geospace storm intervals.

Australian blocking impacts on ocean surface waves

We assess the impact of split-flow blocking in the Australian region on global ocean wind waves using 30 years of high-resolution wave data from the CAWCR wave model hindcast. Statistically significant anomalies in significant wave height (Hs), peak wave period (Tp) and zonal wave energy flux (CgE) are found to covary with local variations in surface zonal wind induced by blocking at 140° E. Strong negative anomalies in these wave fields highlight the role of blocking in promoting calm wave conditions around the Australian coast. A relatively weak but notable remote signal also appears in the Northern Hemisphere extratropics, suggesting a link to the Australian region via tropical forcing. The occurrence of daily low (below the 5th percentile) and high (above the 95th percentile) wave conditions also varies contemporaneously with blocking at 140° E. Considering the ability of global circulation model forecasts to skilfully predict the blocking index out to at least 2 weeks lead time, our results point to blocking as a possible source of predictable wind-wave variability from sub-weekly to seasonal timescales.

Ionospheric Interhemispheric Asymmetry Induced by Planetary Wave Under Solar Minimum Condition

AbstractThe westward quasi‐6‐day planetary wave with zonal wave number 1 (Q6DW) is important in the Earth's atmospheric coupling. The International Global Navigation Satellite System Service total electron content (TEC) maps show that the Q6DW‐related ionospheric perturbations exhibit clear interhemispheric asymmetry, which do not always agree with the interhemispheric asymmetry feature of the background TEC. The National Center for Atmospheric Research Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model is then used to investigate the reasons for planetary wave‐induced interhemispheric asymmetry by imposing Q6DW forcing at the model lower boundary at vernal equinox under solar minimum condition. The interhemispheric asymmetry features of the ionospheric responses to Q6DW are well captured by our model simulations. For example, the southern branch of the Q6DW perturbation in F region is ~26% larger than that in the Northern Hemisphere at 30°E, while the background Model TEC is~4.7% stronger in the Northern Hemisphere. We found that the vertical plasma transportation induced by Q6DW perturbations in the neutral wind fields in F region show the same asymmetric features with TEC, which leads to corresponding ionospheric perturbations by changing the local recombination effect. Our further simulations show that the vertical plasma drifts induced by Q6DW through E × B process can also cause interhemispheric asymmetry in Model TEC. It is also found that the zonal component of Q6DW perturbations contributes more to the asymmetry than the meridional component through both neutral wind direct transportation effects and E region wind dynamo effects. Moreover, the ionospheric asymmetry of the Q6DW‐related oscillations also exhibit clear longitudinal variabilities.

Quasi Two‐, Three‐, and Six‐Day Planetary‐Scale Wave Oscillations in the Upper Atmosphere Observed by TIMED/SABER Over ~17 Years During 2002–2018

AbstractWe present for the first time the global climatology of two‐, three‐, and six‐day planetary waves in the upper atmosphere using nearly 17 years of temperature observations by Thermosphere Ionosphere and Mesosphere Electric Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) during January 2002–December 2018. The observations span the altitude range ~30–110 km and are continuously available over 50°S–50°N latitudes throughout the time interval. The amplitudes and phases of the oscillations with periods in the ranges of 1.5–2.5, 2.5–3.5, and 5.5–6.5 days, and longitudinal zonal wave numbers of W3 (westward propagating wave number‐3) and E1 (eastward propagating wave number‐1) are determined from the two‐dimensional (2‐D) least squares analysis of the data. The study finds that the two‐day W3 waves can penetrate into the equatorial region, although they are mostly observed at higher latitudes. The three‐ and six‐day E1 waves are trapped at low latitudes, and their amplitudes vary with latitude following a Gaussian function with the maximum centered at the equator and the e‐folding width of 27° and 24° latitudes. In the equatorial region, the monthly mean amplitudes are ~3.8–4.8 K for the three‐day waves, and the two‐ and six‐day waves have relatively smaller amplitudes. The waves have the amplitude maximums around solstices and the minimums close to equinoxes. The peak amplitudes are larger during the solar maximum than during the solar minimum, and superimposed with the quasi two‐year cycle. The vertical wavelengths are approximately 30–40 km, suggesting the propagations to the equatorial upper atmosphere. The climatology revealed could be useful for general circulation models.

Nonlinear interactions of stationary planetary waves during February 2016 sudden stratospheric warming

An analysis of nonlinear wave-wave and wave-mean flow interactions has been carried out during a sudden stratospheric warming (SSW) event in February 2016. This approach is based on a study of the perturbed potential enstrophy (Ertel’s potential vorticity squared) balance equation. The results obtained by using UK Met Office reanalysis data are presented. It has been shown that an increase in the nonlinear interactions occurs at higher-middle latitudes of the boreal stratosphere during the SSW. It is noted that it is necessary to include the stationary planetary wave with zonal wave number m=3 in the analysis of major warmings.

Ionospheric Peaked Structures and Their Local Time, Seasonal, and Solar Activity Dependence Based on Global Ionosphere Maps

AbstractThe total electron content (TEC) data from Global Ionosphere Maps are used to obtain the tidal modes through two‐dimensional Fourier transform in both universal time and LT (local time) frames. In the LT frame, a north‐south TEC asymmetry is observed along the longitude, where there is a large displacement of the geomagnetic equator from the geographic equator. The phases of tidal modes lead to a constructive or destructive interference of contributing tidal modes, producing different zonal waves and longitudinal peaked structures at different local time (tLT). The summation of all nonmigrating zonal waves in the low‐latitude region (geomagnetic latitude < 30°) produces a peaked structure with two to four major peaks at most local hours. The amplitude of this peaked structure in June solstice is smaller than those in other seasons. During solar maximum (minimum), the amplitude of peaked structure has a value ~20 (~12) TECu with two to four (one to three) peaks. The locations of peaked structures formed by the nonmigrating tides in 2006 (solar minimum) and in 2014 (solar maximum) are similar. The Weddell Sea Anomaly is found to have a density enhancement in the region from 135°E to 90°W and from 40°S to 80°S with an amplitude ~19.85 (~5.07) TECu during March equinox (December solstice) of solar maximum (minimum).

Diurnal Forcing and Phase Locking of Gravity Waves in the Maritime Continent

Abstract An unfiltered zonal Hovmöller depiction of rainfall in the Maritime Continent (MC) reveals remarkable spatiotemporal continuity of zonally propagating disturbances with a diurnal period, which endure over multiple days and propagate faster than the individual convective storms they coupled with. This phenomenon and its sensitivity to the Madden–Julian oscillation (MJO) during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign is examined here through a well-validated, convection-permitting model simulation conducted on a large domain. We find that these disturbances are zonally propagating diurnal gravity waves excited by vigorous nocturnal mesoscale convective systems over Sumatra and Borneo. These gravity waves are diurnally phase locked: their wavelength very closely matches the distance between these two islands (~1500 km), while their particular zonal phase speed (~±17 m s−1) allows them to propagate this distance in one diurnal cycle. We therefore hypothesize that these waves are amplified by resonant interaction due to diurnal phase locking. While these zonal gravity waves decouple from convection once beyond the MC, their divergent flow signature endures well across the Indian Ocean, provoking the notion that they may influence rainfall at far remote locations. The exact controls over this zonal phase speed remain uncertain; we note, however, that it is roughly consistent with diurnal offshore-propagating modes documented previously. Further study is required to tie this down, and more generally, to understand the sensitivity of these modes to background flow strength and the geography of the MC.

Large‐Amplitude Quasi‐10‐Day Waves in the Middle Atmosphere During Final Warmings

AbstractGeopotential height measurements from the Aura Microwave Limb Sounder between 9‐ and 97‐km altitudes during 2004–2018 are used to examine long‐period (3–20 days) wave activity during the Northern Hemisphere winter and spring, with the primary focus on the response of normal mode Rossby waves in the middle atmosphere to sudden stratospheric warmings (SSWs). Unusually large westward propagating waves with Zonal Wave Number 1 and period ∼10 days are observed at 55° latitude at the stratopause height (∼48 km) and above following final warmings of 2016, 2015, and 2005. In each case, large‐amplitude waves are observed for the duration of two to three wave cycles. Characteristics of the waves are in conformity with the second antisymmetric Rossby normal mode of Zonal Wave Number 1, or the quasi‐10‐day wave. The growth rate of the waves is significantly greater than the classical normal mode in the upper stratosphere (approximately 30–50 km) where instability conditions are met, indicating the amplification or excitation of the waves in that region. The response of the quasi‐10‐day wave during midwinter SSWs, and also during the spring transition without an SSW, is not as obvious as the wave response during final warmings. The results suggest that not only the occurrence of SSW but also the seasonal timing of SSW is an important factor for the transient variability of the quasi‐10‐day wave in the middle atmosphere.

The Interannual Variability of Surface Winds in Antarctica and the Surrounding Oceans: A Climatological Analysis Using the ERA‐Interim Reanalysis Data

AbstractSeason‐dependent interannual variability of surface winds across Antarctica and the Southern Ocean is investigated using European Centre for Medium‐Range Weather Forecasts Interim (ERA‐Interim) reanalysis for 1979–2017 and the empirical orthogonal function (EOF) technique. The first three EOF modes, which account for 18–23%, 11–15%, and 8–10% of the total variances for the four austral seasons, are related to several known modes of Southern Hemisphere large‐scale circulation including the southern annular mode, Pacific South American (PSA), and Zonal Wave 3 (ZW3) and to sea surface temperature anomalies in eastern (Niño 3) and central (Niño 4) tropical Pacific Ocean. Specifically, EOF1 and EOF2 are significantly correlated with southern annular mode in all seasons except for autumn EOF2. Neither EOF1 nor EOF2 are significantly correlated to Niño 3, but Niño 4 is significantly correlated to winter EOF1 and to EOF2 in all seasons but winter. EOF1 is significantly related to ZW3 for all seasons but summer; however, no significant connection is found between EOF2 and ZW3. Significant correlations exist between spring and winter EOF2 and PSA1 and between spring EOF1 and PSA2. EOF3 is significantly correlated with Niño 3, Niño 4, and PSA2 in all seasons but summer. The spatial patterns of all three modes can be largely explained by the anomalous mean sea level pressure fields that strengthen or weaken climatological winds in different regions of the Antarctica and the Southern Ocean, although variations in katabatic forcing also plays a role in the variations of surface winds especially in East Antarctica and during winter season where and when katabatic forcing dominates.

Intraseasonal Snow Cover Variations Over Western Siberia and Associated Atmospheric Processes

AbstractThe present study documents intraseasonal snow cover variations over western Eurasia and associated atmospheric processes using the latest Moderate Resolution Imaging Spectroradiometer/Terra daily snow cover product and National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis. It is found that 9‐ to 30‐day variation dominates total intraseasonal snow cover variations over western Siberia. Composite analysis based on 69 positive snow events over western Siberia reveals that atmospheric circulation anomalies control the 9‐ to 30‐day snow variation over western Siberia. A zonal wave train associated with the North Atlantic Oscillation leads to the development of an anomalous cyclone over western Siberia. The associated anomalous ascending motion, anomalous water vapor convergence, and water vapor increase in the atmosphere provide a favorable condition for snowfall. The snowfall starts when anomalous ascending motion reaches the strongest. The maximum snow cover appears about 1 day after the peak of anomalous descending motion and water vapor flux divergence. The surface air temperature tends to vary out of phase with snow cover over western Siberia. Surface air temperature anomalies over western Siberia are contributed by horizontal advection and diabatic heating. The adiabatic heating has a damping effect in surface air temperature variation.

Investigating an Unusually Large 28‐Day Oscillation in Mesospheric Temperature Over Antarctica Using Ground‐Based and Satellite Measurements

AbstractThe Utah State University Advanced Mesospheric Temperature Mapper was deployed at the Amundsen‐Scott South Pole Station in 2010 to measure OH temperature at ~87 km as part of an international network to study the mesospheric dynamics over Antarctica. During the austral winter of 2014, an unusually large amplitude ~28‐day oscillation in mesospheric temperature was observed for ~100 days from the South Pole Station. This study investigates the characteristics and global structure of this exceptional planetary‐scale wave event utilizing ground‐based mesospheric OH temperature measurements from two Antarctic stations (South Pole and Rothera) together with satellite temperature measurements from the Microwave Limb Sounder on the Aura satellite and the Solar Occultation For Ice Experiment on the Aeronomy of Ice in the Mesosphere satellite. Our analyses have revealed that this large oscillation is a wintertime, high‐latitude phenomenon, exhibiting a coherent zonal wave #1 structure below 80‐km altitude. At higher altitudes, the wave was confined in longitude between 180°E and 360°E. The amplitude of this oscillation reached ~15 K at 85 km, and it was observed to grow with altitude as it extended from the stratosphere into the lower thermosphere in the Southern Hemisphere. The satellite data further established the existence of this oscillation in the Northern Hemisphere during the boreal wintertime. The main characteristics and global structure of this event as observed in temperature are consistent with the predicted 28‐day Rossby Wave (1,4) mode.

Two Types of Heat Wave in Korea Associated With Atmospheric Circulation Pattern

AbstractThis study investigates heat wave variability over Korea during 1979‐2017. It is found that most of heat waves in Korea can be classified into two distinct types based on the spatial patterns of atmospheric circulation anomalies: the zonal wave (Z‐wave) type and the meridional wave (M‐wave) type. The Z‐wave type is accompanied by large‐scale atmospheric waves across the Eurasian continent, while the M‐wave type is associated with convective activities over the subtropical western North Pacific. The Z‐wave type occurs when the high‐pressure node of eastward propagating wave located around Korea and it seems that the associated wave energy could originate from North Atlantic Ocean. The M‐wave type, on the other hand, is driven by northward propagating wave train from subtropical western North Pacific to East Asia, which is triggered by anomalous convective activity over the subtropical western North Pacific. By analyzing thermodynamical as well as dynamical variables, detailed descriptions on the physical characteristics of two types of heat wave are presented in this study along with the possible implications for summer climate variability over Korea.

On the Spectra of Gravity Waves Generated by Two Typical Convective Systems

Spectral characteristics of lower-stratospheric gravity waves generated in idealized mei-yu front and tropical cyclone (TC) are compared by performing high-resolution simulations. The results suggest that the systems which organize convection in different forms can generate waves with distinctly different presentation. The mei-yu front appears as a linear zonal wave source and gravity waves are dominated by cross-frontal (meridional) propagating components. The northward (southward) components have dominant meridional wavelengths of 125–333 km (>250 km), periods of 100–200 min (83–143 min), and phase speeds of 0–15 m s−1 (15–20 m s−1). The TC appears as a point wave source and gravity waves propagate equally in various horizontal directions. The waves exhibit greater power and broader spectral distributions compared with those in the mei-yu front, with dominant horizontal wavelengths longer than 62.5 km, periods of 33–600 min, and phase speeds slower than ~40 m s−1.