Wave Number-1 Research Articles

Significant influence of winter Pacific-North American pattern on spring vegetation in mid-high latitude Asia

Abstract Given that the vegetation over mid-high latitude Asia (MHA) has been more variable in recent years, it is necessary to better understand the physical causes of vegetation variations in this region. Based on the normalized difference vegetation index (NDVI), this study reveals a close linkage of the variability of spring (April–May) vegetation in MHA to the winter (December–January–February) Pacific-North American (PNA) pattern. When the winter PNA pattern lies in the positive phase, the NDVI tends to decrease in most parts of the MHA region during the following spring. Further analysis suggests that the lagged influence of winter PNA on spring atmospheric circulations and hence the vegetation in MHA is accomplished by the stratospheric pathway. The positive PNA phase can enhance the upward transport of wave energy into the stratosphere over the high latitudes in winter through the linear constructive interference of zonal wavenumber 1 (WN1), consequently leading to a weaker polar vortex in the stratosphere during February-March. Subsequently, the weakened polar vortex signal propagates downward from the stratosphere to the troposphere, inducing the negative Arctic Oscillation-like circulation with an anomalous cyclonic circulation dominating the MHA region in spring. The anomalous cyclonic circulation further cools the surface air temperature in MHA via modulating downward solar radiation and temperature advection, resulting in a decrease of spring NDVI in situ.

Prolonged Quasi‐6‐Day Wave Activities in the Northern Hemisphere MLT Region Due To Antarctic Stratospheric Minor Warming Around September Equinox of 2013 and 2014

AbstractThe geopotential height observations from the Aura Microwave Limb Sounder show that the quasi‐6‐day wave (Q6DW) events with westward zonal wavenumber 1 (W1) in the Northern Hemisphere (NH) mesosphere and lower thermosphere (MLT) during September 2013 and 2014 had a prolonged lifetime of ∼45–50 days and reached their maximum amplitudes after the September equinox, while the climatological Q6DW‐W1 is completely dissipated in the background atmosphere before the equinoxes. The Eliassen‐Palm flux diagnostic results indicate that Q6DW‐W1 during September 2013 and 2014 obtained an additional source at ∼60°‐80°S and ∼40–50 km as the September equinox approached, which is related to the baroclinic/barotropic instability in this region. Further investigation on the background atmosphere reveals that several stratospheric minor warming (SMW) events occurred in the Antarctic region during September 2013 and 2014 due to the enhancement of wavenumber 1 activities, accompanied by the increase in the stratospheric temperature, changes in the shape of the polar vortex and the reversal in the zonal mean circulation. The strong planetary wave breaking during the September 2013 and 2014 Antarctic SMW events significantly weakened the strength of the polar night jet (PNJ) and made its peak height descend below ∼35 km, which generated the baroclinic/barotropic instability at the new upper boundary of the PNJ (∼60°‐80°S and ∼40–50 km) by an anomalous double‐jet configuration in the background winds. This unusual instability provided additional wave source and energy for the trans‐equatorial propagation of Q6DW‐W1, which finally led to prolonged wave activities in the NH MLT region.

Unexpected Global Structure of Quasi‐4‐Day Wave With Westward Zonal Wavenumber 2 During the February 2023 Unusual Major Sudden Stratospheric Warming With Elevated Stratopause

AbstractDuring February 2023, the quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) reached its largest amplitude of ∼400 m in the Southern Hemisphere (SH) geopotential height observations since 2004, which occurred simultaneously with an Arctic major sudden stratospheric warming (SSW) with an elevated stratopause (ES). However, the Q4DW‐W2 perturbations in the Northern Hemisphere (NH) were unexpectedly suppressed despite the unstable Arctic stratosphere and mesosphere during the 2023 ES‐SSW. Diagnostic analysis shows that the westward winds at ∼54°N–70°N in the upper stratosphere of ∼‐79 m/s during the 2023 ES‐SSW were the strongest during boreal winters over the past two decades, which benefited from the onset of a preceding minor SSW at the end of January. The strongest westward wind generated a wave geometry configuration of full reflection for Q4DW‐W2 in the NH, while the Q4DW‐W2 enhancement in the SH was induced by the in‐situ amplification of the surviving seeding perturbations.

Southern Hemisphere Circumpolar Wavenumber‐4 Pattern Simulated in SINTEX‐F2 Coupled Model

AbstractInterannual sea surface temperature (SST) variations in the subtropical‐midlatitude Southern Hemisphere are often associated with a circumpolar wavenumber‐4 (W4) pattern. This study is the first attempt to successfully simulate the SST‐W4 pattern using a state‐of‐the‐art coupled model called SINTEX‐F2 and clarify the underlying physical processes. It is found that the SST variability in the southwestern subtropical Pacific (SWSP) plays a key role in triggering atmospheric variability and generating the SST‐W4 pattern during austral summer (December‐February). In contrast, the tropical SST variability has a very limited effect. The anomalous convection and associated divergence over the SWSP induce atmospheric Rossby waves confined in the westerly jet. Then, the synoptic disturbances circumnavigate the subtropical Southern Hemisphere, establishing an atmospheric W4 pattern. The atmospheric W4 pattern has an equivalent barotropic structure in the troposphere, and it interacts with the upper ocean, causing variations in mixed layer depth due to latent heat flux (LHF) anomalies. As incoming climatological solar radiation goes into a thinner (thicker) mixed layer, the shallower (deeper) mixed layer promotes surface warming (cooling). This leads to positive (negative) SST anomalies, developing the SST‐W4 pattern during austral summer. Subsequently, anomalous entrainment due to the temperature difference between the mixed layer and the water below the mixed layer, anomalous LHF, and disappearance of the overlying atmospheric W4 pattern cause the decay of the SST‐W4 pattern during austral autumn. These results indicate that accurate simulation of the atmospheric forcing and the associated atmosphere‐ocean interaction is essential to capture the SST‐W4 pattern in coupled models.

Response of ion velocities of daytime ionospheric wavenumber-4 to solar activity observed by ROCSAT-1 and DEMETER

The electron/ion density/temperature and ion velocities observed by the ROCSAT-1 and DEMETER satellites are used to examine the daytime wavenumber-4 (WN4) feature in the equatorial/low latitude ionosphere during various months and solar activity levels of 1999–2010. A moving median process has been employed to isolate WN4 features and calculate their amplitudes, while the upward ion drift is used to estimate electric fields. The ROCSAT-1 and DEMETER ion density, ion temperature, and ion velocity generally yield prominent WN4 features over the center of Pacific Ocean, the west side of South America, the center of the Atlantic Ocean, and Southern India. The correlation coefficient between the deviation of ion density and upward ion drift is significant during high solar activity of 1999–2004, while it approaches to zero during low solar activity of 2004–2010. This confirms that the longitudinal variation of the upward ion drift is essential during high solar activity, and the associated amplitude of dynamo eastward electric field is in the range of 0.10–0.14 mV/m, which is 15–19% of daily dynamo electric field. By contrast, the deviation of the ion density and the northward field-aligned ion flow show a clear anti-correlation which yields a maximum coefficient in August during low solar activity but no correlation during high solar activity. These indicate that the longitudinal variation of the meridional field-aligned ion flow could play an important role during low solar activity, and its amplitude is in the range of 10.44–13.91 m/s, which is 10–13% of the ambient ion flows.

Quasi-10 d wave activity in the southern high-latitude mesosphere and lower thermosphere (MLT) region and its relation to large-scale instability and gravity wave drag

Abstract. Seasonal variation in westward-propagating quasi-10 d waves (Q10DWs) in the mesosphere and lower thermosphere of the Southern Hemisphere (SH) high-latitude regions is investigated using meteor radar (MR) observations for the period of 2012–2016 and using the Specified Dynamics (SD) version of the Whole Atmosphere Community Climate Model (WACCM). The phase difference in meridional winds measured by two MRs located in Antarctica gives observational estimates of the amplitude and phase of the Q10DW with zonal wavenumber 1 (W1). The amplitude of the observed Q10DW-W1 is large around equinoxes. In order to elucidate the variations in the observed Q10DW-W1 and its possible amplification mechanism, we carry out two SD-WACCM experiments nudged towards the MERRA-2 reanalysis from the surface up to ∼ 60 km (EXP60) and ∼ 75 km (EXP75). Results of the EXP75 indicate that the observed Q10DW-W1 can be amplified around regions of barotropic and/or baroclinic instability in the middle mesosphere around 60–70° S. In the EXP60 experiment, it was also found that the Q10DW-W1 is amplified around the regions of instability, but the amplitude is too large compared to MR observations. The large-scale instability in the EXP60 in the SH summer mesosphere is stronger than that in the EXP75 and Microwave Limb Sounder observations. The larger instability in the EXP60 is related to the large meridional and vertical variations in polar mesospheric zonal winds in association with gravity wave parameterization (GWP). Given uncertainties inherent in GWP, these results can suggest that it is possible for models to spuriously generate traveling planetary waves such as the Q10DW, especially in summer, due to excessively strong large-scale instability in the SH high-latitude mesosphere.

Interannual and Interhemispheric Comparisons of Q2DW Bimodal and Unimodal Structures During the 2003–2020 Summer Period

AbstractBy analyzing Sounding of the Atmosphere using Broadband Emission Radiometry and the Modern‐Era Retrospective analysis for Research Applications Version 2 reanalysis observations from 2003 to 2020, we found that the structural variation of the westward quasi‐2‐day wave (Q2DW) is related to the mean zonal wind in the background atmosphere, and the zonal wavenumber 3 (W3) is more affected by the background atmosphere than the zonal wavenumber 4 (W4). Our study focuses on the spatial structure of ∼50–95 km and ∼50°S–50°N. The spatial structure of W3 and W4 in the Northern and Southern Hemispheres is unimodal and bimodal, while the bimodal structure of W3 in the Southern Hemisphere is more obvious in some years. In the unimodal structure, W3 has a higher (altitude) peak in the Southern Hemisphere (∼82 km) than in the Northern Hemisphere (∼70 km), while W4 peaks at ∼70 km in both hemispheres. In the bimodal structure, W3 in the Southern Hemisphere fluctuates mostly at ∼82 km, followed by ∼68 km, while W3 in the Northern Hemisphere fluctuates mostly at ∼70 km, followed by ∼82 km. The spatial structure of W4 in both hemispheres fluctuates mainly at ∼70 km, followed by ∼82 km. In addition, W3 is stronger in the Southern Hemisphere than the Northern Hemisphere, W4 is stronger in the Northern Hemisphere than the Southern Hemisphere, and peak amplitudes of both W3 and W4 are larger in a bimodal structure than unimodal structure. The diagnostic analysis shows that the interannual and interhemispheric structure changes of Q2DWs may be due to the larger mean flow instabilities, background winds, and refractive index in the mesosphere, which make the bimodal structure of W3 and W4 obtain larger energy for propagation and amplification, resulting in higher (altitude) and larger amplitudes. This indicates that the background atmosphere of the mesosphere can affect the spatial structure of Q2DWs.

On the Influence of ENSO on Sudden Stratospheric Warmings

AbstractUsing the extended ERA5 reanalysis and three state‐of‐the‐art models, this study explores how El Niño‐Southern Oscillation (ENSO) can influence the total frequency, seasonal cycle and preconditioning of sudden stratospheric warmings (SSWs). Reanalysis data shows that in the last seven decades, winters with SSWs were more common than winters without, regardless El Niño (EN) or La Niña (LN) occurrence or the ENSO/SSW definitions. In agreement with previous studies, our models tend to simulate a linear ENSO‐SSW relationship, with more SSWs for EN, around mid‐winter (January–February) as in reanalysis, and less for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave‐like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated, emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO in the North Pacific, favoring a stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO‐forced signal relies on modulating the seasonal‐mean polar vortex strength, becoming weaker and more displaced (stronger and more stable) for EN (LN), while ENSO‐unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are distinct sources of variability of the winter atmospheric circulation operating at different time‐scales and may reconcile previous findings in this context.

Wavenumber-4 Structure in COSMIC-2 Observations: Vertical Plane Perspective

High-rate radio occultation (RO) in COSMIC-2 (FORMOSAT7) enables us to investigate the finer details of the ionosphere owing to the measurements being made at a significantly high spatiotemporal resolution, which was unthinkable a decade ago. In the vertical plane, local-time ionospheric wavenumber-4 (WN4) structures display tilted phase-fronts over the equatorial ionization anomaly (EIA) belt. The longitudinal extent of a tilted WN4 phase-front approximates the zonal wavelength of nonmigrating DE3 tide in the local-time frame of reference, i.e., ~900. The WN4-filtered (residual) component indicates a greater tilt (when visible), with a larger longitudinal extent of a wavenumber structure in the vertical plane. The WN4 structure over the EIA crest region is found to be out of phase (in phase) with respect to that over the EIA trough region during daytime (nighttime), which also depended on the altitude under consideration. Intriguingly, above 400 km, the WN4 structures in the EIA crest and trough regions are seen to be in phase with each other at all local times. The phenomenon of the “longitudinal co-location” of WN4 over the EIA crest and trough regions at altitudes above ~400 km at all local times remains unexplained. Results also highlight that the formation of WN4 is governed by a complex interplay of direct forcing of nonmigrating tides and the zonal electric field whose characteristics within the EIA belt vary drastically with latitude and altitude under consideration.

Carbon Dioxide Retrievals From NOMAD‐SO on ESA's ExoMars Trace Gas Orbiter and Temperature Profile Retrievals With the Hydrostatic Equilibrium Equation: 2. Temperature Variabilities in the Mesosphere at Mars Terminator

AbstractThe Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument scans the Martian atmosphere since 21 April 2018. In this work, we present a subset of the NOMAD SO data measured at the mesosphere. We focused on a spectral range that started to be recorded in Martian year (MY) 35. A total of 968 vertical profiles of carbon dioxide density and temperature covering MY 35 and the beginning of MY 36 were investigated until 135° of solar longitude. We compared 47 profiles with co‐located profiles of the Mars Climate Sounder onboard the Mars Reconnaissance Orbiter. Most profiles show a good agreement as SO temperatures are only 1.8 K higher, but some biases lead to an average absolute difference of 7.4°K. The SO data set is also compared with simulations from the Global Environmental Multiscale‐Mars general circulation model. Both data sets are in good agreement except for the presence of a cold layer in the winter hemisphere and a warm layer at dawn in the Northern hemisphere for solar longitudes between 240° and 360°. Five profiles contain temperatures lower than the limit for CO2 condensation. Strong warm layers were found in 13.5% of the profiles. They are present mainly at dawn and in the winter hemisphere, while the Northern dusks appear featureless. The data set mainly covers high latitudes around 60° and we derived some non‐migrating tides. In the Southern winter hemisphere, we derived apparent zonal wavenumber‐1 (WN‐1) and WN‐3 tidal components with a maximum amplitude of 10% and 5% at 63 km, respectively.

Decadal Quasi-2-Day Wave Observations in the Equatorial Mesopause Region by a Meteor Radar over Kototabang (0.2°S, 100.3°E) and TIMED/TIDI and Comparison with Quasi-2-Day Wave Observations at Mid-Latitudes

We studied the characteristics of quasi-two-day wave (QTDW) using the meridional wind in the mesosphere and lower thermosphere (MLT) obtained from a meteor radar over Kototabang (KB, 0.2°S, 100.3°E) from 2003 to 2012. Atmospheric oscillations have a crucial impact on atmospheric dynamics, which contributes to more accurate space weather forecasting, thus providing a more secure space environment for human space exploration activities such as remote sensing and satellite navigation. QTDWs are typical atmospheric oscillations in the upper stratosphere, mesosphere and lower thermosphere. The occurrence time, amplitudes, periods and vertical wavelengths of QTDW events are analyzed statistically. Data obtained from the TIMED Doppler Interferometer (TIDI), which can measure wind and temperature and is onboard the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite, are used to analyze the global distribution and spatial structure of QTDWs with different zonal wavenumbers. The characteristics of the QTDWs over KB are compared with the QTDWs at the middle latitudes using the meridional wind data from a meteor radar over Wuhan (114.4°E, 30.6°N), Beijing (116.5°E, 39.9°N) and Mohe (121.1°E, 50.1°N). The amplitudes of the QTDW and spectral analysis are calculated by the least squares fitting method. Our results demonstrate that QTDWs are present almost all year around over KB. The occurrence time, amplitudes, periods and vertical wavelengths of QTDW events with different zonal wavenumbers are determined in this study. We also find that the statistical characteristics of the QTDWs in KB are different from those at middle latitudes. The westward zonal wavenumber −4 (W4) events gradually increase with increasing latitude, whereas westward zonal wavenumbers −1, −2, and −3 (W1, W2 and W3, respectively) events all decrease with increasing latitude.

Imprint of Mesospheric Quasi 2‐Day Wave in the Ground Geomagnetic Field Variations at Low Latitudes

AbstractThe strong quasi 2‐day wave (Q2DW) event that occurred during January 2015 was studied by employing medium frequency and meteor radar observations of winds from three different locations at low latitudes with considerable longitudinal separation. The large‐amplitude wave activity that occurred in mid‐January is the focus of this study. Using the advantage offered by the longitudinal separation among the stations, the zonal structure of the Q2DW has been examined. The presence of a dominant westward propagating zonal wavenumber 3 (W3) mode was revealed in the analysis. The availability of geomagnetic field data from a number of stations at low latitudes enabled us to observe the wave event at ionospheric dynamo region heights. The phase‐longitude cross‐section of the quasi 2‐day oscillation in the Y component of the geomagnetic field reiterates the presence of the dominant W3 mode observed at lower altitudes. Similar zonal characteristics and periods of the wave signatures found at mesospheric altitudes and in the manifestation of quiet‐time ionospheric currents in geomagnetic field are in conformity with the notion of the wave coupling of the atmosphere‐ionosphere system.

Decadal Variability of Southern Subtropical SST Wavenumber‐4 Pattern and Its Impact

AbstractA decadal variability is revealed in sea surface temperature (SST) wavenumber‐4 (W4) pattern of southern subtropics (20°–55°S) using wavelet and empirical orthogonal function analysis. The decadal variability of the SST W4 pattern evolves from the decadal modulation of the South Pacific Meridional Mode (SPMM) as seen in SST footprints. The SST residuals of the SPMM create a favorable environment for the frequent occurrence of positive/negative type of the SST W4 pattern and is discussed. Since SST W4 pattern covers the globe, it potentially affects the decadal rainfall variability over Southern continents by modulating local atmospheric circulation. During positive phase, anomalous SST close to South America, Australia, and Southern Africa enforces the wind to go on/offshore and converges/diverges the moisture into/out of the landmass. As a result, specific humidity changes and alters the rainfall over Southern continents on a decadal scale. The scenario is opposite during the negative phase.

Long‐Term Study of Quasi‐16‐day Waves Based on ERA5 Reanalysis Data and EOS MLS Observations From 2005 to 2020

AbstractThis study uses the latest ERA5 reanalysis data and Earth Observing System microwave limb sounder (MLS) observations to investigate the climatological characteristics of quasi‐16‐day waves (Q16DWs) with different zonal wavenumbers. The fitted temperature amplitudes deduced from ERA5 reanalysis data and from MLS observations are highly consistent with each other, even in fine spatial‐temporal structures. The westward‐propagating wave with wavenumber 1 (W1) and the eastward‐propagating waves with wavenumbers 1 (E1) and 2 (E2) are identified as the most dominant propagating components according to the statistical analyses of their fitted temperature and wind amplitudes. All three waves exhibit significant intra‐annual and interannual variations. Their amplitudes in winter/spring and autumn/summer are strong, moderate, or weak. The W1 wave is more intense in boreal winter than in austral winter, while the E2 wave is the opposite, and the E1 wave is equally strong in boreal and austral winters. Their interannual variabilities measured by the ratios of peak amplitudes in temperature, zonal wind, and meridional wind range from 1.64 to 2.15, indicating significant variability. The presence of intense wave activities is always accompanied by a positive squared refractive index. We suggest that the squared refractive index contributes greatly to the global distribution characteristics of the Q16DWs. The W1 wave in the troposphere is more likely to travel upward and reach the stratosphere and mesosphere, while the intense E1 and E2 waves in the stratosphere and mesosphere may come from the upper atmosphere or be excited in situ.

Possible Causes of the Occurrence of a Rare Antarctic Sudden Stratospheric Warming in 2019

A minor Antarctic sudden stratospheric warming (SSW) with the strongest circulation changes since the first major SSW over the Antarctic was recorded in 2002 occurred in early September 2019. The diagnosis demonstrates two possible causes of this SSW. First, the tropical central Pacific warming is identified, which enhanced the amplitude of tropospheric planetary wavenumber 1 (W1) in the extratropics on the seasonal time scale. Second, the impact of intraseasonal convection anomalies similar to previous studies is also suggested here. The enhanced deep convection over the South Pacific Convergence Zone (SPCZ) in late August–early September excited a Rossby wave train to deepen an anomalous ridge, which significantly and persistently strengthened the tropospheric W1. The central Pacific warming and intraseasonal convection anomalies jointly provided the conditions for the occurrence of the Antarctic SSW in 2019 on different time scales. On the other hand, the difference of the stratospheric state between the Antarctic SSWs in 2019 and 2002 may be an important reason why the 2019 event did not meet the major SSW criteria. The stratospheric state before the 2019 SSW event is somewhat not as ideal as that of the 2002 event. Vertical planetary waves are, hence, more difficult to enter into the polar stratosphere, making it more difficult to trigger major events.

A New Mechanism for the Generation of Quasi‐6‐Day and Quasi‐10‐Day Waves During the 2019 Antarctic Sudden Stratospheric Warming

AbstractUsing the Modern‐Era Retrospective analysis for Research and Applications version 2 reanalysis and Aura Microwave Limb Sounder geopotential height and temperature observations, two unusual planetary waves (PWs) with westward zonal wavenumber 1 (W1) and periods of ∼6.0 and ∼9.6 days are identified during a rare Antarctic sudden stratospheric warming (SSW) event in September 2019, which follow a strong W1 PW with period of ∼7.4 days. It is found that the W1 ∼7.4‐day wave is actually the representation of the climatological W1 quasi‐6‐day wave before the September equinox, which peaks on September 10, 2019. Meanwhile, a strong zonally symmetric wave with period of ∼32.2 days occurs in the Southern Hemisphere with a maximum zonal wind amplitude of ∼27 m/s at 65°S and 48 km. Interestingly, the frequencies and zonal wavenumbers of the W1 ∼7.4‐day, ∼6.0‐day, ∼9.6‐day and zonally symmetric ∼32.2‐day waves exactly satisfy the nonlinear interaction theory between PWs. We thus proposed for the first time the occurrence of nonlinear interaction between a W1 ∼7.4‐day wave and a zonally symmetric ∼32.2‐day wave, which generates two W1 PWs with periods of ∼6.0 and ∼9.6 days. The Eliassen‐Palm flux diagnostics suggest that the zonally symmetric ∼32.2‐day wave also contributes to the wind deceleration during this SSW. Moreover, the baroclinic/barotropic instabilities related to the vertical shear in the zonal wind during the SSW considerably enhance the PW activities in the Antarctic stratosphere. The climatological variations of the instabilities and critical layers in the mesosphere can also influence the propagation of these three W1 PWs.

Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes

The longitudinal structure in the altitude of the Sporadic E (Es) was investigated for the first time based on the S4 index provided by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in low latitudes. The longitudinal structure is identified as a symmetrically located wavenumber-4 (WN4) pattern within 30°S–30°N. The WN4 occurs primarily during the daytime at the June solstice and equinoxes, with the largest amplitude at the September equinox and the smallest one at the March equinox. It moves eastward with a speed of ~90°/day. The strongest WN4 appears within 10–20°N and 5–15°S in the Northern and Southern hemispheres, respectively. At the June solstice and the September equinox, the WN4 is stronger in the Northern hemisphere than in the Southern hemisphere, while the situation is reversed at the March equinox. The altitude distribution of the convergence null in the diurnal eastward non-migrating tide with zonal wavenumber-3 (DE3) for the zonal wind is similar to that of the WN4. This and other similar features, such as the seasonal variation, eastward speed, and the symmetrical locations, support the dominant role of the DE3 tide for the formation of the WN4 structure.

On the Westward Quasi‐8‐Day Planetary Waves in the Middle Atmosphere During Arctic Sudden Stratospheric Warmings

AbstractUsing the geopotential height measurements from the Aura Microwave Limb Sounder and the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite, we found eight westward quasi‐8‐day wave (Q8DW) events with zonal wavenumber 1 (W1) during the Arctic sudden stratospheric warmings (SSWs) in 2005–2020. The W1 Q8DW perturbations are mainly confined to the middle and high latitudes in the Northern Hemisphere, and peak at ∼50–80 km. Specified Dynamics Whole Atmosphere Community Climate Model eXtended version simulations are utilized to reproduce the Q8DW activities. The diagnostic results indicate that the W1 Q8DWs originate from the high latitudes in the Northern Hemisphere stratosphere. Their excitation, propagation, and amplification are primarily influenced by the critical layers and baroclinic/barotropic instability, which are dependent on the background conditions during SSWs. Our statistical results show that the W1 planetary waves with periods shorter than 12 days during the Arctic SSWs are most frequently observed at the period of ∼8 days, which is significantly different from the climatological periods of the Rossby (1, 1) and (1, 2) normal modes (6.14 and 9.81 days, respectively). These results indicate that the W1 Q8DW may be a new wave mode. Finally, we found that the reversal (or deceleration) and the recovery of the zonal mean flow during SSWs can result in a zonally symmetric Q8DW, and the W1 Q8DW is likely the child wave generated by the nonlinear interaction between the stationary planetary wave with zonal wavenumber 1 and zonally symmetric Q8DW.

Investigation of the relationship between Euro-Atlantic and West Asia blockings and major stratospheric sudden warmings in the period of 1959–2018

The main aim of this paper is to investigate and compare the possible connection between tropospheric blocking events and major stratospheric sudden warmings (SSWs) in the two periods of 1959−1988 and 1989−2018 to present a dynamical insight into the blocking formation and behaviors. After identifying and characterizing two types of SSWs including wavenumber-1 (W1) and wavenumber-2 (W2) in both the periods, the behaviors of blocking events coincided with major SSWs are examined and compared in the two periods. Then, the relationship between blockings and major SSWs is discussed applying the Empirical Orthogonal Function (EOF) method.In general, 18 and 16 major SSW events were identified in the first and second periods, respectively. The investigation of planetary wave activities indicates that the maximum anomalies of geopotential height amplitude and meridional eddy heat flux in the PRE-SSW phase of both types of SSWs, particularly major W2 warmings, in the second period occurred earlier than those in the first period. The peaks of blocking activities in the second period have also been moved eastward compared with the first period in both prior to and after the onset of SSWs. Moreover, the frequency of blocking event during the PRE-SSW phase of major W1 warmings in Euro-Atlantic and a large part of West Asia in the second period is less than that of the first period, while the occurrence of blocking during the PRE-SSW phase of major W2 warmings in Eastern Europe and West Asia has been increased in the second period. In the POST-SSW phase, blocking activity associated with major W2 warmings is enhanced in West Asia during the second period. In addition, the maximum blocking activities preceding major SSWs in the second period was occurred 5-days prior to the first period. The results of cross-correlation coefficients between blockings and SSWs show significant relationship between them with time lag of about 10-days prior to the onset of warmings in both the periods, especially in the recent years.

Atmospheric wavenumber-4 driven South Pacific marine heat waves and marine cool spells

Marine heat waves (MHW) and cool spells (MCS) can both positively and negatively impact marine ecosystems with potentially large societal and economic impacts. Here, I examine the global teleconnections of MHW/MCS in the southern hemisphere and Tasman Sea. When MHW/MCS are defined with respect to a linear warming trend, there is little evidence that MHW in the Tasman Sea are changing in either frequency or intensity but may be lasting longer. MCS may be becoming weaker and less frequent. I show that MHW/MCS in the Tasman Sea co-occur with corresponding events in the Atlantic, Indian, and eastern-Pacific Oceans, and these southern hemisphere events are likely driven by stalling of a global wavenumber-4 (W4) atmospheric wave, leading to anomalously weak north-easterly winds during MHW or strong south-westerly winds during MCS. Thus, the key to predicting MHW/MCS is in understanding what causes the atmospheric W4 wave to stall.