Quasi-biennial Variations Research Articles

Fire–precipitation interactions amplify the quasi-biennial variability in fires over southern Mexico and Central America

Abstract. Fires have great ecological, social, and economic impact. However, fire prediction and management remain challenges due to a limited understanding of their roles in the Earth system. Fires over southern Mexico and Central America (SMCA) are a good example of this, greatly impacting local air quality and regional climate. Here we report that the spring peak (April–May) of fire activities in this region has a distinct quasi-biennial signal based on multiple satellite datasets measuring different fire characteristics. The variability is initially driven by quasi-biennial variations in precipitation. Composite analysis indicates that strong fire years correspond to suppressed ascending motion and weakened precipitation over the SMCA. The anomalous precipitation over the SMCA is further found to be mostly related to the East Pacific–North Pacific (EP-NP) pattern 2 months prior to the fire season. The positive phase of the EP-NP leads to enhanced precipitation over the eastern US but suppressed precipitation over the SMCA, similar to the spatial pattern of precipitation differences between strong and weak fire years. Meanwhile, the quasi-biennial signals in precipitation and fires appear to be amplified by their interactions through a positive feedback loop at short timescales. Model simulations show that in strong fire years, more aerosol particles are released and transported downstream over the Gulf of Mexico and the eastern US, where suspended light-absorbing aerosols warm the atmosphere and cause the ascending motion of the air aloft. Subsequently, a compensating downward motion is formed over the region of the fire source and ultimately suppresses precipitation and intensifies fires. Statistical analysis shows the different durations of the two-way interaction, where the fire suppression effect of precipitation lasts for more than 20 d, while fire leads to a decrease in precipitation at shorter timescales (3–5 d). This study demonstrates the importance of fire–climate interactions in shaping the fire activities on an interannual scale and highlights how precipitation–fire interactions at short timescales contribute to the interannual variability in both fire and precipitation.

SOLAR AND GEOMAGNETIC ACTIVITY IN 19-25 CYCLES

The paper presents the results of the analysis of changes solar and geomagnetic activity in 19−25 Wolf cycles. A forecast of the maximum geomagnetic activity cycle 25 is made. Solar activity is presented by sunspot number RZ, geomagnetic activity is expressed by the energetic index calculated from the data of the "Lviv" and "Belsk" geomagnetic observatories. By annual averages RZ values, the 11-year window was used to obtain the solar activity minimum (around 1711, 1810, 1901, and 2009). The minimum of 2009 was one of the smallest in the observed data, and we took it as the beginning of the 24th cycle and the 100-year solar activity cycle. The maximum of the 24th cycle occurred in 2014 for RZ and in 2015 for energetic index. Cycle 25 began in 2019 according to RZ, and in 2020 cording to energetic index. For cycles 19−25 solar activity is 1−2 years ahead of magnetic activity. The identified quasi-biennial variations over the studied period and the lag of geomagnetic activity from solar activity allow us to predict the magnitude of geomagnetic activity and its maximum in cycle 25. According to our calculations, the maximum of geomagnetic activity will occur around 2026 and its magnitude will be 1.5−2 times higher than the maximum of the 24th cycle.

The impact of the extratropical North Pacific on the quasi-biennial variability in ENSO

The impact of the extratropical North Pacific on the quasi-biennial variability in ENSO

On the Structure and Variability of the Migrating Diurnal Temperature Tide Observed by SABER

Abstract Temperature observations made by the SABER infrared radiometer from January 2002 through December 2021 are used to study the structure and variability of the migrating diurnal temperature tide in the middle atmosphere (∼17–105 km). In the lower stratosphere, and in the mesosphere and lower thermosphere (MLT), tidal structure is dominated by the gravest latitudinally symmetric mode, with a smaller contribution from the first antisymmetric mode; in the middle and upper stratosphere, vertically nonpropagating modes are prominent. Consistent with previous work, low-frequency variability is mainly semiannual, with maxima at the equinoxes. Quasi-biennial variability is also present and evident in low-passed time series. There are robust relationships between the semiannual and quasi-biennial variability of the tide and the semiannual and quasi-biennial tropical zonal wind oscillations, respectively, which persist throughout the 20-yr dataset. While the physical mechanisms responsible for these relationships cannot be ascertained from the observations, the present results should be useful for hypothesis testing with numerical models. It is also found that the diurnal tide breaks due to convective instability in the MLT. This is reflected in its mean vertical structure, which grows as expected for a nondissipating wave below ∼85 km, but ceases to grow at higher altitudes. Direct confirmation that dissipation is due to breaking is obtained from the potential temperature field, which shows frequent instances of reversed vertical gradient, particularly at the equinoxes. Breaking of the diurnal tide has a major impact on the zonal-mean temperature and zonal wind structure of the MLT at the equinoxes.

Cycle dependence of a quasi-biennial variability in the solar interior

ABSTRACT We investigated the solar cycle dependence on the presence and periodicity of the Quasi-Biennial Oscillation (QBO). Using helioseismic techniques, we used solar oscillation frequencies from the Global Oscillations Network Group (GONG), Michelson Doppler Imager (MDI), and Helioseismic and Magnetic Imager (HMI) in the intermediate-degree range to investigate the frequency shifts over Cycles 23 and 24. We also examined two solar activity proxies, the F10.7 index and the Mg ii index, for the last four solar cycles to study the associated QBO. The analyses were performed using Empirical Mode Decomposition (EMD) and the Fast Fourier Transform (FFT). We found that the EMD analysis method is susceptible to detecting statistically significant Intrinsic Mode Functions (IMFs) with periodicities that are overtones of the length of the data set under examination. Statistically significant periodicities, which were not due to overtones, were detected in the QBO range. We see a reduced presence of the QBO in Cycle 24 compared to Cycle 23. The presence of the QBO was not sensitive to the depth to which the p-mode travelled, nor the average frequency of the p-mode. The analysis further suggested that the magnetic field responsible for producing the QBO in frequency shifts of p-modes is anchored above approximately 0.95 R⊙.

Quasi-Biennial Variability of Indian Ocean Subtropical Mode Water Subduction Driven by Atmospheric Circulation Modes during the Argo Period

Abstract Argo data were used to investigate interannual variability in the subduction rate of the Indian Ocean Subtropical Mode Water (IOSTMW). The IOSTMW subduction rate in the southwestern part of the southern Indian Ocean subtropical circulation varies substantially from year to year without significant trend during the Argo period 2005–19. The variability of the IOSTMW subduction rate is quasi-biennial and dominated by the lateral induction term associated with the wintertime mixed layer depth (MLD). The contribution of vertical pumping to the change of subduction rate is relatively small. This variability in the subduction rate directly contributes to IOSTMW volume with a quasi-biennial variation. The quasi-biennial variations in the IOSTMW subduction and volume reflect the variability of the overlying atmosphere. The wintertime Mascarene high (MH) modulates the winter MLD in the subduction area through changes in heat fluxes and wind forcing. The quasi-biennial variability in MH is associated with the southern annular mode, but it was disturbed during two El Niño events in 2009 and 2015/16. Variability of the wintertime MH plays a key role in driving the quasi-biennial variability of the IOSTMW subduction during the Argo period.

On the high percentage of occurrence of type-B 150-km echoes during the year 2019 and its relationship with mesospheric semi-diurnal tide and stratospheric ozone

• Anomalously high occurrence of 150-km radar echoes over Kototabang during September 2019. • In the years 2017 and 2019, SW2 dominates DW1 due to the larger stratospheric ozone, when compared to other years. • Dominant SW2 along with solar minimum conditions may lead to large echo occurrence through interchange instability. The Kototabang (0.20°S, 100.3°E, 10.36°S dip latitude) Equatorial Atmosphere Radar (EAR) observations of valley region field-aligned irregularities (FAI) reveal maximum percentage of occurrence (PO) of high signal to noise ratio (greater than 3 dB) type-B 150-km echoes during boreal summer (June-August) and winter (December-January) of solar moderate to minimum years 2016–2019. The PO is anomalously high in the solar minimum year 2019 and particularly during the September equinox when a major austral sudden stratospheric warming (SSW) event has occurred. Its possible relation with mesospheric tides and stratospheric ozone is investigated. The space–time spectral analysis of upper mesospheric temperature information obtained from the spaceborne radiometer instrument Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) reveals that migrating semi-diurnal tide (SW2) is dominant during June-August, when the PO of the 150-km echoes maximizes, whereas migrating diurnal tide (DW1) is dominant during equinox months. It also reveals quasi-biennial variability of the DW1 tide probably due to similar variability of the stratospheric ozone. It is suggested that the DW1 tides generated due to solar radiation absorption by stratospheric ozone and tropospheric water vapour can have phases opposite to each other, leading to the suppression of DW1 tide during September 2017 and 2019 resulting in the relative dominance of SW2 tide over DW1 tide. The meridional wind shear associated with the relatively dominant SW2 tide results in an interchange instability developed on the gradient of daytime descending ion layer along with solar minimum conditions leads to plasma irregularities responsible for these echoes. The large eastward propagating diurnal tide with zonal wavenumber 3 (DE3) observed during June-September 2019 is unlikely to cause the instability, due to its weak meridional component.

Impact of Solar Activity on Global Atmospheric Circulation Based on SD-WACCM-X Simulations from 2002 to 2019

In this study, a global atmospheric model, Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (SD-WACCM-X), and the residual circulation principle were used to study the global atmospheric circulation from the lower to upper atmosphere (~500 km) from 2002 to 2019. Our analysis shows that the atmospheric circulation is clearly influenced by solar activity, especially in the upper atmosphere, which is mainly characterized by an enhanced atmospheric circulation in years with high solar activity. The atmospheric circulation in the upper atmosphere also exhibits an ~11 year period, and its variation is highly correlated with the temporal variation in the F10.7 solar index during the same time series, with a maximum correlation coefficient of up to more than 0.9. In the middle and lower atmosphere, the impact of solar activity on the atmospheric circulation is not as obvious as in the upper atmosphere due to some atmospheric activities such as the Quasi-Biennial Oscillation (QBO), El Niño–Southern Oscillation (ENSO), sudden stratospheric warming (SSW), volcanic forcing, and so on. By comparing the atmospheric circulation in different latitudinal regions between years with high and low solar activity, we found the atmospheric circulation in mid- and high-latitude regions is more affected by solar activity than in low-latitude and equatorial regions. In addition, clear seasonal variation in atmospheric circulation was detected in the global atmosphere, excluding the regions near 10−4 hPa and the lower atmosphere, which is mainly characterized by a flow from the summer hemisphere to the winter hemisphere. In the middle and low atmosphere, the atmospheric circulation shows a quasi-biennial oscillatory variation in the low-latitude and equatorial regions. This work provides a referable study of global atmospheric circulation and demonstrates the impacts of solar activity on global atmospheric circulation.

Lyman Decrements of Neutral Hydrogen Lines in the Spectrum of the Sun Based on SDO/EVE Observations. Variations During the 24-th Solar Cycle and in Individual Flares of Classes M and X

Data from satellite observations of fluxes in Lyman series lines and their relative variations, both during the 24-th cycle and in individual class M and X flares, are analyzed. The decrement D1 (D1 = F(Lα)/F(Lβ)) has varied over the 24-th activity cycle by roughly 25% from its maximum values to minimum values during 2014, when there was a secondary maximum in the 24-th cycle according to the main indices. Decrement D2 (D2 = F(Lγ)/F(Lβ)) varies by roughly 10% during the cycle, and decrement D3 (D3 = F(Lδ)/F(Lβ)), by roughly 5% during the cycle. D1, D2, and D3 also vary during the cycle at smaller amplitudes with periods analogous to the periods of the quasi-biennial variations of the main activity indices. In flares of x-ray classes from M to X the variations of the Lyman decrements range from 2 to 20%, with the variations in the decrements being larger on the average for flares with higher energies in the soft x-ray range.

Global analysis for periodic variations in gravity wave squared amplitudes and momentum fluxes in the middle atmosphere

Abstract. Atmospheric gravity waves (GWs) are an important coupling mechanism in the middle atmosphere. For instance, they provide a large part of the driving of long-period atmospheric oscillations such as the Quasi-Biennial Oscillation (QBO) and the semiannual oscillation (SAO) and are in turn modulated. They also induce the wind reversal in the mesosphere–lower thermosphere region (MLT) and the residual mean circulation at these altitudes. In this study, the variations in monthly zonal mean gravity wave square temperature amplitudes (GWSTAs) and, for the first time, absolute gravity wave momentum flux (GWMF) on different timescales such as the annual, semiannual, terannual and quasi-biennial variations are investigated by spectrally analyzing SABER observations from 2002 to 2015. Latitude–altitude cross sections of spectral amplitudes and phases of GWSTA and absolute GWMF in the stratosphere and mesosphere are presented and physically interpreted. It is shown that the time series of GWSTA and GWMF at a certain altitude and latitude results from the complex interplay of GW sources, propagation through and filtering in lower altitudes, oblique propagation superposing GWs from different source locations, and, finally, the modulation of the GW spectrum by the winds at a considered altitude and latitude. The strongest component is the annual variation, dominated in the summer hemisphere by subtropical convective sources and in the winter hemisphere by polar vortex dynamics. At heights of the wind reversal, a 180∘ phase shift also occurs, which is at different altitudes for GWSTA and GWMF. In the intermediate latitudes a semiannual variation (SAV) is found. Dedicated GW modeling is used to investigate the nature of this SAV, which is a different phenomenon from the tropical SAO also seen in the data. In the tropics a stratospheric and a mesospheric QBO are found, which are, as expected, in antiphase. Indication for a QBO influence is also found at higher latitudes. In previous studies a terannual variation (TAV) was identified. In the current study we explain its origin. In particular the observed patterns for the shorter periods, SAV and TAV, can only be explained by poleward propagation of GWs from the lower-stratosphere subtropics into the midlatitude and high-latitude mesosphere. In this way, critical wind filtering in the lowermost stratosphere is avoided and this oblique propagation is hence likely an important factor for MLT dynamics.

Quasi-biennial Rossby and Kelvin waves in the South Indian Ocean: Tropical and subtropical modes and the Indian Ocean Dipole

The quasi-biennial (QB) band (1.2 yr ≤T < 3 yr) accounts for large part of the interannual variability in the South Indian Ocean (SIO). Despite that, it did not receive much attention to date, especially in the subtropical region. In the present work, we investigate the QB variability in sea surface height, the relationship between the tropics and the subtropics in this period band and the influence of the Indian Ocean Dipole (IOD). We use 24 years of altimeter-derived sea surface height anomalies and the MERRA-2 reanalysis winds. We find QB signals propagating westward in the tropical and subtropical SIO and eastward along the Equator; they are very likely to be expressions of Rossby and Kelvin-like waves in the QB frequency. A Complex Empirical Orthogonal Function (CEOF) analysis reveals two distinct reconstructed modes in terms of spatial distribution, which we refer as the tropical and the subtropical modes. The tropical mode explains about half of the QB variance and is given by the leading CEOF; the subtropical mode explains the QB variance in the subtropics and is formed by the superposition of the second to fifth CEOFs. In the subtropics, we show that the vertical structure of the background eastward-flowing South Indian Countercurrent affects the QB signal propagation. The tropical mode contains clear IOD spatial fingerprints, its temporal amplitudes increase in IOD years and has a high correlation with the Dipole Mode Index. Most importantly, the QB tropical waves that emerge in IOD years also appear in non-IOD years albeit with weaker amplitudes. An intriguing finding is the QB eastward propagation along the Equator, resembling a Kelvin wave. This eastward equatorial QB wave also has the highest amplitude in IOD-years, and the Maldives island chain at 73° E seems to disturb its propagation.

Effects of solar activity variations on dynamical processes in the atmosphere: Analysis of empirical data and modeling

The effects of the 11-year, quasi-biennial, and 27-day cycles of solar activity on dynamical processes in the atmosphere are studied using empirical data and results of numerical model calculations. Estimates of changes during the 11-year solar cycle in the wind velocity, potential vorticity, geopotential and its large-scale zonal harmonics are obtained using the ERA-Interim reanalysis data. Features of the response of these atmospheric parameters to the solar cycle in some areas of the atmosphere are revealed for a whole year and depending on season. The results point to the existence of a reliable statistical relation of large-scale dynamical and thermodynamic processes in the troposphere and stratosphere to the 11-year solar cycle. Based on the results of 200-year measurements, the phase synchronization of decadal variations of the North Atlantic Oscillation index with the 11-year solar cycle have been found within time intervals alternating with 45–50-year periodicity. According to the ERA-Interim data, the quasi-biennial oscillation in the equatorial stratospheric zonal wind velocity is phase-synchronized at the stratopause level (∼50 km) with the quasi-biennial variations of UV solar radiation. The proposed reason of the synchronization may be the the inhomogeneous heating of this layer due to the absorption of UV solar radiation by ozone, the meridional gradient of which changes in harmony with the quasi-biennial solar variations. The effect of the 27-day solar cycle on the characteristics of large-scale zonal wave harmonics of the geopotential in the Northern Hemisphere is analyzed using results of calculations by a 3-dimensional chemistry-climatic model. Noticeable correlations of the amplitude of the planetary wave components in winter in the Northern Hemisphere with the 27-day solar cycle have been found. The strongest response is obtained for perturbations with wave number 1 in the middle and high latitudes of the Northern Hemisphere.

Dynamics of Quasi-Biennal Variations of Cosmic Rays and Solar Activity

The quasi-biennial variations in the flux of galactic cosmic rays (GCRs) have been studied based on the data of stratospheric sensing and measurements by neutron monitors, as well as in various manifestations of solar activity and interplanetary medium parameters. It has been shown that quasi-biennial GCR variations are caused by variations with the same period in the mean magnetic field of the Sun that coincide with them over time and have been identified in the anti-phase, which respond to the sign of this field. The variations in the quasi-biennial cosmic ray are caused by quasi-biennial variations in the mean magnetic field of the Sun via the quasi-biennial variations in the interplanetary magnetic field.

Representation of solar tides in the stratosphere and lower mesosphere in state-of-the-art reanalyses and in satellite observations

Abstract. Atmospheric solar tides in the stratosphere and the lower mesosphere are investigated using temperature data from five state-of-the-art reanalysis data sets (MERRA-2, MERRA, JRA-55, ERA-Interim, and CFSR) as well as TIMED SABER and Aura MLS satellite measurements. The main focus is on the period 2006–2012 during which the satellite observations are available for direct comparison with the reanalyses. Diurnal migrating tides, semidiurnal migrating tides, and nonmigrating tides are diagnosed. Overall the reanalyses agree reasonably well with each other and with the satellite observations for both migrating and nonmigrating components, including their vertical structure and the seasonality. However, the agreement among reanalyses is more pronounced in the lower stratosphere and relatively weaker in the upper stratosphere and mesosphere. A systematic difference between SABER and the reanalyses is found for diurnal migrating tides in the upper stratosphere and the lower mesosphere; specifically, the amplitude of trapped modes in reanalyses is significantly smaller than that in SABER, although such difference is less clear between MLS and the reanalyses. The interannual variability and the possibility of long-term changes in migrating tides are also examined using the reanalyses during 1980–2012. All the reanalyses agree in exhibiting a clear quasi-biennial oscillation (QBO) in the tides, but the most significant indications of long-term changes in the tides represented in the reanalyses are most plausibly explained by the evolution of the satellite observing systems during this period. The tides are also compared in the full reanalyses produced by the Japan Meteorological Agency (i.e., JRA-55) and in two parallel data sets from this agency: one (JRA-55C) that repeats the reanalysis procedure but without any satellite data assimilated and one (JRA-55AMIP) that is a free-running integration of the model constrained only by observed sea surface temperatures. Many aspects of the tides are closer in JRA-55C and JRA-55AMIP than these are to the full reanalysis JRA-55, demonstrating the importance of the assimilation of satellite data in representing the diurnal variability of the middle atmosphere. In contrast to the assimilated data sets, the free-running model has no QBO in equatorial stratospheric mean circulation and our results show that it displays no quasi-biennial variability in the tides.

Wild Apple Growth and Climate Change in Southeast Kazakhstan

Wild populations of Malus sieversii [Ldb.] M. Roem are valued genetic and watershed resources in Inner Eurasia. These populations are located in a region that has experienced rapid and on-going climatic change over the past several decades. We assess relationships between climate variables and wild apple radial growth with dendroclimatological techniques to understand the potential of a changing climate to influence apple radial growth. Ring-width chronologies spanning 48 to 129 years were developed from 12 plots in the Trans-Ili Alatau and Jungar Alatau ranges of Tian Shan Mountains, southeastern Kazakhstan. Cluster analysis of the plot-level chronologies suggests different temporal patterns of growth variability over the last century in the two mountain ranges studied. Changes in the periodicity of annual ring-width variability occurred ca. 1970 at both mountain ranges, with decadal-scale variability supplanted by quasi-biennial variation. Seascorr correlation analysis of primary and secondary weather variables identified negative growth associations with spring precipitation and positive associations with cooler fall-winter temperatures, but the relative importance of these relationships varied spatially and temporally, with a shift in the relative importance of spring precipitation ca. 1970 at Trans-Ili Alatau. Altered apple tree radial growth patterns correspond to altered climatology in the Lake Balkhash Basin driven by unprecedented intensified Arctic Oscillations after the late 1970s.

Modeling study of the ionospheric responses to the quasi-biennial oscillations of the sun and stratosphere

The Quasi-biennial Oscillation (QBO) is a persistent oscillation in the zonal mean zonal winds of the low latitude middle atmosphere that is driven by breaking planetary and gravity waves with a period near two years. The atmospheric tides that dominate the dynamics of the mesosphere and lower thermosphere region (MLT, between heights of 70–120 km) are excited in the troposphere and stratosphere, and propagate through QBO-modulated zonal mean zonal wind fields. This allows the MLT tidal response to also be modulated by the QBO, with implications for ionospheric/thermospheric variability. Interannual oscillations in solar radiation can also directly drive the variations in the ionosphere with similar periodicities through the photoionization. Many studies have observed the connection between the solar activity and QBO signal in ionospheric features such as total electron content (TEC).In this research, we develop an empirical model to isolate stratospheric QBO-related tidal variability in the MLT diurnal and semidiurnal tides using values from assimilated TIMED satellite data. Migrating tidal fields corresponding to stratospheric QBO eastward and westward phases, as well as with the quasi-biennial variations in solar activity isolated by the Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD) analysis from Hilbert-Huang Transform (HHT), are then used to drive the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM).The numerical experiment results indicate that the ionospheric QBO is mainly driven by the solar quasi-biennial variations during the solar maximum, since the solar quasi-biennial variation amplitude is directly proportionate to the solar cycle. The ionospheric QBO in the model is sensitive to both the stratospheric QBO and solar quasi-biennial variations during the solar minimum, with solar effects still playing a stronger role.

The SPARC water vapour assessment II: comparison of annual, semi-annual and quasi-biennial variations in stratospheric and lower mesospheric water vapour observed from satellites

Abstract. In the framework of the second SPARC (Stratosphere-troposphere Processes And their Role in Climate) water vapour assessment (WAVAS-II), the amplitudes and phases of the annual, semi-annual and quasi-biennial variation in stratospheric and lower mesospheric water were compared using 30 data sets from 13 different satellite instruments. These comparisons aimed to provide a comprehensive overview of the typical uncertainties in the observational database which can be considered in subsequent observational and modelling studies. For the amplitudes, a good agreement of their latitude and altitude distribution was found. Quantitatively there were differences in particular at high latitudes, close to the tropopause and in the lower mesosphere. In these regions, the standard deviation over all data sets typically exceeded 0.2 ppmv for the annual variation and 0.1 ppmv for the semi-annual and quasi-biennial variation. For the phase, larger differences between the data sets were found in the lower mesosphere. Generally the smallest phase uncertainties can be observed in regions where the amplitude of the variability is large. The standard deviations of the phases for all data sets were typically smaller than a month for the annual and semi-annual variation and smaller than 5 months for the quasi-biennial variation. The amplitude and phase differences among the data sets are caused by a combination of factors. In general, differences in the temporal variation of systematic errors and in the observational sampling play a dominant role. In addition, differences in the vertical resolution of the data, the considered time periods and influences of clouds, aerosols as well as non-local thermodynamic equilibrium (NLTE) effects cause differences between the individual data sets.

Seasonal features of quasi-biennial variations of NO2 stratospheric content derived from ground-based measurements

Seasonal and latitudinal distributions of amplitudes of quasi-biennial variations in total NO2 content (NO2 TC), total ozone content (TOC), and stratospheric temperature are obtained. NO2 TC data from ground-based spectrometric measurements within the Network for the Detection of Atmospheric Composition Change (NDACC), TOC data from satellite measurements, and stratospheric temperature data from ERA-Interim reanalysis are used for the analysis. The differences in the NO2 TC diurnal cycles are identified between the westerly and easterly phases of the quasi-biennial oscillations (QBO) of equatorial stratospheric wind. The QBO effects in the NO2 TC, TOC, and stratospheric temperature in the Northern (NH) and Southern (SH) hemispheres are most significant in the winter–spring periods, with essential differences between the NH and SH. The NO2 TC in the Antarctic is less for the westerly phase of the QBO than that for the easterly phase, and the NO2 TC quasi-biennial variations in the SH mid-latitudes are opposite of the variations in the Antarctic. In the NH, the winter values of the NO2 TC are generally less during the westerly QBO phase than during the easterly phase, whereas in spring, on the contrary, the values for the westerly QBO phase exceed those for the easterly phase. Along with NO2, the features of the quasi-biennial variations of TOC and stratospheric temperature are discussed. Possible mechanisms of the quasi-biennial variations of the analyzed parameters are considered for the different latitudinal zones.

On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23

The activity-related variations in the solar acoustic frequencies have been known for 30 years. However, the importance of the different contributions is still not well established. With this in mind, we developed an empirical model to estimate the spot-induced frequency shifts, which takes into account the sunspot properties, such as area and latitude. The comparison between the model frequency shifts obtained from the daily sunspot records and those observed suggests that the contribution from a stochastic component to the total frequency shifts is about 30%. The remaining 70% is related to a global, long-term variation. We also propose a new observable to investigate the short- and mid-term variations of the frequency shifts, which is insensitive to the long-term variations contained in the data. On the shortest time scales the variations in the frequency shifts are strongly correlated with the variations in the total area covered by sunspots. However, a significant loss of correlation is still found, which cannot be fully explained by ignoring the invisible side of the Sun when accounting for the total sunspot area. We also verify that the times when the frequency shifts and the sunspot areas do not vary in a similar way tend to coincide with the times of the maximum amplitude of the quasi-biennial variations found in the seismic data.

A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies

The frequencies of the solar acoustic oscillations vary over the activity cycle. The variations in other activity proxies are found to be well correlated with the variations in the acoustic frequencies. However, each proxy has a slightly different time behaviour. Our goal is to characterize the differences between the time behaviour of the frequency shifts and of two other activity proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a new observable that is particularly sensitive to the short-term frequency variations. We then compare the observable when computed from model frequency shifts and from observed frequency shifts obtained with the Global Oscillation Network Group (GONG) for cycle 23. Our analysis shows that on the shortest time-scales the variations in the frequency shifts seen in the GONG observations are strongly correlated with the variations in the area covered by sunspots. However, a significant loss of correlation is still found. We verify that the times when the frequency shifts and the sunspot area do not vary in a similar way tend to coincide with the times of the maxima of the quasi-biennial variations seen in the solar seismic data. A similar analysis of the relation between the 10.7cm flux and the frequency shifts reveals that the short-time variations in the frequency shifts follow even more closely those of the 10.7cm flux than those of the sunspot area. However, a loss of correlation between frequency shifts and 10.7cm flux variations is still found around the same times.