Semiannual Variation Research Articles

Semiannual Variation of Total Ion Density of Topside Ionosphere Over Indian Equatorial and Low Latitudes

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Annual and semi-annual variations of TEC over Nepal during the period of 2007–2017 and possible drivers

Annual and semi-annual variations of TEC over Nepal during the period of 2007–2017 and possible drivers

Solar Wind Parameters and Its Geoefficiency During Minimums of Four Solar Cycles

Recently a number of publications have appeared on the long and deep minimum in cycle 23 of solar activity. This interest is due to the fact that it turned out to be the longest and deepest in terms of the number of sunspots in the entire era of space exploration. The features of the minimum of cycle 23 of solar activity and the beginning of cycle 24 made it possible to assume that in the coming decades, a minimum of solar activity similar to the Dalton or Maunder minimum, leading to a global change in the earth's climate, may occur. Such assumptions make a detailed study of the influence of the minimum of solar cycle 23 on the parameters of the solar wind and the interplanetary magnetic field, as well as a comparison of this influence with similar manifestations in the three previous cycles very urgent. The work carried out statistical processing and analysis of data available in print and on the Internet on the indices of solar activity (W and F10.7), on geomagnetic activity, as well as on the parameters of the solar wind and interplanetary field. In contrast to other similar studies, when choosing time intervals for all cycles, only one — 12 months was used, which made it possible to exclude annual and semi-annual variations in solar wind parameters. For the considered minima of solar activity, the geoeffectiveness of the disturbed fluxes ICME, CIR, and Sheath was considered. A monotonic and very significant decrease in the geoeffectiveness of the ICME streams was found. Data processing on the hourly average values of the solar wind parameters at the minima of geomagnetic activity for 4 cycles confirmed the significant difference between cycle 23 and the previous ones in the behavior of the magnetic field. The cycle-by-cycle decrease in the geoeffectiveness of coronal ejections discussed in the press deserves a more detailed analysis using extensive data on magnetic activity indices.

Seasonal and Interhemispheric Effects on the Diurnal Evolution of EIA: Assessed by IGS TEC and IRI-2016 over Peruvian and Indian Sectors

The global total electron content (TEC) map in 2013, retrieved from the International Global Navigation Satellite Systems (GNSS) Service (IGS), and the International Reference Ionosphere (IRI-2016) model are used to monitor the diurnal evolution of the equatorial ionization anomaly (EIA). The statistics are conducted during geomagnetic quiet periods in the Peruvian and Indian sectors, where the equatorial electrojet (EEJ) data and reliable TEC are available. The EEJ is used as a proxy to determine whether the EIA structure is fully developed. Most of the previous studies focused on the period in which the EIA is well developed, while the period before EIA emergence is usually neglected. To characterize dynamics accounting for the full development of EIA, we defined and statistically analyzed the onset, first emergence, and the peaks of the northern crest and southern crest based on the proposed crest-to-trough difference (CTD) profiles. These time points extracted from IGS TEC show typical annual cycles in the Indian sector which can be summarized as winter hemispheric priority, i.e., the development of EIA in the winter hemisphere is ahead of that in the summer hemisphere. However, these same time points show abnormal semiannual cycles in the Peruvian sector, that is, EIA develops earlier during two equinoxes/solstices in the northern/southern hemisphere. We suggest that the onset of EIA is a consequence of the equilibrium between sunlight ionization and ambipolar diffusion. However, the latter term is not considered in modeling the topside ionosphere in IRI-2016, which results in a poor capacity in IRI to describe the diurnal evolution of EIA. Meridional neutral wind’s modulation on the ambipolar diffusion can explain the annual cycle observed in the Indian sector, while the semiannual variation seen in the Peruvian sector might be due to additional competing effects induced by the F region height changes.

Empirical Modeling of Ionospheric Current Using Empirical Orthogonal Function Analysis and Artificial Neural Network

AbstractGiven the potential importance of solar quiet (Sq) ionospheric current in geomagnetic field modeling, it is vital to obtain accurate parameters characterizing its variations, particularly the spatial and temporal variations. In this paper, we derived the Sq current function based on the spherical harmonic analysis (SHA) technique using a 14‐year (2006–2019) quiet geomagnetic field record over the American sector. The empirical orthogonal function (EOF) analysis was then applied to deduce temporal and spatial variations of the Sq current. It is observed that the first EOF mode of the Sq current function is dominated by solar activity, while the second and third EOF modes exhibit annual and semiannual variations, respectively. Also, the artificial neural network (ANN) model of Sq current function was constructed to validate the EOF model predictions. While the Sq current intensity predicted by the ANN model is underestimated by 2.83%, the EOF model underpredicted the Sq current intensity by 1.92% relative to the observation. The root mean square error (RMSE) of the EOF model is 0.64 kA. This RMSE is about 79% smaller than that of the ANN model. In addition, both the EOF and ANN models capture the variation of the total Sq current (Jtotal) intensity with respect to solar activity. In principle, the EOF model had an optimal performance at nearly 98% accuracy, with the ANN model exhibiting almost the same degree of accuracy, which appears to be a reference point for ionospheric conditions when looking for space weather applications.

Seasonal features of geomagnetic activity: a study on the solar activity dependence

Abstract. Seasonal features of geomagnetic activity and their solar-wind–interplanetary drivers are studied using more than five solar cycles of geomagnetic activity and solar wind observations. This study involves a total of 1296 geomagnetic storms of varying intensity identified using the Dst index from January 1963 to December 2019, a total of 75 863 substorms identified from the SuperMAG AL/SML index from January 1976 to December 2019 and a total of 145 high-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) events identified using the AE index from January 1975 to December 2017. The occurrence rates of the substorms and geomagnetic storms, including moderate (-50nT≥Dst>-100nT) and intense (-100nT≥Dst>-250nT) storms, exhibit a significant semi-annual variation (periodicity ∼6 months), while the super storms (Dst≤-250 nT) and HILDCAAs do not exhibit any clear seasonal feature. The geomagnetic activity indices Dst and ap exhibit a semi-annual variation, while AE exhibits an annual variation (periodicity ∼1 year). The annual and semi-annual variations are attributed to the annual variation of the solar wind speed Vsw and the semi-annual variation of the coupling function VBs (where V = Vsw, and Bs is the southward component of the interplanetary magnetic field), respectively. We present a detailed analysis of the annual and semi-annual variations and their dependencies on the solar activity cycles separated as the odd, even, weak and strong solar cycles.

Crustal vertical deformation of Amazon Basin derived from GPS and GRACE/GFO data over past two decades

In this study, the Gravity Recovery and Climate Experiment (GRACE) satellite observations, combining 71 continuous Global Positioning System (CGPS) data, are used to detect surface vertical loading deformation of the Amazon Basin during 2002–2020. The results show that the maximal annual amplitude of the surface mass changes derived by GRACE is more than 80 cm in terms of the equivalent water height (EWH) in the Amazon Basin. Most part of Amazon experiences mass gain, especially the Amazon River, while there is little mass loss in the northern and eastern parts. Through the Pearson correlation analysis, the monthly de-trended time series of GPS-observed vertical deformation and GRACE-derived mass loading are in good agreement with an average correlation coefficient of about 0.75 throughout the Amazon region. The common seasonal signals of GPS vertical displacements and GRACE/GFO loading deformations are extracted using the stack averaging. The two kinds of common seasonal signals show a good consistency, and together indicate approximate 20 mm peak-to-peak seasonal amplitude. Strong annual variations are identified both in the monthly GPS and GRACE/GFO data by the wavelet analysis. However, the time-frequency spectrum of GPS has more signal details and more significant semi-annual variations than that of GRACE/GFO. These results may contribute to the understanding of secular crustal vertical deformation in the Amazon Basin.

Relating 27-Day Averages of Solar, Interplanetary Medium Parameters, and Geomagnetic Activity Proxies in Solar Cycle 24

Correlations between solar, interplanetary-medium parameters, and geomagnetic-activity proxies in 27-day averages (a Bartels rotation) were analyzed for the 2009 – 2016 time interval. In this analysis, two new proxies were considered: i) \(B_{zs}\) GSM (Geocentric Solar Magnetic), calculated as the daily percentage of the IMF southward component along the GSM \(z\)-axis and then averaged every 27 days; ii) four magnetospheric indices (T-indices), calculated from the local north–south (\(x\)) contributions of the magnetosphere’s cross-tail (TAIL), the symmetric ring current (SRC), the partial ring current (PRC), and the Birkeland current (FAC), derived from the Tsyganenko and Sitnov (J. Geophys. Res. 110, A03208, 2005: TS05) semi-empirical magnetospheric model. Our results suggest that, among the parameters tested in this study, solar facular areas, interplanetary magnetic-field intensity and new proxies derived from the TS05 model could be taken into account in an empirical model, with a 27-day resolution, to explain geomagnetic activity felt on the Earth’s surface in terms of solar-surface features and the IMF condition. We further retrieve a clear annual oscillation in series of 27-day-mean values of toward/away asymmetries of geomagnetic-activity indices, which can be interpreted in the light of the Russell–McPherron hypothesis for the semiannual variation of geomagnetic activity.

Geophysical Signal Detection in the Earth’s Oblateness Variation and Its Climate-Driven Source Analysis

This study analyzes the geophysical signals in J2 time series from 1976 to 2020 by using singular spectrum analysis (SSA) and the Lomb-Scargle (L-S) periodogram for the first time. The results of SSA indicate that the secular trend is characterized by a superposition of the secular linear decrease with a rate of approximately (−5.80 ± 0.08) × 10−11/yr and an obvious quadratic rate of (2.38 ± 0.02) × 10−13/yr2. Besides, the annual, semi-annual, and 10.6-year signals with determining for the first time its amplitude of 5.01 × 10−11, are also detected by SSA, where their stochastic behavior can be maintained to the greatest extent. The 18.6-year signal cannot be detected by SSA even when the window size of 18.6 years was selected, while L-S periodogram can detect the signal of 18.6 years after removing the 18.6-year tidal theoretical value and the linear trend, proving the existence of the tidal variations of 18.6 years in the residual time series. Nevertheless, the 10.6-year signal can be found only after removing the secular trend. This fact suggests that the advantages of different methods used will lead to different sensitivity to the particular signals hard to be detected. Finally, the reconstructed ΔJ2 time series through the sum of the climate-driven contributions from glacial isostatic adjustment (GIA), Antarctic ice sheets (ANT), atmosphere (ATM), continental glaciers (GLA), Greenland ice sheets (GRE), ocean bottom pressure (OBP), and terrestrial water storage (TWS) by using GRACE gravity field solution and geophysical models agrees very well with that of the observed ΔJ2 from SLR in terms of the amplitude and phase. About 81.5% of observed ΔJ2 can be explained by the reconstructed value. ATM, TWS, and OBP are the most significant contributing sources for seasonal signals in ΔJ2 time series, explaining up to 40.1%, 31.9%, and 26.3% of the variances of observed ΔJ2. These three components contribute to the annual and semi-annual variations of the observed ΔJ2 up to 30.1% and 1.6%, 30.8% and 1.0%, as well as 25.4% and 0.7%, respectively. GRE, ANT, and GLA have ~3 to ~7-year periodic fluctuations and a positive linear trend, excluding GIA.

Activities of Small‐Scale Gravity Waves in the Upper Mesosphere Observed From Meteor Radar at King Sejong Station, Antarctica (62.22°S, 58.78°W) and Their Potential Sources

AbstractGravity wave (GW) activities in the upper mesosphere (80–100 km) and their potential sources are investigated using meteor radar observations at King Sejong Station, Antarctica (KSS; 62.22°S, 58.78°W) during recent 14 years (2007–2020). GW activities are estimated by horizontal wind variances of small‐scale GWs (periods <2 h, horizontal wavelength <400 km, or vertical wavelength <3–5 km). The wind variances show clear semiannual variations with maxima at solstices, and annual variations are also seen above z = 90 km. The deseasonalized wind variances at z = 96.8 km have a statistically significant periodicity of ∼11 years that can be associated with solar cycle variations. Three major potential GW sources in the lower atmosphere are examined. Orography is a potential source of GWs in winter and autumn, when the basic‐state wind is westerly from the surface up to the mesosphere. The residual of the nonlinear balance equation (RNBE) at 5 hPa, a diagnostic of the GWs associated with jet stream, is the largest in winter and has a secondary maximum in spring. The correlation between the observed GWs and RNBE is significant in equinoxes, while correlation is low in winter. Deep convection in storm tracks is a potential source in autumn and winter. Secondary GWs generated in the mesosphere can also be observed in the upper mesosphere. Ray‐tracing analysis for airglow images observed at KSS indicates that secondary GWs are mostly generated in winter mesosphere, which may be associated with the breaking of orographic GWs.

On the semi-annual variation of relativistic electrons in the outer radiation belt

Abstract. The nature of the semi-annual variation in the relativistic electron fluxes in the Earth's outer radiation belt is investigated using Van Allen Probes (MagEIS and REPT) and Geostationary Operational Environmental Satellite Energetic Particle Sensor (GOES/EPS) data during solar cycle 24. We perform wavelet and cross-wavelet analysis in a broad energy and spatial range of electron fluxes and examine their phase relationship with the axial, equinoctial and Russell–McPherron mechanisms. It is found that the semi-annual variation in the relativistic electron fluxes exhibits pronounced power in the 0.3–4.2 MeV energy range at L shells higher than 3.5, and, moreover, it exhibits an in-phase relationship with the Russell–McPherron effect, indicating the former is primarily driven by the latter. Furthermore, the analysis of the past three solar cycles with GOES/EPS indicates that the semi-annual variation at geosynchronous orbit is evident during the descending phases and coincides with periods of a higher (lower) high-speed stream (HSS) (interplanetary coronal mass ejection, ICME) occurrence.

Simultaneous Observation of Sporadic Potassium and Sodium Layers Over São José dos Campos, Brazil (23.1°S, 45.9°W)

AbstractSporadic potassium (Ks) and sodium (Nas) layers were investigated using more than 1,500 h of simultaneous Na/K LIDAR data at São José dos Campos, Brazil (23.1°S, 45.9°W). These events were observed on more than 88% of the analyzed nights. Although Na and K are both alkali metals, their sporadic layers show different behaviors. The Nas layers were in general 0.7 km wider and occurred ∼2 km higher than Ks layers. Moreover, the Nas layers progression is faster than that of Ks layers not only in the process of reaching their maximum amplitude but also in terms of dissipation. Our study showed that most Ns events (80% of Ks layers and 90% of Nas layers) have a strength factor of upto 5. Moreover, on average, Ks presented a strength factor ∼1.3 times stronger than that of Nas. The relative frequency of occurrence for both Ks and Nas layers exhibits an almost semiannual variation, with maxima in January/October and February/September, respectively. On the other hand, no similar variation was observed in the peak density or in the peak height. The peak density shows an annual variation, with a maximum occurring during winter for Ks events and a semiannual variation with a maxima occurring at the equinoxes for Nas. Different seasonal behaviors were also seen in the peak height of the identified layers, with Ks presenting a semiannual variation with maxima around January/October, while an annual variation occurred in Nas with lower layers occurring around winter.

A comparative study of the northern and southern equatorial ionization anomaly crests in the East-Asian sector during 2006–2015

The GPS observation data measured at GXUN (22.84°N, 108.33°E, MLAT ~ 12.14°N) and BAKO (6.49°S, 106.84°E, MLAT ~ 17.18°S) stations from 2006 to 2015 are utilized to make a comparative study of the northern and southern equatorial ionization anomaly (EIA) crests in the East-Asian sector. The intensity of northern EIA crest is similar to that of southern EIA crest during the solar minimum years, while during the solar maximum years, the intensity of northern EIA crest is a little stronger than that of southern EIA crest except in summer season of northern hemisphere. The geomagnetic latitude (MLAT) of northern EIA crest shows a weak semiannual variation with the highest in equinoctial months and the lowest in solistical months, but the MLAT of southern EIA crest shows a clear annual variation with the highest in summer season of southern hemisphere and the lowest in winter season of southern hemisphere, which is particularly obvious during the solar minimum years. The intensities of northern and southern EIA crests have good correlation, which is independent of the solar activity, but the correlation coefficients of MLAT between northern and southern EIA crests increase with the solar activity. A comprehensive analysis of the previous studies and our results indicate that the intensity, MLAT, and interhemispheric asymmetry of EIA crest are mainly dominated by photochemical process, equatorial fountain effect, and neutral wind, respectively.

Semiannual variation in relativistic electron fluxes of the outer radiation belt: Phases comparison with classical hypotheses predictions

Abstract Relativistic electron fluxes of the outer Van Allen belt present a clear semiannual signature, with maxima fluxes near the equinoxes and minima near the solstices. We investigate the semiannual variation (SAV) of 3.75 MeV and 3.4 MeV electron fluxes from Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and Van Allen probe A (RBSP-A) data, respectively. The fluxes of both missions are binned in McIlwain’s L-shell values and in day of year (DOY). In order to obtain the SAV from the data, a superposed epoch analysis and then a 30-day smoothing is carried out for the entire L-shell range utilized. The results show clear semiannual patterns with maximum differences of ~ 1 and ~ 2 orders of magnitude between values near the equinoxes and solstices, for SAMPEX and RBSP-A fluxes, respectively. The comparison of the maxima-minima dates of the observed SAVs with corresponding dates predicted by the Axial, Equinoctial and Russell & McPherron (RM) theory shows that the latter gives the best estimations for both data sets. However, considerable delays from the theories’ predictions are found. The most important delay occurs in the Mar/Apr maximum and is found to be ~ 26.8 days on average for L ≳ 3.5 RBSP-A data. In the case of SAMPEX, this delay is found for L 3.8 with a maximum of 22.4 days. Accurately determining the Mar/Apr maximum and the Sep/Oct maximum has implications in space weather since it indicates when the SAV will affect the most particular flux enhancements capable of inducing satellite operational anomalies. .

Transient variations of vertical total electron content at low latitude during the period 2013–2017

Abstract We use GPS networks to measure the vertical Total Electron Content (VTEC) variations at low latitude, in three longitude sectors: America, Europe-Africa and Asia, collected during the period 2013–2017. This period corresponds to the increasing phase of the solar cycle 24 (SC#24) observed around 2013–2014 as well as the decreasing phase around 2014–2017. Our results discussed a morphological analysis of regular variations in ionization during different phases of solar activity: daytime variations, seasonal and semi-annual variations and variations based on the solar cycle 24 in three longitude sectors. In all longitude sectors, the highest VTEC values are displayed during the two months of the spring, located after sunrise and before sunset. The lowest values are found during the summer and winter seasons. We found that the winter anomaly and the presence of equinoxial peaks are the most pronounced effects in VTECs in the increasing and decreasing phase of the SC#24. A strong asymmetry is detected between equinoxial peaks and the location of peaks occurring in March/April and October/November at maximum in the solar flux variations during the increase phase. We show that the daily VTEC maximum values were registered between 14:00 and 16:00 LT and the minimum values between 4:00 and 6:00 LT. Double ionization peak in the morning and evening is observed in VTEC annual variations, due to the proximity of the equatorial fountain stations. From the statistical analysis part, we observed practically the same distribution of the different classes of VTEC (two peaks, bell-shaped and plateau-shaped) variations in the three sectors of longitude. These observations indicate longitudinal variation in the presence of the winter anomaly in the Equatorial Ionized Anomaly (EIA) region. Additionally, we can note a longitudinal variation of the spring-autumn VTEC asymmetry in the EIA region during the five years 2013–2017. We observe also that the occurrence of nocturnal peak recorded around 19 local time (LT) shows the same characteristics as the vertical drift E × B (B: magnetic field is perpendicular to E: electric field.) with respect to solar cycle, season and longitude. Three essential characteristics we noted: 1) the occurrence of the nocturnal peak generally follows the solar cycle. 2) The occurrence of the nocturnal peak is generally stronger at the equinoxes than at the solstices. 3) The occurrence of the nocturnal peak is stronger in the Europe-Africa and America sectors than in the Asia sector. As a result, nocturnal peak occurrence is well related to the PRE at the origin of the GNSS signal scintillations.

A Comparative Study of Ionospheric Day‐To‐Day Variability Over Wuhan Based on Ionosonde Measurements and Model Simulations

AbstractIonospheric day‐to‐day variability is essential for understanding the space environment, while it is still challenging to properly quantify and forecast. In the present work, the day‐to‐day variability of F2 layer peak electron densities (NmF2) is examined from both observational and modeling perspectives. Ionosonde data over Wuhan station (30.5°N, 114.5°E; 19.3°N magnetic latitude) are compared with simulations from the specific dynamics Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (SD‐WACCM‐X) and the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) in 2009 and 2012. Both SD‐WACCM‐X and TIEGCM are driven by the realistic 3 h geomagnetic index and daily solar input, and the former includes self‐consistently solved physics and chemistry in the lower atmosphere. The correlation coefficient between observations and SD‐WACCM‐X simulations is much larger than that of the TIEGCM simulations, especially during dusk in 2009 and nighttime in 2012. Both the observed and SD‐WACCM‐X simulated day‐to‐day variability of NmF2 reveal a similar day‐night dependence in 2012 that increases large during the nighttime and decreases during the daytime, and shows favorable consistency of daytime variability in 2009. Both the observations and SD‐WACCM‐X simulations also display semiannual variations in nighttime NmF2 variability, although the month with maximum variability is slightly different. However, TIEGCM does not reproduce the day‐night dependence or the semiannual variations well. The results emphasize the necessity for realistic lower atmospheric perturbations to characterize ionospheric day‐to‐day variability. This work also provides a validation of the SD‐WACCM‐X in terms of ionospheric day‐to‐day variability.

ENSO phase-locking behavior in climate models: from CMIP5 to CMIP6

The phase-locking behavior of El Niño-Southern Oscillation (ENSO) in models from Coupled Model Intercomparison Project (CMIP) phase 5 to phase 6 is assessed in terms of the locking-month of ENSO peak and the sharpness of locking tendency. Overall, a robust improvement exists in CMIP6. Compared to CMIP5, more CMIP6 models truly reproduce the locking-month in November-January. Meanwhile, the sharpness of phase-locking in CMIP6 models also improves, though most of them are still far from the observations. The locking-month is verified to be highly corresponding to the phase of seasonal modulation of ENSO’s instabilities. The sharpness is mainly controlled by the intensity of this modulation and noise. Compared to CMIP5, CMIP6 models generally simulate these affecting factors better. Besides, models displaying an exaggerated semi-annual variation of ENSO’s instabilities simulate the ENSO phase-locking relative-poorly, and these models show no reduction from CMIP5 to CMIP6.

An empirical model of the Saturn thermosphere

We present an empirical model of H2 densities and temperatures above the homopause for the Saturn thermosphere, based on stellar occultations observed by the Cassini/UVIS instrument. These observations span the whole Cassini mission and probe a wide range of latitudes, longitudes and local times. Although the model is based on Cassini stellar occultations only, the results agree well with Voyager occultations and reasonably well with other measurements of temperature and density in the thermosphere. The principal meridional trend that the model captures is an increase in temperature with latitude away from the equator, associated with significantly elevated density level altitudes at latitudes higher than about 40° in both hemispheres. These results confirm the importance of auroral heating in Saturn’s thermosphere and constrain models of global circulation. We estimate the uncertainty in the model based on the standard deviation of the data points around the mean state. We do not attempt to distinguish retrieval errors from spatial and temporal trends. We find that the most important source of uncertainty is an apparent semi-annual variation in H2 density and temperature, associated with expansion and contraction of the pressure levels in the thermosphere. Similar variation occurs in the Earth’s thermosphere where it is likely driven by seasonal changes to global circulation. The model presented here is a useful tool that constrains physical models, helps to interpret existing observations, and supports the planning of future observations and missions.

Semi-annual variation of excited hydroxyl emission at mid-latitudes

Abstract. Ground-based observations show a phase shift in semi-annual variation of excited hydroxyl (OH∗) emissions at mid-latitudes (43∘ N) compared to those at low latitudes. This differs from the annual cycle at high latitudes. We examine this behaviour by utilising an OH∗ airglow model which was incorporated into a 3D chemistry–transport model (CTM). Through this modelling, we study the morphology of the excited hydroxyl emission layer at mid-latitudes (30–50∘ N), and we assess the impact of the main drivers of its semi-annual variation: temperature, atomic oxygen, and air density. We found that this shift in the semi-annual cycle is determined mainly by the superposition of annual variations of temperature and atomic oxygen concentration. Hence, the winter peak for emission is determined exclusively by atomic oxygen concentration, whereas the summer peak is the superposition of all impacts, with temperature taking a leading role.

Effect of meteorology on the variability of ozone in the troposphere and lower stratosphere over a tropical station Thumba (8.5°N, 76.9°E)

Seasonal variability in the vertical distribution of ozone over a tropical station, Thumba (8.5°N, 76.9°E) is investigated using nine-years (2011–2019) of ECC ozonesonde data obtained as part of Tropical Tropopause Dynamics (TTD) campaigns. Ozone exhibits a clear annual variation in the lower troposphere (0–2 km) with winter maximum (49.2 ± 3.6 ppbv) and summer monsoon minimum (21.9 ± 0.7 ppbv). In the middle (2–10 km) and upper (11–17 km) troposphere, ozone is maximum in pre-monsoon and minimum in summer monsoon. Ozone in the lower stratosphere exhibits a clear annual variation with summer monsoon maximum and winter minimum in tandem with the temperature cycle. The relative standard deviation (RSD) of ozone computed at different altitudes exhibits large intra-seasonal variability in the troposphere (~20–30%) compared to the stratosphere (~5%) and is significantly high close to the surface and in the tropical tropopause layer (TTL) (≥30%). Maximum RSD is observed during summer monsoon (June–September) in the upper troposphere (~50%) close to cold point tropopause (CPT). RSD in ozone and temperature profiles show a sharp peak immediately above the CPT in all the seasons, coinciding with the altitude of maximum wind shear. Tropospheric ozone shows an in phase relationship with temperature and is opposite in phase with water vapour. A significant negative correlation is observed between ozone and water vapour in the 2–5 km altitude region. Long-range transport and local convection contribute substantially to the observed variability in tropospheric ozone. The tropospheric column ozone shows a semi-annual variation with maximum contribution to the total columnar ozone during pre-monsoon (~16%) and minimum during summer monsoon (~8%). Mid-tropospheric ozone contributes more to the tropospheric column ozone (40–60%) compared to lower and upper tropospheric ozone. Tropospheric column ozone exhibits a similar seasonality as that of its precursors like NO2, CO and CH4.