Semiannual Variation Research Articles

Intercomparison of In Situ and Satellite Sea Surface Salinity Products for Global Coastal Ocean Studies

Abstract Observing coastal ocean salinity is crucial for understanding the physical and biological processes impacting ocean circulation, ecosystems, and the hydrological cycle. We perform a thorough comparison between in situ and satellite sea surface salinity (SSS) products with a focus on global coastal ocean. Between 40 and 500 km from the coastline, we intercompare SSS from more than two million World Ocean Database (WOD) salinity profiles, gridded Argo data, and satellite products: NASA Multimission Optimally Interpolated Sea Surface Salinity (OISSS), 40- and 70-km Remote Sensing Systems (REMSS) Soil Moisture Active Passive (SMAP), Jet Propulsion Laboratory (JPL) SMAP, Laboratory of Oceanography and Climate: Experiments and Numerical Approaches (LOCEAN) Soil Moisture Ocean Salinity (SMOS), and European Space Agency (ESA) Climate Change Initiative (CCI) salinity. We find good agreement between the satellite products and in situ data with satellite-in situ mean differences (0.5–1.7 pss, where pss is the Practical Salinity Scale), root-mean-square differences (0.5–2.4 pss), and signal-to-noise ratios (1–3.8) generally decreasing at a faster rate from 40- to 100-km distance to the coast and remaining constant past 100 km. NASA OISSS, ESA CCI, 70-km REMSS SMAP, and LOCEAN SMOS have minimum differences with in situ observations. Empirical orthogonal function (EOF) analysis shows that all the satellite and in situ SSS compare well in capturing mode-1 and mode-2 seasonal variability but have notable differences in the amplitude of the mode-3 semiannual variability in coastal SSS. This study has implications for validation and improvement of satellite SSS and emphasizes the need for designing future satellite missions to better resolve the coastal salinity.

Seasonal Cycle in Sea Level Across the Coastal Zone

AbstractData from tide gauges and satellite altimeters are used to provide an up‐to‐date assessment of the mean seasonal cycle in sea level () over most of the global coastal ocean. The tide gauge records, where available, depict a seasonal cycle with complex spatial structure along and across continental boundaries, and an annual oscillation dominating over semiannual variability, except in a few regions (e.g., the northwestern Gulf of Mexico). Comparisons between tide gauge and altimeter data reveal substantial root‐mean‐square differences and only slight improvements in agreement when using along‐track data optimized for coastal applications. Quantification of the uncertainty in the altimeter products, inferred from comparing gridded and along‐track estimates, indicate that differences to tide gauges partly reflect short‐scale features of the seasonal cycle in proximity to the coasts. We additionally probe the seasonal budget using satellite gravimetry‐based manometric estimates and steric terms calculated from the World Ocean Atlas 2023. Focusing on global median values, the sum of the estimated steric and manometric harmonics can explain 65% (respectively 40%) of the annual (semiannual) variance in the coastal observations. We identify several regions, for example, the Australian seaboard, where the seasonal budget is not closed and illustrate that such analysis is mainly limited by the coarse spatial resolution of present satellite‐derived mass change products. For most regions with a sufficiently tight budget closure, we find that although the importance of the manometric term generally increases with decreasing water depth, steric contributions are non‐negligible near coastlines, especially at the annual frequency.

Long-term changes of sodium column abundance at 24.6°S above the Atacama Desert in Chile

Aims. The utilisation of artificial laser guide star (LGS) obviates the necessity for a prominent natural guide star (NGS) within adaptive optics (AO) systems. High-power lasers are fundamental components of most AO systems today. The generation of an LGS relies on the excitation of sodium (denoted by its symbol Na) atoms situated in the upper atmosphere. Therefore, the sodium vertical column density (denoted as CNa) is a crucial parameter. Beyond ensuring the optimal and stable performance of an AO system, knowledge of the return flux from an LGS is imperative during the design phase, aiding in the accurate specification of both the LGS and the AO system. The availability of sodium in the upper atmosphere has been the focal point of diverse studies, exhibiting a pronounced dependence on the specific observatory site. Furthermore, it is well established that CNa varies across multiple timescales, including hours, nights, months, seasons, and even several years. As many of the world’s largest telescopes are located in the Atacama Desert in northern Chile, our objective is to provide CNa statistics pertinent to this specific region. Methods. We used telemetry data from the AO systems operational at the Paranal Observatory (24.6°S, 70.4°W): Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging (GALACSI) and Ground layer Adaptive Optics system Assisted by Lasers (GRAAL). We combined these data with measurements from two space instruments: SCanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) and Optical Spectrograph and Infrared Imaging System (OSIRIS), as well as with simulated data from the Whole Atmosphere Community Climate Model (WACCM). We carefully analysed and compared these datasets to develop a statistical model for the temporal variations of CNa. Results. We validated the use of the AO telemetry data from Paranal systems to retrieve the CNa. The near-continuous measurements encompassing the period from mid-2017 to the end of 2023 facilitated the determination of monthly and yearly abundance and variability of Na in the mesopause region. Throughout the complete years of measurement, the annual and semi-annual variations exhibit consistent characteristics that align with previously documented findings in atmospheric studies. Through meticulous comparison and the fitting of various long-term datasets, we formulated a model depicting the evolution of CNa over time. The validity of our data processing and model is scrutinised, and the results obtained for the Paranal latitude exhibit noteworthy concordance with the findings of other studies.

Exploring ionospheric plasma density trends in the Indian equatorial crest region under varying solar activity conditions

Long-term trends in the evolution of ionospheric plasma at Hyderabad (17.38∘N, 78.48∘E; 8.52∘N Magnetic Latitude), a near-equatorial anomaly (EIA) crest region of the Indian ionospheric sector, have been studied using 9 years of vertical Total Electron Content (TEC) data from 2004 to 2009 and 2011 to 2013 using global positioning satellites (GPS) measurements. The study examined the mean diurnal, monthly, seasonal, and yearly variations of TEC during geomagnetic quiet days in different seasons from 2004 to 2013. The findings reveal that the daytime TEC at the anomaly crest region exhibits semi-annual variations throughout the study period, while midnight TEC shows semi-annual variation only during the high solar activity years of 2011–2013. The winter anomaly was observed in 2004 and 2006. The study also assessed the performance of the International Reference Ionosphere (IRI) 2016 model in reproducing GPS TEC variability at the equatorial crest region. The diurnal and seasonal variation patterns in IRI-TEC show a good correlation with GPS TEC. However, the IRI 2016 model tends to overestimate TEC values during low solar activity conditions (2006–2009) but represents TEC variations reasonably well during high solar activity periods (2011–2013). Nevertheless, the IRI model fails to capture the wide plateau-like structure in the peak TEC, typically occurring between 1200–1600 IST at Hyderabad. Additionally, IRI-TEC consistently indicates very low TEC values during the early morning hours, whereas GPS-TEC measurements suggest a significant presence of plasma density. The study suggests a strong influence of the solar cycle on TEC variations at Hyderabad, evident from the positive correlation (R2= 0.71) with the F10.7 cm index. This characteristic is also well represented by the IRI 2016 model.

Spatial-temporal variation of water vapor scale height and its impact factors in different climate zones of China

Water vapor scale height reflects the vertical distribution of water vapor in the atmosphere, which is a vital parameter in tropospheric zenith wet delay (ZWD) modeling and GNSS meteorology. This study accurately calculated the long-term water vapor scale height using 89 radiosonde stations in China, analyzed the characteristics in temporal and spatial variations and explored the impact factors of the water vapor scale height from the perspective of the climate zones. The numerical results indicate that the water vapor scale height in China exhibits annual and semiannual variations with a rising-then-decreasing trend across the four seasons. A relatively larger value for the water vapor scale height were observed in the subtropical monsoon climate and the temperate continental climate zone, in which the values are greater than 2.35 km, while the values in temperate monsoon climate and the alpine plateau climate zone are relatively small. In addition, the water vapor scale height in different climate zones shows different trends, with a downward trend appeared in the subtropical monsoon climate and the alpine plateau climate zone, while an upward trend was observed in the temperate monsoon climate and the temperate continental climate zone. Furthermore, a positive correlation between water vapor scale height and temperature, pressure, and atmospheric precipitable water was found in China, with a varied degree of correlation in different climate zones.

Intermittency of Gravity Wave Potential Energy Generated by Mountains Revealed from COSMIC-2 Observations

The intermittency of gravity wave potential energy (GWPE) in the upper troposphere and stratosphere was investigated using the Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) temperature data over three typical mountains (Tibetan Plateau, Rocky Mountains, and Andes). These typical mountains have high sea level elevations but different land–sea contrast. The probability density function (PDF) of GWPE has the independent variable of GWPE and dependent variable of occurrence probability of GWPE over a region. Our analysis showed that the PDFs of GWPE over these three mountains roughly followed lognormal distributions in all heights and months. But, the key parameters (mean value and standard deviation) of lognormal distribution varied with heights and months. Above each mountain, the two key parameters exhibited similar temporal and spatial distributions. They had the largest values around the tropopause region, smaller values in the lower stratosphere (~20–30 km), and larger values in the upper stratosphere (~35–45 km). The intermittency of GWs is represented as the ratio of the GWPE at 50th percentile to the GWPE at 90th percentile. The weakest intermittency was at ~20–30 km (above the zonal mean winds of zero) over the Tibetan Plateau and Rocky Mountains in all months and over the Andes from November to March, respectively. Generally, the weakest intermittency (~0.4) occurred in the region where the key parameters were the smallest around summer. The key parameters of lognormal distribution were dominated by annual variation over the Andes throughout the height range, 8–50 km. However, the semiannual variations are also significant in the lower stratosphere over the Tibetan Plateau and Rocky Mountains. The seasonal variations in the intermittency were not as obvious as those of the key parameters. The lognormal distributions and the intermittencies derived here provide an observational constraint on the tunable parameters in GW parameterization schemes.

Exploring ionospheric dynamics: a comprehensive analysis of GNSS TEC estimations during the solar phases using linear function model

Abstract The modeling and forecasting of Total Electron Content (TEC) play a major role in influencing signals from satellite-based navigation systems and impact the performance of diverse satellite-dependent technologies. The intensity of solar ionizing radiation and the state of geomagnetic field activity influence the Global Navigation Satellite System (GNSS)-TEC. This paper uses a Linear TEC Function (LTF) climatology model to understand ionospheric behavior under solar and geomagnetic activities that cause variations in the electron distribution of the ionosphere medium. The LTF model integrates representations of solar EUV photon (MgII) and geomagnetic (SYMH) activities, incorporating solar-modulated oscillations (periodic variations) at four seasonal cycles and a linear trend. The LTF model examined the time series of GPS-TEC at a location (geographic 34.95° N, 134.05° E) with a time resolution of 1 h, from 1997 to 2016, covering solar cycles 23 and 24. The Root Mean Square Deviation (RMSD) and correlation coefficient between the GNSS-TEC and model TEC (LTF) was 5.30 TECU and 95 %. The results indicate that solar components, as well as annual and semi-annual variations, have a significant impact on the daily average TEC. Solar activity appears to be the predominant determining factor of TEC during the solar phases of cycles 23 and 24. In contrast, periodic influences primarily outline TEC during periods characterized by minimal solar activity. The geomagnetic component presents an increased influence, particularly during storm periods. The model demonstrates superior performance in Total TEC modeling compared to other state-of-the-art approaches.

Variability of the equatorial ionization anomaly over the South American sector: Effects of electric field and effective meridional wind

Vertical Total Electron Content (VTEC) Maps over the South American Continent were utilized to investigate the temporal and longitudinal climatology of Equatorial Ionization Anomaly (EIA) using more than 350 Global Navigation Satellite Systems (GNSS) receivers. At a temporal resolution of 10 min, the EIA motions, morphologies, and evolutions were mapped using VTEC keogram along magnetic meridians lines. Between 2014 and 2019, characteristics of the EIA were studied at two different South American magnetic meridians (i.e., 3.36° E and 7.58° E) separated by ∼555 km at an altitude of 300 km. The aim of this study is to examine the EIA’s variability, monthly variations and occurrences at evenly spaced longitudinal sectors. The effects of effective meridional winds component and E × B drift velocity on the daytime asymmetry of EIA anomalies were studied using a physics-based numerical model, Sheffield University Plasmasphere-Ionosphere model at Instituto Nacional de Pesquisas Espaciais (SUPIM-INPE). We found that the EIA parameters such as strength, shape, intensity, and latitudinal positions are affected by the eastward electric field and effective meridional wind. The monthly variations in the EIA over two magnetic meridian sectors demonstrate a semiannual variation. The EIA crests were more symmetric in equinox than in solstice seasons. The asymmetries of the EIA observed during the December solstice are more intense than during the June solstice, whereas September equinox is less symmetric than March equinox seasons. Moreover, this study indicates that the vertical drift and the meridional neutral wind plays a very significant role in the development of the EIA asymmetry by transporting the plasma up the field lines. There was a notable contraction of the EIA southern hemispheric (SH) crests from the December solstice to the June solstice. Meanwhile, the EIA crest positions in the northern hemisphere (NH) expand from the December solstice to the June solstice. According to our observations, the March equinox season had the most EIA occurrences, which were then followed by the September equinox, the December and June solstices. The intensities of the EIA crests also considerably decreased with solar descending phases. Through modeling, this work provides the scientific community with new insights into the evolution/development of EIA and their latitudinal asymmetry, as well as the role of E × B drift and thermospheric neutral wind in assessing the statistical analysis of EIA variability using the largest VTEC database over the South American sector.

120-day variability in migrating tides from CMAM30 winds

Migrating tides and their variability are investigated using Canadian Middle Atmosphere Model (CMAM30) zonal and meridional wind data from 1979 to 2010. Migrating diurnal (DW1) and semi-diurnal (SW2) tides show strong annual and semi-annual variations. The former is stronger in the Southern Hemisphere and the latter is significant over sub-tropical latitudes in the mesosphere. Wavelet analysis in the mesopause region shows a 120-day variability in SW2 tide over equator and ∼±20∘ with no apparent connection with solar activity. A close inspection shows that the 120-day variation is not due to variations in solar energy inputs.

Mesosphere and Lower Thermosphere Temperatures Simulated by WACCM‐X With NAVGEM‐HA Meteorological Analyses and Compared to SABER Observations

AbstractRealistic modeling of the dynamics and variability in the upper mesosphere and lower thermosphere (UMLT) is critical to understand the coupling between different layers of the whole atmosphere system. Here we present simulations of the UMLT temperatures at ∼100 km altitude for one year during 2014 by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere extension (WACCM‐X) constrained below ∼90 km using meteorological analysis products of the high‐altitude version of Navy Global Environmental Model (NAVGEM‐HA). The model results are sampled at the same times and latitudes and longitudes as the satellite observations from Thermosphere Ionosphere and Mesosphere Electric Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER). Comparisons show that the observed and modeled daily zonal mean temperatures are correlated (r ∼0.5–0.7) at most latitudes between ±50°. Both the observations and simulations show an annual variation at mid‐latitudes in two hemispheres with the temperature maximum in summer and the minimum in winter, and at lower latitudes the semiannual variation becomes stronger having the temperature maximums at equinoxes and the minimums during solstices. However, the temperatures observed are on average ∼5–10 K (3%–5%) smaller than the model and the observations show a larger variability. Moreover, migrating tidal amplitudes are mostly overestimated by the model. Though differences are noticed, the WACCM‐X simulations with NAVGEM‐HA meteorological analyses are overall consistent with the SABER observations. These results support that whole atmosphere models informed by high altitude observations would help to simulate the UMLT variability and the atmosphere and ionosphere coupling.

Passive Optical Observation of Mesosphere and Thermosphere Wind Over Three Stations in China

AbstractFabry Perot interferometer (FPI) is an essential ground‐based passive optical observation device to detect middle and upper atmospheric information. Three FPIs are located at Kunming (103.8°E, 25.6°N), Xinglong (117.4°E, 40.2°N), and Mohe (122.3°E, 53.5°N), China. The diurnal and annual variation of night wind at 87, 97, and 250 km are investigated from 2019 to 2021, compared to Horizontal Wind Model 14 (HWM14) to check the prediction accuracy for the local wind field. Our results are as follows: (a) At 87 km, the zonal winds are similar in Kunming and Xinglong, but the meridional winds are generally stronger in Kunming. The zonal and meridional winds in both locations are dominated by semidiurnal variations. (b) At 97 km, the duration of semidiurnal variation of zonal winds and diurnal variation of meridional winds in Kunming is long, which is opposite to that of Xinglong. (c) At 250 km, the wind speed increases with latitude for both zonal and meridional winds, which are both dominated by diurnal variations for all the three sites. Unlike 87 and 97 km, the entire wind field at 250 km is dominated by annual variation, except for a significant semiannual variation at midnight in Mohe. (d) Overall, the HWM14 predictions are stronger than the FPI measurements at peak wind speeds and similar at 87 and 97 km, which surpasses the performance at 250 km. Especially at 250 km, the model results are worse for the zonal winds of Kunming and the meridional winds of Mohe. These results will help to understand the wind field in the middle and upper atmosphere and improve the accuracy of the model.

Seasonal Variation in the Mesospheric Ca Layer and Ca+ Layer Simultaneously Observed over Beijing (40.41°N, 116.01°E)

In March 2020, an all-solid-state dual-wavelength narrow-band lidar system was deployed. A total of 226 nights spanning from March 2020 to July 2022 were employed in order to investigate the seasonal variations of calcium atoms and ions in the mesosphere over Beijing (40.41°N, 116.01°E). The Ca+ layer shows general annual variation, while a semiannual variation is observed on the Ca layer. The calcium atomic column densities ranged from 2.0 × 106 to 1.1 × 108 cm−2, and the calcium ion column densities ranged from 1.6 × 106 to 4.2 × 108 cm−2. The mean centroid heights of Ca+ and Ca are 98.6 km and 93.0 km, respectively, and the centroid heights of Ca+ and Ca are mostly influenced by annual variations. The seasonal variation in the Ca+ and Ca layers in Beijing exhibits similarities to that of Kühlungsborn (54°N). While the peak density of Ca+ in Beijing are similar to those observed in Kühlungsborn, the peak density of the Ca layer in Beijing is about half of that reported in the Ca layer at 54°N. We provide an explanation for the disparities in the column abundance and centroid altitude of the Ca layer between Yanqing and Kühlungsborn, discussing variations in neutralization among different metal ions.

Spatiotemporal distribution and impact factors of GNSS-PWV in China based on climate region

Water vapor plays a pivotal role in the intricate processes of energy exchange and climate dynamics. However, a comprehensive understanding of the spatial distribution and temporal variability of water vapor in distinct climatic regions of China is still limited due to the intricate interplay of complex topography and diverse climatic conditions. This study investigates the multiscale variability of water vapor using 10-year 1-hour data of precipitable water vapor (PWV) from 248 observation stations of the Crustal Movement Observation Network of China (CMONOC). Significant variations in PWV distribution among China's climate regions are observed. Average PWV values vary considerably across regions, with the Tropical and Subtropical Monsoon Climate (TSMC) having the highest mean PWV of 32.09 mm, followed by the Midlatitude Monsoon Climate (MMC) with 15.70 mm, the Humid Continental Climate (HCC) with 9.26 mm, and the Plateau Climate (PC) with 7.66 mm. Seasons exhibit summer concentration and winter reduction across regions. Most stations exhibit increasing PWV trends, while a few show decreasing trends. The amplitude of annual PWV variations is higher in TSMC and MMC regions with a range from 8 to 22 mm, compared to HCC and PC regions with a range from 2 to 15 mm. Semiannual variations range from 0 to 6 mm in each region. PWV negatively correlates with latitude, altitude and pressure, particularly in the TSMC. Positive correlations with temperature exist in all regions, with varying strengths. Thermodynamic factors primarily impact PWV, while dynamic factors have secondary influence, equally significant in the PC region.

A Refined Atmospheric Weighted Average Temperature Model Considering Multiple Factors in the Qinghai–Tibet Plateau Region

The Qinghai–Tibet Plateau region has significant altitude fluctuations and complex climate changes. However, the current global weighted average temperature (Tm) model does not fully consider the impact of meteorological and elevation factors on it, resulting in existing models being unable to accurately predict the Tm in the region. Therefore, this study constructed a weighted average temperature refinement model (XTm) related to surface temperature, water vapor pressure, geopotential height, annual variation, and semi-annual variation based on measured data from 13 radiosonde stations in the Qinghai–Tibet Plateau region from 2008 to 2017. Using the Tm calculated via the numerical integration method of radiosonde observations in the Qinghai–Tibet Plateau region from 2018 to 2019 as a reference value, the quality of the XTm model was tested and compared with the Bevis model and GPT2w (global pressure and temperature 2 wet) model. The results show that for 13 modeling stations, the bias and root-mean-square (RMS) values of the XTm model were −0.02 K and 2.83 K, respectively; compared with the Bevis, GPT2-1, and GPT2w-5 models, the quality of XTm was increased by 47%, 38%, and 47%, respectively. For the four non-modeling stations, the average bias and RMS values of the XTm model were 0.58 K and 2.78 K, respectively; compared with the other three Tm models, the RMS values and the mean bias were both minimal. In addition, the XTm model was also used to calculate the global navigation satellite system (GNSS) precipitable water vapor (PWV), and its average values for the theoretical RMSPWV and RMSPWV/PWV generated by water vapor calculation were 0.11 mm and 1.03%, respectively. Therefore, in the Qinghai–Tibet Plateau region, the XTm model could predict more accurate Tm values, which, in turn, is important for water vapor monitoring.

Constructing a Regional Ionospheric TEC Model in China with Empirical Orthogonal Function and Dense GNSS Observation

Using Global Navigation Satellite Systems (GNSS) observation data for developing a high-precision ionospheric Total Electron Content (TEC) model is one of the essential subjects in ionospheric physics research and the application of satellite navigation correction. In this study, we integrate the Empirical Orthogonal Function (EOF) method with the TEC data provided by the Center for Orbit Determination in Europe (CODE), and observed by the dense GNSS receivers operated by the Crustal Movement Observation Network of China (CMONOC) to construct a regional ionospheric TEC model over China. The EOF analysis of CODE TEC in China from 1998 to 2010 shows that the first-order EOF component accounts for 90.3813% of the total variation of the ionospheric TEC in China. Meanwhile, the average value of CODE TEC is consistent with the spatial and temporal distribution characteristics of the first-order EOF base function, which mainly reflects the latitude and diurnal variations of TEC in China. The first-order coefficient after EOF decomposition shows an obvious 11-year period and semi-annual variations. The maximum amplitude of semi-annual variation mainly appears in March and October, which is closely associated with the variation in geographical longitude, the semi-annual change of the low-latitude electric field, and the ionospheric fountain effect. The second-order coefficient has an evident annual variation, the minimum amplitude mainly occurs in March, August, and September, and the amplitude values in the high solar activity years are more significant than those in the low solar activity years. The third-order coefficient mainly shows the characteristics of annual variation, and the fourth-order coefficient shows the noticeable semi-annual and annual variations. The third and fourth-order coefficients are both modulated by the solar activity index F10.7. The ionospheric TEC model in China, driven by CMONOC real-time GNSS observation data, can better reflect the latitude, local time and seasonal variation characteristics of ionospheric TEC over China. In particular, it can clearly show the spring and autumn asymmetry of ionospheric TEC in the low latitudes. The root mean square error of the absolute error between the model and the actual observation is mainly distributed around 2.45 TECU (1 TECU = 1016 electrons/m2). The values of the TEC model constructed in this study are closer to the actual observed values than those of the CODE TEC in China.

Response of the ionosphere to equatorial electrojet at latitudes near the northern EIA crest in the East African longitudinal sector

Equatorial Electrojet (EEJ) and Equatorial Ionization Anomaly (EIA) are two distinct ionospheric phenomena which are caused by the horizontal configuration of the geomagnetic field at the equator. On the day side of the earth, these activities are controlled by a common zonal electric field, and their interactions must be researched for a number of scientific reasons. In this study, we used multiple Global Navigation Satellite System (GNSS) stations at Al Wajh, Sola village, Nama, and Sheba in 2012 to investigate the correlations between the instantaneous and integrated equatorial electrojet (EEJ) and the variations of vertical total electron content (vTEC) at the northern crest of EIA in the East Africa and Middle East longitudinal sector. The equinoctial and solstitial months have the highest and lowest values of the monthly mean EEJ and vTEC, respectively, and as a result, both parameters show a semi-annual variability. The monthly mean vTEC also demonstrates equinoctial and solstitial asymmetries, resulting in vTEC values that are higher at the September equinox and the December solstice than at the March equinox and the June solstice, respectively. These asymmetries have been also observed in the composition of [O]/[N2] ratio. On days with strong counter electrojet (CEJ), EEJ peaks are smaller and occur later than on days with weak CEJ. Additionally, when CEJ is high, EIA is suppressed and vTEC exhibits low values at all crest points. In the early hours (10:00–12:00 Lt), Nama and Sheb showed higher correlation coefficient values with significant levels than Alwj and Sola. According to the correlation analysis, the anomaly crest develops at the lower latitude stations between 10:00 and 12:00 Lt and then moves to the higher latitude stations later. Generally, the correlations are higher and more significant in the equinoxes than the solstices. The correlations between integrated EEJ (IEEJ) and vTEC were also found to be strong and significant at the lower latitude stations throughout all seasons. These findings suggest that both the instantaneous and integrated EEJ can be used as a proxy for the eastward zonal equatorial electric field and must be considered in regional and global models of total electron content of equatorial and low latitude ionosphere.

Statistical Studies of Auroral Activity and Perturbations of the Geomagnetic Field at Middle Latitudes R.

In this paper, we statistically analyzed substorm activity at auroral latitudes for 2007–2020 and itsrelationship with magnetic disturbances at middle latitudes using the INTERMAGNET, SuperMAG, andIMAGE magnetometer data. The appearance and development of magnetic disturbances at auroral latitudeswas monitored by the IL index (similar to the AL index, but calculated according to IMAGE data). For the2007–2020 period, events that were observed near the meridian of the IMAGE network, in the night sector(2103 MLT), were selected. Two samples of events were used: (1) IL –200 nT for at least 10 min, with anadditional criterion for the presence or absence of positive bays at the Panagyurishte station in Bulgaria, and(2) isolated substorms observed on the IMAGE meridian according to the list of Ohtani and Gjerloev (2020).The distributions of the IL index, as well as the empirical and theoretical cumulative distribution functions,are obtained, and the of the occurrence of extreme events are also estimated. It is shown that, in general, theIL distributions are described well by exponential functions, and out of all events, events accompanied bymid-latitude positive bays were observed in ~65% of cases while their fraction increased with increasing disturbanceintensity. Events with positive bays at midlatitudes of MPB and isolated substorms were betterdescribed by the Weibull distribution for extreme events. From both distributions, annual and semi-annualvariations were identified: annual variations have a summer minimum and a winter maximum, and semiannualvariations have maxima near the equinoxes, which is most likely due to the Russell-McPherron effect.The semi-annual variation is also shown to be more pronounced for events with accompanying mid-latitudinalpositive bays.

Short-Period Variation of the Activity of Atmospheric Turbulence in the MLT Region over Langfang

In this paper, we investigate the activity of atmospheric turbulence in the MLT region and the relationship between the activity of atmospheric turbulence and atmospheric wave activity. We use data from the Langfang MF radar (39.4∘N, 116.7∘E) from July 2019 to June 2020 and NRLMSIS 2.0 to calculate the parameters of atmospheric wave activity and atmospheric turbulence energy dissipation rate (ε). Atmospheric ε is modulated by different periods at different altitudes, and while there are 12 h and 24 h periods at all altitudes, the main period is different at different altitudes. A comparison of the ε with atmospheric tide activity shows that tides have an effect on ε, and the influence of tides on ε may be different at different altitudes. The pattern of variation in ε is similar to that of the atmospheric activity of the gravity wave, with both ε and the atmospheric activity of the gravity wave showing significant semi-annual variation.

Distribution of water masses in the tropical-subtropical convergence off Mexico, using an autonomous profiler (2017–2021).

Data from an autonomous ARGO float launched on December 26, 2016, south of the entrance of the Gulf of California, reveal variations in the distribution of water masses in the tropical-subtropical convergence off Mexico. The float trajectory drifted through three areas of distinct characteristics: i) the southern Gulf of California in January 2017, ii) south of Cabo San Lucas from February 2017 to January 2018, when the float crossed an eddy, and iii) off Cabo Corrientes from February 2018 to April 2021. During the sampling, the vertical distribution of the water masses showed for the first time with “in situ” data a semiannual variability mainly in the salinity as response to poleward propagating semiannual Kelvin wave. The surface water masses had potential densities <26 kg m−3. A shallow salinity minimum (<34.6 g kg−1) observed at ∼ 70 m depth south of Cabo San Lucas provided a proxy for the California Current Water distribution, which had its maximum southern extension during winter. The Tropical Surface Water was observed off Cabo Corrientes mainly above 50 m depth and had maximum northern extension during summer. The Gulf of California Water was scarcely evident in the float trajectory in the southern Gulf. The Transitional Water was located above ∼ 50 m depth and was dominant southwest of Cabo San Lucas during winter and spring, when the California Current Water extended furthest south. The Subtropical Subsurface Water, in spite of its high density (>25 kg m−3) at times extended up to only 20 m below surface in the three areas around the summer as part of a semiannual cycle. The intermittent presence of this oxygen-deficient water close to the surface (∼ 20 m depth) is consistent with the on-going expansion of the shallow oxygen minimum zone. Wavelet analysis highlighted a bimonthly spectral frequency in the StSsW lower limit when the float crossed the eddy in the south of Cabo San Lucas, showing the influence of the mesoscale features of the area, and therefore a possible interchange of properties between deep waters.

Hemispheric Asymmetry of the Annual and Semiannual Variation of Thermospheric Composition

AbstractWe examine hemispheric asymmetry of the annual and semiannual variation of the ratio of O and N2 concentrations (O/N2) using observations by the Global Ultraviolet Imager (GUVI) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite and compare them with Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) model simulations. We found that in the equatorial region, the “equinox peaks” of the observed O/N2 are near the end of March and October, and the two annual lows are near the beginning of July and January. Compared to the equatorial region, in the northern hemisphere (NH) low latitudes, the first “equinox peak” clearly shifts toward the December solstice, whereas in the southern hemisphere (SH) low latitudes, the “equinox peaks” shift toward the June solstice (JS), forming the hemispheric asymmetry characteristics of the annual and semiannual variation. Seasonal variation of O/N2 shows no apparent phase variation with altitude, and the annual and semiannual pattern is consistent from year to year. WACCM‐X reproduces the observed annual and semiannual pattern in NH but in SH, it simulates an annual variation instead of the observed annual and semiannual variation. The largest discrepancy occurs near JS in the lower and middle thermosphere: the simulated O density has an annual high near JS in SH; the simulated N2 density has an annual high near JS in NH but a predominant annual low near JS in SH. These are not in the GUVI data. A weaker thermospheric meridional circulation in the winter hemisphere, or a reduced summer‐to‐winter latitudinal gradient of neutral temperature in WACCM‐X simulations would make model‐data comparisons more consistent.