Seasonal Variation Of Amplitude Research Articles

Propagation Features of Diurnal Internal Tides West of the Luzon Strait Revealed by a Large PIES Array

Abstract Energetic internal tides (ITs) are generated from the Luzon Strait (LS) and propagate westward into the South China Sea (SCS). Owing to the lack of large-scale synchronous measurements, the propagation features and seasonal variations of diurnal ITs remain unclear. From 2018 to 2019, mode-1 diurnal ITs west of the LS were continuously observed using a large-scale moored array of 27 pressure-recording inverted echo sounders (PIESs) and a thermistor chain. Measurements confirmed that diurnal ITs radiate from the LS with a north–south asymmetrical pattern, with the most energetic channel located in the middle and south of the LS. The total energy radiated into the SCS across 120°E is 2.67 GW for the K1 ITs and 1.54 GW for the O1 ITs, approximately 2 times larger than those inferred from satellite observations. K1 dominates among the diurnal ITs, with its maximum isopycnal displacement (amplitude) and energy input to the SCS being the strongest in summer (i.e., 16.3 m and 2.81 GW, respectively). The propagation speed of K1 is higher in summer and autumn along the main channel (i.e., 4.33and 4.36 m s−1, respectively). Seasonal stratification and circulation play important roles in the seasonal variation of amplitude and propagation speed of the K1 ITs. The seasonal variability of diurnal-band ITs, which includes all diurnal constituents, is location-dependent and primarily results from the superposition of the K1 and P1 ITs. In particular, vertical displacement is strong in summer and winter along the main channel of the K1 and P1 ITs. The seasonal amplitude of K1 can modulate this seasonal feature. Significance Statement Internal tides (ITs) are internal waves at tidal frequencies. The Luzon Strait (LS) is one of the most energetic sites to generate large-amplitude ITs. The ITs propagate into the South China Sea (SCS), interact with mesoscale eddies, large-scale circulations, etc., and influence local hydrodynamics as well as ecosystem and sediment transport. This motivated an observation plan to investigate the ITs at the western entrance of the LS. From June 2018 to August 2019, an array of 28 PIESs was deployed in the northeastern SCS, almost covering the western entrance of the LS, to investigate the propagation properties of ITs including their amplitude, phase speed, wavelength, propagation direction, and energy fluxes and their annual and seasonal variations. Here, we primarily focus on the mode-1 diurnal ITs. The new insights enrich our understanding of IT dynamics and seasonal variations and support further improvements in numerical simulations.

Seasonal variations in amplitudes and resonance frequencies of the HVSR amplification peaks linked to groundwater

SUMMARYFollowing the installation of a temporary seismological network in western Greece north of the Gulf of Patras, we determined the quality of the sites of each of the 10 stations in the network. For this, we used the horizontal-to-vertical spectral ratio (HVSR) method and calculated an average curve over randomly selected days between 0 and 10 Hz. The daily HVSR curve is determined by the HVSR 12-hr calculation (1 hr every two) without distinction between seismic ambient noise and earthquake signal. The HVSR curves obtained can be classified in three categories: flat curves without amplification, curves with a amplification peaks covering a large frequency range, and curves with one or more narrow peaks. In this third category C3, one station has one peak, two have two and one has three. On the contrary of what it is commonly assumed, the amplitudes and the resonance frequencies of these narrow peaks are not stable over time in C3. We determined the maximum of the amplitude of each peak with the corresponding central frequency for each day during 2.5 yr. Except for the station with three peaks, which finally appears stable within the uncertainties, the principal peak exhibits a seasonal variation, with a maximum in winter and a minimum in summer, the observations being more dispersed during winter. The second peak, when it exists, varies in the same way except at one station where it varies oppositely. These variations are clearly correlated with the loading and unloading cycle of the underlying aquifers as shown by the comparison with water level and yield measurements from wells located close to the stations. Moreover, they are also correlated with the vertical surface displacements observed at continuously recording GPS stations. The dispersion of the observed maximum amplitude in winter is probably related to the rainfall and the soil moisture modifying the S-wave velocity as revealed by other studies. From this study, we would like to emphasize that the use the HVSR method to constrain the S-wave velocity and the thickness of the sediment layer over the bedrock in the basin, has to be done with caution. Upon further confirmation of its robustness, the HVSR methodology presented here could be a good and easy-to-use tool for a qualitative survey of the aquifer backdrop and its seasonal behaviour, and of the soil moisture conditions.

The effect of using the plant functional type paradigm on a data-constrained global phenology model

Abstract. Leaf seasonality impacts a variety of important biological, chemical, and physical Earth system processes, which makes it essential to represent leaf phenology in ecosystem and climate models. However, we are still lacking a general, robust parametrisation of phenology at global scales. In this study, we use a simple process-based model, which describes phenology as a strategy for carbon optimality, to test the effects of the common simplification in global modelling studies that plant species within the same plant functional type (PFT) have the same parameter values, implying they are assumed to have the same species traits. In a previous study this model was shown to predict spatial and temporal dynamics of leaf area index (LAI) well across the entire global land surface provided local grid cell parameters were used, and is able to explain 96 % of the spatial variation in average LAI and 87 % of the variation in amplitude. In contrast, we find here that a PFT level parametrisation is unable to capture the spatial variability in seasonal cycles, explaining on average only 28 % of the spatial variation in mean leaf area index and 12 % of the variation in seasonal amplitude. However, we also show that allowing only two parameters, light compensation point and leaf age, to be spatially variable dramatically improves the model predictions, increasing the model's capability of explaining spatial variations in leaf seasonality to 70 and 57 % of the variation in LAI average and amplitude, respectively. This highlights the importance of identifying the spatial scale of variation of plant traits and the necessity to critically analyse the use of the plant functional type assumption in Earth system models.

Stability of the seasonal variations in diurnal and semidiurnal components of mid-latitude F2 layer parameters

We report work utilizing 15-min resolution ionospheric data obtained with DPS-4 digisonde in 2003–2011 to study the seasonal variations in amplitudes and phases of the most powerful spectral components of the F2 layer critical frequency (foF2) and peak height (hmF2) fluctuations over Irkutsk (52.5°N, 104.0°E). We show that fluctuations of both parameters contain quasi-harmonic components with periods of Tn=24/nh (n=1–7). The number of distinct spectral peaks varies from 3 in summer to 7 in winter. Amplitude and phase characteristics of the diurnal (n=1) and semidiurnal (n=2) components is studied using the data sets extracted from the original data sets with band-pass filter. It has been found that the amplitudes of diurnal/semidiurnal foF2 and diurnal hmF2 components are maximum in winter and minimum in summer. Amplitudes of the diurnal components vary gradually; those of the foF2 semidiurnal one, abruptly, thus forming a narrow winter maximum in November–January. The phase (local time of maximum) of the diurnal foF2 component increases gradually by 4–6h from winter to summer. The phase of the semidiurnal foF2 component is nearly stable in winter/summer and sharply decreases (increases) by 2–3h near the spring (autumn) equinox. The phase of the diurnal component of hmF2 (local time of minimum) varies slightly between 1130 and 1300 LT; that of the semidiurnal one decreases (increases) by 4–6h from January to March (from September to November). The results obtained show that the main features of seasonal variations in the diurnal and semidiurnal components of the mid-latitude F2 layer parameters recur consistently during the solar activity growth and decline phases.

On the occurrence and variability of the terdiurnal tide in the equatorial mesosphere and lower thermosphere and a comparison with the Kyushu-GCM

[1] We studied the occurrence characteristics and variability of the terdiurnal tide (8 hour period) in the equatorial mesosphere and lower thermosphere (MLT), using a meteor radar at Koto Tabang (0.2°S, 100.3°E) and MF radars at Tirunelveli (8.7°N, 77.8°E) and Pameungpeuk (7.4°S, 107.4°E). These locations, one being located right over the equator and the other two at conjugate points around the equator within ±10°, form a unique experimental setup to study equatorial MLT dynamics. The terdiurnal tide exists as a distinct wave signature at all three locations. While the daily amplitudes can be as large as 15 m s−1, the monthly mean amplitudes lie between 1 and 10 m s−1. The amplitude of the terdiurnal tide at Pameungpeuk is generally smaller than that observed at Tirunelveli and Koto Tabang. The seasonal variation in amplitude shows both annual and semiannual oscillations of ∼1 m s−1 at all three locations. The present observations combined with previous reports indicate that the timing of the primary maximum of the terdiurnal tide amplitude shifts from autumn to late spring and early summer as one moves from high latitudes to the equator (all with respect to the Northern Hemisphere). The amplitudes and seasonal variation in the present observations show good comparison with that simulated by the General Circulation Model (GCM) developed by Kyushu University, Japan. This study supports the occurrence of nonlinear interaction between diurnal and semidiurnal tides and shows that gravity waves play an important role in the generation of the terdiurnal tide.

Seasonal variations of gravity waves revealed in rawinsonde data at Pohang, Korea

Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory, Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their intermittency and locations relative to floating balloons.

Mesospheric temperature inversions over the Indian tropical region

Abstract. To study the mesospheric temperature inversion, daily temperature profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) during the period 1991-2001 over the Indian tropical region (0-30° N, 60-100° E) have been analyzed for the altitude range 34-86km. The frequency of occurrence of inversion is found to be 67% over this period, which shows a strong semiannual cycle, with a maximum occurring one month after equinoxes (May and November). Amplitude of inversion is found to be as high as 40K. Variation of monthly mean peak and bottom heights along with amplitude of inversions also show the semiannual cycle. The inversion layer is detected most frequently in the altitude range of 70-85km, with peak height ranging from 80 to 83km and that of the bottom height from 72 to 74km. A comparison of frequency of temperature inversion with that obtained from Rayleigh lidar observations over Gadanki (13.5° N, 60-100° E) is found to be reasonable. The seasonal variation of amplitude and frequency of occurrence of temperature inversion indicates a good correlation with seasonal variation of average ozone concentration over the altitude range of the inversion layer.

Sea level variability at Ulleung Island in the East (Japan) Sea

Monthly adjusted sea level (ASL) was analyzed to examine the characteristics of sea level variation at Ulleung Island in the East (Japan) Sea from 1979 to 1992. It is remarkable that the nonseasonal variation is comparable with the mean seasonal variation in amplitude at Ulleung Island, whereas the mean seasonal variation is dominant along the eastern coast of the Korean peninsula. At Ulleung Island the variation in the mean seasonal, interannual, and short‐term component variability is 54, 13, and 30% of the total ASL variance, respectively. ASL variability is highly correlated with the upper layer heat content around Ulleung Island. There are two factors that contribute to large nonseasonal variation of the ASL around the island. One is the location of the polar front that divides the northern cold water region and the southern warm water region. The other is the presence of a warm eddy that frequently forms near Ulleung Island. When the northern cold water region extends southward to Ulleung Island, the heat content of the upper layer decreases, and ASL around the island falls. In contrast, when the southern warm water region extends northward to the island, the heat content increases, and ASL rises greatly. The movement of the front with a period of 1–2 years causes interannual variation of ASL. When a warm eddy approaches Ulleung Island, the heat content increases, and ASL rises. The short‐term variation in ASL is caused primarily by the migration of the warm eddy around Ulleung Island.

Continuous Free Oscillations: Atmosphere-Solid Earth Coupling

▪ Abstract In 1997, after almost forty years since the initial attempt by Benioff et al (1959) , continuous free oscillations of the Earth were discovered. Spheroidal fundamental modes between 2 and 7 millihertz are excited continuously with acceleration amplitudes of about 0.3–0.5 nanogals. The signal is now commonly found in virtually all data recorded by STS-1 type broadband seismometers at quiet sites. Seasonal variation in amplitude and the existence of two coupled modes between the atmosphere and the solid Earth support that these oscillations are excited by the atmosphere. Stochastic excitation due to atmospheric turbulence is a favored mechanism, providing a good match between theory and data. The atmosphere has ample energy to support this theory because excitation of these modes require only 500–10000 W whereas the atmosphere contains about 1017 W of kinetic energy. An application of this phenomenon includes planetary seismology, because other planets may be oscillating owing to atmospheric excitation. The interior structure of planets could be learned by determining the eigenfrequencies in the continuous free oscillations. It is especially attractive to pursue this idea for tectonically quiet planets, since quakes may be too infrequent to be recorded by seismic instruments.

Seasonal Variation in Daily Mass Amplitude and Minimum Body Mass: A Test of a Recent Model

Two recent models of body regulation suggest that wintering birds will show one of two energy storage strategies. Both models suggest that birds will increase the amplitude of the daily cycle of body as midwinter approaches and decrease the amplitude toward spring. Seasonal changes in night length and night temperature cause seasonal variation in thermoregulatory demands, hence seasonal variation in amplitude of the daily cycle. The models differ in that only the first but not the second model proposes an increase in dawn body toward midwinter, and a subsequent decrease in toward spring; in the second model, the daily minimum remains seasonally constant. Because fat is a major component of daily and seasonal variation in body mass, these contrasting strategies for body regulation have been termed true winter fattening and constant dawn mass strategies, respectively. In the former case, the seasonal increase in the dawn body/fat is considered to represent an emergency energy supply to be used during sudden poor feeding conditions, i.e. snow and ice storms. We present data on four species of temperate-dwelling passerines that support the first but not the second model. In the Dark-eyed Junco Junco hyemalis wintering in southern Indiana, amplitude of the daily cycle of body mass, evening and dawn body mass, and evening and dawn visible fat class all increased toward midwinter and decreased toward spring. A highly significant linear relationship between visible fat and chemically extracted body lipid in this species verified that the visible fat class system reliably measured body fat content. In addition, the seasonal variation in amplitude was explained by seasonal variation in nighttime roosting temperature and night length. The Northern Cardinal Cardinalis cardinalis and Fox Sparrow Passerella iliaca wintering in southern Indiana and the Song Sparrow Melospiza melodia wintering in southwest British Columbia all displayed a seasonal increase and decrease in dawn body and visible fat class. Results of this study and previous studies indicate that 13 of 14 species studied in Europe, Asia, and North America display this strategy for regulating winter body mass. We conclude that among birds that winter in the temperate zone, there has been widespread natural selection for a seasonal increase in the daily minima of body and fat reserve.

Direct comparisons of prevailing winds and tidal wind fields (24-, 12-h) in the upper middle atmosphere (60–105 km) during 1978–1980 at Saskatoon (52°N, 107°W) and Christchurch (44°S, 173°E

Continuous observations of the upper Middle Atmosphere (60–110 km) have been made since 1978 by M. F. radars at Saskatoon, Canada (52°N, 107°W) and Christchurch, New Zealand (44°S, 173°E). There are important differences in the zonal mean winds between hemispheres and, hence, with CIRA (1972), e.g. summer westward and winter eastward reversal heights. Other differences with CIRA involve the considerable winter variability at each location and the rapidity of the equinoctial transitions. The semi-diurnal tide shows similar strong seasonal variations in amplitudes, phases and wavelengths at each location, e.g. there are generally shorter wavelengths in fall-winter months (~45 km) than in spring-summer (≳100 km). Also, for most months the northward tidal components are closer to being in phase than the eastward near 90 km, favouring the dominance of antisymmetric modes. Recent theoretical models do not match these features well. The weaker diurnal tide also shows marked seasonal variations. Comparisons are also made with tidal and mean wind data from Adelaide (35°S) and Kyoto (35°N) for the same observing interval: there is frequently useful agreement between the four locations.

Microbaroms on a Northern Quarter of the Earth's Surface

MICROBAROMS1 have been studied for about twenty years. Gutenberg and Benioff2 noticed a seasonal variation in the amplitudes of their observations on the west coast of the US, and Daniels3–5 has suggested a possible mechanism for their production by ocean waves. More recently Donn and his co-workers6–9 have examined these waves and their sources in more depth and have shown8 that, in addition to the seasonal variation in amplitude, there is a diurnal variation which is independent of the source. They have also shown8,9 that the long range propagation of these waves to an array of detectors on the east coast of the US is controlled, in the winter, by the winds and temperature at an altitude of 115 km and, in the summer, by these properties at an altitude of 50 km.

Lunar and luni-solar variations of the geomagnetic field in the Indian region

Lunar and luni-solar geomagnetic components have been computed upto four harmonics for low latitude station Alibag, outside equatorial electrojet belt, and the equatorial electrojet stations Annamalainagar, Kodaikanal and Trivandrum in the south Indian region. The computations are confined to data of very high solar activity period 1958–61. Amplitudes of lunar semidiurnal component (L2), in the horizontal intensity (H), undergo an equatorial enhancement. Phase difference of 2 hrs is noticed inL2(H) between nonelectrojet and electrojet stations. In the vertical intensity (Z), L2 is maximum ine andj-seasons at Trivandrum, close to the magnetic equator. Ind-season, however, maximumL2(Z) occurs at Annamalainagar (dip 5°.4N). The phase difference between the electrojet and nonelectrojet stations observed inL2(H) is not noticed inL2(Z). The differential vertical upward drift motion of charged particles may explain the observed phase difference inL2(H). Seasonal variations in amplitudes and times of maxima are noticed at all the stations inL2(H) andL2(Z). Similar variation is also noticed at Alibag inL2 of declination (D).