Significant Annual Variation Research Articles

Developing a yearly warning index to assess the climatic impact on the water resources of Taiwan, a complex-terrain island

Summary Changes in the annual supply and demand ratio with respect to the past mean state are quantified and indexed to evaluate the effect of climate change on the water resources in Taiwan, a complex-terrain island. Four sub-regions were divided based on general terrain structures and major river flows. The changes in future temperature, precipitation and river runoff were projected statistically from monthly mean atmosphere–ocean coupling global circulation model (AOGCM) outputs. The results show that water resources will increase over the unpopulated eastern region, but will decrease over other densely populated and densely industrialized regions. Significant yearly variation is another worrisome factor burdening the vulnerable water management system. The developed yearly warning index for assessing the climatic impact on water resources is very useful to the water management agency and has a universal application potential.

Rural–urban gradient in seasonal forcing of measles transmission in Niger

Seasonally driven cycles of incidence have been consistently observed for a range of directly transmitted pathogens. Though frequently observed, the mechanism of seasonality for directly transmitted human pathogens is rarely well understood. Despite significant annual variation in magnitude, measles outbreaks in Niger consistently begin in the dry season and decline at the onset of the seasonal rains. We estimate the seasonal fluctuation in measles transmission rates for the 38 districts and urban centres of Niger, from 11 years of weekly incidence reports. We show that transmission rates are consistently in anti-phase to the rainfall patterns across the country. The strength of the seasonal forcing of transmission is not correlated with the latitudinal rainfall gradient, as would be expected if transmission rates were determined purely by environmental conditions. Rather, seasonal forcing is correlated with the population size, with larger seasonal fluctuation in more populous, urban areas. This pattern is consistent with seasonal variation in human density and contact rates due to agricultural cycles. The stronger seasonality in large cities drives deep inter-epidemic troughs and results in frequent local extinction of measles, which contrasts starkly to the conventional observation that large cities, by virtue of their size, act as reservoirs of measles.

The impact of large carnivores on the mortality of semi-domesticated reindeer (Rangifer tarandus tarandus L.) calves in Kainuu, southeastern reindeer-herding region of Finland

During 2006-2008 the survival of reindeer calves was studied in the reindeer-herding cooperative of Halla in Kainuu area where totally 546 calves were equipped with radio mortality collars mainly at the age of 1-3 days. The survival was monitored from the calving in May until winter round-ups in October to January. The rate, timing and causes of mortality of reindeer were assessed. In 2006-08 totally 177 radio-collared calves were found dead (mean mortality 32.4%) until mid-January. The results showed significant annual variation in calf mortality and predation. Independent of year the mortality of radio-collared calves was highest during the first two months after birth, and the total mortality was 30.7% at the end of October and reached 34.6% by mid-January. The sex of calves and pelt colour did not affect significantly survival of calves. Predation comprised 70.0% of total mortality. Predation by wolf, bear, lynx and wolverine comprised on average 38.4%, 20.3%, 9.0% and 2.3%, respectively. Birth weight of calves lost or killed by predators did not differ from surviving calves. However, birth weight of calves killed by brown bears was significantly lighter (mean 5.84 kg), whereas calves killed by Eurasian lynx was significantly heavier (mean 6.67 kg) than birth weight of calves that survived (mean 6.26 kg). Bears killed calves mainly in May to July, wolves in July to October and lynx in August to December. Of 209 radio-collared adult females, 17 were found dead (8.0%). These females had calved in May and they were killed mainly by wolves (52.0%) in August to October.

Annual and periovulatory changes in tyrosine hydroxylase activity in the ovary of the catfish Heteropneustes fossilis

In the present study, tyrosine hydroxylase (TH, the rate-limiting enzyme in catecholamine synthesis) activity was demonstrated in the ovary of the catfish to elucidate the possible physiological role of catecholamines in the gonad. The ovary is innervated by seven pairs of nerves, originating from the paired sympathetic chain lying dorsal to the posterior kidney. Ovarian TH activity showed a significant annual variation ( P < 0.001, one-way ANOVA), correlating with gonado-somatic index. Activity was low from December to February (resting phase), increased from March to July (recrudescent phase) and then decreased in post-spawning phase (August–November). The annual pattern was similar to that of the brain. An intraperitoneal injection of 100 IU hCG/fish induced significant periovulatory changes in TH activity with the peak rise at 16 h, and the activity decreased after egg-stripping ( P < 0.001, one-way ANOVA). Brain TH activity showed similar periovulatory changes. The results suggest that catecholamine synthesis is increased during both ovarian recrudescence and spawning of the annual reproductive cycle, implying a functional role in ovarian growth, maturation and ovulation.

Diversity of microcrustacea (Crustacea: Branchiopoda) of Loktak Lake, a Ramsar site, Manipur, India

Microcrustacea of Loktak Lake (collected during Nov. 2002-Oct. 2004) reveal 57 species and show qualitative dominance of Cladocera (51 species). They indicate monthly richness ranging between 33�plus or minus 6 and 32� plus or minus 6 species, record 51.7-82.3 and 53.6-90.0% community similarities during two years respectively, and follow trimodal annual patterns with peaks during winter. The microcrustaceans (112� plus or minus 17 and 124� plus or minus 13 n/l) form an important quantitative component (45.7� plus or minus 4.9 and 43.3� plus or minus 3.7 %) of zooplankton and show broadly trimodal annual patterns with peak abundance during winter. Cladocera > Copepoda mainly contribute to their quantitative variations. ANOVA registers significant monthly variations of microcrustacea richness and significant annual and monthly variations of their abundance. Richness is positively correlated with dissolved oxygen and is negatively correlated with rainfall, hardness, chloride and total dissolved solids while abundance is negatively correlated with pH only. Multiple regressions indicate higher cumulative effect of 15 abiotic factors on richness and abundance. Our results indicate no definite periodicity of richness and abundance of microcrustacea or their constituent groups during two annual cycles and are characterized by their higher species diversity, higher evenness and lower dominance.

Food-web structure and ecosystem services: insights from the Serengeti.

The central organizing theme of this paper is to discuss the dynamics of the Serengeti grassland ecosystem from the perspective of recent developments in food-web theory. The seasonal rainfall patterns that characterize the East African climate create an annually oscillating, large-scale, spatial mosaic of feeding opportunities for the larger ungulates in the Serengeti; this in turn creates a significant annual variation in the food available for their predators. At a smaller spatial scale, periodic fires during the dry season create patches of highly nutritious grazing that are eaten in preference to the surrounding older patches of less palatable vegetation. The species interactions between herbivores and plants, and carnivores and herbivores, are hierarchically nested in the Serengeti food web, with the largest bodied consumers on each trophic level having the broadest diets that include species from a large variety of different habitats in the ecosystem. The different major habitats of the Serengeti are also used in a nested fashion; the highly nutritious forage of the short grass plains is available only to the larger migratory species for a few months each year. The longer grass areas, the woodlands and kopjes (large partially wooded rocky islands in the surrounding mosaic of grassland) contain species that are resident throughout the year; these species often have smaller body size and more specialized diets than the migratory species. Only the larger herbivores and carnivores obtain their nutrition from all the different major habitat types in the ecosystem. The net effect of this is to create a nested hierarchy of subchains of energy flow within the larger Serengeti food web; these flows are seasonally forced by rainfall and operate at different rates in different major branches of the web. The nested structure that couples sequential trophic levels together interacts with annual seasonal variation in the fast and slow chains of nutrient flow in a way that is likely to be central to the stability of the whole web. If the Serengeti is to be successfully conserved as a fully functioning ecosystem, then it is essential that the full diversity of natural habitats be maintained within the greater Serengeti ecosystem. The best way to do this is by controlling the external forces that threaten the boundaries of the ecosystem and by balancing the economic services the park provides between local, national and international needs. I conclude by discussing how the ecosystem services provided by the Serengeti are driven by species on different trophic levels. Tourism provides the largest financial revenue to the national economy, but it could be better organized to provide more sustained revenue to the park. Ultimately, ecotourism needs to be developed in ways that take lessons from the structure of the Serengeti food webs, and in ways that provide tangible benefits to people living around the park while also improving the experience of all visitors.

Seasonal histophysiological study of the pineal gland in relation to gonadal and adrenal gland activities in adult domestic pigeon, Columba livia Gmelin

Some aspects of physiology related to the pineal gland in the adult male domestic pigeon, Columba livia Gmelin have been investigated. It was also correlated with the activity of the gonadal and adrenal glands by changing environmental conditions (e.g., seasonal photoperiods, temperature, humidity and rainfall). The maximum daily temperature was noticed in April and minimum in January, 2003. Pineal gland weights showed significant (F = 52.04, P < 0.01) annual variations with a maximum relative weight of the pineal gland being recorded in the month of July, when testicular relative weight was minimum. Testis and adrenal gland weights showed significant (F = 144.31, P < 0.01; F = 106.36, P < 0.01 respectively) annual variations with a maximum relative adrenal weight being recorded in the month of April, when testicular relative weight reaches a maximum value. Besides, assessment of pineal in relation to seasonal activity showed a completely inverse relationship with gonadal and adrenal gland. During the months of June–August and November–January, the testis remained in a state of atrophy. The rise in temperature contributed to rise in adrenal-gonadal function and reduced pineal function. Testicular weight peaked in March, April and May when the longest days and the highest temperatures prevailed and humidity was moderately low. A second testicular weight peak was also noticed in September and October when the temperature was also moderately high. These observations have been interpreted to suggest that two breeding and two nonbreeding phases are existed in the animal. Ultra structural study of the pineal gland and adrenocortical tissue indicate some features related to the breeding phase of pigeons. Within the pineal gland, mitochondria, dense cored vesicle with material and lipid droplets were found to be maximal within the pinealocyte cell cytoplasm of the regressive phase-I than the primary breeding phase. Golgi body and lysosomes were well defined. Furthermore, rough endoplasmic reticulum sacs were enlarged in size in the regressive phase-I. On the other hand, the number of mitochondria and cytoplasmic secretory granules within both the SCZ and CZ of the adrenocortical regions were found to be maximum during the primary breeding phase than the regressive phase-I. Extensive rough endoplasmic reticulum was found in the cytoplasm also. Furthermore, Golgi complex was enlarged within the breeding season.

Testing of the international reference ionosphere model using the data of dual-frequency satellite altimeters “Topex”/“Poseidon” and “Jason-1”

The international reference ionosphere IRI (IRI-2001 and IRI-2007) model was tested on the basis of a large set of statistical data including the complete 23rd solar cycle. The data of total electron content (TEC) above the ocean surface deduced from measurements by dual-frequency satellite altimeters (“Topex”/“Poseidon” and “Jason-1”) were used for the experimental test. It was found that the model TEC values are systematically less than those obtained experimentally by about 3.1016 m-2. It was shown that the rms deviation of the experimental data from the model shows almost linear dependence on the F10.7 index. The relative errors of IRI-2001 and IRI-2007 have significant 11-year and annual variations. The relative error of IRI-2001 against the altimeter data is less than 40% in 69% of the measurements and of IRI-2007, in 67% of the measurements. The diurnal variation amplitude of the absolute error stipulated by the difference of the model variation and the actual ionization increase during the dawn solar terminator passage is about 5.1016 m-2.

Population Trends in Royal and Sandwich Terns Along the Mid-Atlantic Seaboard, USA, 1975–2005

Abstract. Nest and chick census data gathered from 1975–2005 at all known breeding colonies of Royal (Sterna maxima) and Sandwich (S. sandivicensis) Terns in three mid-Atlantic States are reported. Nest census data were gathered sporadically in North Carolina, Virginia, and Maryland throughout this period. Royal Tern chick counts based on annual banding efforts were obtained in Maryland since 1975 and North Carolina and Virginia since 1977, while Sandwich Tern chick counts date from 1977 in North Carolina and 1980 in Virginia. Royal Tern chick counts show significant annual variation by state, and are positively correlated between North Carolina and Virginia, but negatively correlated between Maryland and both North Carolina and Virginia. Sandwich Tern nest counts showed significant annual variation in North Carolina, as did chick counts in North Carolina and Virginia. These results suggest regional, rather than local, trends in Royal Tern numbers over time. While Royal Tern numbers indicate a gradual dec...

Seasonal pattern of births in patients with multiple sclerosis

The birth data of 816 patients with the diagnosis of multiple sclerosis (MS) according to the McDonald criteria referred to the University Clinic of Neurology Regensburg were assessed for annual periodicity of birth dates. All data were expressed as rates, i.e., number MS births per total number of births per month, prior to descriptive and time series analysis. A significant annual variation of birth rates was found in both genders, with the peak in birth rates per month occurring in late winter and spring months. A cosinor model indicated an acrophase (cosine-model approximation derived peak time) of birth rate in April. These results are consistent with the findings of most of the earlier studies conducted in other countries located in the Northern hemisphere that revealed an excess of MS births in the spring.

Temporal and sex differences in the blubber fatty acid profiles of the New Zealand sea lion Phocarctos hookeri

We determined the fatty acid (FA) composition of the blubber of 82 New Zealand (NZ) sea lions caught as squid fishery by-catch between the years 2000 and 2006 on the Auckland Islands (51° S, 166° E) shelf. A combination of univariate and multivariate analyses showed significant variation in the FA composition between sexes and years. Blubber FA compositions of some males differed significantly from that of females, whereas blubber FA compositions of lactating (LF) and non-lactating females (NLF) were similar. Significant annual FA variation was revealed between the pooled years 2005/2006 and the previous years and between 2000 and 2004. Part of these differences can be attributed to different diets. Indeed, FA variation between the sexes suggests that males feed on deeper species than females, which is consistent with the current knowledge on the different div- ing behaviours between male and female otariids. Concerning annual variation, NZ sea lions are generalist predators, thus their diet is expected to follow the trends of prey stock availability. Nonetheless, FA metabolism is likely to cause some of the FA variation seen between sexes and years, because the deposition and mobilization of FAs would vary according to the nutritional and reproductive states of the individuals.

Tuberculosis Among Foreign-born Persons in Taiwan, 2002–2005

The foreign-born population has been growing in Taiwan. Most foreign-born persons come from countries with a high burden of tuberculosis (TB). Monitoring the trend and characteristics of TB in this population is essential for TB control in Taiwan. Information about foreign-born persons residing in Taiwan and data of all foreign-born TB cases notified during 2002-2005 were obtained from the national authorities and analyzed. A total of 2,444 foreign-born TB cases were notified during 2002-2005, which accounted for 3.6% of all notified TB cases during that period in Taiwan. The proportion of foreign-born TB cases was constant, without any significant yearly variation. The average annual TB notification rate in the foreign-born population was higher than that in the Taiwan-born population (94.0/100,000 vs. 72.0/100,000). There were significant differences in age, sex and regional distribution between foreign-born and Taiwan-born TB cases (p < 0.001). Foreign-born cases were predominantly female (65.4%) and aged 25-44 years (70.9%), whereas the majority of cases among the Taiwan-born population were male (69.4%) and aged > or = 65 years (49.6%). Most foreign-born TB patients (62.7%) lived in northern Taiwan but only about one-third (36.1%) of Taiwan-born TB cases were notified from that region. Among foreign-born TB cases whose original countries were recorded, the majority came from Mainland China and Vietnam, which accounted for 73.0% of all cases, followed by the Philippines (7.4%), Thailand (7.3%) and Indonesia (6.0%). Foreign-born TB patients have different profiles and a higher case rate compared to Taiwan-born patients. Monitoring the epidemiologic trend of TB among foreign-born persons, especially those who come from high TB-burden countries, is essential in the fight against TB in Taiwan.

Patterns of variation in leucocyte counts of female tree swallows, Tachycineta bicolor: Repeatability over time and relationships with condition and costs of reproduction

The heterophil to lymphocyte (HL) ratio in the peripheral blood is increasingly being recognized as a reliable indicator of stress in birds. In this study we examined whether HL ratio, as well as the proportion of heterophils and lymphocytes, varied systematically with sampling time, date, age, climate conditions or with measures of maternal investment in female tree swallows ( Tachycineta bicolor) over three breeding seasons. We found that HL ratios showed significant annual variation, which appeared to be driven by annual changes in the proportion of heterophils. HL ratios were higher among those females laying larger clutches, suggesting a potential cost of reproduction. Variation in body condition also appeared to affect stress levels of females, as decreases in body condition were associated with elevated HL ratios. Among females that we sampled over multiple breeding seasons, we were unable to detect significant repeatability for both HL ratios and proportion of heterophils, although proportion of lymphocytes showed low but significant repeatability within individuals. We therefore suggest that caution should be exercised in using these measures for illustrating the inherent quality or health of individuals over time frames beyond the current breeding attempt, or as predictors of future reproductive potential.

Biliary Atresia: Swiss National Study, 1994–2004

To determine the epidemiology of biliary atresia (BA) in Switzerland, the outcome of the children from diagnosis, and the prognostic factors. The records of all patients with BA born in Switzerland between January 1994 and December 2004 were analyzed. Survival rates were calculated with the Kaplan-Meier method, and prognostic factors evaluated with the log rank test. Median follow up was 58 months (range, 5-124). BA was diagnosed in 48 children. Incidence was 1 in 17,800 live births (95% confidence interval 1/13,900-1/24,800), without significant regional, annual, or seasonal variation. Forty-three children underwent a Kasai portoenterostomy (PE) in 5 different Swiss pediatric surgery units. Median age at Kasai PE was 68 days (range, 30-126). Four-year survival with native liver after Kasai PE was 37.4%. Liver transplantation (LT) was needed in 31 in 48 children with BA, including 5 patients without previous Kasai PE. Four patients (8%, all born before 2001) died while waiting for LT, and 29 LT were performed in 27 patients (28 in Geneva and 1 in Paris). All of the transplanted patients are alive. Four-year overall BA patient survival was 91.7%. Four-year survival with native liver was 75% in patients who underwent Kasai PE before 46 days, 33% in patients operated on between 46 and 75 days, and 11% in patients operated on after 75 days (P = 0.02). Overall survival of patients with BA in Switzerland compares favorably with current international standards, whereas results of the Kasai operation could be improved to reduce the need for LTs in infancy and early childhood.

Variation in sexual reproduction in Taxus cuspidata Sieb. &amp; Zucc

Male and female strobilus production and seed production were measured in multiple years in three natural populations of Taxus cuspidata in Japan. Our goal was to record variation in these parameters among individuals, among populations, and from year to year in this species, which is defined as dioecious. We also wished to determine whether cosexual individuals occurred in the sampled T. cuspidata populations. Male and female strobilus production varied significantly among male and female individuals, respectively, and individuals had significant annual variation in male and female strobilus production. Significant variation in male and female strobilus production was also recorded in comparisons among T. cuspidata populations. In some populations, males were significantly larger than females, but this result was not consistently observed. No cosexual individuals were recorded in any of the populations studied. Correlations among reproductive variables and measured vegetative characters were generally non-significant at all sites and in most years, with a few exceptions. When compared with other Taxus species, strobilus ratios in T. cuspidata were less male biased (2.3 male strobili per female strobilus in T. cuspidata versus 8.7 and 7.0 male strobili per female strobilus in Taxus brevifolia and Taxus canadensis, respectively). Seed production efficiency was comparable to T. brevifolia, but less than half of that recorded for T. canadensis, the only species of the genus that has individuals that are typically cosexual.

Longitudinal and seasonal variations in plasmaspheric electron density: Implications for electron precipitation

The tilt and offset of the Earth's magnetic field can significantly affect the longitudinal and seasonal distribution of electron density in the plasmasphere. Here we show that for the solar maximum conditions of 1990–1991, the largest annual variation determined from CRRES measurements of plasmaspheric equatorial electron density in the range L = 2.5–5.0 occurs at American longitudes (−60°E), while no annual variation occurs at Asian longitudes (+100°E). Plasmaspheric electron density is larger in December than in June at most longitudes, from −180°E eastward to +20°E. At all other longitudes the density ratio from December to June is very close to 1.0. The largest December/June density ratio is at L = 3.0 at American longitudes (−60°E). At L = 4.5 and above, the annual variation disappears. The lowest electron density values for a given L‐shell occur at American longitudes, in June. Ion densities also show significant annual variations, with similar longitudinal and seasonal characteristics in the case of IMAGE EUV He+ measurements. Atomic mass density measurements calculated using the magnetometer cross‐phase technique show significant seasonal variations but also imply composition changes with longitude. Using the quasilinear PADIE code we calculate the bounce‐averaged diffusion rate of electrons by plasmaspheric hiss with a fixed wave intensity. December to June variations in plasmaspheric density, particularly at American longitudes, drive changes in the wave‐particle interactions, increasing diffusion into the loss cone by a factor of ∼3 at 1 MeV at L = 3.0, thus hardening the electron precipitation spectrum during the southern hemisphere winter (in June).

부산 지역 미세먼지 농도의 시간변동 특성 및 기상인자 분석을 통한 먼지생성 해석

In an effort to interpret the characteristics of fine particle concentrations in Busan, time variations of hourly monitored concentrations (Particulate Matter with aerodynamic Diameter ) in Busan are analyzed for the period from 2000 to 2005. The characteristics of aerosol second generation formation process is also interpreted qualitatively, by using the statistical analysis of the meteorological variables including temperature, wind speed, and relative humidity. The result shows some significant annual, seasonal, weekly and diurnal variations of concentrations. In particular, seasonal(i.e., spring) variations are governed by frequency of yellow sand events even for the non-yellow sand cases where yellow-sand days are eliminated in our analysis. However, in seasonal variation, summer season predominate lower concentrations due to the frequent precipitation, and weekly and diurnal variations are both found to be reflecting the emission rate from traffic amount. Correlation coefficients between concentration and meterological variables for non-yellow sand days show overall negative correlation with visibility, wind speed, cloud amounts, and relative humidity. However for non-precipitation days, during non-yellow sand period positive correlation are found clearly with relative humidity, suggesting the importance of secondary aerosol formation in Busan that can be achieved by both homogeneous aerosol formation and heterogeneous transformations resulting from hygroscopic aerosol characteristics.

Rain, prey and predators: climatically driven shifts in frog abundance modify reproductive allometry in a tropical snake

To predict the impacts of climate change on animal populations, we need long-term data sets on the effects of annual climatic variation on the demographic traits (growth, survival, reproductive output) that determine population viability. One frequent complication is that fecundity also depends upon maternal body size, a trait that often spans a wide range within a single population. During an eight-year field study, we measured annual variation in weather conditions, frog abundance and snake reproduction on a floodplain in the Australian wet-dry tropics. Frog numbers varied considerably from year to year, and were highest in years with hotter wetter conditions during the monsoonal season ("wet season"). Mean maternal body sizes, egg sizes and post-partum maternal body conditions of frog-eating snakes (keelback, Tropidonophis mairii, Colubridae) showed no significant annual variation over this period, but mean clutch sizes were higher in years with higher prey abundance. Larger females were more sensitive to frog abundance in this respect than were smaller conspecifics, so that the rate at which fecundity increased with body size varied among years, and was highest when prey availability was greatest. Thus, the link between female body size and reproductive output varied among years, with climatic factors modifying the relative reproductive rates of larger (older) versus smaller (younger) animals within the keelback population.

Short‐Term Temporal Effects on Community Structure of Lepidoptera in Restored and Remnant Tallgrass Prairies

Abstract Understanding the degree to which species assemblages naturally vary over time will be critically important when assessing whether direct management effects or contingency is responsible for species gain or loss. In this study, we tested three predictions related to short‐term variation in prairie moth communities: (1) communities would only exhibit significant temporal variation in newly restored sites (1–3 years old); (2) prairie size and age would positively influence community reassembly, with larger, older restorations sampling a greater proportion of the regional species pool; and (3) older restorations (7–10 years old) would have yet to converge on the community composition of prairie remnants. Moths were sampled from 13 Tallgrass prairie restorations and remnants in central Iowa in 2004–2005. Repeated measures analysis of variance revealed significant effects of sampling year on moth species richness and abundance as well as on the richness of two functional groups, but difference among prairie types was only observed in 2005. Rarefaction analysis revealed that older restorations and prairie remnants supported higher species richness compared to recently planted sites, and nonmetric, multidimensional scaling ordination indicated that restorations older than 7 years were clearly converging on the species composition of remnants. These results suggest that moth communities in restorations and remnants are highly variable in time but that as restorations age, they appear to reaccumulate moth species found in prairie remnants. The long‐term persistence of a particular species assemblage within a given site, however, might be a difficult endpoint to attain in central Iowa prairies because of significant annual variation in species occurrence.

Seasonal variability of GPS‐derived zenith tropospheric delay (1994–2006) and climate implications

The total zenith tropospheric delay (ZTD) is an important parameter of the atmosphere and directly or indirectly reflects the weather and climate processes and variations. In this paper the ZTD time series with a 2‐hour resolution are derived from globally distributed 150 International GPS Service (IGS) stations (1994–2006), which are used to investigate the secular trend and seasonal variation of ZTD as well as its implications in climate. The mean secular ZTD variation trend is about 1.5 ± 0.001 mm/yr at all IGS stations. The secular variations are systematically increasing in most parts of the Northern Hemisphere and decreasing in most parts of the Southern Hemisphere. Furthermore, the ZTD trends are almost symmetrically decreasing with increasing altitude, while the summation of upward and downward trends at globally distributed GPS sites is almost zero, possibly reflecting that the secular ZTD variation is in balance at a global scale. Significant annual variations of ZTD are found over all GPS stations with the amplitude from 25 to 75 mm. The annual variation amplitudes of ZTD near oceanic coasts are generally larger than in the continental inland. Larger amplitudes of annual ZTD variation are mostly found at middle latitudes (near 20°S and 40°N) and smaller amplitudes of annual ZTD variation are located at higher latitudes (e.g., Antarctic) and the equator areas. The phase of annual ZTD variation is about 60° in the Southern Hemisphere (about February, summer) and about 240° in the Northern Hemisphere (about August, summer). The mean amplitude of semiannual ZTD variations is about 10 mm, much smaller than annual variations. The semiannual amplitudes are larger in the Northern Hemisphere than in the Southern Hemisphere, indicating that the semiannual variation amplitudes of ZTD in the Southern Hemisphere are not significant. In addition, the higher‐frequency variability (RMS of ZTD residuals) ranges from 15 to 65 mm of delay, depending on altitude of the station. Inland stations tend to have lower variability and sites at ocean and coasts have higher variability. These seasonal ZTD cycles are due mainly to the wet component variations (ZWD).