Variations In Length Of Day Research Articles

Pacific warm pool excitation, earth rotation and El Niño southern oscillations

The interannual changes in the Earth's rotation rate, and hence in the length of day (LOD), are thought to be caused by the variation of the atmospheric angular momentum (AAM). However, there is still a considerable portion of the LOD variations that remain unexplained. Through analyzing the non‐atmospheric LOD excitation contributed by the Western Pacific Warm Pool (WPWP) during the period of 1970–2000, the positive effects of the WPWP on the interannual LOD variation are found, although the scale of the warm pool is much smaller than that of the solid Earth. These effects are specifically intensified by the El Niño events, since more components of the LOD‐AAM were accounted for by the warm pool excitation in the strong El Niño years. Changes in the Earth's rotation rate has attracted significant attention, not only because it is an important geodetic issue but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere and solid Earth, and hence the global changes.

Cyclic time patterns of death from suicide in northern Finland

Background: Time patterns of suicide have been attributed not only to social and psychological factors but also to direct geophysical effects. Seasonal variations in day length and temperature seem likely to contribute to the timing of the suicide process. Methods: We analysed all suicides ( n=1658) committed in a northern province of Finland during a period of 153 months. Daily data on the number of suicides, local weather conditions and geomagnetic storms were compiled and modelled with Poisson regression using the province population as the denominator, and with the means of harmonic series for seasonal variation. Time series analysis of monthly numbers of suicides was carried out using the seasonal-trend decomposition procedure based on loess. Results: Marked fluctuations in the number of suicides occurred during the study period ( P=0.01). There was significant seasonal variation in death from suicide ( P=0.01), but analysis of the meteorological data showed no evidence of effect on the risk of suicide. Limitations: Assessment of mental disorder or alcohol consumption was missing, since only data derived from death certificate was available for each case. Conclusions: The seasonal effect was significant, but remained modest compared to sex and age as risk factors for suicide. Preventive measures need to be tailored according to time of the year.

Effect of changes in total atmospheric mass on length‐of‐day modeling

The Earth's rotation rate, and the associated length‐of‐day (LOD), show fluctuations at different timescales. At the decadal timescale, this is due mainly to the interaction between the mantle and the liquid core, while the variations at shorter time scales are attributed to interaction with the external fluid layers, in particular the atmosphere, with the oceans and hydrology contributing at about the 1% level. In this paper, we investigate the effect of changes of atmospheric mass on the length‐of‐day modeling, which are associated with evaporation/precipitation and mass closure problems in the atmospheric analyses. We show that an important part of the residuals between observed LOD and the ocean and atmosphere induced LOD variations can be explained by this mass exchange.

Oceanic torques on solid Earth and their effects on Earth rotation

We calculate that the oceanic torque vectors (x, y, z components) acting on the solid Earth using the Parallel Ocean Climate Model (POCM) output data at 3‐day intervals for the period 1988–1995. This study examined three distinct types of torque, with different physical mechanisms: pressure, gravitational, and frictional. The pressure torque is further divided into two parts: that due to Earth's ellipticity and that due to the ocean bottom topography. According to resultant time series the ellipticity pressure torque causes the largest effect for the x and y components. The gravitational torque and the ellipticity pressure torque are shown numerically to be exactly proportional by a factor of −0.49. The topographic pressure torque is somewhat smaller in overall amplitude than the ellipticity‐induced torque (the sum of the ellipticity pressure torque and gravitational torque). The ocean bottom frictional torque is found to be negligible in all three components. To validate the torque calculations, we compare the calculated oceanic torques with effective torque derived from the ocean angular momentum (OAM) variation, both from POCM. It is shown that the effective torque associated with the mass term of the OAM agrees remarkably well with the ellipticity‐induced torque as required by theory. For the motion term the effective torque can be explained mostly by the topographic pressure torque in the intraseasonal periods, but the seasonal and longer‐period components show significant differences between the two. The difference should theoretically be attributed to the exclusion of the wind frictional torque on the ocean. However, a comparison based on the NASA GEOS‐1 atmospheric model shows that the wind frictional torque obtained is insufficient in power in the x and y components. Finally, in the application to the Earth rotation excitation it is found that both the ellipticity and topographic torques have comparable contributions to polar motion excitation for both seasonal and nonseasonal variations. For the length of day variation, our torque approach is presently inadequate because the oceanic contribution is relatively too small to be estimated in the presence of data and model errors.

Male effect in seasonally anovulatory lactating goats depends on the presence of sexually active bucks, but not estrous females

A study was conducted in subtropical northern Mexico (26°N) to determine whether the presence of estrous females can improve the response of seasonally anovulatory goats to the introduction of bucks in the group. The induction of estrous activity was studied in three groups of anovulatory lactating goats during seasonal anestrus. These females were of the Mexican Creole breed. In the control group (sexually inactive (SI), n=20), two control (SI) bucks exposed to normal seasonal daylength variations were used. In the second group (SI+E, n=20+3), two control males were also used, but in addition, three females of the group were in estrus at the time of male introduction. In the third group (sexually active, SA+E, n=19+4), anovulatory females were exposed to two bucks made sexually active by exposure to 2.5 months of long days (16L:8D) followed by two subcutaneous 18 mg melatonin implants, and four estrous females were also present when introducing the bucks. In all groups, males were introduced on 15 March and estrous detection was conducted twice daily for 15 days. The sexual activity of the bucks was observed from 08:00 to 10:00 h during the first five days of exposure to females. More females displayed estrous behavior in the first 15 days following the introduction of the males in the SA+E group (18/19) as compared with the SI or SI+E groups (2/20 and 0/20, respectively; P<0.001). No difference was observed between the two latter groups. Thirteen females of SA+E group showed a second estrus between days 6 and 11 (short estrous cycle duration: 5.4±0.4 days). By contrast, in the SI group none showed a second estrus. The sexual behavior of the males in the SA+E group was greater as compared with that of the males in SI and SI+E groups (over 80% of the total sexual activity recorded in the three groups; P<0.001). By contrast, no differences were found between SI and SI+E males. These results indicate that the presence of estrous females alone at the time of buck introduction is not sufficient to induce an adequate stimulation of seasonally inactive males. The use of sexually active bucks is necessary to induce reproductive activity in anovulatory females, whereas preparation of the bucks with longs days followed by melatonin implants allows them to gain such a capacity.

Atmospheric angular momentum fluctuations and their excitations of earth rotation variations at seasonal scale

We discuss the atmospheric contributions, especially wind contribution, to seasonal variations in the polar wobble and length of day during the period 1985–1995. The atmospheric angular momentum functions, calculated from the output data of GSM9603 GCM of Japan Meteorological Agency (JMA) are compared with those calculated from the reanalysis data of NCEP / NCAR, and from the operational objective analysis data of JMA. The results show that two different vertical wind integration methods lead to different wind contributions to seasonal polar wobble, especially for the component along Greenwich meridian direction in annual variation. However the inter-annual variations of wind contributions estimated by these two methods are almost the same with each other in the two equatorial components. The simulated axial relative AAM function agrees well with the other two as regards the seasonal variation. In addition, its inter-annual amplitude fluctuation of axial relative AAM function at semi-annual scale shows the main characteristics of ENSO evolution better than that estimated from the NCEP reanalysis data, that is, the semi-annual axial relative AAM oscillation becomes weaker during ENSO periods.

Ocean angular momentum signals in a climate model and implications for Earth rotation

Estimates of ocean angular momentum (OAM) provide an integrated measure of variability in ocean circulation and mass fields and can be directly related to observed changes in Earth rotation. We use output from a climate model to calculate 240 years of 3-monthly OAM values (two equatorial terms L 1 and L 2, related to polar motion or wobble, and axial term L 3, related to length of day variations) representing the period 1860–2100. Control and forced runs permit the study of the effects of natural and anthropogenically forced climate variability on OAM. All OAM components exhibit a clear annual cycle, with large decadal modulations in amplitude, and also longer period fluctuations, all associated with natural climate variability in the model. Anthropogenically induced signals, inferred from the differences between forced and control runs, include an upward trend in L 3, related to inhomogeneous ocean warming and increases in the transport of the Antarctic Circumpolar Current, and a significantly weaker seasonal cycle in L 2 in the second half of the record, related primarily to changes in seasonal bottom pressure variability in the Southern Ocean and North Pacific. Variability in mass fields is in general more important to OAM signals than changes in circulation at the seasonal and longer periods analyzed. Relation of OAM signals to changes in surface atmospheric forcing are discussed. The important role of the oceans as an excitation source for the annual, Chandler and Markowitz wobbles, is confirmed. Natural climate variability in OAM and related excitation is likely to measurably affect the Earth rotation, but anthropogenically induced effects are comparatively weak.

Electromagnetic core-mantle coupling-I. Explaining decadal changes in the length of day

Measured changes in the Earth's length of day on a decadal timescale are usually attributed to the exchange of angular momentum between the solid mantle and fluid core. One of several possible mechanisms for this exchange is electromagnetic coupling between the core and a weakly conducting mantle. This mechanism is included in recent numerical models of the geodynamo. The ‘advective torque’, associated with the mantle toroidal field produced by flux rearrangement at the core–mantle boundary (CMB), is likely to be an important part of the torque for matching variations in length of day. This can be calculated from a model of the fluid flow at the top of the outer core; however, results have generally shown little correspondence between the observed and calculated torques. There is a formal non-uniqueness in the determination of the flow from measurements of magnetic secular variation, and unfortunately the part of the flow contributing to the torque is precisely that which is not constrained by the data. Thus, the forward modelling approach is unlikely to be useful. Instead, we solve an inverse problem: assuming that mantle conductivity is concentrated in a thin layer at the CMB (perhaps D″), we seek flows that both explain the observed secular variation and generate the observed changes in length of day. We obtain flows that satisfy both constraints and are also almost steady and almost geostrophic, and therefore assert that electromagnetic coupling is capable of explaining the observed changes in length of day.

Atmospheric, oceanic and hydrological contributions to seasonal variations in length of day

The annual and semiannual residuals derived in the axial angular momentum budget of the solid Earth–atmosphere system reflect significant signals. They must be caused by further excitation sources. Since, in particular, the contribution for the wind term from the atmospheric layer between the 10 and 0.3 hPa levels to the seasonal variations in length of day (LOD) is still missing, it is necessary to extend the top level into the upper stratosphere up to 0.3 hPa. Under the conservation of the total angular momentum of the entire Earth, variations in the oceanic angular momentum (OAM) and the hydrological angular momentum (HAM) are further significant excitation sources at seasonal time scales. Focusing on other contributions to the Earth's axial angular momentum budget, the following data are used in this study: axial atmospheric angular momentum (AAM) data derived for the 10–0.3 hPa layer from 1991 to 1997 for computing the missing wind effects; axial OAM functions as generated by oceanic general circulation models (GCMs), namely for the ECHAM3 and the MICOM models, available from 1975 to 1994 and from 1992 to 1994, respectively, for computing the oceanic contributions to LOD changes, and, concerning the HAM variations, the seasonal estimates of the hydrological contribution as derived by Chao and O'Connor [(1988) Geophys J 94: 263–270]. Using vector representation, it is shown that the vectors achieve a close balance in the global axial angular momentum budget within the estimated uncertainties of the momentum quantities on seasonal time scales.

Induction of flowering in tropical trees by a 30-min reduction in photoperiod: evidence from field observations and herbarium specimens.

During the late rainy season in October 1997 we observed. over a range of >100 km, the highly synchronous emergence of flower buds in several deciduous tree species of the semi-deciduous tropical forest in Guanacaste, Costa Rica. Synchronous flowering soon after the rapid decline in day length around the September equinox and in the absence of any notable climatic cues suggested flower induction by declining photoperiod. By combining field observations and the analysis of flowering herbarium collections, we established highly synchronous flowering periods with low interannual and latitudinal variation predicted for photoperiodic flower induction for more than 25 tree species and a few herbs. We describe morphogenetic changes at the shoot apex of three species during flower induction and the suppression and induction of flowering in several herbaceous species by experimental daylight extension. The combined observations provide strong, mainly indirect evidence for photoperiodic induction of flowering in many tropical tree species. At low latitudes with annual variation in day length of 1 hour, flower induction must be caused by a decline in photoperiod of 30 min or less. This is the first report of photoperiodic control of flowering in trees.

Photoperiodic control of seasonal development and dormancy in tropical stem-succulent trees.

Tropical stem-succulent trees store large quantities of water in their trunks yet remain leafless during the early and mid dry season. In contrast to most other tropical trees, bud break of vegetative buds is not induced in fully hydrated stem succulents between the winter solstice and the spring equinox by leaf abscission, abnormal rain showers or irrigation. Vegetative buds of leafless trees are therefore in a state of endo-dormancy similar to that of temperate perennial plants during early winter. Highly synchronous bud break regularly occurs soon after the spring equinox, often weeks before the first rainfalls of the wet season. These observations suggested that endo-dormancy and bud break might be induced by declining and increasing photoperiods after the autumn and spring equinoxes, respectively. In phenological field observations, we confirmed highly synchronous bud break after the spring equinox in many trees of five stem-succulent species in the northern and southern hemispheres. Shoot growth of potted saplings of Plumeria rubra L. was arrested by a decline in day length below 12 h after the autumn equinox, but continued in saplings maintained in a 13-h photoperiod. Conversely, exposure to a 13-h photoperiod induced bud break of dormant apical buds in saplings and cuttings in January, whereas plants maintained in the natural day length of < 11.7 h remained dormant. Photoperiodic control of endo-dormancy of vegetative buds in stem succulents is thus supported by field observations and experimental variation of the photoperiod. At low latitudes, where annual variation of day length is less than 1 h, bud dormancy is induced and broken by variations in photoperiod of less than 30 min.

A combined length-of-day series spanning 1832–1997: LUNAR97

Universal time (UT) measurements taken by the techniques of lunar occultation, optical astrometry, lunar laser ranging, and very long baseline interferometry have been combined using a Kalman filter to produce a smoothed length-of-day (LOD) series, LUNAR97, spanning 1832.5–1997.5 at yearly intervals. Decadal LOD variations having amplitudes as large as a few milliseconds (ms) are clearly evident in this newly determined series as they have been in previously determined series. Comparing the LUNAR97 LOD series with other available series shows differences that are generally less than the uncertainty in the determination of the LUNAR97 LOD values, which is about 0.6 ms during the early 19th century, improving by more than an order of magnitude to about 0.03 ms during the late 20th century. Decadal LOD variations are, therefore, a robust feature of the lunar occultation, optical astrometric, and space-geodetic measurements of the Earth’s rotation that have been taken since 1832.

General-relativistic theory of elastic deformable astronomical bodies

This paper deals with general perturbations of astronomical bodies in the first post-Newtonian approximation of Einstein's theory of gravity. The formalism starts with a description of some stationary axisymmetric configuration in an asymptotically flat space-time manifold. General perturbations of this configuration are first treated within the Carter-Quintana framework where the bodies are assumed to be composed of some perfectly elastic material. Deviations from the equilibrium configuration are described by means of some displacement field as in the local theory for the global motion of Earth in space. As central results the post-Newtonian perturbation equations for the displacement field, the perturbed field, and local matter variables are presented. Especially for further discussions of global geodynamics (precession, nutation, polar motion, length of day variations) in the framework of general relativity the results presented here might serve as a new starting point. We also derive the perturbed Euler equation for vanishing shear stresses, i.e., for a fluid body, by means of a linearization procedure around the equilibrium state. After taking the Eulerian variations of the local matter variables in terms of the displacement field from the Carter-Quintana framework the two methods are found to produce the same results.

Core Collection of Sorghum: I. Stratification Based on Eco‐Geographical Data

ICRISAT conserves a large (36 719 entries) collection of sorghum [Sorghum bicolor (L.) Moench] accessions in India. This collection comprises cultivated and wild sorghums acquired over the past 25 years from 90 countries. However, it is difficult to characterize and assess a large collection with limited time and resources. To facilitate maintenance, assessment, and utilization of the collection, we considered the establishment of a core collection using stratified sampling strategies. Results from a study of the morpho‐agronomic diversity were used to describe the genetic structure of the collection. Morphological traits, including days to flowering and plant height, can be affected by daylength variation. These two characters were highly correlated with latitudinal and racial distributions of landraces. Thus, stratifying the entire collection for response to photoperiod, estimated by flowering date and plant height, was indicative of a major source of specific adaptation within the collection. This stratification resulted in four clusters, which described the sensitivity of genotypes to photoperiod within the latitudinal range where selection was carried out by farmers. These four clusters may serve as the basis for a random stratified sampling to establish cores in this collection.

Regulation by photoperiod of seasonal changes in body mass and reproductive function in gray mouse lemurs (Microcebus murinus) : Differential responses by sex

Microcebus murinus exhibits highly seasonal biological rhythms to cope with extreme seasonality in availability of resources. To study the role of daylength on seasonal changes in body mass and reproductive function, we exposed male and female gray mouse lemurs to natural, constant, or alternating light cycles for 2 years under constant environmental conditions. When exposed to either constant short (SD: 10 h light/day), long (LD: 14 h light/day), or intermediate (ID: 12 h light/day) daylength, males and females maintained a constant body mass with no spontaneous cyclic variation. We only observed typical seasonal body mass changes in subjects exposed to alternating periods of SD and LD, the weight gain being triggered by SD, whereas weight loss occurred under LD. Reproductive activity in females proceeded from an endogenous rhythm that was expressed under constant daylengths. In contrast, changes in reproductive activity in males depended on daylength variation. In both sexes, SD and LD have direct inhibitory or stimulatory effects on reproductive activity. In females, daylength regulates breeding season by synchronizing an endogenous sexual rhythm with the season, whereas in males, the perception of a critical photoperiod is used to determine the subsequent onset or arrest of their breeding season. These sexual differences in the effect of daylength could be related to sex-specific differences in reproductive constraints.

Phenology of Atlantic Rain Forest Trees: A Comparative Study1

ABSTRACTThis paper describes the phenology of leaf, flower, and fruit phenology in the Atlantic rain forests of southeastern Brazil. For 17 months, we observed the phenological patterns of trees from two Atlantic forest types at four sites: premontane forest (Sites I and IV; the “typical” Atlantic rain forest) and coastal plain forest (Sites II and III). All sites experience a nonseasonal, tropical wet climate, characterized by an annual rainfall usually &gt; 2000 mm and lacking a dry season. We tested for the occurrence (or absence) of seasonal phenological patterns within each site and compared the patterns detected among the four different forest sites using circular statistics. The expected weakly seasonal phenological patterns were not observed for these forests. Flowering and leaf flush patterns of Atlantic rain forest trees were significantly seasonal, concentrated at the beginning of the wettest season, and were significantly correlated with day length and temperature. These results stress the influence that seasonal variation in day length has on ever‐wet forest tree phenology. Fruiting phenologies were aseasonal in all four forests. Flowering patterns did not differ significantly among three of the four forest sites analyzed, suggesting the occurrence of a general flowering pattern for Atlantic rain forest trees.

Decadal correlation between crustal deformation and variation in length of day of the Earth

Recently, in comparing with Earth rotation data, we found some exciting correlation phenomena between length-of-day (LOD) and crustal deformations as well as stress observations in China, the correlation appears most typically on decade time scales and seems to be direction-dependent. As LOD variation induced stress and deformation is far less than the observed ones, we infer that decade variations in crustal deformation and stress are very possibly attributed to the core-mantle coupling process, just like LOD decade variations are attributed to core-mantle coupling. More detailed and extended investigations are going on, and we believe that the similar correlation phenomena should also exist in other part of the world. The LOD-correlated crustal deformations are expected to provide a better constraint on verifying the real core-mantle coupling mechanism, topographic, electromagnetic or gravitational coupling.

Electromagnetic core‐mantle coupling for laterally varying mantle conductivity

Electromagnetic coupling between the Earth's core and mantle is one of the proposed mechanisms to explain length of day (LOD) variations on decadal timescales. Mantle conductivity, a determining parameter in this process, is only poorly known. Earlier work on core‐mantle coupling mostly assumed laterally homogeneous mantle conductivities. However, the lower mantle is a highly inhomogeneous region. Seismic evidence is growing that many of the inhomogeneities must have chemical, compositional, or thermal origins. We consider the effect of different laterally varying conductivity models on electromagnetic mantle torques. Torque amplitudes for a specific epoch can depend strongly on the assumed conductivity distribution. However, when comparing time series of the electromagnetic torque to decadal LOD variations, none of the examined conductivity models improve the agreement significantly. As in the simplest case of a homogeneous mantle conductivity, a minimum average conductance of 108 S is always required to make electromagnetic coupling efficient.

Mars rotation variations induced by atmosphere and ice caps

Because of the conservation of angular momentum, the atmospheric winds and the mass exchange between the Martian ice caps and atmosphere, associated with the sublimation/condensation process (mainly CO2), induce seasonal effects on Mars' polar motion, nutation, and length of day (LOD). These effects are computed using the output of a global circulation model of the Martian atmosphere, providing atmospheric pressure fields, ice cap surface pressure fields, and zonal as well as meridional winds. For the LOD variations, total amplitudes (CO2 and wind effects) of 0.22 ms for the annual wave and of 0.38 ms for the semiannual wave are obtained. These amplitudes are more than one order of magnitude larger than the LOD variations induced by the zonal tides, which are at the level of 10 μm. For the induced polar motion the annual amplitude is ∼11 milliarcseconds (mas), and the semiannual amplitude is ∼3 mas. The effect on the nutations, related to the diurnal forcing, is at the level of 0.1 mas. The differences between the results for a liquid and for a solid core are examined and shown to be &lt;1% of the total effects.

Leaf phenological traits in the mangrove Kandelia candel (L.) Druce

Leaf phenological traits of a sub-tropical Rhizophoracean mangrove, Kandelia candel (L.) Druce, for the period May 1994 to July 1995, are discussed in relation to prevailing environmental factors. Leaf phenologies were characterized by summer leaf production maxima and winter leaf loss maxima. Results of canonical correlation analysis indicate that about 70% of the seasonal variation in leaf production was explained by variation in temperature, humidity and day-length, hence, supporting the hypothesis that leaf phenological traits had a strong environmental component. The strongest single-variable regression model for leaf formation was produced in a temperature model transformed into second order kinetics ( r=0.8, p<0.01). This suggests that temperature (or temperature related effects) was the strongest environmental gradient controlling leaf production in K. candel.