Detection of different-time-scale signals in the length of day variation based on EEMD analysis technique

Scientists pay great attention to different-time-scale signals in the lengllh of day （LOD） variations △LOD, which provide signatures of the Earth＇s interior structure, couplings among different layers, and potential excitations of ocean and atmosphere. In this study, based on the ensemble empirical mode decomposition （EEMD）, we analyzed the latest time series of △LOD data spanning from January 1962 to March 2015. We observed the signals with periods and amplitudes of about 0.5 month and 0.19 ms, 1.0 month and 0.19 ms, 0.5 yr and 0.22 ms, 1.0 yr and 0.18 ms, 2.28 yr and 0.03 ms, 5.48 yr and 0.05 ms, respectively, in coincidence with the results of predecessors. In addition, some signals that were previously not definitely observed by predecessors were detected in this study, with periods and amplitudes of 9.13 d and 0.12 ms, 13.69 yr and 0.10 ms, respectively. The mechanisms of the LOD fluctuations of these two signals are still open.

Time-integrated North Atlantic Oscillation as a proxy for climatic change

The time-integrated yearly values of North Atlantic Oscillation (INAO) are found to be well correlated to the sea surface temperature. The results give the feasibility of using INAO as a good proxy for climate change and contribute to a more complete picture of the full range of variability inherent in the climate system. Moreover, the extrapolation in the future of the well identified 65-year harmonic in INAO suggests a gradual decline in global warming starting from 2005.

Growing Season and Phenological Stages of Small Grain Crops in Response to Climate Change in Alaska

The climate change in Alaska has caused earlier spring snowmelt and the growing season expanded. However, the effect of climate change on crop phenological stages, heading (BBCH 55) and maturity (BBCH 85), is unknown. In this study, the trends of growing-season length (GSL), phenological stages of crops and climatic parameters, and the correlations between climatic parameters and the phenological stages were analyzed using the climate data and crop data over the period of 1978 to 2016. The longer GSL was found in Fairbanks (64.83˚N, 147.77˚W) and in Delta Junction (64.05˚N, 145.60˚W) but not in Palmer (61.60˚N, 149.11˚W). Sowing dates did not change significantly in three locations. The decreasing trends of heading and maturity of crops were observed but varied with location. Heading of barley and oat significantly advanced 3 and 3.1 d decade-1, respectively from 1989 to 2016 in Fairbanks while no change of heading was observed in Delta Junction and Palmer. Maturity of barley, oat and wheat significantly advanced 2.6, 3.8 and 3.9 d decade-1, respectively from 1978 to 2016 in Fairbanks (P < 0.05);maturity of oat and wheat significantly advanced 4.4 and 3.4 d decade-1 from 1978 to 2015, respectively in Delta Junction (P < 0.05). The increasing temperature trends and decreasing precipitation trends were found in Fairbanks and Delta Junction but varied with phenological stages of crops. Sowing was more important for heading than for maturity of crops. The effect of climate change on heading was less important than that on maturity. Earlier maturity of crops in Fairbanks may be attributed to increased temperatures, that in Delta Junction to both increased minimum temperature and decreased precipitation and that in Palmer to temperature and precipitation.

27.3-day and Average 13.6-day Periodic Oscillations in the Earth’s Rotation Rate and Atmospheric Pressure Fields Due to Celestial Gravitation Forcing

Variation in length of day of the Earth （LOD, equivalent to the Earth＇s rotation rate） versus change in atmospheric geopotential height fields and astronomical parameters were analyzed for the years 1962-2006. This revealed that there is a 27.3-day and an average 13.6-day periodic oscillation in LOD and atmospheric pressure fields following lunar revolution around the Earth. Accompanying the alternating change in celestial gravitation forcing on the Earth and its atmosphere, the Earth＇s LOD changes from minimum to maximum, then to minimum, and the atmospheric geopotential height fields in the tropics oscillate from low to high, then to low. The 27.3-day and average 13.6-day periodic atmospheric oscillation in the tropics is proposed to be a type of strong atmospheric tide, excited by celestial gravitation forcing. A formula for a Tidal Index was derived to estimate the strength of the celestial gravitation forcing, and a high degree of correlation was found between the Tidal Index determined by astronomical parameters, LOD, and atmospheric geopotential height. The reason for the atmospheric tide is periodic departure of the lunar orbit from the celestial equator during lunar revolution around the Earth. The alternating asymmetric change in celestial gravitation forcing on the Earth and its atmosphere produces a ＂modulation＂ to the change in the Earth＇s LOD and atmospheric pressure fields.

Relation Between Equatorial Oceanic Activities and LOD Changes

The time series of the length of day (LOD) and the observational Pacific sea level during l962.0-1990.0 are used to study the relation between Earth rotation and equatorial oceanic activities.The results show that (i) the sea level is apparently rising at an average rate of about 1.75±.01mm/a during the past 30 years,(ii) there are large-scale eastward and westward water motions in the upper equatorial Pacific zone,which,according to the dynamical analysis of the angular momentum of the large-scale sea water motion in Pacific Ocean related to the Earth rotation axis accounts for about 30% of the change in ititerannual Eatlh rotation rate; (iii) the interannual changes in Earth rotation also cause changes in the distribution of the water mass in equatorial Pacific,and affect the formation of ENSO events.Based on these results,we give a new model for the interaction between equatorial ocean and Earth rotation.

顾及最小二乘拟合端点效应的日长变化预报

针对日长(Length Of Day,LOD)变化预报中最小二乘(Least Squares,LS)拟合存在端点效应的问题,采用时间序列分析方法对日长变化序列进行端点延拓,形成一个新序列,然后用新序列建立最小二乘模型,最后再结合最小二乘模型和自回归(Autoregressive,AR)模型对原始日长变化序列进行预报。实验结果表明,在日长变化序列两端增加统计延拓数据,能有效减小最小二乘拟合序列的端点畸变,从而提高日长变化的预报精度,尤其对中长期预报精度提高明显。

利用正交本征模态函数分析日长变化的主要激发源

利用本征模态函数的正交分解方法对日长(LOD,length of day)数据序列进行分解,得到了日长变化的101个正交本征模态函数。通过对其中几个主要模态函数进行分析,发现日月地的相对位置与其中几个模态函数极值发生的时间极其吻合,并据此推断出日长变化的主要激发源。同时还发现日长变化存在准周期约为206 d的波动。

Multichannel singular spectrum analysis of the axial atmospheric angular momentum

Earth＇s variable rotation is mainly produced by the variability of the AAM（atmospheric angular momentum）. In particular, the axial AAM component X_3, which undergoes especially strong variations,induces changes in the Earth＇s rotation rate. In this study we analysed maps of regional input into the effective axial AAM from 1948 through 2011 from NCEP/NCAR reanalysis. Global zonal circulation patterns related to the LOD（length of day） were described. We applied MSSA（Multichannel Singular Spectrum Analysis） jointly to the mass and motion components of AAM, which allowed us to extract annual, semiannual, 4-mo nth, quasi-biennial, 5-year, and low-frequency oscillations. PCs（Principal components） strongly related to ENSO（El Nino southern oscillation） were released. They can be used to study ENSO-induced changes in pressure and wind fields and their coupling to LOD. The PCs describing the trends have captured slow atmospheric circulation changes possibly related to climate variability.

Interannual variations in the length of day and ENSO events in 1982-1983 and 1997-1998

The length of day series during the period of 1962.0-2000.0, the atmospheric angular momentum and the Southern Oscillation Index are adopted to analyze the relationships among the EN-SO events that have occurred since 1960, the changes in the length of day and the atmospheric angular momentum. Attention is particularly given to the different effects of the 1982-1983 and 1997-1998 ENSO events on the variations of Earth rotation. The synthetic excitation effects of multi-scale atmospheric oscillations on the anomalous variations of the interannual rates of Earth rotation are revealed by means of the time-frequency spectrum of the wavelet transform.

Premonitory phenomenon of El Nino event reflected in the observations of LOD and sea level

Data sets of the changes of the length of day, the sea surface temperature in the eastern Pacific and of the sea level in Hong Long from tide gauge observations are used to analyze and reveal the reflections in the observations of the length of day and the sea level changes concerned with the premonitory phenomenon of next El Nino event. The results from this study indicate that a new El Nino event has been brewing with the ending of the strong La Nina event that started in early summer of 1998. The estimated formation period of the new El Nino event will begin before the end of 2000, and the peak period may be reached at around the end of 2001.

INTERANNUAL VARIATION OF EARTH ROTATION,El Nio EVENTS AND ATMOSPHERIC ANGULAR MOMENTUM

In this paper the multi-stage digit filter is used to analyse the data of Earth rotation represented by the length of day, ΔLOD. The results show that the interannual variations of Earth rotation, which are in the time scale of several years but not quasi-periodic terms, exist in the long periodic fluctuations. They induce the relative variation in the length of day of 0.3×10^(-8).Comparing the series of length of day with the data of temperature departure of the sea surface in the equatorial area of the eastern Pacific, we found that the deceleration and acceleration of the interannual rate of Earth rotation are consistent with the warming up and down of sea temperature in the equatorial area very well. This means that every El Nio event always occurs after the turning of acceleration of the interannual rate of Earth rotation to deceleration.According to the strong interannual variation in the length of day and strong warming of the sea surface temperature in the equatorial area between 1982 and 1983, we analysed the data from atmospheric angular momentum (AAM) calculated by using the global zonal wind data, and found that the interannual variation in AAM has an excess of two to three months. We suggest that the interannual variations in Earth rotation and the El Nio events are probably responses of solid earth and ocean, respectively, to the anomaly of atmospheric circulation.It is also shown in oar analysis that the minimum of ΔLOD series, which is deduced from UT1 data observed regularly with astrometry, can predict the occurrence of the El Nio events for a long range forecast about one year.

High frequency resolution of earth orientation parametermeasured by GPS

The Global Positioning System (GPS) experiment (IGS’92 Campaign) from June toSeptember of 1992, provided an opportunity to test the ability of GPS for measuring EarthOrientation Parameter (EOP). During that period, IGS series which is a combination ofthe series observed by other space geodetic techniques such as VLBI, SLR and LLR is alsoavailable.

EARTH ROTATION AND DISASTROUS EARTHQU AKES IN XIN JIAN G

Xinjiang, northwest of China, is located in the middle south of Eurasian plate, which collides with the Indian Ocean plate in the south and is pressed southward by the Siberian plate in the north. Therefore, in this region there are inland earthquakes with higher frequency and greater magnitude. It is one of the most active areas in China. There were 104 strong earthquakes with magnitude M_s≥6.0 occurring in Xinjiang since 1600