基於角動量守恆的原理，地球自轉的速度會隨著地表及地球內部質量的流動與分布改變而擾動，可藉由分析日長變化量 (variation in length of day，ΔLOD) 找出改變地球轉速的地球物理現象。本研究利用累積近50年的觀測資料，目的是想得到比先前的研究更精確的結果，以及觀察年際性週期的訊號。本文使用National Centers for Environmental Prediction (NCEP) 提供的1962-2008年LOD做以下兩種研究：第一是計算長週期潮汐相對於彈性地球的zonal response coefficient (κ值) 並與前人做比較。結果得到κ值比較大，範圍在0.315~0.34之間，標準差更小。表示在重力的影響之下，海洋對彈性地球的影響比目前的模式所得更大。第二是計算小波頻譜圖，解釋不同週期的訊息。顯示出El Nino-Southern Oscillation (ENSO)在ΔLOD之中有6年的週期變化相當規律，暗示著ENSO可能存在與其他自然現象某些關聯，進而影響了本身各年份的強弱。此外本文在小波頻譜上發現新的地球物理訊號，週期大約是13~16年，有待此後更多的研究來證明之。

On the basis of conservation of angular momentum, the rotational rate of the earth varies with surface and inner earth’s mass fluid and redistribution. Some geophysical phenomena which change earth’s rotational speed will be discovered by analyzing the variation in length of day (ΔLOD). In this study, nearly 50 years ΔLOD data provided from National Centers for Environmental Prediction (NCEP) are used to calculate more accurate estimates and to observe annual signals. This work has two parts. The first part is to calculate zonal response coefficient (κ) for long-period tides relative to the elastic earth and compare with the result of previous studies. The result κ of this study is larger, between 0.315-0.34, and has smaller standard deviation. It means the ocean impacts the elastic earth greater than ocean models at present days. The second part is revealing signals of different periods with wavelet spectral analysis. It shows that the period of El Nino-Southern Oscillation (ENSO) is very regular. It implies that ENSO may have some association with other natural phenomena, which affect their strength sometimes. In addition, this work finds a new geophysics signal which has a period about 13 to 16 years, and more research is necessary to prove that.

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