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On the Relationships between Low-Frequency Variations of Earth's Rotation and Equatorial Atmospheric Angular Momentum
  • Tri Wahyu Hadi,
  • Faiz Rohman Fajary,
  • Shigeo Yoden
Tri Wahyu Hadi
Bandung Institute of Technology

Corresponding Author:[email protected]

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Faiz Rohman Fajary
Bandung Institute of Technology
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Shigeo Yoden
Institute for Liberal Arts and Sciences, Kyoto University, Kyoto, Japan
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This work mainly concerns low-frequency variations of Atmospheric Angular Momentum (AAM), emphasizing the role of the equatorial region and its relationships with the length of day LOD, whose observed time series indicate an accelerating Earth’s rotation over the last several decades. We applied bivariate and trivariate Empirical Mode Decomposition methods to extract coherent nonstationary signals from the monthly time series of $LOD$ and the two components of AAM, i.e., the mass term MΩ and the motion term Mr. It is found that, over the global domain, a decreasing trend of LOD during the last five decades correlates with an increasing trend in MΩ, whereas the trend in Mr is negligible. However, there is a significantly positive trend in Mr of the equatorial lower troposphere (1000 to 700 hPa), which can be associated with a larger transfer of eastward momentum due to the accelerating Earth. Further analyses of spatio-temporal distribution of Mr anomalies suggest that, at multidecadal time scales, residual changes in the motion term of AAM across the globe tend to be in balance. The long-term positive trend in MΩ, which is dominant over the equatorial latitude belt, is most likely attributed to prolonged effects of the global increase in surface pressure from the mid-1970s until the 1990s. Low-frequency variations of LOD are also found to have a high correlation with the Atlantic Meridional Oscillation index. Our results suggest that long-term changes in the Earth’s rotation rate are partially attributable to the atmospheric and oceanic variability of comparable time scales.