title: The quasi-biennial vertical oscillations at global GPS stations: identification by ensemble empirical mode decomposition.
abstract: Modeling nonlinear vertical components of a GPS time series is critical to separating sources contributing to mass displacements. Improved vertical precision in GPS positioning at stations for velocity fields is key to resolving the mechanism of certain geophysical phenomena. In this paper, we use ensemble empirical mode decomposition (EEMD) to analyze the daily GPS time series at 89 continuous GPS stations, spanning from 2002 to 2013. EEMD decomposes a GPS time series into different intrinsic mode functions (IMFs), which are used to identify different kinds of signals and secular terms. Our study suggests that the GPS records contain not only the well-known signals (such as semi-annual and annual signals) but also the seldom-noted quasi-biennial oscillations (QBS). The quasi-biennial signals are explained by modeled loadings of atmosphere, non-tidal and hydrology that deform the surface around the GPS stations. In addition, the loadings derived from GRACE gravity changes are also consistent with the quasi-biennial deformations derived from the GPS observations. By removing the modeled components, the weighted root-mean-square (WRMS) variation of the GPS time series is reduced by 7.1% to 42.3%, and especially, after removing the seasonal and QBO signals, the average improvement percentages for seasonal and QBO signals are 25.6% and 7.5%, respectively, suggesting that it is significant to consider the QBS signals in the GPS records to improve the observed vertical deformations.
authors.affiliation: School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China. pan_yuanjin@163.com.
authors.forename: Yuanjin
authors.initials: Y
authors.lastname: Pan
authors.affiliation: School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China. wbshen@sgg.whu.edu.cn.|State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, Wuhan 430079, China. wbshen@sgg.whu.edu.cn.
authors.forename: Wen-Bin
authors.initials: WB
authors.lastname: Shen
authors.affiliation: School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China. dhaosgg@126.com.
authors.forename: Hao
authors.initials: H
authors.lastname: Ding
authors.affiliation: Department of Civil Engineering, National Chiao Tung University, Hsinchu 300, Taiwan. cheinway@mail.nctu.edu.tw.
authors.forename: Cheinway
authors.initials: C
authors.lastname: Hwang
authors.affiliation: Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China. lijin@shao.ac.cn.
authors.forename: Jin
authors.initials: J
authors.lastname: Li
authors.affiliation: School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China. txzhang521@163.com.
authors.forename: Tengxu
authors.initials: T
authors.lastname: Zhang
keywords: GPS time series; ensemble empirical mode decomposition (EEMD); loading effects; quasi-biennial vertical oscillations