Indexing the relationship between polar motion and water mass change in a giant river basin

Previous studies on the relationship between polar motion and water mass change have mainly concentrated on the excitation of polar motion via global terrestrial water storage changes (TWSC). In view of the uneven distribution of global terrestrial water storage, the relationship between regional water mass change and polar motion needs to be further explored owing to the lack of documented results. In addition, given the uncertainty in the estimation of TWSC, it is required to develop appropriate indices to describe water mass change from different perspectives. The Amazon River basin (referred to Amazon hereafter), containing the world’s largest river, located at around the 90°W longitude, is selected as the study area. Water vapor flux, precipitation, runoff and TWSC are selected as the indices of water mass changes to reveal the relationship between polar motion and water mass change in this giant basin. The Mann-Kendall (M-K) method, the accumulated anomaly analysis method and the curvature method are used to identify the abrupt change points; the least squares method is used to estimate the trends, and the Fast Fourier Transform (FFT) and the Ensemble Empirical Mode Decomposition (EEMD) are used to perform a periodic analysis, for all the above indices. It is shown that, of all the indices from 1948 to 2011, water vapor flux is the most closely related index to polar motion. In detail, precipitation and water vapor flux contain beat periods of polar motion; water vapor flux, precipitation and polar motion have a common M-K test abrupt change point (occurring in ca. 1968) at the 0.05 significance level; water vapor flux has a similar accumulated anomaly curve with that of polar motion; and water vapor flux is more highly correlated with polar motion than most other indexes. It is found, just like global TWSC, the χ₂ component of the excitation via water vapor flux and water storage change in the Amazon follows that of observed polar motion; χ₁ does not follow. However, the pattern in the Amazon that the χ₂ component of the excitation by water follows that of observed polar motion is at a more significant level than in global. Finally, the new index termed Location of Vapor-based InterTropical Convergence Zone (LVITCZ) we proposed to describe the annual mean latitudinal location of water mass change shows a more close and visual relationship between water mass change and polar motion than other chosen indices do.