Excitation of polar motion by terrestrial water storage: a reappraisal using the WaterGAP hydrological model

Abstract

We assess the role of terrestrial water storage (TWS; that is, land hydrology covering canopies, snow, soil, groundwater, lakes, wetlands, reservoirs, and rivers) and confirm its significant impact in driving polar motion on a wide range of timescales. For this purpose, we use the hydrological model WaterGAP v2.2e in the range 1901–2019 with daily resolution and under climate forcing and direct human influence. TWS-induced polar motion excitation exhibits a prominent long-term trend of ∼4.80 milliarcseconds per year (mas/yr) toward ∼139.25∘E, which is, however, still not in agreement with satellite observations from Gravity Recovery and Climate Experiment (GRACE) and Satellite Laser Ranging (SLR). The mentioned trend is mainly caused by snow water storage (∼4.87 mas/yr toward ∼140.82∘E; driven by changes in snowfall patterns and melting in Greenland), groundwater (∼0.16 mas/yr toward ∼21.16∘E; due to anthropogenic groundwater withdrawal), and reservoir storage (∼0.06 mas/yr toward ∼103.40∘W; resulting mainly from impoundment of reservoirs by humans). Furthermore, in the excitation series derived, there are fluctuations on seasonal and longer timescales (with our focus on periods of up to 6 years), in both prograde and retrograde frequency bands and with amplitudes as large as ∼8.95 mas. We verify these fluctuations against the geodetically observed polar motion excitation, as well as the independent TWS excitation series of GFZ Helmholtz Center for Geosciences, demonstrating significant Pearson correlations (as large as ∼0.81) and coherency (up to ∼0.91). These results improve our understanding of the Earth’s rotational dynamics and have considerable implications for modeling the Earth rotation parameters, their geophysical interpretation, and their inclusion in climate-related studies.