Seasonal and inter-annual gravity changes in the Siberian permafrost region from GRACE

Since its launch in 2002, the GRACE (Gravity Recovery And Climate Experiment) satellite mission has revolutionized the observation of global gravity field and its temporal changes. Now that the time span exceeds 7 years and the spatial resolution becomes as good as ~300 km, GRACE observation can target fine regional gravity changes. Using temporal variation in terrestrial water storage (dW/dt) observed by GRACE, we investigate hydrological regimes in the Siberian permafrost regions, especially in the Lena, Yenisey and Ob river basins by comparing gravity with meteorological quantities of precipitation (P) and evapo-transpiration (E). The hydrological regime in permafrost is different from “ordinary” regions due to low temperature and impermeability of frozen soil. It is important to understand how permafrost reacts to global warming and how it appears in time-variable gravity. In general, soil moisture reflects the time integration of water fluxes of P, E, and runoff (R). For example, comparison between GRACE and P-E suggests that precipitation governs seasonal changes in terrestrial water storage in tropical regions such as the Amazon and Congo river basins. On the other hand, in the Siberian permafrost regions sudden increase in (Arctic-bound) river discharge due to ice thawing (different between the eastern and western Siberian regions due to different permafrost conditions) strongly influences seasonal variability of surface water. On seasonal timescale the mean monthly discharge data in Siberia (http://www.r-arcticnet.sr.unh.edu/v4.0) show a good agreement with dW/dt inferred from GRACE and P-E. In addition, GRACE sees significant inter-annual “secular” mass increases in the three river basins over the last 7 years (30.7, 12.9, and 9.6 Gt/year in the Lena, Yenisey and Ob river basins, respectively.) We also discuss how such increases take place in permafrost regions. Time series of water equivalent thickness in the Lena river basin. Solid circles show monthly mass change from GRACE gravity data. Solid gray curve is modeled by linear, annual and biannual components. Dashed line shows estimated secular trend. Clear seasonal and secular changes are seen.