Secular polar motion observed by GRACE.

A long-term drift in polar motion (PM) has been observed for more than a century, and Glacial Isostatic Adjustment (GIA) has been understood as an important cause. However, observed PM includes contributions from other sources, including contemporary climate change and perhaps others associated with Earth's interior dynamics. It has been difficult to separate these effects, because there is considerable scatter among GIA models concerning predicted PM rates. Here we develop a new method to estimate GIA PM using data from the GRACE mission. Changes in GRACE degree 2, order 1 spherical harmonic coefficients are due both to GIA and contemporary surface mass load changes. We estimate the surface mass load contribution to degree 2, order 1 coefficients using GRACE data, relying on higher-degree GRACE coefficients that are dominantly affected by surface loads. Then the GIA PM trend is obtained from the difference between observed PM trend (which includes effects from GIA and surface mass loads) and the estimated PM trend mostly associated with surface mass loads. A previous estimate of the GIA PM trend from PM observations for the period 1900-1978 is toward 79.90° W at a speed of 3.53 mas/year (10.91 cm/year). Our new estimate for the GIA trend is in a direction of 61.77° W at a speed of 2.18 mas/year (6.74 cm/year), similar to the observed PM trend during the early twentieth century. This is consistent with the view that the early twentieth-century trend was dominated by GIA and that more recently there is an increasing contribution from contemporary surface mass load redistribution associated with climate change. Our GIA PM also agrees with the linear mean pole during 1900-2017. Contributions from other solid Earth process such as mantle convection would also produce a linear trend in PM and could be included in our GIA estimate.

Antarctic ice mass variations from 1979 to 2017 driven by anomalous precipitation accumulation.

Antarctic ice mass balance is determined by precipitation and ice discharge, and understanding their relative contributions to contemporary Antarctic ice mass change is important to project future ice mass loss and resulting sea level rise. There has been evidence that anomalous precipitation affects Antarctic ice mass loss estimates, and thus the precipitation contribution should be understood and considered in future projections. In this study, we revisit changes in Antarctic ice mass over recent decades and examine precipitation contributions over this period. We show that accumulated (time-integrated) precipitation explains most inter-annual anomalies of Antarctic ice mass change during the GRACE period (2003-2017). From 1979 to 2017, accumulated Antarctic precipitation contributes to significant ice mass loss acceleration in the Pacific sector and deceleration in the Atlantic-Indian Sectors, forming a bi-polar spatial pattern. Principal component analysis reveals that such a bi-polar pattern is likely modulated by the Southern Annular Mode (SAM). We also find that recent ice mass loss acceleration in 2007 is related to a variation in precipitation accumulation. Overall ice discharge has accelerated at a steady rate since 1992, but has not seen a recent abrupt increase.

Global sea level change signatures observed by GRACE satellite gravimetry.

Ice mass loss on land results in sea level rise, but its rate varies regionally due to gravitational self-attraction effects. Observing regional sea level rates by ocean mass change using the Gravity Recovery and Climate Experiment (GRACE) gravity solutions is difficult due to GRACE's spatial resolution (~a few hundred km) and other limitations. Here we estimate regional sea level mass change using GRACE data (without contributions from temperature and salinity variations) by addressing these limitations: restoring spatially spread and attenuated signals in post-processed GRACE data; constraining ocean mass distribution to conform to the changing geoid; and judging specific corrections applied to GRACE data including a new geocenter estimate. The estimated global sea level mass trend for 2003-2014 is 2.14 ± 0.12 mm/yr. Regional trends differ considerably among ocean basins, ranging from -0.5 mm/yr in the Arctic to about 2.4 mm/yr in the Indian and South Atlantic Oceans.

Surface mass balance contributions to acceleration of Antarctic ice mass loss during 2003-2013.

Recent observations from satellite gravimetry (the Gravity Recovery and Climate Experiment (GRACE) mission) suggest an acceleration of ice mass loss from the Antarctic Ice Sheet (AIS). The contribution of surface mass balance changes (due to variable precipitation) is compared with GRACE-derived mass loss acceleration by assessing the estimated contribution of snow mass from meteorological reanalysis data. We find that over much of the continent, the acceleration can be explained by precipitation anomalies. However, on the Antarctic Peninsula and other parts of West Antarctica, mass changes are not explained by precipitation and are likely associated with ice discharge rate increases. The total apparent GRACE acceleration over all of the AIS between 2003 and 2013 is -13.6 ± 7.2 Gt/yr2. Of this total, we find that the surface mass balance component is -8.2 ± 2.0 Gt/yr2. However, the GRACE estimate appears to contain errors arising from the atmospheric pressure fields used to remove air mass effects. The estimated acceleration error from this effect is about 9.8 ± 5.8 Gt/yr2. Correcting for this yields an ice discharge acceleration of -15.1 ± 6.5 Gt/yr2.

Field test of the superconducting gravimeter as a hydrologic sensor.

We report on a field test of a transportable version of a superconducting gravimeter (SG) intended for groundwater storage monitoring. The test was conducted over a 6-month period at a site adjacent to a well in the recharge zone of the karstic Edwards Aquifer, a major groundwater resource in central Texas. The purpose of the study was to assess requirements for unattended operation of the SG in a field setting and to obtain a gravimetric estimate of aquifer specific yield. The experiment confirmed successful operation of the SG, but water level changes were small (<0.3 m) leading to uncertainty in the estimate of specific yield. Barometric pressure changes were the dominant cause of both water level variations and non-tidal gravity changes. The specific yield estimate (0.26) is larger than most published values and dependent mainly on low frequency variations in residual gravity and water level time series.