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1. Global groundwater droughts are more severe than they appear in hydrological models: An investigation through a Bayesian merging of GRACE and GRACE-FO data with a water balance model

   https://doi.org/10.1016/j.scitotenv.2023.169476  

   Forootan, Ehsan; Mehrnegar, Nooshin; Schumacher, Maike; Schiettekatte, Leire Anne; Jagdhuber, Thomas; Farzaneh, Saeed; van Dijk, Albert I.J.M.; Shamsudduha, Mohammad; Shum, C.K. (February 2024, Science of The Total Environment)

   Zhang, Kai (Ed.)

   Realistic representation of hydrological drought events is increasingly important in world facing decreased freshwater availability. Index-based drought monitoring systems are often adopted to represent the evolution and distribution of hydrological droughts, which mainly rely on hydrological model simulations to compute these indices. Recent studies, however, indicate that model derived water storage estimates might have difficulties in adequately representing reality. Here, a novel Markov Chain Monte Carlo - Data Assimilation (MCMC-DA) approach is implemented to merge global Terrestrial Water Storage (TWS) changes from the Gravity Recovery And Climate Experiment (GRACE) and its Follow On mission (GRACE-FO) with the water storage estimations derived from the W3RA water balance model. The modified MCMC-DA derived summation of deep rooted soil and groundwater storage estimates is then used to compute 0.5∘ standardized groundwater drought indices globally to show the impact of GRACE/GRACE-FO DA on a global index-based hydrological drought monitoring system. Our numerical assessment covers the period of 2003–2021, and shows that integrating GRACE/GRACE-FO data modifies the seasonality and inter-annual trends of water storage estimations. Considerable increases in the length and severity of extreme droughts are found in basins that exhibited multiyear water storage fluctuations and those affected by climate teleconnections. 

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2. How Salty Is the Global Ocean: Weighing It All or Tasting It a Sip at a Time?

   https://doi.org/10.1029/2021GL092935  

   Ponte, R. M.; Sun, Q.; Liu, C.; Liang, X. (June 2021, Geophysical Research Letters)

   Abstract Global ocean mean salinityis a key indicator of the Earth's hydrological cycle and the exchanges of freshwater between land and ocean, but its determination remains a challenge. Aside from traditional methods based on gridded salinity fields derived from in situ measurements, we explore estimates ofbased on liquid freshwater changes derived from space gravimetry data corrected for sea ice effects. For the 2005–2019 period analyzed, the differentseries show little consistency in seasonal, interannual, and long‐term variability. In situ estimates show sensitivity to choice of product and unrealistic variations. A suspiciously large rise insince ∼2015 is enough to measurably affect halosteric sea level estimates and can explain recent discrepancies in the global mean sea level budget. Gravimetry‐basedestimates are more realistic, inherently consistent with estimated freshwater contributions to global mean sea level, and provide a way to calibrate the in situ estimates. 

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3. Joint Inversion of GNSS and GRACE for Terrestrial Water Storage Change in California

   https://doi.org/10.1029/2021JB023135  

   Carlson, G.; Werth, S.; Shirzaei, M. (March 2022, Journal of Geophysical Research: Solid Earth)

   Abstract Global Navigation Satellite System (GNSS) vertical displacements measuring the elastic response of Earth's crust to changes in hydrologic mass have been used to produce terrestrial water storage change (∆TWS) estimates for studying both annual ∆TWS as well as multi‐year trends. However, these estimates require a high observation station density and minimal contamination by nonhydrologic deformation sources. The Gravity Recovery and Climate Experiment (GRACE) is another satellite‐based measurement system that can be used to measure regional TWS fluctuations. The satellites provide highly accurate ∆TWS estimates with global coverage but have a low spatial resolution of ∼400 km. Here, we put forward the mathematical framework for a joint inversion of GNSS vertical displacement time series with GRACE ∆TWS to produce more accurate spatiotemporal maps of ∆TWS, accounting for the observation errors, data gaps, and nonhydrologic signals. We aim to utilize the regional sensitivity to ∆TWS provided by GRACE mascon solutions with higher spatial resolution provided by GNSS observations. Our approach utilizes a continuous wavelet transform to decompose signals into their building blocks and separately invert for long‐term and short‐term mass variations. This allows us to preserve trends, annual, interannual, and multi‐year changes in TWS that were previously challenging to capture by satellite‐based measurement systems or hydrological models, alone. We focus our study in California, USA, which has a dense GNSS network and where recurrent, intense droughts put pressure on freshwater supplies. We highlight the advantages of our joint inversion results for a tectonically active study region by comparing them against inversion results that use only GNSS vertical deformation as well as with maps of ∆TWS from hydrological models and other GRACE solutions. We find that our joint inversion framework results in a solution that is regionally consistent with the GRACE ∆TWS solutions at different temporal scales but has an increased spatial resolution that allows us to differentiate between regions of high and low mass change better than using GRACE alone. 

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4. Arctic and Sub‐Arctic Mechanisms Explaining Observed Increasing Northward Flow Through the Bering Strait and Why Models May Be Getting It Wrong

   https://doi.org/10.1029/2023GL104697  

   Peralta‐Ferriz, Cecilia; Woodgate, Rebecca A. (December 2023, Geophysical Research Letters)

   Abstract The Pacific oceanic input to the Arctic via the Bering Strait (important for western Arctic ice retreat, water properties, and nutrient supply) has been increasing for three decades. Using satellite Ocean Bottom Pressure (OBP) and Dynamic Ocean Topography (DOT) data, we show that long‐term trends in mooring data for a well‐sampled sub‐period (2003–2014) relate to summer OBP and DOT drop in the Arctic's East Siberian Sea (ESS), in turn caused by stronger westward ESS winds, and increased fall westward winds in the Bering Sea. OBP/DOT differences imply strong (0.17 psu/year) ESS salinization, likely caused by hitherto unappreciated increased Pacific inflow to that region. We find ESS OBP trends are (erroneously) reversed in older data versions, and estimate that ESS salinization may significantly mediate Bering Strait flow increase. These facts may explain why models assimilating older OBP data, or with erroneous Bering Strait salinities, fail to simulate observed Bering Strait flow increase. 

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5. Were Gravity Waves or Lamb Waves Responsible for the Large‐Scale Thermospheric Response to the Tonga Eruption?

   https://doi.org/10.1029/2024AV001470  

   Li, Ruoxi; Lei, Jiuhou; Zhang, Shun‐Rong; Liu, Feifan; Chen, Xuetao; Luan, Xiaoli; Meng, Xing (April 2025, AGU Advances)

   Abstract The extraordinary eruption of the Tonga volcano on 15 January 2022 lofted material to heights exceeding 50 km, marking the highest observed since the satellite era. This eruption caused significant disturbances spanning from the hydrosphere up to the thermosphere. Our recent investigation discovered the dramatic thermospheric responses at satellite altitudes. This study, however, provides physical insights into two main possible processes, secondary gravity waves (GWs) and Lamb waves, which may explain those observed large‐scale thermospheric disturbances. The comparison between the simulations and observations suggests that the MESORAC‐HIAMCM secondary GWs are consistent with GRACE‐FO measured global‐propagation thermospheric density disturbances in timing and amplitude. WACCM‐X simulations suggest that the Lamb wave can reach the thermosphere as a sharp, narrow wave packet, and may contribute about 25% to the total disturbances at 510 km. 

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