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While it is well known that the accelerating melting of the ice sheets of Greenland and Antarctica will increasingly raise global mean sea levels, it is less widely understood how the addition of meltwater from these ice sheets will affect regional patterns of sea level rise. The transfer of water mass from the ice sheets to the ocean will alter Earth's gravity field and rotation, resulting in local changes in sea levels. On time scales from months to decades, the addition of freshwater at high latitudes will alter the mean ocean circulation through a variety of mechanisms that will also alter regional rates of sea level change. The current ocean observing system, including radar and laser altimeters, satellite gravity missions, and the Argo network of profiling floats, has demonstrated the ability to close the sea level budget since 2005, confirming the contributions of ice sheets to contemporary sea level rise. The planned observing system will be capable of monitoring the regional variability of sea level change, which should help improve future projections. [ABSTRACT FROM AUTHOR]

The Poseidon-3 altimeter on board Jason-2 includes a significant new capability with respect to its predecessors, an open-loop [Détermination Immédiate d'Orbite par Doris Embarqué (DIODE)/digital elevation model (DEM)] tracker mode. This innovative mode is capable of successfully tracking the backscatter signal over rapidly varying terrains, and thus it overcomes one of the limitations of the closed-loop Poseidon-2 tracker on board Jason-1. DIODE/DEM achieves this improvement thanks to a predetermined DEM on board that, when combined with DIODE orbit ephemeris, provides improved acquisition timing and reduced data loss in the coastal zone. As a further enhancement, Jason-3 and the Sentinel-3 programs will be capable of autonomously switching to this innovative mode in selected regions. To help recommend how these missions should utilize DIODE/DEM, the authors studied the impact of the tracker mode on the accuracy and precision of wave heights and wind speed, the continuity of the sea level climate data record, and the coverage in coastal regions. The results show close agreement between the open- and closed-loop tracker modes over the open ocean with the exception of some differences at high-tidal variability areas, the coastal zone, and sea ice regions. The DIODE/DEM tracker shows better performance than the closed-loop tracker mode at the coast and in the presence of sea ice. Jason-2, when operating in open-loop mode, allows for an approximately 5% increase of successful acquisitions at the ocean-to-land transition. However, open-loop tracking exhibits more variability in regions of high tides than closed-loop. [ABSTRACT FROM AUTHOR]

In this study, we present an extension to existing numerical retrackers of synthetic-aperture radar (SAR) altimetry signals. To our knowledge at the time of writing this manuscript, it offers the most consistent retrieval of geophysical parameters compared to low-resolution mode (LRM) retracking results. We achieve this by additionally estimating the standard deviation of vertical wave-particle velocities σ v and a new parameter u x , linked to a residual Doppler in the returned radar echoes, which can be related to wind speed and direction. Including this new parameter into the SAR stack retracker mitigates sea surface height estimation errors by up to two centimeters for Sentinel-6MF SAR mode results. Additionally, we found a closed-form equation to describe u x as a function of eastward and northward wind variables, which allows mitigating the effects of this parameter on a SAR stack within level 1B processing and generating a lookup table to correct sea surface height estimates in SAR mode. This additionally opens up the door to estimating the wind speed and direction from SAR altimetry stacks. Additionally, we discuss how this new retracker performs with respect to different planned future baseline processor changes of Sentinel-6MF, namely F09 and F10, by attempting to imitate their level 2 processing. This is achieved by processing cycles 017 to 051 (nearly a full year) of Sentinel-6MF level 1A data on a global scale. We observe that the new retracking method is, on average, more accurate with respect to LRM. However, there is a slight increase in measurement noise due to the introduction of an additional parameter. To ensure that the results of the new retracker are not biased, we retrack using both the new method and the SINCS-OV ZSK retracker on Sentinel-6MF stack data produced in a Monte Carlo simulation. We analyze the simulation results with respect to accuracy, precision, and correlations between estimated parameters. We show that the accuracy of the new retracker is better than SINCS-OV ZSK but less precise, which could be related to higher correlation coefficients—especially with respect to the new parameter u x —between estimated parameters. [ABSTRACT FROM AUTHOR]

Abstract: We have considered the influence of ocean temperature and salinity changes, mass changes of the Greenland ice sheet (GIS) and the isostatic response of the solid earth to the most recent glacial cycle on 20th century sea-level change along the US east coast with the intention of better understanding the observed signal as well as determining the potential of the tide gauge data for constraining the recent (past 50–100 yr) mass balance of the GIS and earth viscosity structure. Our results show that the signal due to steric changes is large and displays a complex spatial variation which can account for a significant portion of the observed signal. In contrast, that due to changes in the GIS is relatively small and insensitive to the specific geometry of the mass balance model adopted. As a consequence, the tide gauge data alone are not capable of providing useful constraints on either the magnitude or form of recent GIS mass balance. Our inference of mantle viscosity structure based on the tide gauge data was affected dramatically when the steric effect was accounted for: An earth model with an upper mantle viscosity of 8×1019 Pa s and a lower mantle viscosity of 5×1022 Pa s produced the best fit to the steric-corrected data; the optimal fit to the uncorrected data was obtained for upper and lower mantle viscosities of 5×1020 Pa s and 1022 Pa s, respectively. [Copyright &y& Elsevier]