Temporal Variations of the Marine Geoid
Creators: name: Siegismund, F.
name: Köhl, A.
name: Rummel, R.
name: Stammer, D.
Publisher: name: FID GEO
Year: 2020

description: The effects of temporal changes in the marine geoid on estimates of the ocean dynamic topography are being investigated. Influences from mass redistribution due to changes of land hydrology, ice sheets, glacial isostatic adjustment (GIA), and ocean and atmospheric dynamics are considered, and the associated crustal deformation is included. The strongest signals are associated with the seasonal cycle caused by changes in terrestrial water storage and ice sheets as well as the redistribution of atmospheric mass. Second to this is the importance of an overall trend caused by GIA and decreasing ice sheets over Greenland and Antarctica. On long spatial scales, the amplitude of regional trends estimated for the geoid height has a sizable fraction of those from sea level anomaly (SLA) for the period 1993–2016, also after subtraction of steric height of the upper 1,000 m to analyze trends in deep ocean geostrophic currents. The estimated strong negative geoid height trend south of Greenland for the period 1993–2016 opposes changes in dynamic sea level for the same period thereby affecting past studies on changes of both the strength of the subpolar gyre based on SLA and the meridional overturning circulation on a section between Cape Farewell and Portugal applying ocean dynamic topography and hydrography. We conclude that temporal geoid height trends should be considered in studies of (multi)decadal trends in sea level and circulation on large spatial scales based on altimetry data referenced to a geoid field.
description: Plain Language Summary: Changes in ocean surface currents are routinely obtained from satellite altimetry data. A correction for changes in the geoid, the equipotential surface of gravity closest to sea level, is considered small and thus usually neglected. We investigate temporal geoid height changes and potential implications on ocean circulation studies using space‐borne gravity data and results from ocean and atmosphere models to discover the individual processes of mass redistribution in the climate system causing thereby changes in the geoid height. We found the largest signals in the seasonal cycle for terrestrial hydrology in the Amazone basin and in negative trends for the Greenland and West Antarctic Ice sheets. For the period 1993–2016 and on spatial scale larger than 1,000 km or so the magnitude of the negative marine geoid height trend south of Greenland is similar to the strength of the negative trend in geocentric sea level from altimetry. This outcome affects past studies on changes in the strength of the subpolar gyre and the Atlantic meridional overturning circulation that neglect geoid height variations. We conclude that temporal geoid height trends should be considered in studies of (multi)decadal trends in sea level and circulation on large spatial scales based on altimetry data.
description: Key Points: The strongest geoid height changes are associated with regionally pronounced seasonal signals and secular trends. In the Subpolar North Atlantic the geoid height trend biases circulation trend estimates based on altimetry. Altimetry data need correction for geoid height change when long‐term variations in ocean dynamics are studied.
description: German Science Foundation (DFG)
description: BMBF funded Verbundprojekt RACE (Regional Atlantic Circulation and Global Change)
description: DFG http://dx.doi.org/10.13039/100004807