Carmen Böning

Validation of GRACE Gravity Fields by In-Situ Data of Ocean Bottom Pressure

GRACE observes the temporally changing gravity field of the Earth with unprecedented accuracy. Compared to the gravity signals of the continental hydrological cycle, local ocean mass variability reflecting ocean current, density and sea level changes are a challenge for the GRACE mission. Hence, validation of GRACE with in-situ observations of ocean bottom pressure is critical to evaluate the capability of GRACE to observe oceanic mass redistribution. Here, GRACE data is compared with in-situ ocean bottom pressure at a hundred sites located in all of the world’s oceans. The advances made by recent GRACE product releases are shown, and gravity fields provided from different data centres are compared. In some regions, particularly at high-latitude sites with comparatively strong ocean bottom pressure variability and dense satellite coverage, GRACE captures oceanic variability quite well. This is a robust feature for all gravity field solutions. In contrast, some other regions show remarkably large differences between different GRACE solutions, suggesting that discrepancies in de-aliasing models play a major role in defining the skill of GRACE to realistically observe local oceanic mass variability.

Antarctic Circumpolar Current Transport Variability in GRACE Gravity Solutions and Numerical Ocean Model Simulations

The Antarctic Circumpolar Current (ACC) is the main contributor to the inter-basin mass exchange in the ocean carrying a mean total transport of about 134 Sv (1 Sv = 106 m3/s) of water around the globe. A large part of its transport is found to be related to the prevailing westerly winds over the Southern Ocean. The Southern Annular Mode (SAM) explains 20–30% of the zonal wind component’s variability and enforces (at least to a certain extent) the variability of the oceanic transport. In this study, the representation of the ACC transport variability in the GRACE time-variable gravity solutions is investigated. Monthly GRACE gravity solutions over the ocean are commonly connected to ocean bottom pressure (OBP) fluctuations. Due to the connection between OBP gradients and geostrophic transport, a part of the ACC transport variability can be explained by gradients of OBP anomalies. Model simulations with the Finite Element Sea Ice-Ocean Model (FESOM) feature variations of about 2–3 Sv on annual and semi-annual periods; more than 50% of the total ACC transport variability is explained by OBP variability. GRACE-derived transport anomalies show variations at semi-annual and annual periods of about 6–9 Sv. Variations detected by GRACE coincide with model results with correlations of about 0.75. Atmospheric variability as described by SAM is found to be related to FESOM- and GRACE-derived OBP on short time scales up to annual. The spatial pattern of SAM is reflected in GRACE OBP by showing an annular band of highly negative correlation around Antarctica south of the southern boundary of the ACC. Model simulations with FESOM confirm that these spatial patterns are related to f/H contours in the Southern Ocean.