ALONG - Atlantic overturning circulation from GRACE/GRACE-FO

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in our global climate system and is the main mechanism of northward heat transport for a warm climate in Northern Europe. Despite its crucial role, the AMOC is only scarcely observed, as observations covering all of the Atlantic Ocean for an extended time are difficult to obtain. Currently, there is an array at 26oN, called RAPID-array, which is the main source of data for monitoring changes in the AMOC. In addition, satellite altimetry is used to monitor near surface currents. Satellite gravimetry offers key advantages compared to these existing data sources by providing ocean bottom pressure anomalies with global coverage, thus allowing to monitor the AMOC in the complete Atlantic Ocean basin. The Gravity Recovery And Climate Experiment (GRACE) satellite mission provided a nearly continuous time series of monthly gravity field snapshots from 2002 up to 2017 and its successor GRACE Follow-On will extend this data record from 2019 onwards. In contrast to in-situ measurements of ocean bottom pressure, which suffer from inherent drift problems, the temporally stable satellite observations allow investigations of the long-term AMOC behavior. Preliminary studies have shown that monitoring changes in the AMOC is possible with observations from GRACE. However, this application is pushing the limits of the current solutions in resolution and accuracy. In order to derive AMOC changes for the complete ocean basin, we build on the extensive knowledge of the Institute of Geodesy in the field of gravity field recovery. To fully exploit the information content in the gravity observations, we will implement a processing chain specifically tailored to the Atlantic Ocean. A thorough investigation of resolution issues will help us to mitigate negative effects of coarse spatial resolution. Using least-squares collocation, consistent degree-1 estimation and stabilization of C20, gridded ocean bottom pressure variations will be computed for the complete data record of GRACE and GRACE Follow-On. This time series, which will span over 15 years, serves as a basis for a novel data product, consisting of ocean mass transport in time, depth, and latitude. It will be the first consistent time series monitoring AMOC changes across all of the Atlantic Ocean basin and will for the first time enable investigations of AMOC coherence from observations across latitudes and depths. Furthermore, the accomplished data set will be extremely valuable for climate studies and climate models, since the AMOC is a key element of heat transport in our changing climate globally, but in particular for Europe.