Monitoring Atlantic Overturning Variability with Satellite Gravimetry

Abstract:

The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism in basin-scale northward heat transport and thus plays an important role for global climate. In the North Atlantic, warmer water from the subequatorial region is transported northward in the upper layers of the ocean, and after cooling at higher latitudes, the water sinks down and is transported back southward. This process has important influence on the climate regime in the Earth’s northern hemisphere, in particular in Northwestern Europe.

Coherence between ocean bottom pressure (OBP) and the AMOC has been characterized in theoretical and simulation studies. Here, we use output from two ocean state estimates, ECCO2 and GECCO2. We use the model data to (1) evaluate to what extent space-based observations of time-variable gravity and the inversion for ocean bottom pressure can be used to observe AMOC variability, and (2) to test algorithms to extract the AMOC signal from GRACE-like OBP observations.

In both ocean state estimates we find a strong correlation between the AMOC signal and local OBP variations, and we are able to reconstruct AMOC variations from OBP anomalies at the model’s resolution. In a next step, the ECCO2 and GECCO2 outputs are smoothed and filtered so that the OBP data resemble the spatial resolution of GRACE gravity solutions.

While the AMOC at GRACE resolution introduces errors due to the decreased spatial sensitivity, first results show that inter-annual AMOC variations can be recovered from GRACE-like OBP observations at some latitudes (e.g., at 28N for the upper ocean transport). At other latitudes, signal leakage can introduce significant errors in the AMOC reconstruction. Finally, the techniques are employed to derive the AMOC signal from recent GRACE OBP solutions.