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Presenter: Johanna Baehr

Seminar Title: Monitoring and detecting changes in the oceanic meridional overturning circulation at 26N in the Atlantic

The meridional overturning circulation (MOC) carries most of the oceanic heat transport in the Atlantic. In 2005, Bryden et al. reported a 30 percent reduction in the MOC at 26N in the Atlantic - based on five shipboard observations over the last 50 years. In 2004, the RAPID/MOCHA array was deployed in the Atlantic with the dedicated purpose to monitor the meridional overturning circulation (MOC) at 26N continuously.

Here, I initially compare the hydrographic characteristics and the MOC estimates from the first deployment period of the RAPID/MOCHA array (March 2004 - March 2005) to two global circulation models: the coupled climate model ECHAM5/MPI-OM, and the oceanic state estimate ECCO-GODAE. In ECHAM5/MPI-OM, I find that the observed and simulated MOC have a similar variability and time-mean within the 99 percent confidence interval. In ECCO-GODAE, I find that the observed and simulated MOC show a significant correlation within the 99 percent confidence interval.

Subsequently, I assess within the coupled climate model ECHAM5/MPI-OM the specific requirements for a single latitude monitoring array at 26N in the Atlantic that could deliver a confident and timely detection of changes in the MOC. The time to detect changes in the MOC using a single (univariate) MOC time series as the detection variable is on the order of decades to a century, while the continuous observation of the (multivariate) east-west density gradients leads to a detection of MOC changes within a few decades.

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