How the Atlantic overturning got its observing system

Jochem Marotzke conceived the basic elements of the RAPID system shortly after taking up a professorship in Southampton, UK, in 1999. But the origin of the idea indirectly dates back much further, to the first days of his PhD work in 1985. He was given the task to construct a simplified model of the AMOC to investigate its stability and multiple equilibria. To justify the simplifying assumptions he had to formulate a dynamically consistent (geostrophic) force balance for the AMOC, in terms of zonally (east–west) averaged quantities only. He failed completely and therefore never lost his obsession with this force balance.

About ten years later, he made theoretical progress by separately predicting the density at the eastern and western boundaries, thus foregoing the insistence on only zonally averaged quantities. He furthermore invoked exact geostrophic balance throughout, including the western boundary current. This combination broke with all previous attempts at formulating simplified yet quantitative characterizations of the AMOC, and later these ideas formed the theoretical basis for the RAPID measurements.

After his move to the UK, a remarkable confluence of individuals and ideas enabled the establishment of the RAPID system, at its core based on monitoring boundary densities and on geostrophy. Seagoing experts then took over and implemented the observing system, even after Jochem Marotzke had left the UK to join the MPI-M. RAPID provided the first-ever time series of the ocean’s overturning; the results showed surprisingly large sub-seasonal variability, implying that the previous method of estimating the AMOC through infrequent ship cruises was prone to misinterpretation. The RAPID results later have encouraged AMOC monitoring approaches at other latitudes, where the monitoring is much more demanding than at 26.5°N.

In his paper Jochem Marotzke also points at two theoretical concepts he proposed in the 1990s and, taking us back to the future, suggests that these concepts together with continued observations and the MPI-M’s global coupled simulations at very high resolution should substantially improve our currently dismal understanding of the causes of AMOC variability.

The paper is unusual in that it weaves together personal and scientific elements, making the result reminiscent of the literary genre of ‘narrative nonfiction’. Jochem Marotzke chose this format because the history of RAPID is so closely connected to important moments in his personal career. The paper is part of a special issue covering the results of a Royal Society Discussion Meeting on ‘Atlantic overturning: new observations and challenges’. Thankfully, the organisers accepted the unusual format, which led to some raised eyebrows among reviewers. But the paper also presents a moral of the tale, which might be useful to early-career scientists and thus would vindicate the format, despite the eyebrow-raising.