Seminar by Director, Axel Timmermann from IBS Center for Climate Physics
24 September 2025
KST 10:00
The Seminar is being held in Room 1010 (Jasmin) – Integrated mechanical engineering building. Click here for the campus map.
In 1961, Klaus Wyrtki published a landmark study [1], in which he laid out the foundation of our modern understanding of the thermohaline circulation. The only problem was that the international oceanographic community largely ignored this study, or in Klaus Wyrtki’s own words, “Nobody at the East Coast of the United States cared about what someone from Australia had to say” (personal communication, 2011). This apparent bias, which is also manifested in the citation report to this study, has delayed research on this important issue, arguably, by several decades. Klaus Wyrtki’s fundamental study on the thermohaline circulation, or Atlantic Meridional Overturning Circulation (AMOC), as we call it now, is a treasure trove of ideas and insights. Using observational data, theoretical and heuristic arguments, the study highlights – among many other things – the role of Southern Ocean upwelling as a key component of the AMOC; an idea that was later popularized through Robby Toggweiler’s very influential study on the effect of the Drake Passage on the AMOC [2].
Furthermore, Klaus Wyrtki’s multi-layer model of the AMOC predicts a total mass transport at 45oN of 10 Sv [1 Sv=106 m3/s]. Modern-day observations of the RAPID- MOCHA array and of ocean reanalysis datasets suggest an amplitude of about 14-22 Sv-26.5 °N with substantial variations on interannual to multidecadal timescales. When comparing the schematics of modern AMOC review papers (e.g., [3]) with Figure 4 in [1], it is striking to see that almost all of the main components of our modern-day understanding of relevant AMOC processes is already described in detail in Klaus Wyrtki’s seminal 1961 paper. One may wonder what would have happened to the research on this important topic if the oceanographic community had recognized Klaus Wyrtki’s paper as relevant already in the 1960s. In 1961, the same year as [1], Henry Stommel wrote an interesting conceptual paper [4] about the potential bistability of a thermohaline frictionally controlled flow between two boxes. Also, this paper’s relevance lay dormant for at least 2 decades, until the first GFDL model simulations by Manabe and Stouffer in 1988 [5] documented evidence for a bistable AMOC. This match between CGCM and box model triggered a worldwide quest to understand the stability and variability of the AMOC. Stommel’s highly idealized non-rotational model and his proposed salt feedback became immediately a key paradigm in the abrupt climate change community. They inspired numerous studies on the issue of AMOC-related tipping points and Dansgaard-Oeschger Events. It is probably fair to say that we know now more about the instability and variability of the AMOC than we know about its long-term mean state and the underlying physical processes. In 2007, a team of oceanographers from the International Pacific Research Center at the University of Hawaii assembled to develop a more comprehensive dynamical understanding of how the AMOC really operates. Together with Prof. Julian McCreary, Assistant Researcher Dr. Ryo Furue, and PhD student Fabian Schloesser, we focused on elucidating the basic mechanisms of the combined 3-dimensional thermohaline and wind-driven flow of the North Atlantic. Our main research findings from this study [6] will be presented in this presentation and contextualized with earlier research. I will in particular, highlight how, in the presence of buoyancy forcing, reduced gravity wave dynamics conspire with diapycnal processes to create a 3-dimensional flow that captures all the main features of the observed AMOC, including the 2-dimensional structure, already known to Klaus Wyrtki more than 60 years ago
References:
- [1] Klaus Wyrtki, The thermohaline circulation in relation to the general circulation in the oceans, Deep-Sea Research, 8, 39-64. (1961). https://www.sciencedirect.com/science/article/pii/0146631361900144
- [2] J.R. Toggweiler, B. Samuels, Effect of Drake passage on the global thermohalinecirculation, Deep Sea Research Part I: Oceanographic Research Papers 42 (4 ), 477-500 (1995).
- [3] Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf, On the driving processes of the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, doi:10.1029/2004RG000166, (2007).
- [4] Henry Stommel, Thermohaline Convection with Two Stable Regimes of Flow”. Tellus. 13 (2): 224–230 (1961).
- [5] Manabe and Stouffer, S. Manabe and R. J. Stouffer, Two Stable Equilibria of a Coupled Ocean-Atmosphere Model, J. Climate 1, 841-866 (1988).
- [6] F. Schloesser, R. Furue, J.P. McCreary, A. Timmermann, Dynamics of the Atlantic meridional overturning circulation. Part 1: Buoyancy-forced response, Progress in Oceanography, Progress in Oceanography, 120, 154-176 (2014).