The contribution of internal waves to the rate of tracer transport in the ocean is investigated using a multi-pronged approach that combines mathematical wave?mean interaction theory, physical modeling, numerical simulation, and comparison with observational field data. Specific topics to be studied include the vertical and horizontal mixing of tracers induced by breaking internal waves, the horizontal dispersion of tracers by non-breaking internal waves, and the structure and transport induced by waves generated by geostrophic flow over topography.
Intellectual merit The proposed research is timely and motivated firstly because of the current and increasing availability of high-resolution ocean data, and secondly because of the current dawning of an era of gravity-wave-permitting ocean models. In such models we can anticipate in the coming years the emergence of numerical gravity waves, albeit at the threshold of the model resolution. This research is anticipated to provide better guidance and an improved framework in which to interpret sub-mesoscale observations both in the field and in high-resolution numerical models. Investigating these issues also raises new mathematical research questions, for instance in the area of non-Gaussian wave theory. So this project is a conceptual two-way link between intertwining mathematical and geophysical research trajectories.
Broader impact Results from this theoretical study may feed into the future design of next-generation ocean forecasting systems for weather and climate, which is of great societal importance. On the educational side, training in advanced mathematics and applied science is paramount to the research elements of this proposal. The proposal includes training of a post-doctoral researcher and a PhD student as well as provision for merit-based stipends for advanced undergraduates interested in research experiences. Experience shows that such early research experiences on topics of fundamental importance can trigger an undergraduate's decision to apply for graduate school in mathematics or in another natural science.
The deep ocean circulation is fed by waters that become dense enough to sink into the ocean abyss in the North Atlantic Ocean and the Southern Ocean around Antarctica. These waters carry dissolved carbon away from the atmosphere and into the deep ocean, thereby playing a crucial part in modulating Earth’s carbon budget and climate. Despite theoretical and observational efforts dating back to the beginning of the twentieth century, we are still struggling to understand how and where these waters return to the ocean surface — in other words, we know how ocean carbon is ‘breathed in’, but are still trying to figure out how it is ‘exhaled’. The work completed as part of this proposal confirmed the hypothesis that bottom waters are mixed up to 2000 meters by breaking internal waves: when internal waves overturn and break they mix the bottom waters with lighter shallower waters, thereby lifting the bottom waters to shallower depths. In particular our work has shown that the internal waves that mix the bottom waters upward are generated by barotropic tides and strong ocean currents impinging over rough bottom topography. These waves were shown to be so energetic that they break within a few hundred meters of the ocean bottom. Thus strong mixing is confined to depths deeper than 2000m, the characteristic depth of the most prominent ocean seamounts and ridges. Above 2000m, the ocean deep waters flow south at a constant depth, until the reach the Southern Ocean where they are lift to the surface by the Roaring Forties, thereby concluding their journey. This is how oceans breath in and out! The work consisted on a combination of analysis of observations, based on new statistical methods developed as part of the proposal, theory and numerical simulations. The results have been published in 10 scientific papers in high profiles journals, including Nature and the Proceedings of the National Academy of Sciences USA. The paradigm shift in our understanding of ocean mixing and its impact on circulation, which emerged from our work, has been highlighted in a News and Views article on our work written by Jennifer McKinnon for Nature (issue 501, pages 321-322).
