The Bulletin of the American Meteorological Society has published a work performed by researchers at the Mediterranean Institute for Advanced Studies - IMEDEA (CSIC-UIB) and the Balearic Islands Coastal Observing and Forecasting System (SOCIB), in Spain, and the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Applied Ocean Physics Laboratory of the University of Washington, in the United States, about the CALYPSO Project. This project, funded by the United States Office of Naval Research (ONR), aims to understand, characterize and predict three-dimensional transport in the ocean, both horizontally and vertically, from its study in the western Mediterranean, a small-scale natural laboratory and one of the ocean's richest biodiversity hotspots. In addition, CALYPSO is aligned with the UN Decade of Ocean Science for Sustainable Development (2021-2030) contributing to the efforts to achieve the Sustainable Development Goals (SDG) and, in particular, the SDG 14 “Life Below Water”.

This is an unsolved global problem, as direct observations of vertical transport in the ocean are not yet possible. “The vertical exchange between the surface ocean and the interior affects the exchange of heat and freshwater, the biological productivity, the export of carbon, and the ventilation of oxygen. Thus, improving the observation and understanding of these processes and their impacts on a climate scale is one of the main challenges being faced in Earth observation”, says Ananda Pascual, researcher at IMEDEA (CSIC-UIB) and co-author of this work. With this objective, CALYPSO covers very small scales of the order of 0.1 to 10 km (submesoscales), the study of which has been intensified in recent years, combining the experience of IMEDEA (CSIC-UIB) and the capabilities of SOCIB.

Scientists are investigating this vertical transport and the exchange of water masses at the eastern end of the Alboran Sea, the westernmost part of the Mediterranean Sea, an area of great oceanographic interest where the Atlantic and Mediterranean water masses are converging, giving rise to the Almeria–Oran front.

 

In order to observe, understand, and predict the three-dimensional pathways by which water from the surface ocean makes its way into the deeper ocean, scientists use an innovative set of observational techniques, along with process study models, predictive models, and data synthesis. “Although numerical models can capture the horizontal circulation of the ocean, the vertical exchange rates are very sensitive to the resolution of the model and to the forcing of the surface and are difficult to corroborate with the existing observations,” points out Pascual. Thus, better measurements, quantification and dynamic understanding of such transport are needed to advance the prediction of where (and how much) vertical transport occurs, and possibly infer it from satellite observations in the future.

 

 

Furthermore, the CALYPSO Project addresses the ongoing debate between the roles of mesoscale (10-100 km scales) and submesoscale (0.1-10 km) processes in vertical transport. “Water travels thousands of times faster horizontally than vertically, and discovering the pathways by which it is transported vertically below the surface is a challenge,” highlights the researcher.

 

In 2018 and 2019 scientists have found that in summer (May-June) thermal stratification isolates the surface layers so that pathways from the surface do not penetrate further than the depth of the mixed layer—where temperature and salinity are constantly changing due to direct contact with the air—. Likewise, the numerical models reveal that the vertical speeds in winter are greater than in summer but, in both cases, the deformation of the mesoscale and the areas of frontogenesis (where the encounter between two water masses occurs) are relevant for a mass of water to sink under another as a result of differences in temperature and salinity that determine that one mass is denser than another.

Now, the researchers continue with the intensive and detailed assessment of the data collected during the last campaigns. In parallel, they are working on the preparation of the final field campaign scheduled for March-April 2022, from the research vessels L'Atalante, Pelagia and SOCIB. This campaign will include a greater number of observation platforms and will complete the data set collected to understand, characterize and predict three-dimensional transport in the ocean.