Presenter: Caroline Comby (Mediterranean Institute of Oceanography, CNRS)

Description:
Vertical velocities knowledge is essential to study fine-scale dynamics in the ocean, to explain their key role in global oceanic balance and to understand their impact on the vertical transfer of nutrients, biological production mechanisms and carbon budget, despite their low intensity. With the increasing global warming issues linked to the forcing of the carbon cycle by anthropogenic activities, the estimation of vertical velocities becomes an essential information for a better representation of biogeochemical budgets. However, these vertical velocities have long been neglected, simply parameterized, or considered as not measurable, due mainly to their order of magnitude (mm s^-1 to cm s^-1), generally much lower than the one of the horizontal velocities (several cm s^-1). Consequently, direct in situ measurement of vertical velocities is still currently one of the biggest challenges in physical oceanography. We have been developing a new method for direct in situ measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs, conventional with four beams vs new generation with five beams) following different sampling techniques (lowered vs free falling). We collected the data during the FUMSECK cruise (2019, Ligurian Sea, NorthWestern Mediterranean Sea), and our analyses provided profiles of vertical velocities of the order of mm s^-1, as expected, with standard deviations of a few mm s^-1. While the fifth beam of the Sentinel V has shown a better accuracy than conventional ADCPs, the free-fall technique has provided a more accurate measurement compared to the lowered technique. Finally, this innovative study paves the way to perform simple and direct in situ measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP. Consequently, we plan to apply these findings in areas characterized by intense fine-scale ocean dynamics to improve the observational component of small-scale processes.