Abstract
Estimating vertical velocity in the oceanic upper layers is a key issue for understanding ocean dynamics and the transport of biogeochemical elements. This paper aims to identify the physical sources of vertical velocity associated with sub-mesoscale dynamics (fronts, eddies) and mixed-layer depth (MLD) structures, using (a) an ocean adaptation of the generalized Q-vector form of the ω-equation deduced from a primitive equation system which takes into account the turbulent buoyancy and momentum fluxes and (b) an application of this diagnostic method for an ocean simulation of the Programme Océan Multidisciplinaire Méso Echelle (POMME) field experiment in the North-Eastern Atlantic. The approach indicates that w-sources can play a significant role in the ocean dynamics and strongly depend on the dynamical structure (anticyclonic eddy, front, MLD, etc.). Our results stress the important contribution of the ageostrophic forcing, even under quasi-geostrophic conditions. The turbulent w-forcing was split into two components associated with the spatial variability of (a) the buoyancy and momentum (Ekman pumping) surface fluxes and (b) the MLD. Process (b) represents the trapping of the buoyancy and momentum surface energy into the MLD structure and is identified as an atmosphere/oceanic mixed-layer coupling. The momentum-trapping process is 10 to 100 times stronger than the Ekman pumping and is at least 1,000 times stronger than the buoyancy w-sources. When this decomposition is applied to a filamentary mixed-layer structure simulated during the POMME experiment, we find that the associated vertical velocity is created by trapping the surface wind-stress energy into this structure and not by Ekman pumping.
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Acknowledgements
We are grateful to Rosemary Morrow, Anna Pirani and our two anonymous reviewers for their help in revising this manuscript and to Youcef Amar for his valuable technical assistance.
This work was supported by the French programs PATOM and PROOF (CNRS/INSU).
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Giordani, H., Prieur, L. & Caniaux, G. Advanced insights into sources of vertical velocity in the ocean. Ocean Dynamics 56, 513–524 (2006). https://doi.org/10.1007/s10236-005-0050-1
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DOI: https://doi.org/10.1007/s10236-005-0050-1