Seminar: Diagnosing Frontal Dynamics from Observations using a Variational Approach
Esporles, February 15, 2021. Intensive hydrographic and horizontal velocity measurements collected in the Alboran Sea allowed to diagnose the 3D dynamics of a frontal system. The sampled domain was characterized by the presence of an anticyclonic eddy of ~40 km diameter, with an intense front on its eastern side, separating Atlantic and Mediterranean waters. Here, we implement a multi-variate variational analysis (VA) to reconstruct the hydrographic fields combining the high-resolution measurements of underway CTD (1 km) and the information about the flow shape from the Acoustic Dopler Currentmeter (ADCP) velocities. Thanks to the possibilities offered by the VA our multi-variate analysis is based on a physical constraint and better preserve submesoscale gradients in comparison with the classical Optimal interpolation (OI). A comparison between real drifter trajectories and virtual particles advected in the reconstructions quantifies the improvements of the VA over the OI, with a skill-score 15% bigger. Quasi-Geostrophic and Semi-Geostrophic omega equations allowed us to estimate the vertical velocities w that reach 40 m/day in the dense side of the front. A further 3D advection showed a favorable agreement with the biological samplings made through CTD casts in two eddy locations. We can then estimate how nutrients and other passive tracers leave the mixed layer and subduct. Downwelling temperature filaments also revealed evidence of subduction that is remarkably compatible with the w from SG while not with QG. Some hypothesis on what are the advantages of SG in this scenario were made based on w components decomposition and Spice analysis. The multi-platform approach of this work and the use of variational methods allowed us to improve the characterization and understanding of (sub)-mesoscale frontal dynamics.
Date and Time: Thursday, February 25, 12:00h
YOUTUBE LIVE: https://www.youtube.com/user/ImedeaComDiv (The chat option on YouTube will be enabled to animate the discussion and compile the questions for the speaker)
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Source: IMEDEA (CSIC-UIB)
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