On the Joint Use of High Resolution Tracer Image and Altimetric Field for the Control of Ocean Circulations

Abstract:

During the past 20 years, altimetric satellites have revealed the turbulent ocean dynamics at the mesoscale. Additionally, high resolution sensors of tracers such as the Sea Surface Temperature or the Ocean Color reveal even smaller structures at the submesoscale, which are not detected by altimetry. Therefore, the two types of complementary observations are expected to refine the estimation of the ocean circulation.

The goal of this study is to explore the feasibility of inverting tracer information to possibly control ocean dynamics emerging from altimeter data analysis. To do so, we choose an image data assimilation strategy in which a cost-function is built that aims at minimizing the misfits between some image of submesoscale flow structure and tracer images. In the present work, we have explored the extent to which the Finite-Size Lyapunov Exponents (FSLE) can be considered as a proxy characterizing the submesoscale flow structure.

A prerequisite to the study is the investigation of the sensitivity of FSLE horizontal patterns to velocity errors. Indeed, the misfit between a FSLE derived from a velocity with errors, and the FSLE derived from an error free velocity is successfully minimized. The next step is the inversion of sub-mesoscale tracer information to correct a mesoscale altimetric field using real observation (from AVISO for the velocity and from MODIS sensor for the tracer). The ocean dynamical variable to be corrected is the mesoscale altimetric velocity field using a high resolution tracer image. The strategy is similar to the one used to invert FSLE. The cost function measures the misfit between the FSLE derived from the altimetric velocity and the high resolution tracer image. Then, the corrected velocity field is compared with along track altimetric data to confirm the improvement of the estimation of the dynamics. Several test cases have been studied and demonstrating the success of the inversion of sub-mesoscale tracer information to correct a mesoscale altimetric velocity field.

These results show the feasibility of assimilating tracer submesoscales into ocean models for the control of mesoscale dynamics and larger scales as deduced from altimetry and therefore the benefit of the joint use of tracer image and altimetric data for the control of ocean circulations.