Variational Analysis to reconstruct the 3D dynamics of an anticyclonic eddy, possible use of SWOT observations.

Abstract:

Inferring the full 3D dynamics of structures belonging to the mesoscale and submesoscale is still a challenging task for both remote and in situ observations. In this work, horizontal and vertical velocity fields of a 40km diameter anticyclonic eddy have been calculated from underway CTD (uCTD) data obtained during a cruise that took place in the Alboran Sea in 2019. A multivariate reconstruction technique has been developed and tested to complement the hydrographic data with the total horizontal velocities measured by an ADCP. The reconstruction method is based on a variational analysis which has been demonstrated to be a better technique compared to the optimal interpolation (OI) due to the possibility of preserving the gradients and the simpler possibility of implementing a multi-variable approach.

This method uses the ADCP velocities and the thermal wind balance to avoid an artificial curvature of the isopycnal when reconstructing the density field from uCTD. The shape of the reconstructed features is important when, as in this case, the vertical velocity is calculated through the QG and SG omega equations. Drifter trajectories have been used to evaluate the improvements of this reconstruction method compared to the OI.

The along-isopycnal component of the vertical velocity has also been estimated by projecting the non-divergent component of the ADCP velocities to an isopycnal surface. These three different estimations of the vertical velocity field from observational data provide an advance in understanding the dynamics associated with mesoscale features. Indirect validation of these vertical velocity fields has been observed in the biological samples from CTD casts which present subduction evidence in agreement with the negative values of w.

The available altimeters were helpful to locate the eddy during the cruise planning but had limitations in defining its shape due to the lack of resolution, which confirms the need for the SWOT high-resolution observations. The role of the ADCP data in the multivariate analysis used in this study can be in the future played by the horizontal velocities derived from SWOT observations. The combination of a multivariate reconstruction method with the SWOT observations can thus provide a better estimation of nutrients fluxes and heat transport can on global scales.