Mesoscale oceanic structures have a key role on the oceanic global circulation through the enhancement of large scale fluxes of heat, salt, momentum, and biogeochemical tracers. Moreover, mesoscale vertical motions may have an important contribution on the nutrient replenishment of the euphotic layer, and, hence, on marine ecosystems.

Furthermore, mesoscale coherent features, or eddies, have the capacity to trap fluid of their formation regions within their cores and propagate offshore during long distances. However, despite their importance, the mechanisms through which mesoscale structures, as mesoscale eddies, influence marine ecosystems is still under discussion.

In this dissertation we aim to shed some light on the thermohaline and dynamical structure, including ageostrophic secondary circulation, of mesoscale features through remote sensing observations and in situ data, paying special attention to mesoscale eddies and induced vertical motions.

To achieve this goal, the thesis is developed in four chapters self-contained, three of them are already published.

First, we demonstrate the importance of mesoscale vertical motions on nutrient distribution in an oligotrophic region of the global ocean through synthetic observation-based data. Vertical motions may account for local increases of nitrate uptake rates of up to 30% in regions with mesoscale activity.

Then, we take advantage of the high-resolution in situ data obtained from the intensively sampling of a mesoscale eddy within the Canary Eddy Corridor, the main pathway for long-lived eddies in the northeast Atlantic which constitutes a natural laboratory for the study of these structures. With these data we analyze in detail the anatomy of the eddy, revealing its main hydrographic and dynamic characteristics.

Thirdly, as the eddy is moderately ageostrophic, we estimate and analyze its ageostrophic secondary circulation, revealing for the first time the distribution of the vertical velocity field within a mesoscale eddy through high-resolution in situ data.

Finally, we perform a comparison of the synthetic observation-based product with the in situ data obtained from the cruise, evaluating the capacity of the synthetic fields to reproduce the eddy hydrodynamic structure.