On the complexity of upper-ocean mesoscale dynamics

Abstract

At the ocean surface many physical processes coexist contributing to an extremely complex dynamics. Fortunately, recent advances in measuring systems and the increasing computational capability allow to study more realistically the mechanisms and interactions able to create the great diversity of ocean structures: eddies, filaments, fronts, waves and areas of intense mixing. Combining observations with numerical model recent developments it is possible to infer the dominant processes that govern the surface dynamics, the spatio-temporal variability and to estimate the potential impacts on the biological diversity and on human activities such as transport, fisheries and tourism. In this PhD dissertation physical processes at the ocean surface are studied from different standpoints, merging datasets and with a special focus on the Western Mediterranean Sea. First, the surface circulation in the western Mediterranean Sea is depicted departing from fields of ocean currents. These currents are obtained from a 3-year numerical simulation. By computing a Lagrangian descriptor, the Finite Size Lyapunov Exponent, it has been possible to identify and to quantify the intrusion of Atlantic waters into the Balearic Sea. Then it has been analyzed the wind induced mass transport in the upper layers of the Western Mediterranean Sea. Thus, departing from 16-year daily wave model outputs, the current generated by the Stokes drift is estimated as well as its interaction with Ekman transport terms. Mass transport is vertically integrated along the column of water that is directly affected by wind. It is shown that the interaction between Stokes and Ekman terms can be very important at some locations, where even contribute to more than 40% of the total wind induced mass transport. The availability of operational ocean models, waves and winds allow to design a Lagrangian tool to make short-term forecasts and Search ans Rescue operations at the ocean surface. This tool departs from the deployment of massless Lagrangian particles and it includes a spatiallydependent diffusivity and a wind drag term. The Lagrangian forecasting modulus estimates the kernel density of probability over the final positions of the launched particles, providing contours of accumulated probability. This is very useful to manage human and material resources involved in the resolution of crisis at sea. To conclude, it has been assessed the mechanisms by which the atmosphere responds to an ocean surface mesoscale gradient of temperature. In particular, it is studied the impact of ocean mesoscale eddies in the northwest tropical Pacific Ocean. To this aim, almost 20 years of satellite-based observations are combined with numerical simulations using an atmospheric model. Observations suggest that the response is leaded by the vertical momentum-mixing mechanism. However, numerical simulations also show the existence under certain conditions -sea surface temperature, wind or eddy propertiesof the mechanism based on pressure gradient adjustment, indicating that this mechanism should not be set aside. Results of this dissertation are a step further in our comprehension of the surface ocean complexity where, besides the geostrophic dynamics, wind has a key role generating currents, inducing mixing and waves, as well as their interactions. It has also been shown that the ocean affects the atmosphere, with a considerable influence in certain conditions. It highlights the need for studying the ocean and the atmosphere as a coupled system to acquire a deep understanding of the underlying physics.