A multi‐scale energy and vorticity analysis of coupled atmosphere‐ocean dynamics in the Mediterranean Sea

Abstract

The Mediterranean Sea is a small yet interesting basin of study. The number of external forcings acting on it, together with its topography and other local factors confer unique characteristics to the basin. As a consequence, its general circulation is complex and highly variable, with features at diverse and interacting scales. One way to explain the dynamics of ocean and atmospheric processes is trough energy analysis. At this respect, the description of the energy budgets of the Mediterranean is a relatively unexplored topic. The aim of this study is to improve the understanding of some physical processes involving ocean-atmosphere interaction in the Mediterranean by looking at the energy budgets.

Here we make use of two methodologies to evaluate the energy balances based in two different scale divisions. A decomposition of the fields in a mean and its deviation is the base of what we call “classic energy analysis”, since this is the most used method. A more innovative method based on spectral decomposition (Multi-Scale Energy analysis) is also used. The approach gives us a means to determine the partition and the transfers of energy between the different scales, and to identify the role of other processes involved in the energy balance (external forcing, advection, conversion between kinetic and potential energy).

The local energy patterns and balances are inspected for a series of increasingly complex experiments. The methodologies were first validated using an idealized instability model. Two cases are inspected: one with a growing perturbation and a stable flow. The energy balances evidenced three processes: a redistribution of energy among the domain via advection and pressure work, an energy conversion from APE to KE, and a source of energy via baroclinic conversion as the triggering mechanism for the growing perturbation. The analyses were coherent between the types of methodologies used. Second, an idealized simulation performed with a state of the art ocean model (NEMO) is used for a first evaluation of energy balances under controlled conditions. The simulation reproduces the generation of a coastal upwelling by a favorable wind forcing a motionless ocean. The energy balance evidences three processes important during the formation of a coastal upwelling: horizontal advection, buoyancy and scale transferences. Finally, as a realistic study case, we will assess the dynamics of some upwelling events along the Mediterranean coasts. For this purpose, we will use a high-resolution, coupled atmosphere-ocean regional model (NEMO-COSMO) to simulate the Mediterranean Region. This last case will surely contribute to improve our understanding of the Mediterranean Region environmental characteristics. Moreover, we expect the results of this study to evidence the needs of future developments of the regional coupled system in terms of resolution, parameterizations and representation of physical processes.