Air-Sea Interaction and Coupling with Submesoscale structures in the MEDiterranean – ASICS-MED

From January to March 2013, a field campaign has been conducted within the framework of the ASICS-MED project, implementing on alert and autonomous platforms allowing to simultaneously collect atmospheric, oceanic and modeling data (several model outputs). This campaign has been the ocean component of the international HyMeX initiative dedicated to the Mediterranean water cycle.

Lessons learnt from ASIC-MED
Studies on the coupling mechanisms between the ocean and the atmosphere have shown that the energy involved in the interactions between the wind and frontal and eddy structures is ten times stronger than the energy loss associated with the surface fluxes and that these processes contribute in up to 30% to the dispersion of the newly formed dense water. So small ocean scales have a major role.
ASICS-MED has provided an ideal framework to develop an innovative representation of vertical mixing and convection in the ocean which improves the chronology of the incorporation of the intermediate water masses and the intensity of winter cooling.
Coupled models have shown their added value : sudden changes in surface temperatures, dense water formation and flows, ocean-wind interactions are significantly more realistic in coupled mode.
Observations of the turbulent field in the marine atmospheric boundary layer has revealed the presence of circulations that redistribute heat and momentum with non-local effects on air-sea interface exchanges. This major result highlights that ocean-atmosphere coupling is not limited to the air-sea interface, but includes the atmospheric and oceanic boundary layers.

ASICS-MED has shed light on fine-scale processes and dynamical mechanisms that must be represented in operational and climate models.

Scientific production
The project has led to nearly a hundred of communications and to 26 publications, of which an overview paper presenting the field campaign and a joint special issue in JGR:Oceans - JGR:Atmospheres. The datasets collected are accessible to the scientific community through the HyMeX/MISTRALS database. The ASICS-MED field campaign also represents experience gained for future experiments of this kind. Finally, the developments carried out within the projects are being incorporated into operational ocean forecasting models.

Facts
The ASICS-MED project is a basic research project associating the National Centre for Meteorological Research (CNRM), the Mediterranean Institute of Oceanology (MIO), the Laboratory of Oceanography and Climate: Experiments and Numerical Approaches (LOCEAN), the Laboratory of Dynamical Meteorology (LMD), and the Laboratory of Aerology (LA). The project started in November 2012 and lasted 60 months. It was allocated a grant of 775 k€ by ANR for a global cost of 4,733 k€.

The North Western Mediterranean is prone to strong wind events that may lead – through their interaction with the upper ocean dynamics – to dense water formation. The Gulf of Lion is thereby a major key spot of dense water formation through the deep ocean convection mechanism that results from the interaction of the Northern Current with the cyclonic gyre under strong winds (Mistral, Tramontana). Although this mechanism is of crucial importance to accurately simulate both the water masses and the basin-scale thermohaline circulation, its representation in state-of-the-art ocean models remains rudimentary.
The main objective of ASICS-MED is to deeply investigate the ocean-atmosphere processes that contribute to dense water formation to improve their representation in ocean models. The process studies focus on the mesoscale and sub-mesoscale that were already shown to significantly contribute to the basin scale dense water formation.
The methodology is to:
(i) simultaneously collecting observations in the ocean and the marine atmospheric boundary layer (MABL) over the Gulf of Lion during late winter 2013, focusing on strong wind and ocean convective events;
(ii) making use of several high-resolution limited-area ocean models over the North-Western Mediterranean together with a dedicated version of the AROME atmospheric model to simulate the evolution of both the upper ocean and MABL during the preconditioning phase and the dense water formation;
(iii) developing a mass-flux parameterization scheme for ocean convection and improving air-sea fluxes parameterizations;
and (iv) identifying (sub)mesoscale ocean/atmosphere processes involved in the upper ocean and MABL evolution and leading to dense water formation.
The ASICS-MED project will take advantage from a strong national and international collaborative environment as part of the much wider HyMeX research initiative. ASICS-MED, which will deeply contribute to a major issue in physical oceanography (namely dense water formation), should also strongly benefit to operational oceanography as well as to climate studies focusing on the interannual to decadal variability of the Mediterranean system, especially in the context of climate change.