Ocean convection is a key process in the exchanges between the upper and deep ocean layers and is a driver of the thermohaline circulation. It occurs in only few specific regions on the globe, of which the north-western Mediterrenean during the Winter period. The ASICS-MED project has addressed the role of the interactions between the ocean and the atmosphere in these events.
The north-western Mediterranean is prone to strong wind events (Mistral, Tramontane) which may, when interacting with the upper ocean dynamics, lead to dense water formation and convective vertical mixing. The goal of ASICS-MED has been to study in detail the processes of kilometric scale that contribute to the formation and propagation of dense water in the Gulf of Lion in order to improve their representations in ocean models.
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.
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 ac
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.
Monsieur Hervé GIORDANI (Groupe d'Etude de l'Atmosphère Météorologique) – Herve.firstname.lastname@example.org
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
GAME Groupe d'Etude de l'Atmosphère Météorologique
MIO Institut Méditerranéen d'Océanologie
IPSL Institut Pierre Simon Laplace
LA Laboratoire d'Aérologie
Help of the ANR 774,998 euros
Beginning and duration of the scientific project: October 2012 - 48 Months