Preface to the Special Section: Dense Water Formations in the Northwestern Mediterranean: From the Physical Forcings to the Biogeochemical Consequences

Abstract

The northwestern Mediterranean Sea is one of the few sites of open-sea deep convection and dense water formation. The area is characterized by intense air-sea exchanges favored by the succession of strong northerly and northwesterly winds during autumn and winter that eventually induce deep convection episodes and the formation of the Western Mediterranean Deep Water. This region exhibits a significant spring phytoplankton bloom, which appears to be largely influenced in intensity and diversity by the winter mixing. To understand and resolve the interplay between the atmosphere and ocean, and the impact of ocean circulation and mixing on biogeochemistry, we carried out an unprecedented observational effort during a large experiment between July 2012 and July 2013. A multiplatform approach-combining aircraft, balloons, ships, moorings, floats, and gliders -aimed both at characterizing the dynamics of the atmosphere and at quantifying the physical, biogeochemical, and biological properties of the water masses. Beyond a better understanding of the wind dynamics, the interannual variability of the deep convection, and the seasonal variability of the nutrient distribution and plankton structure in the pelagic ecosystem, the experiment provided a reference data set that was used as a benchmark for advancing the modeling of the surface fluxes, convective processes, dense water formation rates, and physical-biogeochemical coupling processes. It also represented an opportunity for complementary investigations such as evaluating model parameterizations or studying the role of submesoscale eddies in dense water spreading and biogeochemistry. High-resolution, realistic, three-dimensional atmospheric and oceanic models are essential for assessing the intricacy of buoyancy fluxes, horizontal advection, and convective processes, but they need to be validated by data that describe the variability of convection at small spatial scales (a few kilometers) and at high frequency (typically a day), and by accurate concomitant observations throughout the water column and atmosphere, especially during strong wind events. Likewise, the lack of in situ observations simultaneously inferring the main physical and biogeochemical variables was a major obstacle to unify a CONAN ET AL. 6983

Thermohaline Evolution of the Western Mediterranean Deep Waters Since 2005: Diffusive Stages and Interannual Renewal Injections

“…In 2013, an intense deep water formation event occurred again (Houpert et al., ; Waldman et al., ), but no cascading signal was detected. This event was the subject of detailed observational and modeling studies presented in Journal of Geophysical Research‐Oceans and Journal of Geophysical Research‐Atmospheres special issues (Conan et al., ). After this period no further convective events were reported except for one of minor intensity in 2015 (Durrieu de Madron et al., ).…”

Oxygen budget of the north-western Mediterranean deep- convection region

“…By analysing the carbon, nitrogen and oxygen cycles, the NW deep-convection region is always found to be an autotrophic ecosystem. The discrepancies in magnitude obtained depending on the element considered reflect different dynamics for these elements in the euphotic layer, possibly due to variable O 2 : C : N ratios in biogeochemical production and consumption processes as shown by Copin-Montégut (2000) in the Ligurian Sea using high-frequency measurements.…”

“…One of the objectives of the DEWEX (DEnse Water EXperiment) project carried out in 2012/13 was to investigate the deep-convection process, the formation of north-western Mediterranean Deep Waters and the impact of deep convection on biogeochemical fluxes (Conan et al, 2018). Three cruises and the deployment of autonomous platforms (glider, Argo floats) provided an unprecedented intensive observation of this region before, during and after a deep-convection event and completed the observation effort during the stratified period operated since 2010 in the framework of the MOOSE-GE (Mediterranean Ocean Observing System for the Environment-Grande Échelle) programme (Estournel et al, 2016b).…”

Oxygen budget for the north-western Mediterranean deep convection region

“…One of the objectives of the DEWEX (DEnse Water EXperiment) project carried out in 2012/2013 was to investigate the deep convection process, the formation of North-Western Mediterranean Deep Waters and the impact of deep convection on biogeochemical fluxes (Conan et al, 2018). Three cruises and the deployment of autonomous platforms (glider, Argo floats) provided an unprecedented intensive observation of this region before, during and after a deep convection event, and completed the observation effort during the stratified period operated since 2010 in the framework of the MOOSE-GE (Mediterranean Ocean Observing System for the Environment-Grande Échelle) program (Estournel et al, 2016b).…”