Louis Prieur

“Almofront-1” (April–May 1991): an interdisciplinary study of the Almeria-Oran geostrophic front, SW Mediterranean Sea

Abstract This paper provides an introduction to the following 12 papers of this special issue of the journal. In addition to a brief historical review and a description of the Almeria-Oran front (Eastern Alboran Sea), a summary is given here of the “Almofront-1” cruise as regards objectives, strategies and methods. The main physical, chemical and biological features of the front as encountered during the “Almofront-1” cruise are presented, illustrated and discussed.

Spreading of Levantine Intermediate Waters by submesoscale coherent vortices in the northwestern Mediterranean Sea as observed with gliders

Since 2007, gliders have been regularly deployed in the northwestern Mediterranean Sea, a crucial region regarding the thermohaline circulation of the Mediterranean Sea. It revealed for the first time very warm (10.48C) and saline (10.1) submesoscale anticyclones at intermediate depth characterized by a small radius (

High resolution modeling of dense water formation in the north-western Mediterranean during winter 2012-2013: Processes and budget

5 km), high Rossby (

Gradients of phytoplankton abundance, composition and photosynthetic pigments across the Almeria-Oran front (SW Mediterranean Sea)

0.3), and Burger (

Observing mixed layer depth, nitrate and chlorophyll concentrations in the northwestern Mediterranean: A combined satellite and NO3 profiling floats experiment

0.7) numbers. They are likely order of 10 to be formed each year, have a life time order a year and certainly contribute significantly to the spreading of the Levantine Intermediate Waters (LIW) toward the whole subbasin, thus potentially impacting wintertime vertical mixing through hydrographical and dynamical preconditioning. They could be mainly formed by the combined action of turbulent mixing and flow detachment of the northward flow of LIW at the northwestern headland of Sardinia. Upwelling conditions along the western coast of Sardinia associated with a southward geostrophic flow within the upper layers seem to play a key role in their formation process.

Estimating dense water volume and its evolution for the year 2012–2013 in the Northwestern Mediterranean Sea: An observing system simulation experiment approach

The evolution of the stratification of the north-western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air - sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time-integrated buoyancy budget over the autumn - winter period. The volume of dense water formed in winter was estimated to be about 50,000 km 3 with a density anomaly larger than 29.113 kg m -3 . The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58 % of the effect of surface buoyancy fluxes.

Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean

As a part of the interdisciplinary study of the geostrophic front of the eastern Alboran Sea (“Almofront-1”, April–May 1991), several characteristics of phytoplankton biomass have been measured at the regional scale to evaluate the gradients between the frontal jet and the surrounding water masses. Microplanktonic diatoms, chlorophyll a and fucoxanthin were the most abundant in the front by 1–2 orders of magnitude whereas pico- and nanoplankton, which consist mostly of prymnesiophytes, and 19′hexanoyloxufucoxanthin tended to be the most abundant in the adjacent waters. Correlations between the various phytoplankton components and tracers are examined. The Almeria-Oran front behaves typically as a fertilisation site in an otherwise oligotrophic environment. Frontal fertilisation favored the growth of one or a few opportunistic, autochthonous diatom species, the remainder of the Alboran Sea being occupied by a diversified population of the smallest size-classes of phytoplankton.

Scales and dynamics of submesoscale coherent vortices formed by deep convection in the northwestern Mediterranean Sea

Two profiling floats, equipped with nitrate concentration sensors were deployed in the northwestern Mediterranean from summer 2012 to summer 2013. Satellite ocean color data were extracted to evaluate surface chlorophyll concentration at float locations. Time series of mixed layer depths and nitrate and chlorophyll concentrations were analyzed to characterize the interplay between the physical-chemical and biological dynamics in the area. Deep convection (mixed layer depth > 1000 m) was observed in January–February, although high-nitrate surface concentrations could be already observed in December. Chlorophyll increase is observed since December, although high values were observed only in March. The early nitrate availability in subsurface layers, which is likely due to the permanent cyclonic circulation of the area, appears to drive the bloom onset. The additional nitrate supply associated to the deep convection events, although strengthening the overall nitrate uptake, seems decoupled of the December increase of chlorophyll.

Deep microbial communities evidenced in the Liguro-Provençal front by their ETS activity

The Northwestern Mediterranean (NWMed) Sea includes one of the best observed ocean deep convection sites in the World. An observing system simulation experiment (OSSE) is developed to provide a methodology for estimating observing network errors. It is applied to quantify dense water volumes in the NWMed during 2012–2013 with their observation error from MOOSE network. Results from the OSSE show low spatiotemporal sampling errors, which confirms MOOSE network ability to measure dense waters. However, results are highly sensitive to instrumental stability. The dense water volume is then estimated in observations from four ship cruises between summers 2012 and 2013. A large seasonal cycle is found, maximal in spring 2013 and dominated by the area west of 6.5°E. The dense water volume ( σ0>29.11 kg/m3) is stable between summer 2012 ( 13.3±0.6 × 1013 m3) and winter 2013 ( 13.7±1.3 × 1013 m3). It increases dramatically in spring 2013 ( 17.7±0.9 × 1013 m3) due to an intense convective event, and it finally decreases rapidly in summer 2013 ( 15.1±0.6 × 1013 m3) due to restratification and spreading. We estimate an open-sea dense water formation (DWF) rate of 1.4±0.3 Sv between summer 2012 and spring 2013 over the studied area, extrapolated to 2.3±0.5 Sv over the whole NWMed Sea and for the optimal timing. This is to our knowledge the highest measured DWF rate, suggesting winter 2013 was exceptionally convective. The observed restratification rate between spring and summer 2013 is −0.8±0.4 Sv. This study provides robust quantifications of deep convection during an exceptional event that will allow to evaluate numerical simulations.

A 1 year sea surface heat budget in the northeastern Atlantic basin during the POMME experiment: 2. Flux optimization

Upward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the Western Mediterranean while physical evidence of such upward advections is seldom reported. The goal of this study is to define biological and chemical criteria, which can be used to localize such upward motions zones. We use a one-dimensional, time-dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen. Simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth. In conformance with the continuity equation, boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer. Low upward advections (≤3 m d -1 ) led to a shallowing and sharpening of the nitracline, oxycline. and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass. By confining the phytoplankton-nutrient system in the surface mixed layer, higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer, the nitracline and the oxycline being then at the top of the pycnocline. Data collected during the Prolig 2 cruise (May 1985) on the heavy side of the Liguro-Provencal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almeria-Oran front during the Almofront 1 cruise (April 1991) in a similar situation. In both fronts, upward advections of 1-2 m d -1 would be sufficient to account for the observed vertical distributions and the increased primary production. Ecological implications for the phytoplankton-nutrient system are discussed, particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts.