Double diffusion in the Mediterranean Sea: Observation and parameterization of salt finger convection

Onken, Reiner; Brambilla, Elena

doi:10.1029/2002jc001349

Cited by 18 publications

(13 citation statements)

References 26 publications

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“…al., 1987). As an essential prerequisite for staircase formation in the Tyrrhenian Sea and the Algerian Basin, the probability distribution of local density ratios fits to low conditional values (1-1.7) under the LIW core (Onken and Brembilla, 2003).…”

Section: Thermohaline Staircasesmentioning

confidence: 83%

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

Taillandier¹,

Prieur²,

D’Ortenzio³

et al. 2020

Preprint

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Abstract. Characterizing the spatio-temporal arrangements of inorganic nutrients is critical to improve our understanding of the marine biological primary production. Among the processes contributing to nutrient distributions, diapycnal diffusion plays a crucial role for the supply of nutrients to the surface productive zone, and for the equilibration of vertical differences in nutrient concentrations induced by large scale thermohaline circulation. This is the case in the western Mediterranean Sea, where Levantine intermediate waters (LIW), that circulate below the surface layer, regionally distribute the nutrient stocks conveyed from the eastern basin or provided by terrestrial inputs, atmospheric deposition, and remineralization of organic matter. In the present study, we focus on the role played by diffusive processes in the LIW fertilization, considering long-term observations of thermohaline staircases. In association with the unprecedented contribution of profiling floats to explore their structural changes, the fine characterization of western Mediterranean thermohaline staircases sampled during the cruise PEACETIME can be carried out from a different perspective. Observations revealed that thermohaline staircases develop over epicentral regions confined inside large scale circulation features and sustained by saltier LIW inflows on the periphery. As observed in the Algerian Basin, these epicentral regions are thought to be site of active mixing, with changes of seawater properties by about + 0.06 °C in temperature and + 0.02 in salinity during the four years of observation. In-situ lateral density ratios are analysed in the view of theoretical predictions to identify and untangle (i) salt fingering as driver of water mass conversion, with (ii) isopycnal diffusion as spreader of heat and salt from the surrounding sources. In the Tyrrhenian Sea, the resulting nutrient fluxes bring upward from deep waters 5 μmol/m2/d in nitrate, which represents one fourth of LIW fertilization by diapycnal diffusion, but remains a secondary contributor to the enrichment of Ionian water inflows.

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Section: Thermohaline Staircasesmentioning

confidence: 83%

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

Taillandier¹,

Prieur²,

D’Ortenzio³

et al. 2020

Preprint

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“…In the finger regime, salty and warm water is overlying fresher and cooler water, leading to the formation of blobs of rising freshwater and sinking salty water (Kunze 2003). The finger regime enhances mixing in several regions of the ocean (Schmitt et al 2005) and leads to an increased apparent diffusivity of salt as compared to heat, which has been exemplified in the Mediterranean Sea by Onken and Brambilla (2003). Conversely, in the diffusive regime, cooler and fresher water is lying on top of warmer and saltier water, with the apparent diffusivity of heat being significantly enhanced compared to that of salts and other dissolved substances (Kelley et al 2003).…”

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confidence: 99%

Double‐diffusive convection in Lake Kivu

Schmid

Busbridge

est

2009

Limnology & Oceanography

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Double-diffusive staircases with a total of 230-350 mixed layers and sharp interfaces were observed in nine microstructure temperature profiles measured during February 2004 in Lake Kivu. The presence of these staircases at depths . 120 m indicates that diapycnal turbulent mixing is weak and vertical diffusive transport is dominated by double diffusion. Contrary to previously investigated natural or laboratory double-diffusive systems, the dissolved gases CO 2 and CH 4 contribute significantly to the density stratification, thereby influencing the formation and the structure of the staircases. The density ratio (i.e., the ratio of the stabilizing effect of dissolved substances to the destabilizing effect of temperature) ranges between 2.0 and 4.5 in large sections of the deep waters, implying a high susceptibility to the formation of staircases. The mixed layers (average thickness 0.48 m) are shown to be in a state of active convection. The average thickness of the interfaces (0.18 m) is surprisingly constant and independent of the large-scale stratification. The vertical heat fluxes correlate well with the temperature steps across the interfaces. Lake Kivu receives inflows from subaquatic springs at several depths that maintain the large-scale structure of the density stratification and disturb the staircases. In comparison to earlier observations from 1972, the double-diffusive heat fluxes appear to have been reduced, leading to a heat accumulation in the deep waters. Conversely, the strengthening of the main chemocline indicates an increased discharge of the subaquatic springs that could be responsible for recent changes in the nutrient cycling and methane production in the lake.

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“…The stratification in the Mediterranean Sea is especially favourable for double diffusion because of the high-salinity core of Levantine Intermediate Water below the main thermocline (Millot, 1999). Onken and Brambilla (2003) have shown that in the Algerian Basin (the model domain is embedded in that basin) and below about 300 m depth, the vertical diffusivity of salt may be up to twice as high as the diffusivity of heat. This 25 leads to an enhanced diffusion of buoyancy and similarly may affect the entire circulation pattern and the different skill scores for temperature and salinity.…”

Section: Discussionmentioning

confidence: 99%

Forecast skill score assessment of a relocatable ocean prediction system, using a simplified objective analysis method

Onken¹

2017

Preprint

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A Relocatable Ocean Prediction System (ROPS) was employed to an observational data set which was collected in June 2014 in the waters to the west of Sardinia (Western Mediterranean) in the mainframe of the REP14-MED experiment. The observational data, comprising almost 5000 temperature and salinity profiles from a fleet of underwater gliders and shipborne probes, were assimilated in the Regional Ocean Modeling System (ROMS) which is the heart of ROPS, and verified against independent observations from ScanFish tows by means of the forecast skill score as defined by Murphy (1993). A simplified 5 objective analysis (OA) method was utilised for assimilation, taking account of only those profiles which were located within a predetermined time window W . As a result of a sensitivity study, the highest skill score was obtained for a correlation length scale C = 12.5 km, W = 24 hours, and r = 1, where r is the ratio between the error of the observations and the background error, both for temperature and salinity. Additional ROPS runs showed that (i) the skill score of assimilation runs was mostly higher than the score of a control run without assimilation, (i) the skill score increased with increasing forecast range, and (iii) 10 the skill score for temperature was higher than the score for salinity in the majority of cases. Further on, it is demonstrated that the vast number of observations can be managed by the applied OA method without data reduction, enabling timely operational forecasts even on a commercially available Personal Computer or a laptop. IntroductionA Relocatable Ocean Prediction System (ROPS) is presented which enables rapid nowcasts and forecasts of ocean environmen-15 tal parameters in limited regions. In this study, ROPS was implemented for the waters west of Sardinia (Western Mediterranean Sea) within the framework of the REP14-MED experiment (Onken et al. (2014, 2017a)).The major components of ocean operational systems are observations and ocean circulation models coupled with data assimilation systems, to combine the observations with dynamics and issue nowcasts and forecasts which are delivered to the customers. While systems on global scale are utilised to provide estimates on large-scale circulation patterns and associated 20 features, regional operational systems are focusing more on societally relevant oceanographic information for e.g. search and rescue operations, pollutant dispersal, fishery management , and military applications. Meanwhile, quite a number of real-time ocean operational systems are available, spanning the scales of ocean horizontal circulation patterns from global to coastal (Dombrowsky (2011), Zhu (2011)). significant improvement of the forecast skill. For extremely small forecast ranges, the skill score even became negative, because the assimilation disequilibrated the balance of forces in the dynamical model.In all ROPS runs, including the run without assimilation, the skill score for temperature was mostly higher than the corresponding score for salinity. This is in a...

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Double diffusion in the Mediterranean Sea: Observation and parameterization of salt finger convection

Cited by 18 publications

References 26 publications

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

Double‐diffusive convection in Lake Kivu

Forecast skill score assessment of a relocatable ocean prediction system, using a simplified objective analysis method

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