Wind-forced submesoscale symmetric instability around deep convection in the NW Mediterranean Sea

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500m in the NW Mediterranean. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders, mainly as part of the MOOSE observatory of the NW Mediterrnean Sea (https://www.moose-network.fr/). Subduction down to 200-300m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution (SYMPHONIE model, https://sirocco.obs-mip.fr/ocean-models/s-model/).During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1-5km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open-sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

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Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

Fluids ◽

10.3390/fluids6030123 ◽

2021 ◽

Vol 6 (3) ◽

pp. 123

Author(s):

Anthony Bosse ◽

Pierre Testor ◽

Pierre Damien ◽

Claude Estournel ◽

Patrick Marsaleix ◽

...

Keyword(s):

Mediterranean Sea ◽

Mixed Layer ◽

Deep Convection ◽

Realistic Model ◽

Ligurian Sea ◽

Vertical Movements ◽

Northwestern Mediterranean Sea ◽

Wind Events ◽

Northwestern Mediterranean ◽

Symmetric Instability

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

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Importance of the variability of hydrographic preconditioning for deep convection in the Gulf of Lion, NW Mediterranean

Ocean Science ◽

10.5194/os-6-573-2010 ◽

2010 ◽

Vol 6 (2) ◽

pp. 573-586 ◽

Cited By ~ 26

Author(s):

L. Grignon ◽

D. A. Smeed ◽

H. L. Bryden ◽

K. Schroeder

Keyword(s):

Interannual Variability ◽

Deep Convection ◽

Mixed Layer Depth ◽

Surface Fluxes ◽

Ligurian Sea ◽

Intermediate Layers ◽

Gulf Of Lion ◽

Nw Mediterranean ◽

Convective Mixed Layer ◽

Correlation Tests

Abstract. We study the variability of hydrographic preconditioning defined as the heat and salt contents in the Ligurian Sea before convection. The stratification is found to reach a maximum in the intermediate layer in December, whose causes and consequences for the interannual variability of convection are investigated. Further study of the interannual variability and correlation tests between the properties of the deep water formed and the winter surface fluxes support the description of convection as a process that transfers the heat and salt contents from the top and intermediate layers to the deep layer. A proxy for the rate of transfer is given by the final convective mixed layer depth, that is shown to depend equally on the surface fluxes and on the preconditioning. In particular, it is found that deep convection in winter 2004–2005 would have happened even with normal winter conditions, due to low pre-winter stratification.

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Importance of the variability of hydrographic preconditioning for deep convection in the Gulf of Lion, NW Mediterranean

Ocean Science Discussions ◽

10.5194/osd-7-51-2010 ◽

2010 ◽

Vol 7 (1) ◽

pp. 51-90

Author(s):

L. Grignon ◽

D. A. Smeed ◽

H. L. Bryden ◽

K. Schroeder

Keyword(s):

Interannual Variability ◽

Deep Convection ◽

Mixed Layer Depth ◽

Surface Fluxes ◽

Ligurian Sea ◽

Intermediate Layers ◽

Gulf Of Lion ◽

Nw Mediterranean ◽

Convective Mixed Layer ◽

Correlation Tests

Abstract. We study the variability of hydrographic preconditioning defined as the heat and salt contents in the Ligurian Sea before convection. The stratification is found to reach a maximum in the intermediate layer in December, whose causes and consequences for the interannual variability of convection are investigated. Further study of the interannual variability and correlation tests between the properties of the deep water formed and the winter surface fluxes support the description of convection as a process that transfers the heat and salt contents from the top and intermediate layers to the deep layer. A proxy for the rate of transfer is given by the final convective mixed layer depth, that is shown to depend equally on the surface fluxes and on the preconditioning. In particular, it was found that deep convection in winter 2004–2005 would have happened even with normal winter conditions, due to low pre-winter stratification.

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Study of fine-scale dynamics and their coupling with biogeochemistry - FUMSECK cruise

10.5194/egusphere-egu21-7199 ◽

2021 ◽

Author(s):

Stéphanie Barrillon ◽

Caroline Comby ◽

Jean-Luc Fuda ◽

Anne Petrenko ◽

Melilotus Thyssen ◽

...

Keyword(s):

Mediterranean Sea ◽

Free Fall ◽

Storm Event ◽

Fine Scale ◽

Carbon Export ◽

Ligurian Sea ◽

Micro Particles ◽

Nw Mediterranean ◽

Spatio Temporal

FUMSECK (Facilities for Updating the Mediterranean Submesoscale - Ecosystem Coupling Knowledge) is a one-week cruise, which took place in spring 2019, in the gulf of Genoa (NW Mediterranean Sea), onboard the R/V T&#233;thys II. It was conducted in preparation of the BioSWOT-Med cruise in the SW Mediterranean Sea in 2022, planned as part of the ``Adopt a Cross Over'' initiative organising simultaneous oceanographic cruises around the world during the SWOT fast sampling phase. During FUMSECK we tested various technological innovations for the study of fine-scale dynamics and their coupling with biogeochemistry.By their interactions, the fine scales could induce some ageostrophic and tridimensional dynamics, which are a critical point for the understanding of the vertical exchanges and their effect on biogeochemistry. Therefore, the fine scales play a key role in the oceans global balance and, despite their low intensity, clearly impact processes such as nutriment vertical transfer and carbon export. However, their ephemeral nature complicates their in situ measurements, which are nevertheless essential for their understanding and for the confirmation of the models&#8217; prediction and the satellite observations. Furthermore, measuring vertical velocities in situ represents a real challenge since they are several orders of magnitude below the horizontal ones.The FUMSECK cruise benefited from the automatic Lagrangian SPASSO treatment of the satellite data with an onshore team providing a daily bulletin of analysis and guidance on the fine-scale structures in the studied area. The distribution of phytoplankton functional groups at a small spatio-temporal scale was measured by automated flow cytometry with imaging. This technology allows to address the distribution of phytoplankton at fine scales within its hydrodynamic context. Several methods of measuring vertical velocities have been deployed, using different ADCP at fixed depth and in profile, FF-ADCP (Free Fall ADCP), the VVP (Vertical Velocities Profiler) prototype developed at MIO, and a SeaExplorer glider. These methods have shown promising results for in situ measurement of vertical velocities. Overall results show an abrupt change of population associated with a fine-scale structure appearance in relation with a storm event.In addition, in order to study the physical part of the biological carbon pump, we experienced the release, following, pumping and detection by cytometry of a sample of biodegradable micro-particles that mimic the phytoplankton, and established a proof-of-concept for this method. Finally, we studied the MVP (Moving Vessel Profiler) instruments behaviour and reduced significantly a rotative effect.We will describe the instrumental and analysis methodology deployed during FUMSECK in the study area of the Ligurian Sea, including the Northern Current, and present the results on the fine-scale dynamics and their impact on biology.

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Winter Mixed Layer Development in the Central Irminger Sea: The Effect of Strong, Intermittent Wind Events

Journal of Physical Oceanography ◽

10.1175/2007jpo3678.1 ◽

2008 ◽

Vol 38 (3) ◽

pp. 541-565 ◽

Cited By ~ 67

Author(s):

Kjetil Våge ◽

Robert S. Pickart ◽

G. W. K. Moore ◽

Mads Hvid Ribergaard

Keyword(s):

Mixed Layer ◽

Deep Convection ◽

Layer Model ◽

Pressure System ◽

Nao Index ◽

Irminger Sea ◽

Wind Events ◽

Upper Level ◽

The Impact ◽

Mixed Layer Model

Abstract The impact of the Greenland tip jet on the wintertime mixed layer of the southwest Irminger Sea is investigated using in situ moored profiler data and a variety of atmospheric datasets. The mixed layer was observed to reach 400 m in the spring of 2003 and 300 m in the spring of 2004. Both of these winters were mild and characterized by a low North Atlantic Oscillation (NAO) index. A typical tip jet event is associated with a low pressure system that is advected by upper-level steering currents into the region east of Cape Farewell and interacts with the high topography of southern Greenland. Heat flux time series for the mooring site were constructed that include the enhancing influence of the tip jet events. This was used to force a one-dimensional mixed layer model, which was able to reproduce the observed envelope of mixed layer deepening in both winters. The deeper mixed layer of the first winter was largely due to a higher number of robust tip jet events, which in turn was caused by the steering currents focusing more storms adjacent to southern Greenland. Application of the mixed layer model to the winter of 1994–95, a period characterized by a high-NAO index, resulted in convection exceeding 1700 m. This prediction is consistent with hydrographic data collected in summer 1995, supporting the notion that deep convection can occur in the Irminger Sea during strong winters.

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Winter Atmospheric Buoyancy Forcing and Oceanic Response during Strong Wind Events around Southeastern Greenland in the Regional Arctic System Model (RASM) for 1990–2010*

Journal of Climate ◽

10.1175/jcli-d-15-0592.1 ◽

2016 ◽

Vol 29 (3) ◽

pp. 975-994 ◽

Cited By ~ 15

Author(s):

Alice K. DuVivier ◽

John J. Cassano ◽

Anthony Craig ◽

Joseph Hamman ◽

Wieslaw Maslowski ◽

...

Keyword(s):

Mixed Layer ◽

Deep Convection ◽

Heat Fluxes ◽

System Model ◽

Strong Wind ◽

Self Organizing Map ◽

Ocean Mixed Layer ◽

Irminger Sea ◽

Wind Events ◽

Wind Patterns

Abstract Strong, mesoscale tip jets and barrier winds that occur along the southeastern Greenland coast have the potential to impact deep convection in the Irminger Sea. The self-organizing map (SOM) training algorithm was used to identify 12 wind patterns that represent the range of winter [November–March (NDJFM)] wind regimes identified in the fully coupled Regional Arctic System Model (RASM) during 1990–2010. For all wind patterns, the ocean loses buoyancy, primarily through the turbulent sensible and latent heat fluxes; haline contributions to buoyancy change were found to be insignificant compared to the thermal contributions. Patterns with westerly winds at the Cape Farewell area had the largest buoyancy loss over the Irminger and Labrador Seas due to large turbulent fluxes from strong winds and the advection of anomalously cold, dry air over the warmer ocean. Similar to observations, RASM simulated typical ocean mixed layer depths (MLD) of approximately 400 m throughout the Irminger basin, with individual years experiencing MLDs of 800 m or greater. The ocean mixed layer deepens over most of the Irminger Sea following wind events with northerly flow, and the deepening is greater for patterns of longer duration. Seasonal deepest MLD is strongly and positively correlated to the frequency of westerly tip jets with northerly flow.

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