Open-ocean deep convection explored in the Mediterranean

Abstract. Deep convection in open ocean polynyas are common sources of error on the representation of Antarctic Bottom Water (AABW) formation in Ocean General Circulation Models. Even though those events are well described in non-assimilatory ocean simulations, recent appearance of open ocean polynya in Estimating the Circulation and Climate of the Ocean Phase II reanalysis product raises a question if this spurious event is also found in state-of-art reanalysis products. In order to answer this question, we evaluate how three recently released high-resolution ocean reanalysis form AABW in their simulations. We found that two of them (ECCO2 and SoSE) create AABW by open ocean deep convection events in Weddell Sea, showing that assimilation of sea ice has not been enough to avoid open ocean polynya appearance. The third reanalysis – My Ocean University Reading – actually creates AABW by a rather dynamically accurate mechanism, depicting both continental shelf convection, and exporting of Dense Shelf Water to open ocean. Although the accuracy of the AABW formation in this reanalysis allows an advance in represent this process, the differences found between the real ocean and the simulated one suggests that ocean reanalysis still need substantial improvements to accurately represent AABW formation.

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Transport by deep convection in basin-scale geostrophic circulation: turbulence-resolving simulations

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.64 ◽

2019 ◽

Vol 865 ◽

pp. 681-719

Author(s):

Catherine A. Vreugdenhil ◽

Bishakhdatta Gayen ◽

Ross W. Griffiths

Keyword(s):

Boundary Layer ◽

Thermal Boundary Layer ◽

Large Scale ◽

Deep Convection ◽

Ocean Basin ◽

Open Ocean ◽

Small Scale ◽

Large Scale Circulation ◽

Buoyancy Forcing ◽

Basin Scale

Direct numerical simulations are used to investigate the nature of fully resolved small-scale convection and its role in large-scale circulation in a rotating $f$-plane rectangular basin with imposed surface temperature difference. The large-scale circulation has a horizontal geostrophic component and a deep vertical overturning. This paper focuses on convective circulation with no wind stress, and buoyancy forcing sufficiently strong to ensure turbulent convection within the thermal boundary layer (horizontal Rayleigh numbers $Ra\approx 10^{12}{-}10^{13}$). The dynamics are found to depend on the value of a convective Rossby number, $Ro_{\unicode[STIX]{x0394}T}$, which represents the strength of buoyancy forcing relative to Coriolis forces. Vertical convection shifts from a mean endwall plume under weak rotation ($Ro_{\unicode[STIX]{x0394}T}>10^{-1}$) to ‘open ocean’ chimney convection plus mean vertical plumes at the side boundaries under strong rotation ($Ro_{\unicode[STIX]{x0394}T}<10^{-1}$). The overall heat throughput, horizontal gyre transport and zonally integrated overturning transport are then consistent with scaling predictions for flow constrained by thermal wind balance in the thermal boundary layer coupled to vertical advection–diffusion balance in the boundary layer. For small Rossby numbers relevant to circulation in an ocean basin, vertical heat transport from the surface layer into the deep interior occurs mostly in ‘open ocean’ chimney convection while most vertical mass transport is against the side boundaries. Both heat throughput and the mean circulation (in geostrophic gyres, boundary currents and overturning) are reduced by geostrophic constraints.

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The impact of SST front on the surface wind in the southern Indian Ocean

10.5194/egusphere-egu2020-7987 ◽

2020 ◽

Author(s):

Xuhua Cheng

Keyword(s):

Indian Ocean ◽

Deep Convection ◽

Surface Wind ◽

Polar Front ◽

Open Ocean ◽

Heat Fluxes ◽

Southern Indian Ocean ◽

Sst Front ◽

Microwave Imager ◽

The Impact

&#160;Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) overthe southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter,located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55oE-78oE,&#160;and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layeraccompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing,thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphereby deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode(SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, whilehas negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to thereduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere.

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The M3A network of open ocean observatories in the Mediterranean Sea

2013 MTS/IEEE OCEANS - Bergen ◽

10.1109/oceans-bergen.2013.6607996 ◽

2013 ◽

Cited By ~ 4

Author(s):

Roberto Bozzano ◽

Sara Pensieri ◽

Laura Pensieri ◽

Vanessa Cardin ◽

Fabio Brunetti ◽

...

Keyword(s):

Mediterranean Sea ◽

Open Ocean ◽

The Mediterranean

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Comparison between the Large-Scale Environments of Moderate and Intense Precipitating Systems in the Mediterranean Region

Monthly Weather Review ◽

10.1175/2009mwr2922.1 ◽

2009 ◽

Vol 137 (11) ◽

pp. 3933-3959 ◽

Cited By ~ 38

Author(s):

Beatriz M. Funatsu ◽

Chantal Claud ◽

Jean-Pierre Chaboureau

Keyword(s):

Mediterranean Region ◽

Large Scale ◽

Deep Convection ◽

Extreme Rainfall ◽

Convective Precipitation ◽

Target Area ◽

Weather Forecasts ◽

Microwave Sounding ◽

The Mediterranean ◽

Upper Level

Abstract A characterization of the large-scale environment associated with precipitating systems in the Mediterranean region, based mainly on NOAA-16 Advanced Microwave Sounding Unit (AMSU) observations from 2001 to 2007, is presented. Channels 5, 7, and 8 of AMSU-A are used to identify upper-level features, while a simple and tractable method, based on combinations of channels 3–5 of AMSU-B and insensitive to land–sea contrast, was used to identify precipitation. Rain occurrence is widespread over the Mediterranean in wintertime while reduced or short lived in the eastern part of the basin in summer. The location of convective precipitation shifts from mostly over land from April to August, to mostly over the sea from September to December. A composite analysis depicting large-scale conditions, for cases of either rain alone or extensive areas of deep convection, is performed for selected locations where the occurrence of intense rainfall was found to be important. In both cases, an upper-level trough is seen to the west of the target area, but for extreme rainfall the trough is narrower and has larger amplitude in all seasons. In general, these troughs are also deeper for extreme rainfall. Based on the European Centre for Medium-Range Weather Forecasts operational analyses, it was found that sea surface temperature anomalies composites for extreme rainfall are often about 1 K warmer, compared to nonconvective precipitation conditions, in the vicinity of the affected area, and the wind speed at 850 hPa is also stronger and usually coming from the sea.

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Interannual variability of the Mediterranean trophic regimes from ocean color satellites

Biogeosciences Discussions ◽

10.5194/bgd-12-14941-2015 ◽

2015 ◽

Vol 12 (17) ◽

pp. 14941-14980 ◽

Cited By ~ 4

Author(s):

N. Mayot ◽

F. D'Ortenzio ◽

M. Ribera d'Alcalà ◽

H. Lavigne ◽

H. Claustre

Keyword(s):

Spatial Distribution ◽

Mediterranean Sea ◽

Interannual Variability ◽

Deep Convection ◽

Regional Scale ◽

Ocean Color ◽

Run Off ◽

The Mediterranean ◽

The One ◽

The Impact

Abstract. D'Ortenzio and Ribera d'Alcalà (2009, DR09 hereafter) divided the Mediterranean Sea into "bioregions" based on the climatological seasonality (phenology) of phytoplankton. Here we investigate the interannual variability of this bioregionalization. Using 16 years of available ocean color observations (i.e. SeaWiFS and MODIS), we analyzed the spatial distribution of the DR09 trophic regimes on an annual basis. Additionally, we identified new trophic regimes, with seasonal cycles of phytoplankton biomass different from the DR09 climatological description and named "Anomalous". Overall, the classification of the Mediterranean phytoplankton phenology proposed by DR09 (i.e. "No Bloom", "Intermittently", "Bloom" and "Coastal"), is confirmed to be representative of most of the Mediterranean phytoplankton phenologies. The mean spatial distribution of these trophic regimes (i.e. bioregions) over the 16 years studied is also similar to the one proposed by DR09. But at regional scale some annual differences, in their spatial distribution and in the emergence of "Anomalous" trophic regimes, were observed compared to the DR09 description. These dissimilarities with the DR09 study were related to interannual variability in the sub-basin forcing: winter deep convection events, frontal instabilities, inflow of Atlantic or Black Sea Waters and river run-off. The large assortment of phytoplankton phenologies identified in the Mediterranean Sea is thus verified at interannual level, confirming the "sentinel" role of this basin to detect the impact of climate changes on the pelagic environment.

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

Biogeosciences ◽

10.5194/bg-18-937-2021 ◽

2021 ◽

Vol 18 (3) ◽

pp. 937-960

Author(s):

Caroline Ulses ◽

Claude Estournel ◽

Marine Fourrier ◽

Laurent Coppola ◽

Fayçal Kessouri ◽

...

Keyword(s):

Dissolved Oxygen ◽

Mediterranean Sea ◽

Atmospheric Oxygen ◽

Deep Convection ◽

Western Mediterranean ◽

Biogeochemical Model ◽

The North ◽

Oxygen Budget ◽

The Mediterranean ◽

North Western

Abstract. The north-western Mediterranean deep convection plays a crucial role in the general circulation and biogeochemical cycles of the Mediterranean Sea. The DEWEX (DEnse Water EXperiment) project aimed to better understand this role through an intensive observation platform combined with a modelling framework. We developed a three-dimensional coupled physical and biogeochemical model to estimate the cycling and budget of dissolved oxygen in the entire north-western Mediterranean deep-convection area over the period September 2012 to September 2013. After showing that the simulated dissolved oxygen concentrations are in a good agreement with the in situ data collected from research cruises and Argo floats, we analyse the seasonal cycle of the air–sea oxygen exchanges, as well as physical and biogeochemical oxygen fluxes, and we estimate an annual oxygen budget. Our study indicates that the annual air-to-sea fluxes in the deep-convection area amounted to 20 molm-2yr-1. A total of 88 % of the annual uptake of atmospheric oxygen, i.e. 18 mol m−2, occurred during the intense vertical mixing period. The model shows that an amount of 27 mol m−2 of oxygen, injected at the sea surface and produced through photosynthesis, was transferred under the euphotic layer, mainly during deep convection. An amount of 20 mol m−2 of oxygen was then gradually exported in the aphotic layers to the south and west of the western basin, notably, through the spreading of dense waters recently formed. The decline in the deep-convection intensity in this region predicted by the end of the century in recent projections may have important consequences on the overall uptake of atmospheric oxygen in the Mediterranean Sea and on the oxygen exchanges with the Atlantic Ocean, which appear necessary to better quantify in the context of the expansion of low-oxygen zones.

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Monitoring open-ocean deep convection from space

Geophysical Research Letters ◽

10.1029/2008gl036422 ◽

2009 ◽

Vol 36 (3) ◽

pp. n/a-n/a ◽

Cited By ~ 28

Author(s):

Marine Herrmann ◽

Jérome Bouffard ◽

Karine Béranger

Keyword(s):

Deep Convection ◽

Open Ocean

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