scholarly journals Water Mass Transformation in the Greenland Sea during the Period 1986–2016

2019 ◽  
Vol 49 (1) ◽  
pp. 121-140 ◽  
Author(s):  
Ailin Brakstad ◽  
Kjetil Våge ◽  
Lisbeth Håvik ◽  
G. W. K. Moore
Keyword(s):  
Water Column ◽  
Water Mass ◽  
Deep Convection ◽  
Deep Ocean ◽  
Intermediate Water ◽  
Greenland Sea ◽  
Near Surface ◽  
Water Column Stability ◽  
Dense Class ◽  
Mixed Layers

AbstractHydrographic measurements from ships, autonomous profiling floats, and instrumented seals over the period 1986–2016 are used to examine the temporal variability in open-ocean convection in the Greenland Sea during winter. This process replenishes the deep ocean with oxygen and is central to maintaining its thermohaline properties. The deepest and densest mixed layers in the Greenland Sea were located within its cyclonic gyre and exhibited large interannual variability. Beginning in winter 1994, a transition to deeper (>500 m) mixed layers took place. This resulted in the formation of a new, less dense class of intermediate water that has since become the main product of convection in the Greenland Sea. In the preceding winters, convection was limited to <300-m depth, despite strong atmospheric forcing. Sensitivity studies, performed with a one-dimensional mixed layer model, suggest that the deeper convection was primarily the result of reduced water-column stability. While anomalously fresh conditions that increased the stability of the upper part of the water column had previously inhibited convection, the transition to deeper mixed layers was associated with increased near-surface salinities. Our analysis further suggests that the volume of the new class of intermediate water has expanded in line with generally increased depths of convection over the past 10–15 years. The mean export of this water mass from the Greenland Sea gyre from 1994 to present was estimated to be 0.9 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection.

scholarly journals Why did deep convection persist over four consecutive winters (2015–2018) southeast of Cape Farewell?

Ocean Science ◽  
2020 ◽  
Vol 16 (1) ◽  
pp. 99-113
Author(s):  
Patricia Zunino ◽  
Herlé Mercier ◽  
Virginie Thierry
Keyword(s):  
Water Column ◽  
Deep Convection ◽  
Buoyancy Flux ◽  
Intermediate Water ◽  
Shallow Convection ◽  
Local Cooling ◽  
Argo Data ◽  
Irminger Sea ◽  
Mixed Layers ◽  
Buoyancy Loss

Abstract. After more than a decade of shallow convection, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional convection depths (> 1500 m) in 2015 and 2016. Additionally, deep mixed layers deeper than 1600 m were also reported southeast of Cape Farewell in 2015. In this context, we used Argo data to show that deep convection occurred southeast of Cape Farewell (SECF) in 2016 and persisted during two additional years in 2017 and 2018 with a maximum convection depth deeper than 1300 m. In this article, we investigate the respective roles of air–sea buoyancy flux and preconditioning of the water column (ocean interior buoyancy content) to explain this 4-year persistence of deep convection SECF. We analyzed the respective contributions of the heat and freshwater components. Contrary to the very negative air–sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the SECF region during the winters of 2016, 2017 and 2018 were close to the climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end-of-summer water column to homogenize it down to a given depth. B was lower for the winters of 2016–2018 than for the 2008–2015 winter mean, especially due to a vanishing stratification from 600 down to ∼1300 m. This means that less air–sea buoyancy loss was necessary to reach a given convection depth than in the mean, and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1300 m. We show that the decrease in B was due to the combined effects of the local cooling of the intermediate water (200–800 m) and the advection of a negative S anomaly in the 1200–1400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016–2018 despite the atmospheric forcing being close to the climatological average.

scholarly journals Relationship between spatial distribution of chaetognaths and hydrographic conditions around seamounts and islands of the tropical southwestern Atlantic

2014 ◽  
Vol 86 (3) ◽  
pp. 1151-1165 ◽  
Author(s):  
CHRISTIANE S. DE SOUZA ◽  
JOANA A.G. LUZ ◽  
PAULO O. MAFALDA JUNIOR
Keyword(s):  
Spatial Distribution ◽  
Water Column ◽  
Water Mass ◽  
South Atlantic ◽  
Northeastern Brazil ◽  
Southwestern Atlantic ◽  
The South ◽  
Water Column Stability ◽  
South Atlantic Central Water ◽  
Hydrographic Conditions

Relationship between spatial distribution of chaetognaths and hydrographic conditions around seamounts and islands off Northeastern Brazil were analyzed from 133 oceanographic stations during the months of January – April of 1997 and April – July of 1998. Oblique zooplankton tows, using 50 cm diameter Bongo nets with 500µm mesh with a flowmeter to determine the filtered volume, were carried out to a maximum of 200m depth. The Superficial Equatorial Water, which had a salinity > 36 PSU and temperature > 20°C, occupied the top 80 to 200m depth. Below this water mass was the South Atlantic Central Water with salinity ranging from 34.5 to 36 PSU and temperature from 6 to 20°C. The community of chaetognaths showed six species: Pterosagitta draco, Flaccisagitta enflata, Flaccisagitta hexaptera, Pseudosagitta lyra, Serratosagitta serratodentata, and Sagitta helenae. Of these species, F. enflata was the most abundant (32.05% in 1997 and 42.18% in 1998) and the most frequent (87.88% in 1997 and 95% in 1998) during both periods. A mesopelagic specie was identified (P. lyra). This specie was more abundant in 1997 (3.42%), when the upwelling was more intense. P. lyra occurred in 22% of the samples during 1997. The abundance of F. enflata, an epiplanktonic species, increased, associated with greater water-column stability.

scholarly journals Two superimposed cold and fresh anomalies enhanced Irminger Sea deep convection in 2016–2018

2019 ◽  
Author(s):  
Patricia Zunino ◽  
Herlé Mercier ◽  
Virginie Thierry
Keyword(s):  
Water Column ◽  
North Atlantic Ocean ◽  
Deep Convection ◽  
Intermediate Water ◽  
Argo Data ◽  
The North ◽  
Irminger Sea ◽  
The Mean ◽  
Earth System Models ◽  
Buoyancy Loss

Abstract. While Earth system models project a reduction, or even a shut-down, of deep convection in the North Atlantic Ocean in response to anthropogenic forcing, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional depths > 1,500 m in 2015 and 2016. In this context, we used Argo data to show that deep convection persisted in the Irminger Sea during two additional years in 2017 and 2018 with maximum convection depth > 1,300 m. In this article, we investigate the respective roles of air-sea flux and preconditioning of the water column to explain this exceptional 4-year persistence of deep convection; we quantified them in terms of buoyancy and analyzed both the heat and freshwater components. Contrary to the very negative air-sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the Irminger Sea during winters 2016, 2017 and 2018 were close to climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end of summer water column to homogenize the water column down to a given depth. B was lower for winters 2016–2018 than for the mean 2008–2015, including a vanishing stratification from 600 m down to ~1,300 m. It means that less air-sea buoyancy loss was necessary to reach a given convection depth than in the mean and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1,300 m. We showed that the decrease in B was due to the combined effects of a cooling of the intermediate water (200–800 m) and a decrease in salinity in the 1,200–1,400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016–2018 despite the atmospheric forcing was close to the climatological average.

scholarly journals Role of cabbeling in water densification in the Greenland Basin

Ocean Science ◽  
2009 ◽  
Vol 5 (3) ◽  
pp. 247-257 ◽  
Author(s):  
Y. Kasajima ◽  
T. Johannessen
Keyword(s):  
Barents Sea ◽  
Water Mass ◽  
Cold Water ◽  
Maximum Velocity ◽  
The Arctic ◽  
Intermediate Water ◽  
Greenland Sea ◽  
Velocity Magnitude ◽  
Basin Scale ◽  
Greenland Basin

Abstract. The effects of cabbeling mixing on water mass modification in the Greenland Sea were explored by hydrographic observations across the Greenland Basin in summer 2006. The neutral surface was chosen as a reference frame, and the strength of cabbeling mixing was quantified by the dianeutral velocity magnitude. Active cabbeling spots were detected with the criterion of the velocity magnitude >1 m/day, and four active cabbeling areas were identified; the west of Bear Island (SB), the Arctic Frontal Zone (AFZ), the central Greenland Sea (CG) and the western Greenland Sea (WG). The most vigorous cabbeling mixing was found at SB, where warm North Atlantic Water (NAW) mixed with cold water from the Barents Sea, inducing a maximum velocity of 7.5 m/day and a maximum density gain of 4.7×10−3 kg/m3. At AFZ and CG, the mixing took place between NAW, modified NAW and Arctic Intermediate Water (AIW), and the density gain at these fronts were 1.5×10−3 kg/m3 (AFZ) and 1.3×10−3 kg/m3 (CG). In the western Greenland Sea, the active cabbeling spots were widely separated and mixing appeared to be rather weak, with a maximum velocity of 2.5 m/day. The mixing source waters at WG were modified NAW, AIW and even denser water, and the density gain in this area was 0.4×10−3 kg/m3. The deepest mixing produced water whose density is equivalent to that of the dense water of the basin, indicating that cabbeling in the western Greenland Sea contributed directly to basin-scale water densification. The water mass modification rate was the highest at AFZ (about 8.0 Sv), suggesting that cabbeling may play an important role in water transformation in the Greenland Basin.

Sensitivity of convective overturning and turbulent mixing of dissolved gases in the Labrador Sea to atmospheric forcing

2021 ◽  
Author(s):  
Romina Piunno ◽  
Kent Moore
Keyword(s):  
Carbon Dioxide ◽  
Water Column ◽  
Deep Convection ◽  
Deep Ocean ◽  
Labrador Sea ◽  
Dissolved Gases ◽  
Atmospheric Forcing ◽  
Surface Cooling ◽  
Anthropogenic Carbon ◽  
The Impact

&lt;p&gt;Deep oceanic convection occurs in few locations around the globe. One such location is found in the Labrador Sea where dense waters can subside to depths in excess of 2km below the surface. The weak stratification preconditions the water column for deep convection, triggered by wintertime surface cooling associated with high wind speed events. The convected water brings with it dissolved gases, such as Carbon Dioxide, which are in constant flux between ocean and atmosphere. It is thought that this process of turbulent boundary layer interactions coupled with deep convection is responsible for mixing these gases into the deep ocean, making the ocean the largest sink of anthropogenic carbon.&lt;/p&gt;&lt;p&gt;The convective overturning process depends on the temperature and salinity profiles which, together dictate density and thus the static stability of the water column. We have adapted a widely used one-dimensional mixed-layer model, referred to as PWP, to include a parameterization of the air-sea flux of gases such as Oxygen and Carbon Dioxide. &amp;#160;The model is forced with surface meteorological fields from the ERA5 reanalysis as well as the higher resolution operational reanalysis from the ECMWF.&lt;/p&gt;&lt;p&gt;With the model, we investigate the sensitivity of deep-water formation and the vertical profile of these gases to various atmospheric forcing scenarios. Overturning in the Labrador Sea is most active during the winter months when heat flux out of the ocean is at its maximum. It is found that overturning is far more sensitive to thermal forcing than it is to freshwater forcing within the range of forcings typical to the Labrador Sea. We explore the impact of this sensitivity, including the dependence of the atmospheric forcing on modes of climate variability such as the NAO, &amp;#160;has on the role that the Labrador Sea plays as a marine sink for anthropogenic carbon.&lt;/p&gt;

scholarly journals Water mass evolution of the Greenland Sea since lateglacial times

2013 ◽  
Vol 9 (4) ◽  
pp. 5037-5075
Author(s):  
M. M. Telesiński ◽  
R. F. Spielhagen ◽  
H. A. Bauch
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
Water Mass ◽  
Deep Convection ◽  
Sediment Cores ◽  
Atlantic Water ◽  
Global Ocean ◽  
Circulation System ◽  
Greenland Sea ◽  
Sst Cooling

Abstract. Four sediment cores from the central and northern Greenland Sea, a crucial area for the global ocean circulation system, were analyzed for planktic foraminiferal fauna, planktic and benthic stable oxygen and carbon isotopes as well as ice-rafted debris. During the Last Glacial Maximum, the Greenland Sea was dominated by cold and ice-bearing water masses. Meltwater discharges from the surrounding ice sheets affected the area during the deglaciation, influencing the water mass circulation. The Younger Dryas was the last major freshwater event in the area. The onset of the Holocene interglacial was marked by an improvement of the environmental conditions and rising sea surface temperatures (SST). Although the thermal maximum was not reached simultaneously across the basin, due to the reorganization of the specific water mass configuration, benthic isotope data indicate that the overturning circulation reached a maximum in the central Greenland Sea around 7 ka. After 6–5 ka the SST cooling and increasing sea-ice cover is noted alongside with decreasing insolation. Conditions during this Neoglacial cooling, however, changed after 3 ka due to further sea-ice expansion which limited the deep convection. As a result, a well stratified upper water column amplified the warming of the subsurface waters in the central Greenland Sea which were fed by increased inflow of Atlantic Water from the eastern Nordic Seas. Our data reconstruct a variety of time- and space-dependent oceanographic conditions. These were the result of a complex interplay between overruling factors such as changing insolation, the relative influence of Atlantic, Polar and meltwater, sea-ice processes and deep water convection.

scholarly journals Evolution of <sup>231</sup>Pa and <sup>230</sup>Th in overflow waters of the North Atlantic

2018 ◽  
Vol 15 (23) ◽  
pp. 7299-7313 ◽  
Author(s):  
Feifei Deng ◽  
Gideon M. Henderson ◽  
Maxi Castrillejo ◽  
Fiz F. Perez ◽  
Reiner Steinfeldt
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Water Mass ◽  
Deep Ocean ◽  
Water Age ◽  
Meridional Transport ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic

Abstract. Many palaeoceanographic studies have sought to use the 231Pa∕230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, 230Th and 231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231Pa and 230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. 230Th and 231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 µBq kg−1 and between 0.37 and 4.80 µBq kg−1, respectively. Both 230Th and 231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Assessing this dataset using extended optimum multi-parameter (eOMP) analysis and CFC-based water mass age indicates that the low values of 230Th and 231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. The importance of water age is confirmed for 230Th by a strong correlation between 230Th and water mass age (though this relationship with age is less clear for 231Pa and the 231Pa∕230Th ratio). Scavenged 231Pa and 230Th were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more 230Th is scavenged (∼80 %) than 231Pa (∼40 %), consistent with the relatively higher particle reactivity of 230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of the meridional transport of 230Th and 231Pa with this new GEOVIDE dataset enables a complete budget for 230Th and 231Pa for the North Atlantic. Results suggest that net transport southward of 230Th and 231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicate that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230Th and 74 % of the production for 231Pa. This result confirms a significantly higher advective loss of 231Pa to the south relative to 230Th and supports the use of 231Pa∕230Th to assess meridional transport at a basin scale.

Deep convection in the Lofoten Basin: ARGO vs MITgcm

2020 ◽  
Author(s):  
Aleksandr Fedorov ◽  
Belonenko Tatyana
Keyword(s):  
Mixed Layer ◽  
Water Mass ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Intermediate Water ◽  
Model Data ◽  
Layer Depth ◽  
Russian Science ◽  
Intermediate Water Mass

&lt;p&gt;The Lofoten basin (the LB) contains relatively warm and salty waters regarding border basins such as Greenland and Barents Seas. Variability of the processes inside occurring in the basin reflects on the climate as on the mesoscales as on the interannual scales. We use a term mixed layer depth (MLD) as a border of the pycnocline in the water column, this parameter lets us evaluate the intensity of the convection in the region. Several methods of MLD calculations are tested in the current study: Kara, Montegut, and Dukhovskoy. The convection in the basin destructs stratification and forms massive intermediate water mass. The MITgcm data for 1993-2012 and over 5000 in-situ Argo T, S profiles for 2001-2017 were used in the calculations of the MLD.&lt;/p&gt;&lt;p&gt;We consider the maximum MLD (mMLD) in the region and its spatial distribution. The mMLD is higher in the central part of the LB and corresponds to the location of the Lofoten basin eddy (the LBE). Here the mMLD reaches 1000 meters, the medium maximum is 400 meters based both on the in-situ and model data. The maximum mixed layer depth &amp;#8203;&amp;#8203;varies in the range of 400-1000 meters according to both datasets were used. The MLD over 400 meters is observed from January to May every year.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgments: &lt;/strong&gt;The authors acknowledge the support of the Russian Science Foundation (project No. 18-17-00027). The results of the MITgcm were provided by D.L. Volkov, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, USA.&lt;/p&gt;

scholarly journals Evolution of <sup>231</sup>Pa and <sup>230</sup>Th in overflow waters of the North Atlantic

2018 ◽  
Author(s):  
Feifei Deng ◽  
Gideon M. Henderson ◽  
Maxi Castrillejo ◽  
Fiz F. Perez
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Water Mass ◽  
Deep Ocean ◽  
Water Age ◽  
Meridional Transport ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic

Abstract. Many paleoceanographic studies have sought to use the 231Pa / 230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As yet, however, no study has fully assessed the concentration of, or controls on, 230Th and 231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231Pa and 230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. Th-230 and 231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.004 and 0.738 dpm/1000l, and between 0.023 and 0.295 dpm/1000l, respectively. Both 230Th and 231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Analyzing this dataset with Extended Optimum Multi-Parameter (eOMP) Analysis and CFC-based water mass age indicates that the low values of 230Th and 231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. This importance of water age is confirmed for 230Th by a strong correlation between 230Th and water mass age (though this relationship is less clear, for 231Pa and 231Pa / 230Th ratio). Scavenged 231Pa and 230Th were estimated and compared to their Potential Total concentrations in the water column. The result shows that more 230Th is scavenged (~ 80 %) relative to 231Pa (~ 40 %), consistent with the relatively higher particle-reactivity of 230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of meridional transport of 230Th and 231Pa with this new GEOVIDE dataset enables a complete budget for 230Th and 231Pa for the North Atlantic. Results suggest that net transport southward of 230Th and 231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicates that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230Th, and 77 % of the production for 231Pa. This result confirms a significantly higher advective loss of 231Pa to the south relative to 230Th and supports the use of 231Pa / 230Th to assess meridional transport at a basin scale.

scholarly journals Long-Term Changes in the Water Mass Properties in the Balearic Channels Over the Period 1996–2019

2021 ◽  
Vol 8 ◽  
Author(s):  
Manuel Vargas-Yáñez ◽  
Mélanie Juza ◽  
M. Carmen García-Martínez ◽  
Francina Moya ◽  
Rosa Balbín ◽  
...  
Keyword(s):  
Water Column ◽  
Water Mass ◽  
Eastern Mediterranean ◽  
Western Mediterranean ◽  
Intermediate Water ◽  
Deep Waters ◽  
Mass Properties ◽  
Long Term Changes ◽  
Intermediate Waters

The analysis of a 24-year time series of Conductivity-Temperature-Depth (CTD) casts collected in the Balearic Channels (1996–2019) has allowed detecting and quantifying long-term changes in water mass properties in the Western Mediterranean. For the complete period, the intermediate waters have experienced warming and salting at rates of 1.4°C/100yr and 0.3–0.6/100yr for the Western Intermediate Water, and 1°C/100yr and 0.3–0.4/100yr for the Levantine Intermediate Water. The density of these two water masses has not changed. The deep waters, defined as those denser than 29.1 kg/m3, showed positive trends in temperature, salinity, and density (0.8°C/100yr, 0.2/100yr, and 0.02 kg.m–3/100yr, respectively). The high temporal variability of the upper layer makes the detection of long-term changes more difficult. Nevertheless, combining CTD data with temperature data from the oceanographic station at L’Estartit and simulated data from the NCEP/NCAR reanalysis, it can be established that the Atlantic Water increased its temperature at a rate of 2.1–2.8°C/100yr and likely its salinity at a rate of 0.6/100yr. The water column absorbed heat at a rate equivalent to 1–1.2 W/m2. All these trends are much higher than those reported in previous works (more than double in some cases). The warming of the water column produced an increase in the thermosteric component of sea level. However, this increase was compensated by the decrease in the halosteric component. Besides these changes, other alterations related to the Western Mediterranean Transition have been observed over shorter periods. The temperature and salinity of the intermediate waters increased before the winter of 2004/2005 and then the temperature and salinity of the deep waters increased dramatically in 2005. The density of the deep water reached values unprecedented before 2005. Deep and intermediate waters were uplifted by the presence of such dense deep waters. The arrival of warmer and saltier intermediate waters from the Eastern Mediterranean is also observed, mainly after 2010.

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