scholarly journals Ocean Convection

Fluids ◽  
2021 ◽  
Vol 6 (10) ◽  
pp. 360
Author(s):  
Catherine Vreugdenhil ◽  
Bishakhdatta Gayen
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Mixed Layer ◽  
Water Mass ◽  
Current Knowledge ◽  
Open Ocean ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Global Circulation Models ◽  
Ocean Convection

Ocean convection is a key mechanism that regulates heat uptake, water-mass transformation, CO2 exchange, and nutrient transport with crucial implications for ocean dynamics and climate change. Both cooling to the atmosphere and salinification, from evaporation or sea-ice formation, cause surface waters to become dense and down-well as turbulent convective plumes. The upper mixed layer in the ocean is significantly deepened and sustained by convection. In the tropics and subtropics, night-time cooling is a main driver of mixed layer convection, while in the mid- and high-latitude regions, winter cooling is key to mixed layer convection. Additionally, at higher latitudes, and particularly in the sub-polar North Atlantic Ocean, the extensive surface heat loss during winter drives open-ocean convection that can reach thousands of meters in depth. On the Antarctic continental shelf, polynya convection regulates the formation of dense bottom slope currents. These strong convection events help to drive the immense water-mass transport of the globally-spanning meridional overturning circulation (MOC). However, convection is often highly localised in time and space, making it extremely difficult to accurately measure in field observations. Ocean models such as global circulation models (GCMs) are unable to resolve convection and turbulence and, instead, rely on simple convective parameterizations that result in a poor representation of convective processes and their impact on ocean circulation, air–sea exchange, and ocean biology. In the past few decades there has been markedly more observations, advancements in high-resolution numerical simulations, continued innovation in laboratory experiments and improvement of theory for ocean convection. The impacts of anthropogenic climate change on ocean convection are beginning to be observed, but key questions remain regarding future climate scenarios. Here, we review the current knowledge and future direction of ocean convection arising from sea–surface interactions, with a focus on mixed layer, open-ocean, and polynya convection.

Climatological Annual Cycle of the Salinity Budgets of the Subtropical Maxima

2016 ◽  
Vol 46 (10) ◽  
pp. 2981-2994 ◽  
Author(s):  
Benjamin K. Johnson ◽  
Frank O. Bryan ◽  
Semyon A. Grodsky ◽  
James A. Carton
Keyword(s):  
Ocean Circulation ◽  
Mixed Layer ◽  
Annual Cycle ◽  
Salinity Gradient ◽  
Circulation Model ◽  
Open Ocean ◽  
Surface Cooling ◽  
The North ◽  
The Pacific ◽  
Diffusive Fluxes

AbstractSix subtropical salinity maxima (Smax) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) Smax lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (E − P) flux over the Smax is asymmetric about the equator. Over the Northern Hemisphere Smax, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere Smax, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six Smax reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six Smax. The NI Smax budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean Smax budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These Smax have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the Smax into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.

scholarly journals Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years: a model study

2016 ◽  
Vol 12 (2) ◽  
pp. 339-375 ◽  
Author(s):  
K. Wallmann ◽  
B. Schneider ◽  
M. Sarnthein
Keyword(s):  
Southern Ocean ◽  
Sea Level ◽  
Ocean Circulation ◽  
Dissolved Inorganic Carbon ◽  
Nutrient Utilization ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Total Alkalinity ◽  
Global Ocean ◽  
Atmospheric Pco2

Abstract. We have developed and employed an Earth system model to explore the forcings of atmospheric pCO2 change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus (DP), reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), 13C-DIC, and 14C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial pCO2 drawdown may be attributed to marine regressions. The glacial sea-level low-stands implied steepened ocean margins, a reduced burial of particulate organic carbon, phosphorus, and neritic carbonate at the margin seafloor, a decline in benthic denitrification, and enhanced weathering of emerged shelf sediments. In turn, low-stands led to a distinct rise in the standing stocks of DIC, TA, and nutrients in the global ocean, promoted the glacial sequestration of atmospheric CO2 in the ocean, and added 13C- and 14C-depleted DIC to the ocean as recorded in benthic foraminifera signals. The other half of the glacial drop in pCO2 was linked to inferred shoaling of Atlantic meridional overturning circulation and more efficient utilization of nutrients in the Southern Ocean. The diminished ventilation of deep water in the glacial Atlantic and Southern Ocean led to significant 14C depletions with respect to the atmosphere. According to our model, the deglacial rapid and stepwise rise in atmospheric pCO2 was induced by upwelling both in the Southern Ocean and subarctic North Pacific and promoted by a drop in nutrient utilization in the Southern Ocean. The deglacial sea-level rise led to a gradual decline in nutrient, DIC, and TA stocks, a slow change due to the large size and extended residence times of dissolved chemical species in the ocean. Thus, the rapid deglacial rise in pCO2 can be explained by fast changes in ocean dynamics and nutrient utilization whereas the gradual pCO2 rise over the Holocene may be linked to the slow drop in nutrient and TA stocks that continued to promote an ongoing CO2 transfer from the ocean into the atmosphere.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

&lt;p&gt;The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (&lt; 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization &amp;#8211; based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems&lt;sup&gt;&amp;#160;&lt;/sup&gt;&amp;#8211; to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges &amp;#8211; an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.&lt;/p&gt;

Characteristics and variability of ocean ventilation in the high-latitude North Atlantic in an eddy-permitting ocean model

2021 ◽  
Author(s):  
Helen L. Johnson ◽  
Graeme MacGilchrist ◽  
David P. Marshall ◽  
Camille Lique ◽  
Matthew Thomas ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Mixed Layer ◽  
North Atlantic ◽  
High Latitude ◽  
Circulation Model ◽  
Open Ocean ◽  
Labrador Sea ◽  
Surface Mixed Layer ◽  
Atmospheric Forcing ◽  
Boundary Current

&lt;p&gt;A substantial fraction of the deep ocean is ventilated in the high latitude North Atlantic. As a result, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a set of comprehensive Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined &amp;#8216;North Atlantic Deep Water&amp;#8217; (NADW) reveal the times and locations of subduction from the surface mixed layer at high temporal and spatial resolution. The major fraction (&amp;#8764;60%) of NADW ventilation results from subduction directly into the Labrador Sea boundary current, with a smaller fraction (&amp;#8764;25%) arising from open ocean deep convection in the Labrador Sea. There is a notable absence of ventilation arising from subduction in the Greenland&amp;#8211;Iceland&amp;#8211;Norwegian Seas, due to the re-entrainment of those waters as they move southward. Temporal variability in ventilation arises both from changes in subduction &amp;#8212; driven by large-scale atmospheric forcing &amp;#8212; and from year-to-year changes in the subsurface retention of newly subducted water, the result of an inter-annual equivalent of Stommel&amp;#8217;s mixed layer demon. This interannual demon operates most effectively in the open ocean where newly subducted water is slow to escape its region of subduction. Thus, while subduction in the boundary current dominates NADW ventilation, processes in the open ocean set the variability, mediating the translation of inter-annual variations in atmospheric forcing to the ocean interior.&lt;/p&gt;

The Norwegian node for the European Multidisciplinary Seafloor and water column Observatory

2020 ◽  
Author(s):  
Thibaut Barreyre ◽  
Ilker Fer ◽  
Bénédicte Ferré
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
Large Scale ◽  
Water Mass ◽  
Critical Role ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Fram Strait ◽  
Nordic Seas

&lt;p&gt;NorEMSO is a coordinated, large-scale deep-ocean observation facility to establish the Norwegian node for the European Multidisciplinary Seafloor and water column Observatory (EMSO). The project aims to explore the under-sampled Nordic Seas to gain a better understanding of the critical role that they play in our climate system and global ocean circulation. An overarching scientific objective is to better understand the drivers for the temporal and spatial changes of water mass transformations, ocean circulation, acidification and thermo-chemical exchanges at the seafloor in the Nordic Seas, and to contribute to improvement of models and forecasting by producing and making available high quality, near real time data. NorEMSO will achieve this by combining expansion of existing and establishment of new observatory network infrastructure, as well as its coordination and integration into EMSO.&lt;/p&gt;&lt;p&gt;NorEMSO comprises of three main components: moored observatories, gliders, and seafloor and water column observatory at the Mohn Ridge (EMSO-Mohn).&lt;/p&gt;&lt;p&gt;Moored observation systems include an array of four moored observatories located at key positions in the Nordic Seas (Svin&amp;#248;y, Station M, South Cape, and central Fram Strait).&lt;/p&gt;&lt;p&gt;Gliders will be operated along five transects across both the Norwegian and the Greenland Seas to monitor circulation and water mass properties at those key locations. Transects in the Norwegian and Lofoten basins will focus on monitoring the Norwegian Atlantic Current, and a transect in Fram Strait will monitor properties and variability in the return Atlantic Water along the Polar Front in the northern Nordic Seas. In addition, transects in the Greenland and Iceland Seas will address the water mass transformation processes through wintertime open ocean convection, and the southbound transport of surface water from the Arctic Ocean and dense water that feeds the lower limb of the Atlantic Meridional Overturning Circulation in the East Greenland Current.&lt;/p&gt;&lt;p&gt;EMSO-Mohn will establish, at the newly discovered hydrothermal site on the Mohn Ridge, a fixed-point seabed-water-column-coupled and wireless observatory with a multidisciplinary approach &amp;#8211; from geophysics and physical oceanography to ecology and microbiology. It is primarily directed at understanding hydrothermal fluxes and associated hydrothermal plume dynamics in the water column and how it disperses in an oceanographic front over the Mohn Ridge.&lt;/p&gt;&lt;p&gt;Following EMSO philosophy, NorEMSO will provide data and platforms to a large and diverse group of users, from scientists and industries to institutions and policy makers. The observations will serve climate research, ocean circulation understanding, numerical operational models, design of environmental policies, and education.&lt;/p&gt;

How CMEMS INSTAC contributes to the monitoring of the ocean?

2020 ◽  
Author(s):  
Mélanie Juza ◽  
Tanguy Szekely ◽  
Jérôme Gourrion ◽  
Inmaculada Ruiz-Parrado ◽  
Sylvie Pouliquen ◽  
...  
Keyword(s):  
Climate Change ◽  
Ocean Circulation ◽  
Water Mass ◽  
Extreme Event ◽  
Marine Ecosystem ◽  
Mesoscale Eddy ◽  
Heat Content ◽  
Temporal Scales ◽  
Essential Information

&lt;p&gt;&lt;span&gt;CMEMS IN SITU TAC (INSTAC) collects &lt;em&gt;in situ&lt;/em&gt; observations from various platforms (e.g. Argo floats, gliders, drifters, ships, fixed stations, marine mammals, high-frequency radar). It provides free, open and quality-controlled physical and biogeochemical ocean data in both delayed mode and near-real time, in support to the operational oceanography, the ocean health and the climate change. Monitoring the 4-dimensional ocean at various spatial and temporal scales, the INSTAC multi-year products provide an essential information on the ocean state, variability and changes. Hence, the INSTAC group contributes substantially to the elaboration of the annual CMEMS Ocean State Report (&lt;em&gt;Von Schuckmann et al.&lt;/em&gt;, 2016, 2018, 2019, 2020), in collaboration with internal and external scientific experts, as well as with other CMEMS TACs and MFCs.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;A general overview of the INSTAC contributions to the CMEMS Ocean State Report is presented, highlighting its capacity to describe, analyze and understand the ocean state and variability of both physical and biogeochemical components from the sea surface to the deep ocean, from the coastal to open sea waters at both short-term (event) and long-term temporal scales. The INSTAC team contributes to the CMEMS Ocean Monitoring Indicators (e.g. temperature, salinity, ocean heat content, water mass and heat exchange, extreme event detection), investigates the ocean circulation variability (e.g. cold and fresh blob in North Atlantic, mesoscale eddy anomaly), analyses the impacts of climate change on marine ecosystem and ocean circulation (e.g. water mass responses to climate warming, cyclones), and develops operational applications and services (pollution risk, search-and-rescue, storm and wave alerts, river discharges).&lt;/span&gt;&lt;/p&gt;

scholarly journals The shallow meridional overturning circulation of the South China Sea

2014 ◽  
Vol 11 (2) ◽  
pp. 1191-1212 ◽  
Author(s):  
N. Zhang ◽  
J. Lan ◽  
F. Cui
Keyword(s):  
South China Sea ◽  
Formation Mechanism ◽  
Wind Stress ◽  
Mixed Layer ◽  
South China ◽  
Open Ocean ◽  
The South China Sea ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract. In this paper, the structure and formation mechanism of the annual-mean shallow meridional overturning circulation of the South China Sea (SCS) are investigated. A distinct clockwise overturning circulation is present above 400 m in the SCS on the climatological annual mean scale. The shallow meridional overturning circulation consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling in the southern SCS and a northward return flow of surface water. The formation mechanism is explored by studying causes of the branches constituting the meridional overturning circulation. The surface branch is driven by the annual mean zonal component of the wind stress which is predominantly westward. Another effect of the wind is Ekman pumping related subduction in the north hence the main source of downwelling there. The mixed layer depth reaches its maximum in winter and shoals in spring, which causes the thermocline to outcrop and ventilate. Part of the water mass from the bottom of the mixed layer subducts into the thermocline and flows southward along the isopycnals. The upwelling region is mainly along the Vietnam coast and in the open-ocean off it. In summer, the alongshore component of wind stress off Vietnam can cause coastal upwelling and the increase of alongshore wind off the coast can also cause great upwelling in the open-ocean off the Vietnam coast.

scholarly journals 2017 program of studies: ice-ocean interactions

2018 ◽  
Author(s):  
Claudia Cenedese ◽  
Mary-Louise Timmermans
Keyword(s):  
Climate Change ◽  
Fluid Dynamics ◽  
Ocean Circulation ◽  
High Latitude ◽  
Ocean Dynamics ◽  
Geophysical Fluid Dynamics ◽  
Study Program ◽  
Ice Sheet Dynamics ◽  
Summer Study

The 2017 Geophysical Fluid Dynamics Summer Study Program theme was Ice-Ocean Interactions. Three principal lecturers, Andrew Fowler (Oxford), Adrian Jenkins (British Antarctic Survey) and Fiamma Straneo (WHOI/Scripps Institution of Oceanography) were our expert guides for the first two weeks. Their captivating lectures covered topics ranging from the theoretical underpinnings of ice-sheet dynamics, to models and observations of ice-ocean interactions and high-latitude ocean circulation, to the role of the cryosphere in climate change. These icy topics did not end after the first two weeks. Several of the Fellows' projects related to ice-ocean dynamics and thermodynamics, and many visitors gave talks on these themes.

scholarly journals The Dynamics of Mixed Layer Deepening during Open-Ocean Convection

2020 ◽  
Vol 50 (6) ◽  
pp. 1625-1641
Author(s):  
Taimoor Sohail ◽  
Bishakhdatta Gayen ◽  
Andrew McC. Hogg
Keyword(s):  
Mixed Layer ◽  
Large Scale ◽  
Numerical Models ◽  
Mixed Layer Depth ◽  
Baroclinic Instability ◽  
Open Ocean ◽  
Scale Model ◽  
Quasi Equilibrium ◽  
Layer Depth ◽  
Ocean Convection

AbstractOpen-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.

scholarly journals Effects of eustatic sea-level change, ocean dynamics, and iron fertilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years

2015 ◽  
Vol 11 (3) ◽  
pp. 2405-2481
Author(s):  
K. Wallmann ◽  
B. Schneider ◽  
M. Sarnthein
Keyword(s):  
Southern Ocean ◽  
Sea Level ◽  
Ocean Circulation ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Total Alkalinity ◽  
Global Ocean ◽  
Dust Deposition ◽  
Iron Fertilization ◽  
Atmospheric Pco2

Abstract. We developed and employed an earth system model to explore the forcings of atmospheric pCO2 change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus, reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), 13C-DIC and 14C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial pCO2 drawdown may be attributed to marine regressions. The glacial sea-level low-stands implied steepened ocean margins, a reduced burial of particulate organic carbon, phosphorus, and neritic carbonate at the margin seafloor, a decline in benthic denitrification, and enhanced weathering of emerged shelf sediments. In turn, they led to a distinct rise in the standing stocks of DIC, TA, and nutrients in the global ocean, promoted the glacial sequestration of atmospheric CO2 in the ocean, and added 13C- and 14C-depleted DIC to the ocean as recorded in benthic foraminifera signals. The other half of the glacial drop in pCO2 was linked to reduced deep ocean dynamics, a shoaling of Atlantic meridional overturning circulation, and a rise in iron fertilization. The increased transit time of deep waters in the glacial ocean led to significant 14C depletions with respect to the atmosphere. The deglacial rapid and stepwise rise in atmospheric pCO2 was induced by upwelling both in the Southern Ocean and subarctic North Pacific and promoted by a drop in dust-borne iron discharge to the Southern Ocean. The deglacial sea-level rise led to a gradual decline in nutrient, DIC, and TA stocks, a slow change due to the large size and extended residence times of dissolved chemical species in the ocean. Thus, the rapid deglacial rise in pCO2 was dominated by fast changes in ocean dynamics and reduced dust deposition whereas the gradual pCO2 rise over the Holocene may be linked to the slow drop in nutrient and TA stocks that continued to promote an ongoing CO2 transfer from the ocean into the atmosphere.

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