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 Water mass evolution of the Greenland Sea since late glacial times

2014 ◽  
Vol 10 (1) ◽  
pp. 123-136 ◽  
Author(s):  
M. M. Telesiński ◽  
R. F. Spielhagen ◽  
H. A. Bauch
Keyword(s):  
Sea Ice ◽  
Water Mass ◽  
Environmental Variability ◽  
Deep Convection ◽  
Sediment Cores ◽  
Late Glacial ◽  
Atlantic Water ◽  
Water Masses ◽  
Greenland Sea ◽  
The Holocene

Abstract. Four sediment cores from the central and northern Greenland Sea basin, a crucial area for the renewal of North Atlantic deep water, were analyzed for planktic foraminiferal fauna, planktic and benthic stable oxygen and carbon isotopes as well as ice-rafted debris to reconstruct the environmental variability in the last 23 kyr. During the Last Glacial Maximum, the Greenland Sea was dominated by cold and sea-ice bearing surface water masses. Meltwater discharges from the surrounding ice sheets affected the area during the deglaciation, influencing the water mass circulation. During the Younger Dryas interval the last major freshwater event occurred in the region. The onset of the Holocene interglacial was marked by an increase in the advection of Atlantic Water and a rise in sea surface temperatures (SST). Although the thermal maximum was not reached simultaneously across the basin, benthic isotope data indicate that the rate of overturning circulation reached a maximum in the central Greenland Sea around 7 ka. After 6–5 ka a SST cooling and increasing sea-ice cover is noted. Conditions during this so-called "Neoglacial" cooling, however, changed after 3 ka, probably due to enhanced 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 reveal that the Holocene oceanographic conditions in the Greenland Sea did not develop uniformly. These variations were a response to a complex interplay between the Atlantic and Polar water masses, the rate of sea-ice formation and melting and its effect on vertical convection intensity during times of Northern Hemisphere insolation changes.

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

Antarctic Climate History and Global Climate Changes

2017 ◽  
Author(s):  
Pontus Lurcock ◽  
Fabio Florindo
Keyword(s):  
Ocean Circulation ◽  
Climate Changes ◽  
Numerical Models ◽  
Global Climate ◽  
Sediment Cores ◽  
Ice Sheets ◽  
Global Ocean ◽  
Climate History ◽  
Global Climate Changes ◽  
Planetary Albedo

Antarctic climate changes have been reconstructed from ice and sediment cores and numerical models (which also predict future changes). Major ice sheets first appeared 34 million years ago (Ma) and fluctuated throughout the Oligocene, with an overall cooling trend. Ice volume more than doubled at the Oligocene-Miocene boundary. Fluctuating Miocene temperatures peaked at 17–14 Ma, followed by dramatic cooling. Cooling continued through the Pliocene and Pleistocene, with another major glacial expansion at 3–2 Ma. Several interacting drivers control Antarctic climate. On timescales of 10,000–100,000 years, insolation varies with orbital cycles, causing periodic climate variations. Opening of Southern Ocean gateways produced a circumpolar current that thermally isolated Antarctica. Declining atmospheric CO2 triggered Cenozoic glaciation. Antarctic glaciations affect global climate by lowering sea level, intensifying atmospheric circulation, and increasing planetary albedo. Ice sheets interact with ocean water, forming water masses that play a key role in global ocean circulation.

scholarly journals Relative timing of precipitation and ocean circulation changes in the western equatorial Atlantic over the last 45 kyr

2018 ◽  
Vol 14 (9) ◽  
pp. 1315-1330 ◽  
Author(s):  
Claire Waelbroeck ◽  
Sylvain Pichat ◽  
Evelyn Böhm ◽  
Bryan C. Lougheed ◽  
Davide Faranda ◽  
...  
Keyword(s):  
Mass Transport ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Wavelet Transforms ◽  
Water Mass ◽  
Sediment Cores ◽  
Equatorial Atlantic ◽  
The North ◽  
Atmospheric Changes ◽  
Circulation Changes

Abstract. Thanks to its optimal location on the northern Brazilian margin, core MD09-3257 records both ocean circulation and atmospheric changes. The latter occur locally in the form of increased rainfall on the adjacent continent during the cold intervals recorded in Greenland ice and northern North Atlantic sediment cores (i.e., Greenland stadials). These rainfall events are recorded in MD09-3257 as peaks in ln(Ti ∕ Ca). New sedimentary Pa ∕ Th data indicate that mid-depth western equatorial water mass transport decreased during all of the Greenland stadials of the last 40 kyr. Using cross-wavelet transforms and spectrogram analysis, we assess the relative phase between the MD09-3257 sedimentary Pa ∕ Th and ln(Ti ∕ Ca) signals. We show that decreased water mass transport between a depth of ∼1300 and 2300 m in the western equatorial Atlantic preceded increased rainfall over the adjacent continent by 120 to 400 yr at Dansgaard–Oeschger (D–O) frequencies, and by 280 to 980 yr at Heinrich-like frequencies. We suggest that the large lead of ocean circulation changes with respect to changes in tropical South American precipitation at Heinrich-like frequencies is related to the effect of a positive feedback involving iceberg discharges in the North Atlantic. In contrast, the absence of widespread ice rafted detrital layers in North Atlantic cores during D–O stadials supports the hypothesis that a feedback such as this was not triggered in the case of D–O stadials, with circulation slowdowns and subsequent changes remaining more limited during D–O stadials than Heinrich stadials.

Eddy formation by dense flows on slopes in a rotating fluid

1998 ◽  
Vol 363 ◽  
pp. 229-252 ◽  
Author(s):  
GREGORY F. LANE-SERFF ◽  
PETER G. BAINES
Keyword(s):  
Ocean Circulation ◽  
Laboratory Experiments ◽  
Lower Layer ◽  
Ekman Layer ◽  
Global Ocean ◽  
Circulation System ◽  
Geostrophic Adjustment ◽  
Viscous Effects ◽  
Dense Fluid ◽  
Global Ocean Circulation

Properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated with a variety of laboratory experiments. The dense fluid may initially flow down the slope but it turns (under the influence of rotation) to flow along the slope, and initial geostrophic adjustment gives it an anticyclonic velocity profile. Some of the dense fluid drains downslope in a viscous Ekman layer, which may become unstable to growing waves. Provided that the viscous draining is not too strong, cyclonic vortices form periodically in the upper layer and the dense flow breaks up into a series of domes. Three processes may contribute to the formation of these eddies. First, initial downslope flow of the dense current may stretch columns of ambient fluid by the ‘Taylor column’ process (which we term ‘capture’). Secondly, the initial geostrophic adjustment implies lower-layer collapse which may stretch the fluid column, and thirdly, viscous drainage will progressively stretch and spin up a captured water column. Overall this last process may be the most significant, but viscous drainage has contradictory effects, in that it progressively removes dense lower-layer fluid which terminates the process when the layer thickness approaches that of the Ekman layer. The eddies produced propagate along the slope owing to the combined effects of buoyancy–Coriolis balance and ‘beta-gyres’. This removes fluid from the vicinity of the source and causes the cycle to repeat. The vorticity of the upper-layer cyclones increases linearly with Γ=Lα/D (where L is the Rossby deformation radius, α the bottom slope and D the total depth), reaching approximately 2f in the experiments presented here. The frequency at which the eddy/dome structures are produced also increases with Γ, while the speed at which the structures propagate along the slope is reduced by viscous effects. The flow of dense fluid on slopes is a very important part of the global ocean circulation system and the implications of the laboratory experiments for oceanographic flows are discussed.

scholarly journals North Atlantic deep water formation and AMOC in CMIP5 models

Ocean Science ◽  
2017 ◽  
Vol 13 (4) ◽  
pp. 609-622 ◽  
Author(s):  
Céline Heuzé
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Deep Water ◽  
Deep Convection ◽  
The Arctic ◽  
Subpolar Gyre ◽  
Deep Water Formation ◽  
Nordic Seas ◽  
Water Formation

Abstract. Deep water formation in climate models is indicative of their ability to simulate future ocean circulation, carbon and heat uptake, and sea level rise. Present-day temperature, salinity, sea ice concentration and ocean transport in the North Atlantic subpolar gyre and Nordic Seas from 23 CMIP5 (Climate Model Intercomparison Project, phase 5) models are compared with observations to assess the biases, causes and consequences of North Atlantic deep convection in models. The majority of models convect too deep, over too large an area, too often and too far south. Deep convection occurs at the sea ice edge and is most realistic in models with accurate sea ice extent, mostly those using the CICE model. Half of the models convect in response to local cooling or salinification of the surface waters; only a third have a dynamic relationship between freshwater coming from the Arctic and deep convection. The models with the most intense deep convection have the warmest deep waters, due to a redistribution of heat through the water column. For the majority of models, the variability of the Atlantic Meridional Overturning Circulation (AMOC) is explained by the volumes of deep water produced in the subpolar gyre and Nordic Seas up to 2 years before. In turn, models with the strongest AMOC have the largest heat export to the Arctic. Understanding the dynamical drivers of deep convection and AMOC in models is hence key to realistically forecasting Arctic oceanic warming and its consequences for the global ocean circulation, cryosphere and marine life.

Zonal Wave 3 Pattern in the Southern Hemisphere generated by tropical convection

2021 ◽  
Author(s):  
Rishav Goyal ◽  
Martin Jucker ◽  
Alex Sen Gupta ◽  
Harry Hendon ◽  
Matthew England
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Hadley Cell ◽  
Wave Source ◽  
The Tropics

Abstract A distinctive feature of the Southern Hemisphere (SH) extratropical atmospheric circulation is the quasi-stationary zonal wave 3 (ZW3) pattern, characterized by three high and three low-pressure centers around the SH extratropics. This feature is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the ZW3 pattern has significant impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the existence of three major land masses in the SH extratropics. Here we use an atmospheric general circulation model to show that the stationery ZW3 pattern is instead driven by zonal asymmetric deep atmospheric convection in the tropics, with little to no role played by the orography or land masses in the extratropics. Localized regions of deep convection in the tropics form a local Hadley cell which in turn creates a wave source in the subtropics that excites a poleward and eastward propagating wave train which forms stationary waves in the SH high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, will impact the zonal wave 3 pattern, with implications for Southern Hemisphere climate, ocean circulation, and sea-ice.

Evaluating Met Office uncoupled and coupled forecasts during the Iceland Greenland Seas Project field campaign in 2018

2021 ◽  
Author(s):  
Chris Barrell ◽  
Ian Renfrew ◽  
Steven Abel ◽  
Andrew Elvidge ◽  
John King
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
Rapid Change ◽  
Coupled Model ◽  
Vital Role ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Near Surface ◽  
Field Campaign ◽  
Important Region

<div> <p>During a cold-air outbreak (CAO) a cold polar airmass flows from the frozen land or ice surface, over the marginal ice zone (MIZ), then out over the comparatively warm open ocean. This constitutes a dramatic change in surface temperature, roughness and moisture availability, typically causing rapid change in the atmospheric boundary layer. Consequently, CAOs are associated with a range of severe mesoscale weather phenomena and accurate forecasting is crucial. Over the Nordic Seas CAOs also play a vital role in global ocean circulation, causing densification and sinking of ocean waters that form the headwaters of the Atlantic meridional overturning circulation. </p> </div><div> <p>To tackle the lack of observations during wintertime CAOs and improve scientific understanding in this important region, the Iceland Greenland Seas Project (IGP) undertook an extensive field campaign during February and March 2018. Aiming to characterise the atmospheric forcing and the ocean response, particularly in and around the MIZ, the IGP made coordinated ocean-atmosphere measurements, involving a research vessel, a research aircraft, a meteorological buoy, moorings, sea gliders and floats.  </p> </div><div> <p>The work presented here employs these novel observational data to evaluate output from the UK Met Office global operational forecasting system and from a pre-operational coupled ocean-ice-atmosphere system. The Met Office aim to transition to a coupled operational forecast in the coming years, thus verification of model versions in development is essential. Results show that this coupled model’s sea ice is generally more accurate than a persistent field. However, it can also suffer from cold-biased sea surface temperatures around the MIZ, which influences the modelled near-surface meteorology. Both these effects demonstrate the crucial importance of accurate sea ice simulation in coupled model forecasting in the high latitudes. Hence, an ice edge metric is then used to quantify the accuracy of the coupled model MIZ edge at two ocean grid resolutions. </p> </div>

scholarly journals Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

2018 ◽  
Author(s):  
Svein Østerhus ◽  
Rebecca Woodgate ◽  
Héðinn Valdimarsson ◽  
Bill Turrell ◽  
Laura de Steur ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Bering Strait ◽  
Data Set ◽  
Considerable Uncertainty ◽  
Denmark Strait

Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Into this region, water enters through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow) and then modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat, salt, and other substances into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, has the observational coverage become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993–2015. The total AM-import (oceanic inflows plus freshwater) is found to be 9.1 ± 0.7 Sv (1 Sv = 106 m3 s−1) and has a seasonal variation of amplitude close to 1 Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remaining two thirds leaving as overflow. The overflow is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater, but is still ~ 2/3rds from modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s and this allows us to estimate trends for the AM-exchanges as a whole. At the 95 % level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM-inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM-exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the Meridional Overturning Circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM-exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflows through the Denmark Strait, the overflow across the Iceland-Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

scholarly journals Sustaining observations in the polar oceans

2014 ◽  
Vol 372 (2025) ◽  
pp. 20130337 ◽  
Author(s):  
E. P. Abrahamsen
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
Ice Cover ◽  
Global Ocean ◽  
Shelf Seas ◽  
Polar Oceans ◽  
Global Ocean Circulation ◽  
Coordination And Collaboration ◽  
Temperature Depth

Polar oceans present a unique set of challenges to sustained observations. Sea ice cover restricts navigation for ships and autonomous measurement platforms alike, and icebergs present a hazard to instruments deployed in the upper ocean and in shelf seas. However, the important role of the poles in the global ocean circulation provides ample justification for sustained observations in these regions, both to monitor the rapid changes taking place, and to better understand climate processes in these traditionally poorly sampled areas. In the past, the vast majority of polar measurements took place in the summer. In recent years, novel techniques such as miniature CTD (conductivity–temperature–depth) tags carried by seals have provided an explosion in year-round measurements in areas largely inaccessible to ships, and, as ice avoidance is added to autonomous profiling floats and gliders, these promise to provide further enhancements to observing systems. In addition, remote sensing provides vital information about changes taking place in sea ice cover at both poles. To make these observations sustainable into the future, improved international coordination and collaboration is necessary to gain optimum utilization of observing networks.

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