Effects of Mountains and Ice Sheets on Global Ocean Circulation*

2011 ◽  
Vol 24 (11) ◽  
pp. 2814-2829 ◽  
Author(s):  
Andreas Schmittner ◽  
Tiago A. M. Silva ◽  
Klaus Fraedrich ◽  
Edilbert Kirk ◽  
Frank Lunkeit
Keyword(s):  
Ocean Circulation ◽  
Deep Water ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Deep Water Formation ◽  
The Pacific ◽  
Water Formation ◽  
Meridional Overturning

Abstract The impact of mountains and ice sheets on the large-scale circulation of the world’s oceans is investigated in a series of simulations with a new coupled ocean–atmosphere model [Oregon State University–University of Victoria model (OSUVic)], in which the height of orography is scaled from 1.5 times the actual height (at T42 resolution) to 0 (no mountains). The results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation. Higher mountains reduce water vapor transport from the Pacific and Indian Oceans into the Atlantic Ocean and contribute to increased (decreased) salinities and enhanced (reduced) deep-water formation and meridional overturning circulation in the Atlantic (Pacific). Orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress. The presence of the Antarctic ice sheet cools winter air temperatures by more than 20°C directly above the ice sheet and sets up a polar meridional overturning cell in the atmosphere. The resulting increased meridional temperature gradient strengthens midlatitude westerlies by ~25% and shifts them poleward by ~10°. This leads to enhanced and poleward-shifted upwelling of deep waters in the Southern Ocean, a stronger Antarctic Circumpolar Current, increased poleward atmospheric moisture transport, and more advection of high-salinity Indian Ocean water into the South Atlantic. Thus, it is the current configuration of mountains and ice sheets on earth that determines the difference in deep-water formation between the Atlantic and the Pacific.

scholarly journals Arctic–North Atlantic Interactions and Multidecadal Variability of the Meridional Overturning Circulation

2005 ◽  
Vol 18 (19) ◽  
pp. 4013-4031 ◽  
Author(s):  
Johann H. Jungclaus ◽  
Helmuth Haak ◽  
Mojib Latif ◽  
Uwe Mikolajewicz
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Multidecadal Variability ◽  
Water Formation ◽  
Meridional Overturning

Abstract Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation.

scholarly journals The global ocean circulation on a retrograde rotating earth

2011 ◽  
Vol 7 (2) ◽  
pp. 487-499 ◽  
Author(s):  
V. Kamphuis ◽  
S. E. Huisman ◽  
H. A. Dijkstra
Keyword(s):  
Ocean Circulation ◽  
Deep Water ◽  
North Pacific ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Global Ocean Circulation ◽  
The North Pacific

Abstract. To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterised by an Atlantic Meridional Overturning Circulation (MOC) with a deep water formation at northern latitudes and the absence of such a deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the Atlantic as a whole is a basin with net evaporation, while the Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed, but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is relatively weak but intermittently strong deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model the stability properties of the Atlantic MOC on a retrograde rotating earth are also investigated, showing a similar regime of multiple equilibria as in the present-day case. These results indicate that the present-day asymmetry in surface freshwater flux is not the most important factor setting the Atlantic-Pacific salinity difference and, thereby, the asymmetry in the global MOC.

scholarly journals The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation

Ocean Science ◽  
2014 ◽  
Vol 10 (2) ◽  
pp. 227-241 ◽  
Author(s):  
K. Lohmann ◽  
J. H. Jungclaus ◽  
D. Matei ◽  
J. Mignot ◽  
M. Menary ◽  
...  
Keyword(s):  
Deep Water ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Deep Water Formation ◽  
Multidecadal Variability ◽  
Nordic Seas ◽  
Denmark Strait ◽  
Water Formation ◽  
Meridional Overturning

Abstract. We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is partly done by analysing long (order of 1000 years) control simulations with five coupled climate models. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55 to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland Ridge. The latter might, however, be underestimated, as the models in general do not realistically simulate the flow path of the Iceland–Scotland overflow water south of the Iceland–Scotland Ridge. The influence of variations in subpolar deep water formation is, on multimodel average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multimodel average, about half and one-third respectively of the decadal to multidecadal AMOC variance. Apart from analysing multimodel control simulations, we have performed sensitivity experiments with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. The sensitivity experiments indicate that variations in subpolar deep water formation and Nordic Seas overflows are not completely independent. We further conclude from these experiments that the decadal to multidecadal AMOC variability north of about 50° N is mainly related to variations in Nordic Seas overflows. At 45° N and south of this latitude, variations in both subpolar deep water formation and Nordic Seas overflows contribute to the AMOC variability, with neither of the processes being very dominant compared to the other.

Inference of Submarine Meltwater in the Boundary Current System around Greenland in 2015-2019

2021 ◽  
Author(s):  
Dagmar Kieke ◽  
Oliver Huhn ◽  
Christian Mertens ◽  
Monika Rhein ◽  
Reiner Steinfeldt ◽  
...  
Keyword(s):  
Deep Water ◽  
Large Scale ◽  
Noble Gas ◽  
Current System ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Deep Water Formation ◽  
Boundary Current ◽  
Water Formation

<p>Melting of the Greenland Ice Sheet is one of the major causes that adds to the ice sheet mass loss and subsequently to the global sea level rise. The accelerated melting observed in recent decades is mainly caused by surface melting due to atmospheric warming and submarine melting caused by the increased inflow of warm Atlantic Water into the glacier-inhabited fjords of Greenland. This water reaches the front of marine terminating glaciers or the base of floating ice tongues inducing submarine melting. However, knowledge about submarine melt rates is limited and often inferred from indirect or remote sensing methods. Open questions exist regarding the processes that control the interaction of the oceans with marine terminating glaciers and the subsequent pathway of glacially modified water. The increasing release of this meltwater into the ocean is expected to have an impact on the deep water formation in the North Atlantic causing it to decrease. Since the deep water formation and spreading contribute to the deep limb of the Atlantic Meridional Overturning Circulation, identifying, tracking, and quantifying the oceanic submarine meltwater content and its variability is of high interest. The noble gases helium and neon provide a useful tool to identify and to quantify the fraction of glacially modified water in the oceanic water column. In this study we evaluate hydrographic, velocity and noble gas measurements from a number of cruises conducted across the boundary current system around Greenland between 2015 and 2019. With focus on the East and West Greenland Current systems, we aim at obtaining a large-scale view on the submarine meltwater distribution around Greenland and discuss the different regional regimes in two Greenlandic fjord systems and the boundary current around Greenland.</p>

scholarly journals AMOC recovery in a multi-centennial scenario using a coupled atmosphere-ocean-ice sheet model

2020 ◽  
Author(s):  
Lars Ackermann ◽  
Paul Gierz ◽  
Gerrit Lohmann
Keyword(s):  
Deep Water ◽  
Climate Models ◽  
Ice Sheet ◽  
Coupled Model ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Deep Water Formation ◽  
Future Global Warming ◽  
Water Formation

<p>Future global warming will affect ocean conditions by different mechanisms. One mechanism is the melting of the Greenland Ice Sheet (GIS), which may lead to a freshening of regions of deep water formation and eventually contribute to a possible slowdown of the Atlantic Meridional Overturning Circulation (AMOC). We simulate the two Coupled Model Intercomparison Project (CMIP) scenarios RCP4.5 and RCP8.5, to assess the effects of melt-induced fresh water on the AMOC. We use a newly developed coupled multi-resolution atmosphere-ocean-ice sheet model with high resolution at the coasts resolving the complex ocean dynamics. Our results show an AMOC recovery for both scenarios in simulations run with and without an included ice sheet model. We find that the ice sheet is not only acting as a source of freshwater to the ocean but also as a sink. This leads to local storage and redistribution of freshwater and largely compensates for the meltwater release. This physical consistency is missing in climate models without dynamic ice sheets. Therefore, we argue that freshwater hosing experiments should be assessed critically, as they might overestimate the North Atlantic freshening, induced by ice sheet melting. Because of the compensating effect, we find little effect of the included ice sheet model on the AMOC. Our results show a main freshwater release in West Greenland. There, the freshwater might be trapped in the Labrador Current and transported away from regions of deep water formation. Our results show an AMOC recovery, starting within the first half of the 22nd century. We assume the increase in net evaporation over the Atlantic and the resulting increase in ocean salinity, to be the main driver of this recovery.</p>

scholarly journals Breaking the Linkage Between Labrador Sea Water Production and Its Advective Export to the Subtropical Gyre

2016 ◽  
Vol 46 (7) ◽  
pp. 2169-2182 ◽  
Author(s):  
Sijia Zou ◽  
M. Susan Lozier
Keyword(s):  
Deep Water ◽  
Sea Water ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Subtropical Gyre ◽  
Global Ocean ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
Water Formation ◽  
Labrador Sea Water

AbstractDeep water formation in the northern North Atlantic has been of long-standing interest because the resultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not collectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.

scholarly journals Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

2017 ◽  
Vol 98 (4) ◽  
pp. 737-752 ◽  
Author(s):  
M. Susan Lozier ◽  
Sheldon Bacon ◽  
Amy S. Bower ◽  
Stuart A. Cunningham ◽  
M. Femke de Jong ◽  
...  
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Deep Water ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Intergovernmental Panel ◽  
Water Formation ◽  
Meridional Overturning

Abstract For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Atlantic Dominance of the Meridional Overturning Circulation

2008 ◽  
Vol 38 (2) ◽  
pp. 435-450 ◽  
Author(s):  
A. M. de Boer ◽  
J. R. Toggweiler ◽  
D. M. Sigman
Keyword(s):  
Deep Water ◽  
Hydrological Cycle ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Ocean Temperature ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Salinity Gradients ◽  
Water Formation ◽  
Meridional Overturning

Abstract North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.

scholarly journals Coupled ice sheet–climate modeling under glacial and pre-industrial boundary conditions

2014 ◽  
Vol 10 (1) ◽  
pp. 563-624
Author(s):  
F. A. Ziemen ◽  
C. B. Rodehacke ◽  
U. Mikolajewicz
Keyword(s):  
Boundary Conditions ◽  
Deep Water ◽  
General Circulation ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Deep Water Formation ◽  
Ice Streams ◽  
Proxy Data ◽  
Ice Stream ◽  
Water Formation

Abstract. We studied the climate of the last glacial maximum (LGM) in a set of coupled ice sheet–climate model experiments. They are based on the standard Paleoclimate Modelling Intercomparison Project Phase 2 (PMIP-2) experiments and extend the PMIP-2 (and PMIP-3) protocol by explicitly modeling the ice sheets. This adds a new layer of complexity and yields a set of ice sheets and climate that interact and are consistent with each other. We studied the behavior of the ice sheets and the climate system and compared our results to proxy data. The setup consists of the atmosphere-ocean-vegetation general circulation model ECHAM5/MPIOM/LPJ bidirectionally coupled with the Parallel Ice Sheet Model (PISM). We validated the setup by comparing the LGM experiment results with proxy data and by performing a pre-industrial control run. In both cases, the results agree reasonably well with reconstructions and observations. This shows that the model system adequately represents large, non-linear climate perturbations. Under LGM boundary conditions, the surface air temperature decreases by 3.5 K, and the precipitation north of 45° N by 0.12 m yr−1 (−18%) compared to the pre-industrial conditions. The North Atlantic Deep Water cell strengthens from 17.0 to 22.1 Sv (1 Sv = 106 m3 s−1) and the deep water formation shifts from the Labrador and GIN Seas to southeast of Iceland. Under LGM boundary conditions, different ice sheet configurations imply different locations of deep water formation. The major ice streams form in topographic troughs. In large parts, the modeled ice stream locations agree with sedimentary seafloor deposits. Most ice streams show recurring surges. The Hudson Strait Ice Stream surges with an ice volume equivalent to about 5 m sea level and a recurrence interval of about 7000 yr.

scholarly journals The global ocean circulation on a retrograde rotating earth

2010 ◽  
Vol 6 (6) ◽  
pp. 2455-2482
Author(s):  
V. Kamphuis ◽  
S. E. Huisman ◽  
H. A. Dijkstra
Keyword(s):  
Ocean Circulation ◽  
Deep Water ◽  
North Pacific ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Global Ocean Circulation ◽  
The North Pacific

Abstract. To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterized by an Atlantic Meridional Overturning Circulation (MOC) with deep water formation at northern latitudes and the absence of such deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the North Atlantic is a basin with net evaporation, while the North Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is strong and highly variable deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model also the stability properties of the Atlantic MOC on a retrograde rotating earth are investigated, showing a similar regime of multiple equilibria as in the present-day case. These results demonstrate that the present-day asymmetry in surface freshwater flux is not a crucial factor for the Atlantic-Pacific asymmetry in the global MOC.

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