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.

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

2017 ◽  
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 turns, 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 forecast Arctic oceanic warming and its consequences on the global ocean circulation, cryosphere and marine life.

Influence of Nordic Seas dynamics on the Atlantic Water propagation and its impacts on sea ice concentration.

2021 ◽  
Author(s):  
Sourav Chatterjee ◽  
Roshin P Raj ◽  
Laurent Bertino ◽  
Nuncio Murukesh
Keyword(s):  
Sea Ice ◽  
Deep Water ◽  
Atlantic Water ◽  
The Arctic ◽  
Deep Water Formation ◽  
Sea Ice Concentration ◽  
Nordic Seas ◽  
Ice Concentration ◽  
Water Formation

<p>Enhanced intrusion of warm and saline Atlantic Water (AW) to the Arctic Ocean (AO) in recent years has drawn wide interest of the scientific community owing to its potential role in ‘Arctic Amplification’. Not only the AW has warmed over the last few decades , but its transfer efficiency have also undergone significant modifications due to changes in atmosphere and ocean dynamics at regional to large scales. The Nordic Seas (NS), in this regard, play a vital role as the major exchange of polar and sub-polar waters takes place in this region. Further, the AW and its significant modification on its way to AO via the Nordic Seas has large scale implications on e.g., deep water formation, air-sea heat fluxes. Previous studies have suggested that a change in the sub-polar gyre dynamics in the North Atlantic controls the AW anomalies that enter the NS and eventually end up in the AO. However, the role of NS dynamics in resulting in the modifications of these AW anomalies are not well studied. Here in this study, we show that the Nordic Seas are not only a passive conduit of AW anomalies but the ocean circulations in the Nordic Seas, particularly the Greenland Sea Gyre (GSG) circulation can significantly change the AW characteristics between the entry and exit point of AW in the NS. Further, it is shown that the change in GSG circulation can modify the AW heat distribution in the Nordic Seas and can potentially influence the sea ice concentration therein. Projected enhanced atmospheric forcing in the NS in a warming Arctic scenario and the warming trend of the AW can amplify the role of NS circulation in AW propagation and its impact on sea ice, freshwater budget and deep water formation.</p>

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.

Assessing the stability of the AMOC during past warm climates

2021 ◽  
Author(s):  
Megan Murphy O' Connor ◽  
Christophe Colin ◽  
Audrey Morley
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
Nordic Seas ◽  
Water Formation ◽  
Rockall Trough ◽  
Eastern North Atlantic ◽  
Warm Climates

<p>There is emergent evidence that abrupt shifts of the Atlantic Meridional Overturning Circulation (AMOC) have occurred during interglacial periods, with recent observations and model simulations showing that we may have over-estimated its stability during warm climates. In this study, we present a multi-proxy reconstruction of deep-water characteristics from the Rockall Trough in the Eastern North Atlantic to assess the variability of Nordic seas and Labrador Sea deep-water formation during past interglacial periods MIS 1, 5, 11, and 19. To test the warm climate stability hypothesis and to constrain the variability of deep-water formation for past warm climates, we performed geochemical analysis on planktic (Nd isotopes) and benthic foraminifera (δ<sup>18</sup>O and δ<sup>13</sup>C) along with sedimentological analysis. This approach allows us to reconstruct paleocurrent flow strength, as well as the origin and contribution of different water masses to one of the deep-water components of the AMOC in the Rockall Trough. We found that deep-water properties varied considerably during each of our chosen periods. For example during the Holocene εNd variability is smaller (1.8 per mil) when compared to variability during MIS 19 (3.3 per mil), an interglacial that experienced very similar orbital boundary conditions. Our results confirm that deep-water variability in the eastern North Atlantic basin was more variable in previous interglacial periods when compared to our current Holocene and provide new insight into the relative contribution of Nordic Seas Deep Water and Labrador Sea Water in the Rockall trough.</p>

Constraining sources and relative flow rates of bottom waters in the Late Cretaceous Pacific Ocean

Geology ◽  
2020 ◽  
Vol 48 (5) ◽  
pp. 509-513 ◽  
Author(s):  
Shannon J. Haynes ◽  
Kenneth G. MacLeod ◽  
Jean-Baptiste Ladant ◽  
Andrew Vande Guchte ◽  
Masoud A. Rostami ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Deep Water ◽  
Deep Convection ◽  
Source Region ◽  
Source Area ◽  
Geochemical Data ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Bottom Waters

Abstract Geochemical data suggest that ocean circulation patterns changed over a period of long-term cooling during the last 10 m.y. of the Cretaceous (late Campanian–Maastrichtian). Proposed changes include enhanced deep-water formation in the South Atlantic and/or Indian sectors of the Southern Ocean, initiation or enhanced deep-water formation in the North Atlantic, and alternating regions of deep convection in the North and South Pacific. Existing geochemical data do not allow simple confirmation or rejection of any of these scenarios. To test Pacific circulation during the Maastrichtian, we measured neodymium isotopic (εNd) values from four Pacific Deep Sea Drilling Project and Ocean Drilling Program sites and compare results both to Earth system model simulations using Maastrichtian paleogeography and to previous studies. Pacific εNd results consistently show a small negative εNd excursion during a well-documented, ∼1–3 m.y. early Maastrichtian cooling pulse (EMCP) but no other consistent trends across the late Campanian–late Maastrichtian interval (∼10 m.y.). Model results show that different CO2 forcings lead to changes in rates, but not patterns, of circulation. These combined results support the existence of a sustained source region for intermediate and deep waters in the southwestern Pacific throughout the late Campanian–Maastrichtian and indicate that changes in εNd values during the EMCP reflect an increased rate of overturning in the Pacific rather than changes in the source area of Pacific bottom waters.

scholarly journals Freshening of the Labrador Sea as a trigger for Little Ice Age development

2017 ◽  
Vol 13 (4) ◽  
pp. 317-331 ◽  
Author(s):  
Montserrat Alonso-Garcia ◽  
Helga (Kikki) F. Kleiven ◽  
Jerry F. McManus ◽  
Paola Moffa-Sanchez ◽  
Wallace S. Broecker ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Deep Water ◽  
Little Ice Age ◽  
Ice Age ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
The North Atlantic

Abstract. Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea ice can have a large impact on ocean circulation dynamics in the North Atlantic, modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29° N, 48.37° W; 3432 m water depth). IRD from the fraction 63–150 µm shows particularly high concentrations during the intervals  ∼  AD 1000–1100,  ∼  1150–1250,  ∼  1400–1450,  ∼  1650–1700 and  ∼  1750–1800. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling that occurred during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes. This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.

scholarly journals Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models

2020 ◽  
Author(s):  
Céline Heuzé
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Convection ◽  
North Atlantic Deep Water ◽  
Global Ocean ◽  
Antarctic Bottom Water ◽  
Deep Water Formation ◽  
Water Formation

Abstract. Deep water formation is the driver of the global ocean circulation, yet it was poorly represented in the previous generation of climate models. We here quantify biases in Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) formation, properties, transport and global extent in 35 climate models that participated in the latest Climate Model Intercomparison Project (CMIP6). Several CMIP6 models are correctly forming AABW via shelf processes, but in both hemispheres, the large majority of climate models form deep water via open ocean deep convection, too deep, too often, over too large an area. Models that convect the least form the most accurate AABW, but the least accurate NADW. The four CESM2 models with their pipe/overflow parameterisation are among the most accurate models. In the Atlantic, the colder AABW, the stronger the abyssal overturning at 30° S, and the further north the AABW layer extends. The saltier NADW, the stronger the Atlantic Meridional Overturning Circulation (AMOC), and the further south the NADW layer extends. In the Indian and Pacific oceans in contrast, the fresher models are the ones who extend the furthest regardless of the strength of their abyssal overturning, most likely because they also are the models with the weakest fronts in the Antarctic Circumpolar Currents. There are clear improvements since CMIP5: several CMIP6 models correctly represent or parameterise Antarctic shelf processes, fewer models exhibit Southern Ocean deep convection, more models convect at the right location in the Labrador Sea, bottom density biases are reduced, and abyssal overturning is more realistic. But more improvements are required, e.g. by generalising the use of overflow parameterisations or by coupling to interactive ice sheet models, before deep water formation, and hence heat and carbon storage, are represented accurately.

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