The AMOC at 47° North in Observations and a High Resolution Ocean Model

2020 ◽  
Author(s):  
Simon Wett ◽  
Monika Rhein ◽  
Arne Biastoch ◽  
Claus W. Böning ◽  
Klaus Getzlaff
Keyword(s):  
High Resolution ◽  
Global Climate ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Model Simulations ◽  
Main Driver ◽  
First Results ◽  
Continuous Current ◽  
Meridional Overturning ◽  
To Come

<p>The Atlantic Meridional Overturning Circulation (AMOC) is the main driver of northward oceanic volume and heat transport in the Atlantic. Due to its definition via the streamfunction the exact calculation of the AMOC requires knowledge of the full velocity field. Since the early 2000s, observations of the AMOC are available at 47° North in the form of hydrographic sections across the Atlantic and continuous current measurements from moored instruments at specific locations. However, the spatial resolution of current measurements is coarse and shipbased hydrographic sections are mostly done only once a year. Also the observational timeseries still remain too short to come to conclusions about decadal trends in the AMOC variability. Thus, today our knowledge about the role of the AMOC in the global climate system is mainly based on model simulations. Comparing these model simulations against observations remains an important task to accurately predict the future of the AMOC and adapt to changes.</p><p>We present first results of a model observations comparison in the subpolar North Atlantic between observations at 47° North and the high resolution ocean model VIKING20X. The model has a 1/20° nest in the Atlantic embedded in a global 1/4° model. It covers the years from 1980 to 2018 and thus overlaps with the whole observational period. This comparison will help assessing different methods of estimating the AMOC strength from observations.</p>

AMOC Evolution at 47°N in the Last Decades in Observations and a High-Resolution Ocean Model

2021 ◽  
Author(s):  
Simon Wett ◽  
Monika Rhein ◽  
Arne Biastoch ◽  
Claus Böning ◽  
Klaus Getzlaff
Keyword(s):  
High Resolution ◽  
Model Simulation ◽  
Saline Water ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Model Simulations ◽  
Improve Model ◽  
Meridional Overturning ◽  
The Individual

<p><span>The Atlantic Meridional Overturning Circulation (AMOC) plays an important role for the climate system of Europe and the Arctic. It is responsible for the northward transport of warm and saline water in the upper water column and the southward transport of cold and fresh water in the deep.</span></p><p><span>Since the early 2000s, observations from ship-based measurements and moorings are available which allow estimates of the individual components of the AMOC. However, the spatial resolution of mooring measurements is coarse and ship-based surveys are mostly done only once a year, adding to the uncertainty of these measurements. Earlier observational studies in the subpolar North Atlantic have found decadal trends of individual AMOC components. However, whether the entirety of the AMOC exhibits a trend remains unclear. Due to the observational limitations, most knowledge about the recent AMOC development is based on model simulations. Comparing these model simulations with observations remains an important task to understand the changes in the AMOC strength in the last decades and improve model representations of the AMOC.</span></p><p><span>We analyze a realization of the high-resolution VIKING20X ocean model from 1980 to 2019 offering a large overlap with the available observations. We compare it to measurements of the NOAC array at 47°N and sections obtained from repeated ship surveys. We aim to merge observations and model simulation to better estimate recent AMOC changes and increase our understanding of the underlying processes.</span></p>

Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic

2021 ◽  
Author(s):  
Claus W. Böning ◽  
Arne Biastoch ◽  
Klaus Getzlaff ◽  
Patrick Wagner ◽  
Siren Rühs ◽  
...  
Keyword(s):  
High Resolution ◽  
North Atlantic ◽  
Deep Convection ◽  
Alternative Interpretation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
First Results ◽  
Meridional Overturning

<p>A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by “sinking” (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.</p>

Preliminary biostratigraphy of IODP Expedition 383 sites

2021 ◽  
Author(s):  
Mariem Saavedra-Pellitero ◽  
Anieke Brombacher ◽  
Oliver Esper ◽  
Alexandre de Souza ◽  
Elisa Malinverno ◽  
...  
Keyword(s):  
High Resolution ◽  
Water Mass ◽  
Global Climate ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
The Pacific ◽  
Meridional Overturning ◽  
Circumpolar Current ◽  
Oceanic Carbon ◽  
The Antarctic

<p>The Antarctic Circumpolar Current (ACC) is a major driver of global climate. It connects all three ocean basins, integrating global climate variability, and its vertical water mass structure plays a key role in oceanic carbon storage. The Atlantic and Indian sectors of the ACC are well studied, but the Pacific sector lacks deep-sea drilling records. Therefore, past water mass transport through the Drake Passage and its effect on Atlantic Meridional Overturning Circulation are not well understood. To fill this gap, IODP Expedition 383 recovered sediments from three sites in the central South Pacific and three sites from the southern Chilean Margin.</p><p>Here we present the preliminary biostratigraphy developed during the expedition. The sediments contained abundant nannofossils, foraminifera, radiolarians, diatoms and silicoflagellates which produced age models that were in excellent agreement with the shipboard magnetostratigraphy. Two sites contain high-resolution Pleistocene records, one site goes back to the Pliocene, and two others reach back to the late Miocene. Post-cruise research will further refine these age models through high-resolution bio-, magneto- and oxygen isotope stratigraphies that are currently being generated.</p>

scholarly journals Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model

Ocean Science ◽  
2019 ◽  
Vol 15 (3) ◽  
pp. 489-512 ◽  
Author(s):  
Siren Rühs ◽  
Franziska U. Schwarzkopf ◽  
Sabrina Speich ◽  
Arne Biastoch
Keyword(s):  
High Resolution ◽  
Upper Limb ◽  
Atlantic Meridional Overturning Circulation ◽  
Ocean Model ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
The South ◽  
Overturning Circulation ◽  
The Pacific ◽  
Meridional Overturning

Abstract. The northward flow of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC) system and by waters entering from the Pacific through Drake Passage (DP), commonly referred to as the “warm” and “cold” water routes, respectively. However, there is no final consensus on the relative importance of these two routes for the upper limb's volume transport and thermohaline properties. In this study we revisited the AC and DP contributions by performing Lagrangian analyses between the two source regions and the North Brazil Current (NBC) at 6∘ S in a realistically forced high-resolution (1∕20∘) ocean model. Our results agree with the prevailing conception that the AC contribution is the major source for the upper limb transport of the AMOC in the tropical South Atlantic. However, they also suggest a non-negligible DP contribution of around 40 %, which is substantially higher than estimates from previous Lagrangian studies with coarser-resolution models but now better matches estimates from Lagrangian observations. Moreover, idealized analyses of decadal changes in the DP and AC contributions indicate that the ongoing increase in Agulhas leakage indeed may have induced an increase in the AC contribution to the upper limb of the AMOC in the tropics, while the DP contribution decreased. In terms of thermohaline properties, our study highlights the fact that the AC and DP contributions cannot be unambiguously distinguished by their temperature, as the commonly adopted terminology may imply, but rather by their salinity when entering the South Atlantic. During their transit towards the NBC the bulk of DP waters experiences a net density loss through a net warming, whereas the bulk of AC waters experiences a slight net density gain through a net increase in salinity. Notably, these density changes are nearly completely captured by Lagrangian particle trajectories that reach the surface mixed layer at least once during their transit, which amount to 66 % and 49 % for DP and AC waters, respectively. This implies that more than half of the water masses supplying the upper limb of the AMOC are actually formed within the South Atlantic and do not get their characteristic properties in the Pacific and Indian Oceans.

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

2021 ◽  
Author(s):  
Tomas Jonathan ◽  
Mike Bell ◽  
Helen Johnson ◽  
David Marshall
Keyword(s):  
Global Climate ◽  
Dominant Role ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Relative Role ◽  
Overturning Circulation ◽  
Near Surface ◽  
Boundary Density ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. </p><p>The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.</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.

scholarly journals Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

2013 ◽  
Vol 9 (4) ◽  
pp. 4771-4806 ◽  
Author(s):  
L. Menviel ◽  
A. Timmermann ◽  
T. Friedrich ◽  
M. H. England
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
External Boundary ◽  
3 Dimensional ◽  
Ice Volume ◽  
Freshwater Fluxes ◽  
Meridional Overturning ◽  
The Continuum

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger (DO) and Heinrich events (HE) is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-dimensional intermediate complexity Earth system model covering the period 50 ka BP to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing Northern Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our analysis is consistent with the idea that variations in ice sheet calving and related changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of DO and HE variability seen in paleorecords across the globe.

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