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  • 1
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    Air‐Sea Interactions and Water Mass Transformation During a Katabatic Storm in the Irminger Sea (2022)
    Details
    Publication Date: 2022-10-04
    Description: We use a global 5‐km resolution model to analyze the air‐sea interactions during a katabatic storm in the Irminger Sea originating from the Ammassalik valleys. Katabatic storms have not yet been resolved in global climate models, raising the question of whether and how they modify water masses in the Irminger Sea. Our results show that dense water forms along the boundary current and on the shelf during the katabatic storm due to the heat loss caused by the high wind speeds and the strong temperature contrast. The dense water contributes to the lightest upper North Atlantic Deep Water as upper Irminger Sea Intermediate Water and thus to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The katabatic storm triggers a polar low, which in turn amplifies the near‐surface wind speed due to the superimposed pressure gradient, in addition to acceleration from a breaking mountain wave. Overall, katabatic storms account for up to 25% of the total heat loss (20 January 2020 to 30 September 2021) over the Irminger shelf of the Ammassalik area. Resolving katabatic storms in global models is therefore important for the formation of dense water in the western boundary current of the Irminger Sea, which is relevant to the AMOC, and for the large‐scale atmospheric circulation by triggering polar lows.
    Description: Plain Language Summary: Katabatic storms are outbursts of cold air associated with strong winds from coastal valleys of Greenland, in particular from the Ammassalik valleys in southeast Greenland. These storms are not resolved in global climate models because of their small spatial extent. However, they are important for the formation of dense water on the Irminger Sea shelf, because they induce a substantial heat loss from the coastal water. In this study, we resolve katabatic storms for the first time in a global climate model and analyze the water transformation caused by a single storm before quantifying the importance of katabatic storms for the entire simulation period. We find that a water mass is formed during the katabatic storm that is dense enough to contribute to the cooling and sinking of the global conveyor belt in the subpolar North Atlantic. Overall, katabatic storms account for up to 25% of the heat loss over the Irminger shelf of the Ammassalik area.
    Description: Key Points: For the first time, the direct effect of a katabatic storm on the Irminger Sea has been simulated in a global climate model. The katabatic storm induces strong heat loss and dense water formation over the Irminger shelf (Sermilik Trough) and in the boundary current. Dense water forming in the western boundary current during katabatic storms contributes to the lightest upper North Atlantic Deep Water.
    Description: Collaborative Research Centre TRR181 funded by DFG
    Description: Max Planck Society for Advancement of Science
    Description: NextGEMS
    Description: European Union’s Horizon 2020
    Description: https://hdl.handle.net/21.11116/0000-0008-ECF1-E
    Description: https://cera-www.dkrz.de/WDCC/ui/Compact.jsp?acronym=DKRZ_LTA_033_ds00010
    Description: https://mpimet.mpg.de/en/science/modeling-with-icon/code-availability
    Keywords: ddc:551.5
    Language: English
    Type: doc-type:article
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    CITATION GEO-LEO
  • 2
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    ICON‐O: The Ocean Component of the ICON Earth System Model—Global Simulation Characteristics and Local Telescoping Capability (2022)
    Details
    Publication Date: 2023-01-25
    Description: We describe the ocean general circulation model Icosahedral Nonhydrostatic Weather and Climate Model (ICON‐O) of the Max Planck Institute for Meteorology, which forms the ocean‐sea ice component of the Earth system model ICON‐ESM. ICON‐O relies on innovative structure‐preserving finite volume numerics. We demonstrate the fundamental ability of ICON‐O to simulate key features of global ocean dynamics at both uniform and non‐uniform resolution. Two experiments are analyzed and compared with observations, one with a nearly uniform and eddy‐rich resolution of ∼10 km and another with a telescoping configuration whose resolution varies smoothly from globally ∼80 to ∼10 km in a focal region in the North Atlantic. Our results show first, that ICON‐O on the nearly uniform grid simulates an ocean circulation that compares well with observations and second, that ICON‐O in its telescope configuration is capable of reproducing the dynamics in the focal region over decadal time scales at a fraction of the computational cost of the uniform‐grid simulation. The telescopic technique offers an alternative to the established regionalization approaches. It can be used either to resolve local circulation more accurately or to represent local scales that cannot be simulated globally while remaining within a global modeling framework.
    Description: Plain Language Summary: Icosahedral Nonhydrostatic Weather and Climate Model (ICON‐O) is a global ocean general circulation model that works on unstructured grids. It rests on novel numerical techniques that belong to the class of structure‐preserving finite Volume methods. Unstructured grids allow on the one hand a uniform coverage of the sphere without resolution clustering, and on the other hand they provide the freedom to intentionally cluster grid points in some region of interest. In this work we run ICON‐O on an uniform grid of approximately 10 km resolution and on a grid with four times less degrees of freedom that is stretched such that in the resulting telescoping grid within the North Atlantic the two resolutions are similar, while outside the focal area the grid approaches smoothly ∼80 km resolution. By comparison with observations and reanalysis data we show first, that the simulation on the uniform 10 km grid provides a decent mesoscale eddy rich simulation and second, that the telescoping grid is able to reproduce the mesoscale rich circulation locally in the North Atlantic and on decadal time scales. This telescoping technique of unstructured grids opens new research directions.
    Description: Key Points: We describe Icosahedral Nonhydrostatic Weather and Climate Model (ICON‐O) the ocean component of ICON‐ESM 1.0, based on the ICON modeling framework. ICON‐O is analyzed in a globally mesoscale‐rich simulation and in a telescoping configuration. In telescoping configuration ICON‐O reproduces locally the eddy dynamics with less computational costs than the uniform configuration.
    Description: https://swiftbrowser.dkrz.de/public/dkrz_07387162e5cd4c81b1376bd7c648bb60/kornetal2021
    Description: https://mpimet.mpg.de/en/science/modeling-with-icon/code-availability
    Keywords: ddc:551.46 ; ocean modeling ; ocean dynamics ; unstructured grid modeling ; local refinement ; structure preservation numerics
    Language: English
    Type: doc-type:article
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    CITATION GEO-LEO
  • 3
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    What causes seasonal variations in meridional overturning and heat transport of the Indian Ocean? (1996)
    In:  Other Sources
    Details
    Publication Date: 2018-06-08
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    NASA TECHNICAL REPORTS
  • 4
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    Corrigendum to "Observed and simulated estimates of the meridional overturning circulation at 26.5 degrees N in the Atlantic" published in Ocean Sci., 5, 575–589, 2009 (2010)
    Copernicus Publications (EGU)
    In:  Ocean Science, 6 (1). p. 1.
    Details
    Publication Date: 2013-01-31
    Type: Article , PeerReviewed
    Format: text
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    PAPER (OCEANREP)
  • 5
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    A monitoring design for the Atlantic meridional overturning circulation (2003)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 30 (7). p. 1314.
    Details
    Publication Date: 2018-02-20
    Description: Current hydrographic data can provide snapshots but no continuous timeseries of the meridional overturning circulation (MOC). Using output from two eddy-permitting numerical ocean models we test the feasibility of a monitoring system for the MOC in the North Atlantic. The results suggest that a relatively simple arrangement, using moorings placed across a longitude-depth section and the zonal wind stress, is able to capture most of the MOC strength and vertical structure as a function of time. Being closely related to the transport of energy to the North Atlantic, measuring the MOC would open the prospect of having continuous information about a key element of northern hemisphere climate.
    Type: Article , PeerReviewed
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    PAPER (OCEANREP)
  • 6
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    The present and future system for measuring the Atlantic meridional overturning circulation and heat transport (2010)
    OceanObs'09
    In:  In: Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society" Conference. , ed. by Hall, J., Harrison, D. E. and Stammer, D. ESA Publication, WPP-306 . OceanObs'09, Venice, Italy.
    Details
    Publication Date: 2012-07-06
    Description: of the global combined atmosphere-ocean heat flux and so is important for the mean climate of the Atlantic sector of the Northern Hemisphere. This meridional heat flux is accomplished by both the Atlantic Meridional Overturning Circulation (AMOC) and by basin-wide horizontal gyre circulations. In the North Atlantic subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are also important. Climate models suggest the AMOC will slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number of latitudes, to establish the dynamics and variability at each latitude and then their meridional connectivity. Modeling studies indicate that adaptations of the 26.5°N type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models.
    Type: Book chapter , NonPeerReviewed
    Format: text
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  • 7
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    Observed Flow Compensation Associated with the MOC at 26.5 N in the Atlantic (2007)
    American Association for the Advancement of Science (AAAS)
    In:  Science, 317 (5840). pp. 938-941.
    Details
    Publication Date: 2016-09-08
    Description: The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5°N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by ±5.7 × 106 cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.
    Type: Article , PeerReviewed
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  • 8
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    Seasonal variability of the Atlantic meridional overturning circulation at 26.5°N (2010)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 23 (21). pp. 5678-5698.
    Details
    Publication Date: 2020-08-04
    Description: The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, forty-eight-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008 the AMOC had a mean strength of 18.7 ±2.1 Sv with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic mid-ocean and Gulf Stream transports of 2.2 Sv and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.
    Type: Article , PeerReviewed
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  • 9
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    The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5° N (2010)
    Copernicus Publications (EGU)
    In:  Ocean Science, 6 (2). pp. 475-490.
    Details
    Publication Date: 2013-01-31
    Description: We study the contribution of eastern-boundary density variations to sub-seasonal and seasonal anomalies of the strength and vertical structure of the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N, by means of the RAPID/MOCHA mooring array between April 2004 and October 2007. The major density anomalies are found in the upper 500 m, and they are often coherent down to 1400 m. The densities have 13-day fluctuations that are apparent down to 3500 m. The two strategies for measuring eastern-boundary density – a tall offshore mooring (EB1) and an array of moorings on the continental slope (EBH) – show little correspondence in terms of amplitude, vertical structure, and frequency distribution of the resulting basin-wide integrated transport fluctuations, implying that there are significant transport contributions between EB1 and EBH. Contrary to the original planning, measurements from EB1 cannot serve as backup or replacement for EBH: density needs to be measured directly at the continental slope to compute the full-basin density gradient. Fluctuations in density at EBH generate transport variability of 2 Sv rms in the AMOC, while the overall AMOC variability is 4.8 Sv rms. There is a pronounced deep-reaching seasonal cycle in density at the eastern boundary, which is apparent between 100 m and 1400 m, with maximum positive anomalies in spring and maximum negative anomalies in autumn. These changes drive anomalous southward upper mid-ocean flow in spring, implying maximum reduction of the AMOC, and vice-versa in autumn. The amplitude of the seasonal cycle of the AMOC arising from the eastern-boundary densities is 5.2 Sv peak-to-peak, dominating the 6.7 Sv peak-to-peak seasonal cycle of the total AMOC. Our analysis suggests that the seasonal cycle in density may be forced by the strong near-coastal seasonal cycle in wind stress curl.
    Type: Article , PeerReviewed
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    PAPER (OCEANREP)
  • 10
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    A prototype system for observing the Atlantic Meridional Overturning Circulation - scientific basis, measurement and risk mitigation strategies, and first results (2008)
    IMarEST
    In:  Journal of Operational Oceanography, 1 (1). pp. 19-28.
    Details
    Publication Date: 2017-10-05
    Description: The Atlantic Meridional Overturning Circulation (MOC) carries up to one quarter of the global northward heat transport in the Subtropical North Atlantic. A system monitoring the strength of the MOC volume transport has been operating since April 2004. The core of this system is an array of moored sensors measuring density, bottom pressure and ocean currents. A strategy to mitigate risks of possible partial failures of the array is presented, relying on backup and complementary measurements. The MOC is decomposed into five components, making use of the continuous moored observations, and of cable measurements across the Straits of Florida, and wind stress data. The components compensate for each other, indicating that the system is working reliably. The year-long average strength of the MOC is 18.7±5.6 Sv, with wind-driven and density-inferred transports contributing equally to the variability. Numerical simulations suggest that the surprisingly fast density changes at the western boundary are partially linked to westward propagating planetary waves
    Type: Article , PeerReviewed
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