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  • 1
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    Online Resource
    Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean
    American Meteorological Society ; 2018
    In:  Journal of Climate Vol. 31, No. 10 ( 2018-05-15), p. 4157-4174
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 31, No. 10 ( 2018-05-15), p. 4157-4174
    Abstract: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-17-0654.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2018
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 2
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    An assessment of global and regional sea level for years 1993–2007 in a suite of interannual CORE-II simulations
    Elsevier BV ; 2014
    In:  Ocean Modelling Vol. 78 ( 2014-06), p. 35-89
    Details
    In: Ocean Modelling, Elsevier BV, Vol. 78 ( 2014-06), p. 35-89
    Type of Medium: Online Resource
    ISSN: 1463-5003
    URL: Article
    DOI: 10.1016/j.ocemod.2014.03.004
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2014
    detail.hit.zdb_id: 1126496-2
    detail.hit.zdb_id: 1498544-5
    SSG: 14
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  • 3
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    Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model
    Copernicus GmbH ; 2019
    In:  Ocean Science Vol. 15, No. 3 ( 2019-05-09), p. 489-512
    Details
    In: Ocean Science, Copernicus GmbH, Vol. 15, No. 3 ( 2019-05-09), p. 489-512
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 1812-0792
    URL: Article
    URL: Article
    DOI: 10.5194/os-15-489-2019
    DOI: 10.5194/os-15-489-2019-corrigendum
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2019
    detail.hit.zdb_id: 2183769-7
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  • 4
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    Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods
    Copernicus GmbH ; 2021
    In:  Ocean Science Vol. 17, No. 4 ( 2021-08-16), p. 1067-1080
    Details
    In: Ocean Science, Copernicus GmbH, Vol. 17, No. 4 ( 2021-08-16), p. 1067-1080
    Abstract: Abstract. The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 (1/20∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.
    Type of Medium: Online Resource
    ISSN: 1812-0792
    URL: Article
    DOI: 10.5194/os-17-1067-2021
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
    detail.hit.zdb_id: 2183769-7
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  • 5
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    Online Resource
    Analysis of the position and strength of westerlies and trades with implications for Agulhas leakage and South Benguela upwelling
    Copernicus GmbH ; 2019
    In:  Earth System Dynamics Vol. 10, No. 4 ( 2019-12-05), p. 847-858
    Details
    In: Earth System Dynamics, Copernicus GmbH, Vol. 10, No. 4 ( 2019-12-05), p. 847-858
    Abstract: Abstract. The westerlies and trade winds over the South Atlantic and Indian Ocean are important drivers of the regional oceanography around southern Africa, including features such as the Agulhas Current, the Agulhas leakage, and the Benguela upwelling. Agulhas leakage constitutes a fraction of warm and saline water transport from the Indian Ocean into the South Atlantic. The leakage is stronger during intensified westerlies. Here, we analyze the wind stress of different observational and modeled atmospheric data sets (covering the last 2 millennia, the recent decades, and the 21st century) with regard to the intensity and position of the southeasterly trades and the westerlies. The analysis reveals that variations of both wind systems go hand in hand and that a poleward shift of the westerlies and trades and an intensification of westerlies took place during the recent decades. Furthermore, upwelling in South Benguela is slightly intensified when trades are shifted poleward. Projections for strength and position of the westerlies in the 21st century depend on assumed CO2 emissions and on their effect relative to the ozone forcing. In the strongest emission scenario (RCP8.5) the simulations show a further southward displacement, whereas in the weakest emission scenario (RCP2.6) a northward shift is modeled, possibly due to the effect of ozone recovery dominating the effect of anthropogenic greenhouse forcing. We conclude that the Agulhas leakage has intensified during the last decades and is projected to increase if greenhouse gas emissions are not reduced. This will have a small impact on Benguela upwelling strength and may also have consequences for water mass characteristics in the upwelling region. An increased contribution of Agulhas water to the upwelling water masses will import more preformed nutrients and oxygen into the upwelling region.
    Type of Medium: Online Resource
    ISSN: 2190-4987
    URL: Article
    URL: Article
    DOI: 10.5194/esd-10-847-2019
    DOI: 10.5194/esd-10-847-2019-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2019
    detail.hit.zdb_id: 2578793-7
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  • 6
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    Pacific Ocean Contribution to the Asymmetry in Eastern Indian Ocean Variability
    American Meteorological Society ; 2013
    In:  Journal of Climate Vol. 26, No. 4 ( 2013-02-15), p. 1152-1171
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 26, No. 4 ( 2013-02-15), p. 1152-1171
    Abstract: Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where interannual atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by an anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of an anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing, and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-11-00673.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2013
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 7
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    Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?
    American Meteorological Society ; 2019
    In:  Journal of Physical Oceanography Vol. 49, No. 5 ( 2019-05), p. 1141-1157
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 49, No. 5 ( 2019-05), p. 1141-1157
    Abstract: To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m 2 s −1 in the Cape Basin.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-18-0152.1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2019
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
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  • 8
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    Spatio‐temporal variation in ocean current‐driven hatchling dispersion: Implications for the world's largest leatherback sea turtle nesting region
    Wiley ; 2017
    In:  Diversity and Distributions Vol. 23, No. 6 ( 2017-06), p. 604-614
    Details
    In: Diversity and Distributions, Wiley, Vol. 23, No. 6 ( 2017-06), p. 604-614
    Abstract: The lives of juvenile leatherback turtles are amongst the most enigmatic of all marine mega‐vertebrates. For these cryptic organisms, ocean models provide important insights into their dispersion from natal sites. Here, corroborated by fisheries bycatch data, we simulate spatio‐temporal variation in hatchling dispersion patterns over five decades from the World's largest leatherback turtle nesting region. Location Equatorial Central West Africa (3.5°N to −6°S) spanning the Gulf of Guinea in the North, Gabon and the Republic/Democratic Republic of the Congo in the South. Results Due to dynamic oceanic conditions at these equatorial latitudes, dispersion scenarios differed significantly: (1) along the north to south gradient of the study region, (2) seasonally and (3) between years. From rookeries to the north of the equator, simulated hatchling retention rates within the Gulf of Guinea were very high ( 〉 99%) after 6 months of drift, whilst south of the equator, retention rates were as low as c . 6% with the majority of simulated hatchlings dispersing west into the South Atlantic Ocean with the South Equatorial Current. Seasonal dispersion variability was driven by wind changes arising from the yearly north/southward migration of the intertropical convergence zone resulting in the increasing westerly dispersion of hatchlings throughout the hatching season. Annual variability in wind stress drove a long‐term trend for decreased retention within the Gulf of Guinea and increased westerly dispersion into habitats in the South Atlantic Ocean. Main conclusions Shifts in dispersion habitats arising from spatio‐temporal oceanic variability expose hatchlings to different environments and threats that will influence important life history attributes such as juvenile growth/survival rates; anticipated to impact the population dynamics and size/age structure of populations into adulthood. The impacts of local and dynamic oceanic conditions thus require careful considerations, such as subregional management, when managing marine populations of conservation concern.
    Type of Medium: Online Resource
    ISSN: 1366-9516 , 1472-4642
    URL: Issue
    URL: Article
    DOI: 10.1111/ddi.2017.23.issue-6
    DOI: 10.1111/ddi.12554
    Language: English
    Publisher: Wiley
    Publication Date: 2017
    detail.hit.zdb_id: 2020139-4
    detail.hit.zdb_id: 1443181-6
    SSG: 12
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  • 9
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    Regional imprints of changes in the Atlantic Meridional Overturning Circulation in the eddy-rich ocean model VIKING20X
    Copernicus GmbH ; 2021
    In:  Ocean Science Vol. 17, No. 5 ( 2021-09-02), p. 1177-1211
    Details
    In: Ocean Science, Copernicus GmbH, Vol. 17, No. 5 ( 2021-09-02), p. 1177-1211
    Abstract: Abstract. A hierarchy of global 1/4∘ (ORCA025) and Atlantic Ocean 1/20∘ nested (VIKING20X) ocean–sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5∘ N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53∘ N, 26.5∘ N and 11∘ S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5∘ N, it is shown that in particular the outflow of North Atlantic Deepwater at 53∘ N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.
    Type of Medium: Online Resource
    ISSN: 1812-0792
    URL: Article
    DOI: 10.5194/os-17-1177-2021
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
    detail.hit.zdb_id: 2183769-7
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  • 10
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    The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations
    Copernicus GmbH ; 2019
    In:  Geoscientific Model Development Vol. 12, No. 7 ( 2019-07-29), p. 3329-3355
    Details
    In: Geoscientific Model Development, Copernicus GmbH, Vol. 12, No. 7 ( 2019-07-29), p. 3329-3355
    Abstract: Abstract. The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.
    Type of Medium: Online Resource
    ISSN: 1991-9603
    URL: Article
    DOI: 10.5194/gmd-12-3329-2019
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2019
    detail.hit.zdb_id: 2456725-5
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