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  • 1
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    Online Resource
    Eddy Diffusivity Estimates from Lagrangian Trajectories Simulated with Ocean Models and Surface Drifter Data—A Case Study for the Greater Agulhas System
    American Meteorological Society ; 2018
    In:  Journal of Physical Oceanography Vol. 48, No. 1 ( 2018-01), p. 175-196
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 48, No. 1 ( 2018-01), p. 175-196
    Abstract: The Lagrangian analysis of sets of particles advected with the flow fields of ocean models is used to study connectivity, that is, exchange pathways, time scales, and volume transports, between distinct oceanic regions. One important factor influencing the dispersion of fluid particles and, hence, connectivity is the Lagrangian eddy diffusivity, which quantifies the influence of turbulent processes on the rate of particle dispersal. Because of spatial and temporal discretization, turbulence is not fully resolved in modeled velocities, and the concept of eddy diffusivity is used to parameterize the impact of unresolved processes. However, the relations between observation- and model-based Lagrangian eddy diffusivity estimates, as well as eddy parameterizations, are not clear. This study presents an analysis of the spatially variable near-surface lateral eddy diffusivity estimates obtained from Lagrangian trajectories simulated with 5-day mean velocities from an eddy-resolving ocean model (INALT01) for the Agulhas system. INALT01 features diffusive regimes for dynamically different regions, some of which exhibit strong suppression of eddy mixing by mean flow, and it is consistent with the pattern and magnitude of drifter-based eddy diffusivity estimates. Using monthly mean velocities decreases the estimated diffusivities less than eddy kinetic energy, supporting the idea that large and persistent eddy features dominate eddy diffusivities. For a noneddying ocean model (ORCA05), Lagrangian eddy diffusivities are greatly reduced, particularly when the Gent and McWilliams parameterization of mesoscale eddies is employed.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    URL: Article
    DOI: 10.1175/JPO-D-17-0048.1
    DOI: 10.1175/JPO-D-17-0048.s1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2018
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
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  • 2
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    Changing Spatial Patterns of Deep Convection in the Subpolar North Atlantic
    American Geophysical Union (AGU) ; 2021
    In:  Journal of Geophysical Research: Oceans Vol. 126, No. 7 ( 2021-07)
    Details
    In: Journal of Geophysical Research: Oceans, American Geophysical Union (AGU), Vol. 126, No. 7 ( 2021-07)
    Abstract: Eddy‐rich ocean model simulations (1958–2019) feature large variability of spatial deep convection patterns in subpolar North Atlantic In 2015–2018, deep convection showed exceptional large and small relative contributions of eastern and western subpolar gyre, respectively Small western contribution is potentially associated with enhanced Greenland melting and recent eastern North Atlantic fresh anomaly
    Type of Medium: Online Resource
    ISSN: 2169-9275 , 2169-9291
    URL: Article
    DOI: 10.1029/2021JC017245
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2021
    detail.hit.zdb_id: 2016804-4
    detail.hit.zdb_id: 161667-5
    detail.hit.zdb_id: 3094219-6
    SSG: 16,13
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  • 3
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    Exceptional freshening and cooling in the eastern subpolar North Atlantic caused by reduced Labrador Sea surface heat loss
    Copernicus GmbH ; 2022
    In:  Ocean Science Vol. 18, No. 5 ( 2022-10-21), p. 1507-1533
    Details
    In: Ocean Science, Copernicus GmbH, Vol. 18, No. 5 ( 2022-10-21), p. 1507-1533
    Abstract: Abstract. Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Published theories for the mechanisms driving the freshening include: reduced transport of saltier, warmer surface waters northwards from the subtropics associated with reduced meridional overturning; shifts in the pathways of fresher, cooler surface water from the Labrador Sea driven by changing patterns of wind stress; and the eastward expansion of the subpolar gyre. Using output from a high-resolution hindcast model simulation, we propose that the primary cause of the exceptional freshening and cooling is reduced surface heat loss in the Labrador Sea. Tracking virtual fluid particles in the model backwards from the eastern subpolar North Atlantic between 1990 and 2020 shows the major cause of the freshening and cooling to be an increased outflow of relatively fresh and cold surface waters from the Labrador Sea; with a minor contribution from reduced transport of warmer, saltier surface water northward from the subtropics. The cooling, but not the freshening, produced by these changing proportions of waters of subpolar and subtropical origin is mitigated by reduced along-track heat loss to the atmosphere in the North Atlantic Current. We analyse modelled boundary exchanges and water mass transformation in the Labrador Sea to show that since 2000, while inflows of lighter surface waters remain steady, the increasing output of these waters is due to reduced surface heat loss in the Labrador Sea beginning in the early 2000s. Tracking particles further upstream reveals that the primary source of the increased volume of lighter water transported out of the Labrador Sea is increased recirculation of water, and therefore longer residence times, in the upper 500–1000 m of the subpolar gyre.
    Type of Medium: Online Resource
    ISSN: 1812-0792
    URL: Article
    DOI: 10.5194/os-18-1507-2022
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2022
    detail.hit.zdb_id: 2183769-7
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  • 4
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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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  • 5
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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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  • 6
    Online Resource
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    Indian Ocean sources of Agulhas leakage
    American Geophysical Union (AGU) ; 2017
    In:  Journal of Geophysical Research: Oceans Vol. 122, No. 4 ( 2017-04), p. 3481-3499
    Details
    In: Journal of Geophysical Research: Oceans, American Geophysical Union (AGU), Vol. 122, No. 4 ( 2017-04), p. 3481-3499
    Abstract: A study of pathways, timescales, and water transformations of Indian Ocean sources of Agulhas leakage On‐route, Indonesian Through‐Flow cools and salinificates, while Tasman leakage is deeper and experiences no change in properties The Pacific contributes at least 7.9 Sv to the total Agulhas leakage of about 14 Sv
    Type of Medium: Online Resource
    ISSN: 2169-9275 , 2169-9291
    URL: Issue
    URL: Article
    DOI: 10.1002/jgrc.v122.4
    DOI: 10.1002/2016JC012676
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2017
    detail.hit.zdb_id: 2016804-4
    detail.hit.zdb_id: 161667-5
    detail.hit.zdb_id: 3094219-6
    SSG: 16,13
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  • 7
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    On the suitability of North Brazil Current transport estimates for monitoring basin‐scale AMOC changes
    American Geophysical Union (AGU) ; 2015
    In:  Geophysical Research Letters Vol. 42, No. 19 ( 2015-10-16), p. 8072-8080
    Details
    In: Geophysical Research Letters, American Geophysical Union (AGU), Vol. 42, No. 19 ( 2015-10-16), p. 8072-8080
    Abstract: Set of ocean model hindcasts yields a robust interannual to decadal variability of the NBC AMOC trends are manifested in the NBC but superimposed by wind‐driven gyre changes An NBC transport array can be a useful component of an AMOC monitoring system
    Type of Medium: Online Resource
    ISSN: 0094-8276 , 1944-8007
    URL: Issue
    URL: Article
    DOI: 10.1002/grl.v42.19
    DOI: 10.1002/2015GL065695
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2015
    detail.hit.zdb_id: 2021599-X
    detail.hit.zdb_id: 7403-2
    SSG: 16,13
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  • 8
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    A 334-year coral record of surface temperature and salinity variability in the greater Agulhas Current region
    Copernicus GmbH ; 2022
    In:  Climate of the Past Vol. 18, No. 6 ( 2022-06-27), p. 1453-1474
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 18, No. 6 ( 2022-06-27), p. 1453-1474
    Abstract: Abstract. The Agulhas Current (AC) off the southern tip of Africa is one of the strongest western boundary currents and a crucial choke point of inter-ocean heat and salt exchange between the Indian Ocean and the southern Atlantic Ocean. However, large uncertainties remain concerning the sea surface temperature (SST) and salinity (SSS) variability in the AC region and their driving mechanisms over longer timescales, due to only short observational datasets being available and the highly dynamic nature of the region. Here, we present an annual coral skeletal Sr/Ca composite record paired with an established composite oxygen isotope record from Ifaty and Tulear reefs in southwestern Madagascar to obtain a 334-year (1661–1995) reconstruction of δ18Oseawater changes related to surface salinity variability in the wider Agulhas Current region. Our new annual δ18Oseawater composite record from Ifaty traces surface salinity of the southern Mozambique Channel and AC core region from the SODA reanalysis between 1958 and 1995. δ18Oseawater appears to be mainly driven by large-scale wind forcing in the southern Indian Ocean on interannual to decadal timescales. The δ18Oseawater and SST at Ifaty show characteristic interannual variability of between 2 and 4 years and interdecadal variability of 8 to 16 years, coherent with El Niño–Southern Oscillation (ENSO) records. Lagged correlations with the multivariate ENSO index reveals a 1–2-year lag of δ18Oseawater and salinity at Ifaty and the AC region, suggesting that propagation of anomalies by ocean Rossby waves may contribute to salinity changes in the wider southwestern Indian Ocean. The δ18Oseawater and SST reconstructions at Ifaty reveal the highest interannual variability during the Little Ice Age, especially around 1700 CE, which is in agreement with other Indo-Pacific coral studies. Our study demonstrates the huge potential to unlock past interannual and decadal changes in surface ocean hydrology and ocean transport dynamics from coral δ18Oseawater beyond the short instrumental record.
    Type of Medium: Online Resource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-18-1453-2022
    DOI: 10.5194/cp-18-1453-2022-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2022
    detail.hit.zdb_id: 2217985-9
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  • 9
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    Advective timescales and pathways of Agulhas leakage : AGULHAS LEAKAGE PATHWAYS
    American Geophysical Union (AGU) ; 2013
    In:  Geophysical Research Letters Vol. 40, No. 15 ( 2013-08-16), p. 3997-4000
    Details
    In: Geophysical Research Letters, American Geophysical Union (AGU), Vol. 40, No. 15 ( 2013-08-16), p. 3997-4000
    Type of Medium: Online Resource
    ISSN: 0094-8276
    URL: Article
    DOI: 10.1002/grl.50782
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2013
    detail.hit.zdb_id: 2021599-X
    detail.hit.zdb_id: 7403-2
    SSG: 16,13
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  • 10
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    Robust estimates for the decadal evolution of Agulhas leakage from the 1960s to the 2010s
    Springer Science and Business Media LLC ; 2022
    In:  Communications Earth & Environment Vol. 3, No. 1 ( 2022-12-14)
    Details
    In: Communications Earth & Environment, Springer Science and Business Media LLC, Vol. 3, No. 1 ( 2022-12-14)
    Abstract: Agulhas leakage, the transport of warm and salty waters from the Indian Ocean into the South Atlantic, has been suggested to increase under anthropogenic climate change, due to strengthening Southern Hemisphere westerly winds. The resulting enhanced salt transport into the South Atlantic may counteract the projected weakening of the Atlantic overturning circulation through warming and ice melting. Here we combine existing and new observation- and model-based Agulhas leakage estimates to robustly quantify its decadal evolution since the 1960s. We find that Agulhas leakage very likely increased between the mid-1960s and mid-1980s, in agreement with strengthening winds. Our models further suggest that increased leakage was related to enhanced transport outside eddies and coincided with strengthened Atlantic overturning circulation. Yet, it appears unlikely that Agulhas leakage substantially increased since the 1990s, despite continuously strengthening winds. Our results stress the need to better understand decadal leakage variability to detect and predict anthropogenic trends.
    Type of Medium: Online Resource
    ISSN: 2662-4435
    URL: Article
    DOI: 10.1038/s43247-022-00643-y
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2022
    detail.hit.zdb_id: 3037243-4
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