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  • 1
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    OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project (2016)
    Copernicus Publications (EGU)
    In:  Geoscientific Model Development, 9 (9). pp. 3231-3296.
    Details
    Publication Date: 2019-02-01
    Description: The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, sea-ice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, HighResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.5194/gmd-9-3231-2016
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models (2017)
    Copernicus Publications (EGU)
    In:  Ocean Science Discussions .
    Details
    Publication Date: 2021-03-04
    Description: The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations.
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.5194/os-2017-24
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    OceanRep: Article in a Scientific Journal - without review
  • 3
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    Pacific Decadal Oscillation and recent oxygen decline in the eastern tropical Pacific Ocean (2018)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2023-11-08
    Description: The impact of the positive and negative phases of the Pacific Decadal Oscillation (PDO) on the extension of the poorly oxygenated regions of the eastern Pacific Ocean was assessed using a coupled ocean circulation–biogeochemical model. We show that during a “typical” PDO-positive phase the volume of the suboxic regions expands by 7 % over 50 years due to a slowdown of the large-scale circulation related to the decrease in the intensity of the trade winds. Changes in oxygen levels are mostly controlled by advective processes between 10∘ N and 10∘ S, whereas diffusive processes are dominant poleward of 10∘: in a “typical” PDO-positive phase the sluggish equatorial current system provides less oxygen to the eastern equatorial part of the basin while the oxygen transport by diffusive processes significantly decreases south of 10∘ S. The suboxic region located north of 10∘ N displays less sensitivity to the phase of the PDO as the local upwelling-related processes play a dominant role compared to the large-scale circulation in setting the oxygen concentration. Our study suggests that the prevailing PDO-positive conditions since 1975 may explain a significant part of the current deoxygenation occurring in the eastern Pacific Ocean.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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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 (2019)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2022-04-06
    Description: 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: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Multidecadal Indian Ocean variability linked to the Pacific and implications for preconditioning Indian Ocean dipole events (2017)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 1739-1751, doi:10.1175/JCLI-D-16-0200.1.
    Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multidecadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the twentieth century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multidecadal variations associated with the Pacific decadal oscillation, and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multidecadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward-propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.
    Description: This research was supported by a Research Fellowship by the Alexander von Humboldt Foundation, as well as the Ocean Climate Change Institute and the Investment in Science Fund at WHOI.
    Description: 2017-08-15
    Keywords: Indian Ocean ; Ocean dynamics ; Climate variability ; Multidecadal variability ; Pacific decadal oscillation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin-mode resonance (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 3011-3029, doi:10.1175/JPO-D-15-0248.1.
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semiannual variability is also pronounced, despite weak forcing at that period. This study uses multiyear, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semiannual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the fourth mode and the semiannual cycle is dominated by the second mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semiannual cycles of the respective dominant baroclinic modes are associated with characteristic basinwide structures. Using an idealized, linear, reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e., square basin vs realistic coastlines) or forcing (i.e., spatially uniform vs spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Description: This study was supported by the Deutsche Forschungsgemeinschaft as part of the Sonderforschungsbereich 754 (SFB754) ‘‘Climate–Biogeochemistry Interactions in the Tropical Ocean’’ and through several research cruises with R/V Meteor, R/V Maria S. Merian, andR/VL’Atalante by the German Federal Ministry of Education and Research as part of the cooperative projects RACE (03F0605B) and SACUS (03G0837A) and by European Union 7th Framework Programme (FP7 2007–13) under Grant Agreement 603521 PREFACE project.
    Keywords: Atlantic Ocean ; Ocean circulation ; In situ oceanic observations ; Ocean models ; Seasonal cycle ; Tropical variability
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 7
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    Influences of Pacific climate variability on decadal subsurface ocean heat content variations in the Indian Ocean (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 4157-4174, doi:10.1175/JCLI-D-17-0654.1.
    Description: 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.
    Description: This research was supported by a scholarship from the China Scholarship Council (CSC) to X. J., a research fellowship by the Alexander von Humboldt Foundation to C. C. U., an NSF OCE PO Grant (OCE- 1242989) to Y.-O. K., the ONR Young Investigator Award (N00014-15-1-2588) to H. S., and a research grant from the Ministry of Science and Technology of the People’s Republic of China to Tsinghua University (2017YFA0603902).
    Description: 2018-10-30
    Keywords: Atmosphere-ocean interaction
    Repository Name: Woods Hole Open Access Server
    Type: Article
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