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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Climate dynamics 9 (1994), S. 221-234 
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract. Using a multivariate model testing procedure that distinguishes between model inadequacies and data uncertainties, we investigate the ability of the LODYC GCM to simulate the evolution of the 20° C isotherm depth during the 1982–1984 FOCAL/SEQUAL experiment in the equatorial Atlantic. Two different versions of the model are considered: the “Ri” version which has a Richardson number dependent parameterization of vertical mixing and the new “TKE” version which uses a local estimation of the turbulent kinetic energy to parameterize vertical mixing. Some effects of the forcing uncertainties are considered by forcing the TKE version with three equally plausible wind stress fields whose differences are consistent with the measurement and sampling errors, and the drag coefficient indeterminacy. The resulting uncertainties in the model response are substantial and can be as large as the differences between simulations with the two GCM versions, which stresses the need to take the forcing uncertainties into account. Although only one Ri run is available, it is shown that the “TKE” parameterization significantly improves the representation of the equatorial upwelling and the simulation of the depth of the thermocline in the eastern Atlantic. However, there remain significant differences with the observations which cannot be explained by the forcing uncertainties that were considered. The two model versions perform better in the equatorial wave guide than in the 12° N–12° S domain, and they are better distinguished over large domains than along sections, which shows that a global multivariate view point must be used in model-reality comparisons. Finally, a comparison with a linear multimode model emphasizes the need for greater model complexity to properly simulate the equatorial upwelling and the thermocline variability in the tropical Atlantic.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Climate dynamics 9 (1994), S. 221-234 
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Using a multivariate model testing procedure that distinguishes between model inadequacies and data uncertainties, we investigate the ability of the LODYC GCM to simulate the evolution of the 20°C isotherm depth during the 1982–1984 FOCAL/SEQUAL experiment in the equatorial Atlantic. Two different versions of the model are considered: the “Ri” version which has a Richardson number dependent parameterization of vertical mixing and the new “TKE” version which uses a local estimation of the turbulent kinetic energy to parameterize vertical mixing. Some effects of the forcing uncertainties are considered by forcing the TKE version with three equally plausible wind stress fields whose differences are consistent with the measurement and sampling errors, and the drag coefficient indeterminacy. The resulting uncertainties in the model response are substantial and can be as large as the differences between simulations with the two GCM versions, which stresses the need to take the forcing uncertainties into account. Although only one Ri run is available, it is shown that the “TKE” parameterization significantly improves the representation of the equatorial upwelling and the simulation of the depth of the thermocline in the eastern Atlantic. However, there remain significant differences with the observations which cannot be explained by the forcing uncertainties that were considered. The two model versions perform better in the equatorial wave guide than in the 12°N-12°S domain, and they are better distinguished over large domains than along sections, which shows that a global multivariate view point must be used in model-reality comparisons. Finally, a comparison with a linear multimode model emphasizes the need for greater model complexity to properly simulate the equatorial upwelling and the thermocline variability in the tropical Atlantic.
    Type of Medium: Electronic Resource
    Location Call Number Limitation Availability
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  • 3
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (−5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (〉 5 mm day −1), and fewer light showers (≤1 mm day−). Rainy days with rates between 1 and 5 mm day−1 were almost unchanged.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract. We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (–5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (〉5 mm day–1), and fewer light showers (≤1 mm day–1). Rainy days with rates between 1 and 5 mm day–1 were almost unchanged.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 2021-08-25
    Description: Lack of constraint on spatial and long-term temporal variability of the El Niño southern Oscillation (ENSO) and its sensitivity to external forcing limit our ability to evaluate climate models and ENSO future projections. Current knowledge of Holocene ENSO variability derived from paleoclimate reconstructions does not separate the role of insolation forcing from internal climate variability. Using an updated synthesis of coral and bivalve monthly resolved records, we build composite records of seasonality and interannual variability in four regions of the tropical Pacific: Eastern Pacific (EP), Central Pacific (CP), Western Pacific (WP) and South West Pacific (SWP). An analysis of the uncertainties due to the sampling of chaotic multidecadal to centennial variability by short records allows for an objective comparison with transient simulations (mid-Holocene to present) performed using four different Earth System models. Sea surface temperature and pseudo-δ18O are used in model-data comparisons to assess the potential influence of hydroclimate change on records. We confirm the significance of the Holocene ENSO minimum (HEM) 3-6ka compared to low frequency unforced modulation of ENSO, with a reduction of ENSO variance of ∼50 % in EP and ∼80 % in CP. The approach suggests that the increasing trend of ENSO since 6ka can be attributed to insolation, while models underestimate ENSO sensitivity to orbital forcing by a factor of 4.7 compared to data, even when accounting for the large multidecadal variability. Precession-induced change in seasonal temperature range is positively linked to ENSO variance in EP and to a lesser extent in other regions, in both models and observations. Our regional approach yields insights into the past spatial expression of ENSO across the tropical Pacific. In the SWP, today under the influence of the South Pacific Convergence Zone (SPCZ), interannual variability was increased by ∼200 % during the HEM, indicating that SPCZ variability is independent from ENSO on millennial time scales.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 6
    Publication Date: 2022-10-26
    Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Stammer, D., Bracco, A., AchutaRao, K., Beal, L., Bindoff, N. L., Braconnot, P., Cai, W., Chen, D., Collins, M., Danabasoglu, G., Dewitte, B., Farneti, R., Fox-Kemper, B., Fyfe, J., Griffies, S. M., Jayne, S. R., Lazar, A., Lengaigne, M., Lin, X., Marsland, S., Minobe, S., Monteiro, P. M. S., Robinson, W., Roxy, M. K., Rykaczewski, R. R., Speich, S., Smith, I. J., Solomon, A., Storto, A., Takahashi, K., Toniazzo, T., & Vialard, J. Ocean climate observing requirements in support of climate research and climate information. Frontiers in Marine Science, 6, (2019): 444, doi:10.3389/fmars.2019.00444.
    Description: Natural variability and change of the Earth’s climate have significant global societal impacts. With its large heat and carbon capacity and relatively slow dynamics, the ocean plays an integral role in climate, and provides an important source of predictability at seasonal and longer timescales. In addition, the ocean provides the slowly evolving lower boundary to the atmosphere, driving, and modifying atmospheric weather. Understanding and monitoring ocean climate variability and change, to constrain and initialize models as well as identify model biases for improved climate hindcasting and prediction, requires a scale-sensitive, and long-term observing system. A climate observing system has requirements that significantly differ from, and sometimes are orthogonal to, those of other applications. In general terms, they can be summarized by the simultaneous need for both large spatial and long temporal coverage, and by the accuracy and stability required for detecting the local climate signals. This paper reviews the requirements of a climate observing system in terms of space and time scales, and revisits the question of which parameters such a system should encompass to meet future strategic goals of the World Climate Research Program (WCRP), with emphasis on ocean and sea-ice covered areas. It considers global as well as regional aspects that should be accounted for in designing observing systems in individual basins. Furthermore, the paper discusses which data-driven products are required to meet WCRP research and modeling needs, and ways to obtain them through data synthesis and assimilation approaches. Finally, it addresses the need for scientific capacity building and international collaboration in support of the collection of high-quality measurements over the large spatial scales and long time-scales required for climate research, bridging the scientific rational to the required resources for implementation.
    Description: This work was partly supported by the DFG funded excellence center CliSAP of the Universituat Hamburg (DS). AB was supported by the National Science Foundation through award NSF-1658174 and by the NOAA through award NA16OAR4310173. SM was supported by the Earth Systems and Climate Change Hub of the Australian Government’s National Environmental Science Program.
    Keywords: Ocean observing system ; Ocean climate ; Earth observations ; In situ measurements ; Satellite observations ; Ocean modeling ; Climate information
    Repository Name: Woods Hole Open Access Server
    Type: Article
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