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  • 1
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: Online-Ressource (PDF-Datei: 34 Seiten = 1,4 MB)
    Language: German
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 2
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (44 Blatt = 900 KB)
    Language: German
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 3
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (87 Blatt = 6 MB)
    Language: German
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 4
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    In:  EPIC3Annals of glaciology, 44, pp. 310-316
    Publication Date: 2019-07-17
    Description: In the Arctic, sea ice motion and ice export are prominent processes and good indicators of Arctic climate system variability. Sea ice drift is simulated using a dynamic-thermodynamic sea ice model, validated with retrievals from SSM/I satellite observations. Both data sets agree well in reproducing the main Arctic drift patterns. In order to study inner Arctic transports and ice volume anomalies, the Arctic Ocean is splitted by ten boarderlines, separating the marginal seas from the central Arctic Ocean and the Nordic Seas. It is found that the already dominant sea ice export through the Fram Strait has increased at the expense of export through the Barents Sea in recent years. Furthermore, ice export from the Eurasian marginal seas increases slightly, followed by a larger ice production during the winter season. In contrast to this the sea ice volume moved within the Beaufort Gyre decreases distinctly. In total, the ice volume in the central Arctic decreases during the 40 year period covered by this study. The changes in the ice volume are going along with two wind-driven circulation regimes of the Arctic sea ice motion, which recur approximately every 11 years. For the volume anomalies we derive a correlation of 0.59 to the NAO index lagging the NAO by two years.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 5
    Publication Date: 2023-08-01
    Description: Simulations with a free-running coupled climate model show that heat release associated with Southern Ocean deep convection variability can drive centennial-scale Antarctic temperature variations of up to 2.0 °C. The mechanism involves three steps: Preconditioning: heat accumulates at depth in the Southern Ocean; Convection onset: wind and/or sea-ice changes tip the buoyantly unstable system into the convective state; Antarctic warming: fast sea-ice–albedo feedbacks (on annual–decadal timescales) and slow Southern Ocean frontal and sea-surface temperature adjustments to convective heat release (on multidecadal–century timescales) drive an increase in atmospheric heat and moisture transport toward Antarctica. We discuss the potential of this mechanism to help drive and amplify climate variability as observed in Antarctic ice-core records.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 6
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    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (13). pp. 6951-6958.
    Publication Date: 2020-02-06
    Description: Global climate models exhibit large biases in the Southern Ocean. For example, in models Antarctic bottom water is formed mostly through open-ocean deep-convection rather than through shelf convection. Still, the timescale, region, and intensity of deep-convection variability vary widely among models. We investigate the physical controls of this variability in the Atlantic sector of the Southern Ocean, where most of the models simulate recurring deep-convection events. We analyzed output from eleven exemplary CMIP5 models and four versions of the Kiel Climate Model (KCM). Of several potential physical control parameters that we tested, the ones shared by all these models are: Stratification in the convection region influences the timescale of the deep-convection variability, i.e. models with a strong (weak) stratification vary on long (short) timescales. And, sea ice volume affects the modeled mean state in the Southern Ocean: large (small) sea ice volume is associated with a non-convective (convective) predominant regime.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 7
    Publication Date: 2020-02-06
    Description: The loss of Arctic sea ice is a conspicuous example of climate change. Climate models project ice-free conditions during summer this century under realistic emission scenarios, reflecting the increase in seasonality in ice cover. To quantify the increased seasonality in the Arctic-Subarctic sea ice system, we define a non-dimensional seasonality number for sea ice extent, area, and volume from satellite data and realistic coupled climate models. We show that the Arctic-Subarctic, i.e. The northern hemisphere, sea ice now exhibits similar levels of seasonality to the Antarctic, which is in a seasonal regime without significant change since satellite observations began in 1979. Realistic climate models suggest that this transition to the seasonal regime is being accompanied by a maximum in Arctic amplification, which is the faster warming of Arctic latitudes compared to the global mean, in the 2010s. The strong link points to a peak in sea-ice-related feedbacks that occurs long before the Arctic becomes ice-free in summer.
    Type: Article , PeerReviewed
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  • 8
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    Springer
    In:  Current Climate Change Reports, 3 (3). pp. 163-173.
    Publication Date: 2020-02-06
    Description: The Southern Ocean featured some remarkable changes during the recent decades. For example, large parts of the Southern Ocean, despite rapidly rising atmospheric greenhouse gas concentrations, depicted a surface cooling since the 1970s, whereas most of the planet has warmed considerably. In contrast, climate models generally simulate Southern Ocean surface warming when driven with observed historical radiative forcing. The mechanisms behind the surface cooling and other prominent changes in the Southern Ocean sector climate during the recent decades, such as expanding sea ice extent, abyssal warming, and CO2 uptake, are still under debate. Observational coverage is sparse, and records are short but rapidly growing, making the Southern Ocean climate system one of the least explored. It is thus difficult to separate current trends from underlying decadal to centennial scale variability. Here, we present the state of the discussion about some of the most perplexing decadal climate trends in the Southern Ocean during the recent decades along with possible mechanisms and contrast these with an internal mode of Southern Ocean variability present in state-of-the art climate models.
    Type: Article , PeerReviewed
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  • 9
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    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 121 (3). pp. 1931-1952.
    Publication Date: 2019-04-04
    Description: The Arctic sea ice cover is thinning and retreating, causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean. New model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades. Simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress (0.003 N/m2 per decade, 4.6%) due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift. In contrast, with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of -0.002 N/m2 per decade (3.1%) over the period 1980-2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover. The effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions. This reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2019-09-24
    Description: A quasi-oscillatory multi-centennial mode of open ocean deep convection in the Atlantic sector of the Southern Ocean in the Kiel Climate Model is described. The quasi-periodic occurrence of the deep convection causes variations in regional and global surface air temperature, Southern Hemisphere sea ice coverage, Southern Ocean and North Atlantic sea surface height, the Antarctic Circumpolar Current and the Atlantic Meridional Overturning Circulation (AMOC). The deep convection is stimulated by a strong built-up of heat at mid-depth. When the heat reservoir is virtually depleted a coincidental strong freshening event at the sea surface shuts down the convection. The heat originates from relatively warm deep water formed in the North Atlantic. The several decades lasting recharge process of the heat reservoir depends on the AMOC and the Weddell Gyre and sets a minimum delay for the deep convection to recur. While the strength of the AMOC increases, the Weddell Gyre weakens during the non-convective regime. Convection onset and shutdown also depend on the stochastic occurrence of favorable sea surface conditions, which contributes to the multi-centennial period of the phenomenon. The shutdown triggers a century-long deviation in AMOC strength caused by significant reductions in bottom water formation and surface salinity in the Southern Ocean’s Atlantic sector. Additional numerical experimentation reveals that sea ice has an important effect on the frequency of occurrence and intensity of the deep convection. Further, we find intriguing similarities to the Weddell Polynya observed during the 1970s
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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