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  • 1
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    Southern Ocean deep convection as a driver of Antarctic warming events (2016)
    AGU (American Geophysical Union) | Wiley
    Details
    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
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Physical controls of Southern Ocean deep-convection variability in CMIP5 models and the Kiel Climate Model (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (13). pp. 6951-6958.
    Details
    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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    DOI: 10.1002/2017GL074087
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    The Arctic-Subarctic sea ice system is entering a seasonal regime: Implications for future Arctic amplification (2017)
    Nature Research
    In:  Scientific Reports, 7 (1). p. 4618.
    Details
    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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    DOI: 10.1038/s41598-017-04573-0
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Southern Ocean Decadal Variability and Predictability (2017)
    Springer
    In:  Current Climate Change Reports, 3 (3). pp. 163-173.
    Details
    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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    DOI: 10.1007/s40641-017-0068-8
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    The impact of variable sea ice roughness on changes in Arctic Ocean surface stress: A model study (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 121 (3). pp. 1931-1952.
    Details
    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
    Format: text
    DOI: 10.1002/2015JC011186
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Assessing the potential and side effects of ocean albedo modification in the Arctic (2015)
    In:  [Poster] In: AGU Fall Meeting 2015, 14.-18.12.2015, San Francisco, USA .
    Details
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Poster
  • 7
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    Southern Ocean Forcing of the North Atlantic at Multi-centennial Timescales in the Kiel Climate Model (2015)
    Elsevier
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 114 . pp. 39-48.
    Details
    Publication Date: 2020-08-05
    Description: Internal multi-centennial variability of open ocean deep convection in the Atlantic sector of the Southern Ocean impacts the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the Kiel Climate Model. The northward extent of Antarctic Bottom Water (AABW) strongly depends on the state of Weddell Sea deep convection. The retreat of AABW results in an enhanced meridional density gradient that drives an increase in the strength and vertical extent of the North Atlantic Deep Water (NADW) cell. This shows, for instance, as a peak in AMOC strength at 30°N about a century after Weddell Sea deep convection has ceased. The stronger southward flow of NADW is compensated by an expansion of the North Atlantic subpolar gyre and an acceleration of the North Atlantic Current, indicating greater deep water formation. Contractions of the North Atlantic subpolar gyre enable warm water anomalies, which evolved in response to deep convection events in the Southern Ocean, to penetrate farther to the north, eventually weakening the AMOC and closing a quasi-centennial cycle. Gyre contractions are accompanied by increases in sea level of up to 20 cm/century in some areas of the North Atlantic. In the Southern Ocean itself, the heat loss during the convective regime results in a sea surface height decrease on the order of 10 cm/century, with a maximum of 30 cm/century in the Weddell Sea. Hence, the impact of the Southern Ocean Centennial Variability (SOCV) on regional as well as North Atlantic sea level is of the same order of magnitude as the rise of global average sea level during the 20th century, which amounts to about 15–20 cm. This suggests that internal variability on a centennial time scale cannot be neglected a priori in assessments of 20th and 21st century AMOC and regional sea level change.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1016/j.dsr2.2014.01.018
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Assessing climate impacts and risks of ocean albedo modification in the Arctic (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 121 (5). pp. 3044-3057.
    Details
    Publication Date: 2020-11-23
    Description: The ice albedo feedback is one of the key factors of accelerated temperature increase in the high northern latitudes under global warming. This study assesses climate impacts and risks of idealized Arctic Ocean albedo modification (AOAM), a proposed climate engineering method, during transient cli- mate change simulations with varying representative concentration pathway (RCP) scenarios. We find no potential for reversing trends in all assessed Arctic climate metrics under increasing atmospheric CO2 con- centrations. AOAM only yields an initial offset during the first years after implementation. Nevertheless, sea ice loss can be delayed by 25(60) years in the RCP8.5(RCP4.5) scenario and the delayed thawing of perma- frost soils in the AOAM simulations prevents up to 40(32) Pg of carbon from being released by 2100. AOAM initially dampens the decline of the Atlantic Meridional Overturning and delays the onset of open ocean deep convection in the Nordic Seas under the RCP scenarios. Both these processes cause a subsurface warming signal in the AOAM simulations relative to the default RCP simulations with the potential to desta- bilize Arctic marine gas hydrates. Furthermore, in 2100, the RCP8.5 AOAM simulation diverts more from the 2005–2015 reference state in many climate metrics than the RCP4.5 simulation without AOAM. Considering the demonstrated risks, we conclude that concerning longer time scales, reductions in emissions remain the safest and most effective way to prevent severe changes in the Arctic.
    Type: Article , PeerReviewed
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    DOI: 10.1002/2015JC011433
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Southern Ocean deep convection in global climate models: A driver for variability of subpolar gyres and Drake Passage transport on decadal timescales (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 121 (6). pp. 3905-3925.
    Details
    Publication Date: 2019-02-01
    Description: We investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA-UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water-like water mass and affect the large-scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a precondition for subsequent strong convection activity. The combination of enhanced-gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport, and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport, and Ross Gyre strength variability are evident in most models, whether or not they host open ocean convection.
    Type: Article , PeerReviewed
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    DOI: 10.1002/2015JC011286
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Southern Ocean deep-convection as a driver of Antarctic warming events (2016)
    In:  [Poster] In: IPICS 2. Open Science Conference, 07.-11.03.2016, Hobart, Australia .
    Details
    Publication Date: 2016-11-23
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Poster
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