Southern Ocean deep convection as a driver of Antarctic warming events

Pedro, J. B.; Martin, Torge; Steig, Eric J.; Jochum, Markus; Park, Wonsun; Rasmussen, S. O.

doi:10.1002/2016gl067861

Cited by 45 publications

(47 citation statements)

References 58 publications

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“…3) we tentatively correlate this period in our record to the end of DO7 and the minimum between AIM6 and AIM7. This corroborates the overlaps and time lags that have been postulated for DO and AIM events (Markle et al, 2016;Pedro et al, 2018;WAIS Divide Project Members, 2015). At 34 ka we note a temperature peak at the onset of AIM6 (Figs.…”

Section: A Detailed Hydroclimate Scenario For the Central South Atlanticsupporting

confidence: 84%

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Björck¹,

Sjolte²,

Ljung³

et al. 2019

Clim. Past

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Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

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Section: A Detailed Hydroclimate Scenario For the Central South Atlanticsupporting

confidence: 84%

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Björck¹,

Sjolte²,

Ljung³

et al. 2019

Clim. Past

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“…Spurious open ocean deep convection creates AABW through a rather abrupt mechanism and enhances bottom layer ventilation and heat/carbon uptake, which inserts errors in climate estimates (e.g., Heuzé et al, 2013). The appearance of spurious open ocean deep convection in the Southern Ocean simulations can go even further and cause the warming of the abyssal layer, the cooling of surface waters and atmospheric warming Pedro et al, 2016). The mechanisms of bottom water formation in some of the reanalyses investigated here agree with those ideas: two of the three reanalysis products formed AABW by deep convection through the appearance of open ocean polynyas.…”

Section: Discussionmentioning

confidence: 99%

On deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

2017

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Abstract. Open ocean deep convection is a common source of error in the representation of Antarctic Bottom Water (AABW) formation in ocean general circulation models. Although those events are well described in non-assimilatory ocean simulations, the recent appearance of a massive open ocean polynya in the Estimating the Circulation and Climate of the Ocean Phase II reanalysis product (ECCO2) raises questions on which mechanisms are responsible for those spurious events and whether they are also present in other state-of-the-art assimilatory reanalysis products. To investigate this issue, we evaluate how three recently released high-resolution ocean reanalysis products form AABW in their simulations. We found that two of the products create AABW by open ocean deep convection events in the Weddell Sea that are triggered by the interaction of sea ice with the Warm Deep Water, which shows that the assimilation of sea ice is not enough to avoid the appearance of open ocean polynyas. The third reanalysis, My Ocean University Reading UR025.4, creates AABW using a rather dynamically accurate mechanism. The UR025.4 product depicts both continental shelf convection and the export of Dense Shelf Water to the open ocean. Although the accuracy of the AABW formation in this reanalysis product represents an advancement in the representation of the Southern Ocean dynamics, the differences between the real and simulated processes suggest that substantial improvements in the ocean reanalysis products are still needed to accurately represent AABW formation.

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“…Marine sediment cores in the North Atlantic have revealed the fluctuations of AMOC in the last glacial cycle (Böhm et al, 2015;Henry et al, 2016), which have been regarded as the leading hypotheses in interpreting D-O cycles (Rahmstorf, 2002;Henry et al, 2016). In climate models, the fluctuations of North Atlantic ocean circulation, either realized by freshwater fluxes or spontaneously occurring, are tightly associated with the change of Greenland temperature that mimics D-O events (Ganopolski and Rahmstorf, 2001;Menviel et al, 2014;Peltier and Vettoretti, 2014). Such variations of AMOC pose challenges on climate models, and those with a reasonable AMOC representation are best suited for studying paleoclimates, especially for the abrupt climate change events that are tightly associated with AMOC variations.…”

Section: Introductionmentioning

confidence: 99%

Description and evaluation of NorESM1-F: a fast version of the Norwegian Earth System Model (NorESM)

et al. 2019

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A new computationally efficient version of the Norwegian Earth System Model (NorESM) is presented. This new version (here termed NorESM1-F) runs about 2.5 times faster (e.g., 90 model years per day on current hardware) than the version that contributed to the fifth phase of the Coupled Model Intercomparison project (CMIP5), i.e., NorESM1-M, and is therefore particularly suitable for multimillennial paleoclimate and carbon cycle simulations or large ensemble simulations. The speed-up is primarily a result of using a prescribed atmosphere aerosol chemistry and a tripolar ocean-sea ice horizontal grid configuration that allows an increase of the ocean-sea ice component time steps. Ocean biogeochemistry can be activated for fully coupled and semi-coupled carbon cycle applications. This paper describes the model and evaluates its performance using observations and NorESM1-M as benchmarks. The evaluation emphasizes model stability, important large-scale features in the ocean and sea ice components, internal variability in the coupled system, and climate sensitivity. Simulation results from NorESM1-F in general agree well with observational estimates and show evident improvements over NorESM1-M, for example, in the strength of the meridional overturning circulation and sea ice simulation, both important metrics in simulating past and future climates. Whereas NorESM1-M showed a slight global cool bias in the upper oceans, NorESM1-F exhibits a global warm bias. In general, however, NorESM1-F has more similarities than dissimilarities compared to NorESM1-M, and some biases and deficiencies known in NorESM1-M remain.

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Southern Ocean deep convection as a driver of Antarctic warming events

Cited by 45 publications

References 58 publications

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

On deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

Description and evaluation of NorESM1-F: a fast version of the Norwegian Earth System Model (NorESM)

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