scholarly journals The Driving Mechanisms on Southern Ocean Upwelling Change during the Last Deglaciation

Geosciences ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 266
Author(s):  
Wei Liu ◽  
Zhengyu Liu ◽  
Shouwei Li
Keyword(s):  
Southern Ocean ◽  
Climate Model ◽  
Model Simulation ◽  
Early Stage ◽  
Surface Wind ◽  
Ekman Pumping ◽  
Last Deglaciation ◽  
Buoyancy Forcing ◽  
Local Topography ◽  
The Last Deglaciation

We explore the change in Southern Ocean upwelling during the last deglaciation, based on proxy records and a transient climate model simulation. Our analyses suggest that, beyond a conventional mechanism of the Southern Hemisphere westerlies shift, Southern Ocean upwelling is strongly influenced by surface buoyancy forcing and the local topography. Over the Antarctic Circumpolar Current region, the zonal mean and local upwelled flows exhibited distinct evolution patterns during the last deglaciation, since they are driven by different mechanisms. The zonal mean upwelling is primarily driven by surface wind stress via zonal mean Ekman pumping, whereas local upwelling is driven by both wind and buoyancy forcing, and is tightly coupled to local topography. During the early stage of the last deglaciation, the vertical extension of the upwelled flows increased downstream of submarine ridges but decreased upstream, which led to enhanced and diminished local upwelling, downstream and upstream of the submarine ridges, respectively.

Poleward shift in the Southern Ocean westerlies synchronous with the deglacial rise in atmospheric CO2

2020 ◽  
Author(s):  
William Gray ◽  
Robert Wills ◽  
Elisabeth Michel ◽  
Masa Kageyama
Keyword(s):  
Southern Ocean ◽  
Feedback Loop ◽  
Climate Model ◽  
Last Deglaciation ◽  
Westerly Winds ◽  
Poleward Shift ◽  
The Last Deglaciation ◽  
Climate Model Simulations ◽  
The Last Glacial Maximum ◽  
The Last Glacial

<p>The Southern Ocean westerly winds are hypothesised to play a key role in regulating atmospheric CO<sub>2</sub> over glacial-interglacial cycles; constraints on the paleo-latitude of the westerly winds have, however, remained allusive.  Here we use changes in the spatial pattern of planktic foraminiferal ∂<sup>18</sup>O to track changes in the latitude of the Southern Ocean polar and subtropical fronts over the last deglaciation, which are closely tied to the position of the westerly winds. We find a ~5° equator-ward shift in the position of the fronts (and thus westerlies) during the last glacial maximum relative to their Holocene position. Our reconstruction shows the poleward shift in the westerlies over deglaciation closely mirrors the sub-millennial scale variability seen in the rise in atmospheric CO<sub>2</sub>. We propose that changes in the position of the westerly winds modulate CO<sub>2</sub> via changes in the extent of Southern Ocean sea ice and circulation of the abyssal ocean. Using climate model simulations, we explore the possibility of a feedback loop by which these CO<sub>2</sub>/climatic changes may lead to further changes in the position of the westerly winds.</p>

scholarly journals Hydrographic shifts south of Australia over the last deglaciation and possible interhemispheric linkages

2021 ◽  
pp. 1-12
Author(s):  
Matthias Moros ◽  
Patrick De Deckker ◽  
Kerstin Perner ◽  
Ulysses S. Ninnemann ◽  
Lukas Wacker ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Critical Role ◽  
Temperature Response ◽  
South Australia ◽  
Last Deglaciation ◽  
Tight Coupling ◽  
Oceanic Fronts ◽  
The Last Deglaciation ◽  
Circumpolar Current

Abstract Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2 increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2 upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2 uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2 capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2 state.

scholarly journals Different ocean states and transient characteristics in Last Glacial Maximum simulations and implications for deglaciation

2013 ◽  
Vol 9 (5) ◽  
pp. 2319-2333 ◽  
Author(s):  
X. Zhang ◽  
G. Lohmann ◽  
G. Knorr ◽  
X. Xu
Keyword(s):  
Last Glacial Maximum ◽  
Model Comparison ◽  
Climate Model ◽  
Deep Ocean ◽  
Glacial Maximum ◽  
Meridional Overturning Circulation ◽  
Last Deglaciation ◽  
New Paradigm ◽  
Last Glacial ◽  
The Last Deglaciation

Abstract. The last deglaciation is one of the best constrained global-scale climate changes documented by climate archives. Nevertheless, understanding of the underlying dynamics is still limited, especially with respect to abrupt climate shifts and associated changes in the Atlantic meridional overturning circulation (AMOC) during glacial and deglacial periods. A fundamental issue is how to obtain an appropriate climate state at the Last Glacial Maximum (LGM, 21 000 yr before present, 21 ka BP) that can be used as an initial condition for deglaciation. With the aid of a comprehensive climate model, we found that initial ocean states play an important role on the equilibrium timescale of the simulated glacial ocean. Independent of the initialization, the climatological surface characteristics are similar and quasi-stationary, even when trends in the deep ocean are still significant, which provides an explanation for the large spread of simulated LGM ocean states among the Paleoclimate Modeling Intercomparison Project phase 2 (PMIP2) models. Accordingly, we emphasize that caution must be taken when alleged quasi-stationary states, inferred on the basis of surface properties, are used as a reference for both model inter-comparison and data model comparison. The simulated ocean state with the most realistic AMOC is characterized by a pronounced vertical stratification, in line with reconstructions. Hosing experiments further suggest that the response of the glacial ocean is dependent on the ocean background state, i.e. only the state with robust stratification shows an overshoot behavior in the North Atlantic. We propose that the salinity stratification represents a key control on the AMOC pattern and its transient response to perturbations. Furthermore, additional experiments suggest that the stratified deep ocean formed prior to the LGM during a time of minimum obliquity (~ 27 ka BP). This indicates that changes in the glacial deep ocean already occur before the last deglaciation. In combination, these findings represent a new paradigm for the LGM and the last deglaciation, which challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from 21 ka BP boundary conditions.

scholarly journals <sup>10</sup>Be in last deglacial climate simulated by ECHAM5-HAM – Part 1: Climatological influences on <sup>10</sup>Be deposition

2013 ◽  
Vol 9 (4) ◽  
pp. 3681-3709 ◽  
Author(s):  
U. Heikkilä ◽  
S. J. Phipps ◽  
A. M. Smith
Keyword(s):  
Sea Ice ◽  
Climate Model ◽  
Mass Conservation ◽  
High Latitudes ◽  
Last Deglaciation ◽  
Surface Cooling ◽  
Climate System Model ◽  
The Last Deglaciation ◽  
The Impact ◽  
Atmospheric Concentrations

Abstract. Reconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation two solar proxies (10Be and 14C) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on 10Be deposition by means of ECHAM5-HAM atmospheric aerosol-climate model simulations, forced by sea surface temperatures and sea ice extent created by the coupled climate system model CSIRO Mk3L. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same 10Be production rate was used in each simulation to isolate the impact of climate on 10Be deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of 10Be in the stratosphere by 30 (10k and 11k) to 80 (12k) days, heavily increasing the atmospheric concentrations. Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric 10Be concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the 10Be deposition rate changes by no more than 20% at mid- to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on 10Be deposition remain small even though atmospheric concentrations can vary significantly. Averaged over a longer period all 10Be produced has to be deposited by mass conservation. This dominates over any climatic influences on 10Be deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on 10Be in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkilä et al., 2013).

scholarly journals Major Forest Changes in Subtropical China since the Last Ice Age

Forests ◽  
2021 ◽  
Vol 12 (10) ◽  
pp. 1314
Author(s):  
Qiuchi Wan ◽  
Xiao Zhang ◽  
Yaze Zhang ◽  
Yuanfu Yue ◽  
Kangyou Huang ◽  
...  
Keyword(s):  
Forest Ecosystems ◽  
Model Simulation ◽  
Southern China ◽  
Ice Age ◽  
Evergreen Forest ◽  
Pollen Record ◽  
Seasonal Temperature ◽  
Last Deglaciation ◽  
Subtropical Zone ◽  
The Last Deglaciation

In the subtropical zone of southern China, there was a considerable conversion of forests from deciduous to evergreen broadleaf in the early Holocene. However, the exact timing of this vegetation change and its relationship to climate are still unclear. We examined a high-resolution pollen record collected in the mid-subtropical zone and then performed a correlation with regional data to reconstruct the history of forest ecosystems since the last deglaciation. Our data show that the expansion of the evergreen plant component already occurred at low elevations during the last deglaciation. The subtropical mountain landscape was not recolonized by evergreen forests until the mid-Holocene at about 8.1 ka BP. Based on fossil pollen reconstruction and climate model simulation, we conclude that the primary increase in evergreen components of subtropical ecosystems was triggered by postglacial temperature increase, and that a complete conversion from deciduous to evergreen forest ecosystems did not occur until Holocene winter temperatures and seasonal temperature contrast reached a threshold suitable for the growth and persistence of evergreen tree species.

scholarly journals Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events

2020 ◽  
Vol 6 (42) ◽  
pp. eabb3807
Author(s):  
Tao Li ◽  
Laura F. Robinson ◽  
Tianyu Chen ◽  
Xingchen T. Wang ◽  
Andrea Burke ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Deep Sea ◽  
Deep Ocean ◽  
Biological Pump ◽  
Last Deglaciation ◽  
Proxy Data ◽  
Ocean Ventilation ◽  
Carbon Release ◽  
The Last Deglaciation

The Southern Ocean plays a crucial role in regulating atmospheric CO2 on centennial to millennial time scales. However, observations of sufficient resolution to explore this have been lacking. Here, we report high-resolution, multiproxy records based on precisely dated deep-sea corals from the Southern Ocean. Paired deep (∆14C and δ11B) and surface (δ15N) proxy data point to enhanced upwelling coupled with reduced efficiency of the biological pump at 14.6 and 11.7 thousand years (ka) ago, which would have facilitated rapid carbon release to the atmosphere. Transient periods of unusually well-ventilated waters in the deep Southern Ocean occurred at 16.3 and 12.8 ka ago. Contemporaneous atmospheric carbon records indicate that these Southern Ocean ventilation events are also important in releasing respired carbon from the deep ocean to the atmosphere. Our results thus highlight two distinct modes of Southern Ocean circulation and biogeochemistry associated with centennial-scale atmospheric CO2 jumps during the last deglaciation.

The Impact of Poleward Moisture and Sensible Heat Flux on Arctic Winter Sea Ice Variability*

2015 ◽  
Vol 28 (13) ◽  
pp. 5030-5040 ◽  
Author(s):  
Hyo-Seok Park ◽  
Sukyoung Lee ◽  
Seok-Woo Son ◽  
Steven B. Feldstein ◽  
Yu Kosaka
Keyword(s):  
Sea Ice ◽  
Climate Model ◽  
Model Simulation ◽  
Early Stage ◽  
Sensible Heat Flux ◽  
The Arctic ◽  
Sea Ice Concentration ◽  
Sea Ice Thickness ◽  
Surface Warming ◽  
The Impact

Abstract The surface warming in recent decades has been most rapid in the Arctic, especially during the winter. Here, by utilizing global reanalysis and satellite datasets, it is shown that the northward flux of moisture into the Arctic during the winter strengthens the downward infrared radiation (IR) by 30–40 W m−2 over 1–2 weeks. This is followed by a decline of up to 10% in sea ice concentration over the Greenland, Barents, and Kara Seas. A climate model simulation indicates that the wind-induced sea ice drift leads the decline of sea ice thickness during the early stage of the strong downward IR events, but that within one week the cumulative downward IR effect appears to be dominant. Further analysis indicates that strong downward IR events are preceded several days earlier by enhanced convection over the tropical Indian and western Pacific Oceans. This finding suggests that sea ice predictions can benefit from an improved understanding of tropical convection and ensuing planetary wave dynamics.

Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation

Science ◽  
2014 ◽  
Vol 346 (6214) ◽  
pp. 1223-1227 ◽  
Author(s):  
Bette L. Otto-Bliesner ◽  
James M. Russell ◽  
Peter U. Clark ◽  
Zhengyu Liu ◽  
Jonathan T. Overpeck ◽  
...  
Keyword(s):  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Abrupt Onset ◽  
Meridional Overturning Circulation ◽  
Last Deglaciation ◽  
African Rainfall ◽  
Meridional Overturning ◽  
The Last Deglaciation ◽  
Precipitation Changes ◽  
Humid Period

During the last deglaciation, wetter conditions developed abruptly ~14,700 years ago in southeastern equatorial and northern Africa and continued into the Holocene. Explaining the abrupt onset and hemispheric coherence of this early African Humid Period is challenging due to opposing seasonal insolation patterns. In this work, we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical African precipitation changes. Our results show that meltwater-induced reduction in the Atlantic meridional overturning circulation (AMOC) during the early deglaciation suppressed precipitation in both regions. Once the AMOC reestablished, wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas (GHG) concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase.

Intercomparison of the Southern Ocean Circulations in IPCC Coupled Model Control Simulations

2006 ◽  
Vol 19 (18) ◽  
pp. 4560-4575 ◽  
Author(s):  
Joellen L. Russell ◽  
Ronald J. Stouffer ◽  
Keith W. Dixon
Keyword(s):  
Southern Ocean ◽  
Large Scale ◽  
Heat Storage ◽  
Climate Model ◽  
Model Simulation ◽  
Coupled Model ◽  
Heat Fluxes ◽  
Wind Forcing ◽  
Transient Climate Response ◽  
Model Control

Abstract The analyses presented here focus on the Southern Ocean as simulated in a set of global coupled climate model control experiments conducted by several international climate modeling groups. Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from it being the region where most of the transformation of the World Ocean’s water masses occurs. In climate change experiments that simulate greenhouse gas–induced warming, Southern Ocean air–sea heat fluxes and three-dimensional circulation patterns make it a region where much of the future oceanic heat storage takes place, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. Factors such as the Southern Ocean’s wind forcing, heat, and salt budgets are linked to the structure and transport of the ACC in ways that have not been expressed clearly in the literature. These links are explored here in a coupled model context by analyzing a sizable suite of preindustrial control experiments associated with the forthcoming Intergovernmental Panel on Climate Change’s Fourth Assessment Report. A framework is developed that uses measures of coupled model simulation characteristics, primarily those related to the Southern Ocean wind forcing and water mass properties, to allow one to categorize, and to some extent predict, which models do better or worse at simulating the Southern Ocean and why. Hopefully, this framework will also lead to increased understanding of the ocean’s response to climate changes.

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