scholarly journals Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial-interglacial cycle

2002 ◽  
Vol 17 (2) ◽  
pp. 7-1-7-12 ◽  
Author(s):  
Andreas Schmittner ◽  
Amy C. Clement
Keyword(s):  
High Latitude ◽  
Thermohaline Circulation ◽  
Last Glacial ◽  
Freshwater Forcing ◽  
The Last Glacial

High-Latitude Forcing of the South American Summer Monsoon During the Last Glacial

Science ◽  
2012 ◽  
Vol 335 (6068) ◽  
pp. 570-573 ◽  
Author(s):  
L. C. Kanner ◽  
S. J. Burns ◽  
H. Cheng ◽  
R. L. Edwards
Keyword(s):  
Summer Monsoon ◽  
High Latitude ◽  
South American ◽  
The South ◽  
Last Glacial ◽  
The Last Glacial

scholarly journals South African speleothems reveal influence of high- and lowlatitude forcing over the past 113.5 k.y.

Geology ◽  
2021 ◽  
Author(s):  
Brian Chase ◽  
Chris Harris ◽  
Maarten J. de Wit ◽  
Jan Kramers ◽  
Sean Doel ◽  
...  
Keyword(s):  
South African ◽  
High Latitude ◽  
Austral Summer ◽  
Summer Rainfall ◽  
Last Glacial ◽  
Change Dynamics ◽  
The Past ◽  
Stage 4 ◽  
Insolation Forcing ◽  
The Last Glacial

Variation in δ18O and δ13C values in a speleothem from the Cango Caves in southernmost South Africa enable the construction of coherent regional composite records spanning the past 113,500 yr. Novel for the region in terms of both their length and detail, these records indicate environmental and climatic changes that both are consistent with records from the wider region and show a clear evolution from low- to high-latitude forcing dominance across the last glacial period. Prior to ca. 70 ka, the influence of direct low-latitude insolation forcing is expressed through increases in summer rainfall during austral summer insolation maxima. With the onset of Marine Isotope Stage 4, cooler global conditions and the development of high-latitude ice sheets appear to have supplanted direct insolation forcing as the dominant driver pacing patterns of environmental change, with records from the Southern and Northern Hemisphere tropics exhibiting a positive relationship until after the Last Glacial Maximum. These results highlight the complexity of South African climate change dynamics as a response to changing global boundary conditions and provide a critical reference for regional and global comparisons.

scholarly journals Ocean Gyres and Abrupt Change in the Thermohaline Circulation: A Conceptual Analysis

2005 ◽  
Vol 18 (13) ◽  
pp. 2403-2416 ◽  
Author(s):  
Hannah Longworth ◽  
Jochem Marotzke ◽  
Thomas F. Stocker
Keyword(s):  
High Latitude ◽  
General Circulation ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Abrupt Change ◽  
General Circulation Models ◽  
Water Vapor Transport ◽  
Bifurcation Structure ◽  
Conceptual Foundation ◽  
Freshwater Forcing

Abstract The implications are investigated of representing ocean gyre circulations by a diffusion term in the Stommel and Rooth box models of the thermohaline circulation (THC) in one and two hemispheres, respectively. The approach includes mostly analytical solution and study of the bifurcation structure, but also numerical integration and feedback analysis. Sufficient diffusion (gyre strength) eliminates multiple equilibria from either model, highlighting the need for accurate gyre circulation strength in general circulation models (GCMs) when considering the potential for abrupt climate change associated with THC shutdown. With diffusion, steady-state flow strength in the Rooth model depends on freshwater forcing (i.e., implied atmospheric water vapor transport) in both hemispheres, not only on that in the upwelling hemisphere, as in the nondiffusive case. With asymmetric freshwater forcing, two solutions (strong stable and weak unstable) are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere. Under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle-node bifurcation (if diffusion is sufficiently strong to prevent a subcritical Hopf bifurcation first), followed by flow reversal. Thus, the bifurcation structure with respect to freshwater forcing of the diffusive Rooth model of two-hemisphere THC is similar to that of the Stommel model of single-hemisphere THC, albeit with a very different dynamical interpretation. Gyre circulations stabilize high-latitude sinking in the Stommel model. In the Rooth model, gyre circulations only stabilize high-latitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere, by an amount that increases with diffusion. The values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with GCMs, suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric THC, and also for the potential of the Atlantic THC to undergo abrupt change.

scholarly journals Major diversification of caribou shaped by glacial cycles before the Last Glacial Maximum

2020 ◽  
Author(s):  
Rebecca S. Taylor ◽  
Micheline Manseau ◽  
Cornelya F. C. Klütsch ◽  
Jean L. Polfus ◽  
Audrey Steedman ◽  
...  
Keyword(s):  
Last Glacial Maximum ◽  
High Latitude ◽  
Glacial Maximum ◽  
Effective Population ◽  
Glacial Cycles ◽  
Last Glacial ◽  
Genetic Patterns ◽  
Population Sizes ◽  
The Last Glacial Maximum ◽  
The Last Glacial

AbstractPleistocene glacial cycles influenced the diversification of high-latitude wildlife species through recurrent periods of range contraction, isolation, divergence, and expansion from refugia and subsequent admixture of refugial populations. For many taxa, research has focused on genetic patterns since the Last Glacial Maximum (LGM), however glacial cycles before the LGM likely impacted genomic variation which may influence contemporary genetic patterns. We investigate diversification and the introgressive history of caribou (Rangifer tarandus) in western Canada using 33 high-coverage whole genomes coupled with larger-scale mitochondrial data. Contrary to the well-established paradigm that caribou ecotypes and contemporary genetic diversity arose from two major lineages in separate refugia during the LGM, a Beringian-Eurasian (BEL) and a North American (NAL) lineage, we found that the major diversifications of caribou occurred much earlier at around 110 kya, the start of the last glacial period. Additionally, we found effective population sizes of some caribou reaching ~700,000 to 1,000,000 individuals, one of the highest recorded historical effective population sizes for any mammal species thus far. Mitochondrial analyses dated introgression events prior to the LGM dating to 20-30 kya and even more ancient at 60 kya, coinciding with colder periods with extensive ice coverage, further demonstrating the importance of glacial cycles and events prior to the LGM in shaping demographic history. Reconstructing the origins and differential introgressive history has implications for predictions on species responses under climate change. Our results highlight the need to investigate pre-LGM demographic patterns to fully reconstruct the origin of species diversity, especially for high-latitude species.

Convulsed western Mediterranean thermohaline circulation during the last glacial-interglacial transition from neodymium isotope ratios

2020 ◽  
Author(s):  
Jaime Frigola ◽  
Leopoldo D. Pena ◽  
Albert Català ◽  
Jose Noel Pérez-Asensio ◽  
Ester Garcia-Solsona ◽  
...  
Keyword(s):  
Water Column ◽  
Deep Water ◽  
Thermohaline Circulation ◽  
Deep Convection ◽  
Sediment Cores ◽  
Western Mediterranean ◽  
Early Holocene ◽  
Last Glacial ◽  
Water Column Stratification ◽  
The Last Glacial

<p>Severe changes in the western Mediterranean (WMed) thermohaline circulation occurred during the last major glacial-interglacial transition, mostly due to oceanographic and atmospheric re-adjustments. Changes in water column stratification or deep water convection are controlled by the evaporation-precipitation balance of the basin but also by temperature and the intensity of local winds.</p><p>A general freshening of the inflowing Atlantic waters through the Strait of Gibraltar during the central phase of the Heinrich Stadial (HS) 1 due to the intense iceberg melting favoured increased water column stratification in the WMed and reduced deep convection, as observed by synchronous depletions in both the benthic δ<sup>13</sup>C and grain-size signals from a location close to the deep convection source. Post-glacial sea level rise further enhanced stratification of the WMed, with minimum deep convection achieved at a later time, and leading to the formation of the last Organic Rich Layer (ORL) in the Alboran Sea. These stratified conditions in the surface WMed were also favoured by more stable atmospheric conditions related to summer insolation maxima.</p><p>The integrated study of benthic foraminifera assemblages and TOC records from three sediment cores distributed between 900 m to 2400 m water depths in the basin corroborate the deterioration of deep water ventilation conditions during this period but also provide a first glimpse on an earlier re-ventilation of the intermediate layer. However, the Deep Western Mediterranean Waters did not re-ventilated until the early Holocene, when deep water convection in the Gulf of Lion was resumed by a major cold event with intensified north-westerly winds.</p><p>We review these key changes in the western Mediterranean thermohaline system by means of neodymium isotopic ratios (ε<sub>Nd</sub>) measurements on Fe-Mn encrusted planktonic foraminifera from 4 sediment cores covering a depth profile from 650 mwd to 2400 mwd. Neodymium isotope ratios are used here as a conservative water mass mixing proxy thus allowing to investigate changes of the western Mediterranean thermohaline circulation during the last 22 kyr. Validation of Fe-Mn encrusted foraminifera ε<sub>Nd</sub> as tracer of deep water masses in the Mediterranean has been carried out by means of analysis in core-top samples from depths representing main Mediterranean water masses and comparison with published seawater ε<sub>Nd</sub> values.</p><p>Our results show a quite homogeneous ε<sub>Nd</sub> signal for intermediate and deep waters during the end of the last glacial period, prior to the deposition of the ORL, likely suggesting a very well homogenized water column. However, significant differences can be observed between the deepest record and those of intermediate depths during the ORL time interval and the early Holocene (14-6 kyr), pointing to important changes in the western Mediterranean thermohaline circulation and likely suggesting different waters mass sources for deep and intermediate levels.</p>

Joint influence of surface erosion and high-latitude ice-sheet extent on Asian dust cycle during the last glacial maximum

2019 ◽  
Vol 157 (5) ◽  
pp. 777-789 ◽  
Author(s):  
Xinzhou Li ◽  
Xiaodong Liu ◽  
Haibo Zhou
Keyword(s):  
Last Glacial Maximum ◽  
High Latitude ◽  
Ice Sheet ◽  
Glacial Maximum ◽  
Gas Content ◽  
Surface Erosion ◽  
Dust Emissions ◽  
Last Glacial ◽  
The Last Glacial Maximum ◽  
The Last Glacial

AbstractThe dust cycle plays an important role in the long-term evolution of the climate and environment. In this paper, an improved climate model including aerosol processes was used to carry out a set of sensitivity experiments and comparative analyses of the effects of high-latitude ice-sheet extent and abnormal dust erosion, as well as Earth’s orbital parameters and atmospheric greenhouse gas content, on dust activities during the last glacial maximum. The comparative analysis found that incorporating the abnormal surface erosion factor alone could increase dust emissions by 2.77-fold and 3.77-fold of the present-day global and Asian dust emissions, respectively. The high-latitude ice-sheet factor caused global dust emissions to increase by 1.25-fold that of the present day. Sensitivity experiments showed that increased surface erosion in Asia during the last glacial maximum made the greatest contribution to the increased dust emissions in Asia, followed by the high-latitude ice-sheet factor, while the contributions of the greenhouse gas content and orbital parameters were relatively weak. Strong dust emissions during the glacial period were therefore not only dependent on the development of the high-latitude ice sheets but were strongly associated with the underlying surface characteristics of local dust source regions.

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