scholarly journals Masked millennial-scale climate variations in South West Africa during the last glaciation

2011 ◽  
Vol 7 (5) ◽  
pp. 3511-3540
Author(s):  
I. Hessler ◽  
L. Dupont ◽  
D. Handiani ◽  
A. Paul ◽  
U. Merkel ◽  
...  
Keyword(s):  
Climatic Conditions ◽  
Meridional Overturning Circulation ◽  
Pollen Record ◽  
Oceanic Circulation ◽  
Vegetation Development ◽  
Climate Reconstructions ◽  
South West Africa ◽  
Meridional Overturning ◽  
Abrupt Shifts ◽  
Millennial Scale

Abstract. Large and abrupt shifts between extreme climatic conditions characterise the last glacial and deglacial period and are thought to be transmitted by the atmospheric and oceanic circulation. Millennial-scale climatic shifts associated with North Atlantic Heinrich Stadials (HSs) are thought to be closely related to a reduction of the Atlantic Meridional Overturning Circulation (AMOC), which lead to the accumulation of heat in the South Atlantic and a southward shift of the Intertropical Convergence Zone (ITCZ). Due to the linkage between the oceans and the atmosphere it is assumed that HSs also influence the vegetation composition in the African tropics. To address the connection between tropical African vegetation development and high-latitude climate change we present a high-resolution marine pollen record from ODP Site 1078 (off Angola) covering the period 50–10 ka BP. Although several tropical African vegetation and climate reconstructions indicate an impact of HSs on the African subcontinent, our vegetation record shows no response. Model simulations conducted with an Earth System Model of Intermediate Complexity (EMIC) including a dynamical vegetation component lead to the hypothesis that the vegetation response during HSs might have been muted by mechanisms that partly cancel each other.

scholarly journals Masked millennial-scale climate variations in South West Africa during the last glaciation

2012 ◽  
Vol 8 (2) ◽  
pp. 841-853 ◽  
Author(s):  
I. Hessler ◽  
L. Dupont ◽  
D. Handiani ◽  
A. Paul ◽  
U. Merkel ◽  
...  
Keyword(s):  
Climate Change ◽  
Pollen Record ◽  
Climate Variations ◽  
Vegetation Composition ◽  
Climatic Response ◽  
Tropical Africa ◽  
Vegetation Development ◽  
Climate Reconstructions ◽  
South West Africa ◽  
Vegetation And Climate

Abstract. To address the connection between tropical African vegetation development and high-latitude climate change we present a high-resolution pollen record from ODP Site 1078 (off Angola) covering the period 50–10 ka BP. Although several tropical African vegetation and climate reconstructions indicate an impact of Heinrich Stadials (HSs) in Southern Hemisphere Africa, our vegetation record shows no response. Model simulations conducted with an Earth System Model of Intermediate Complexity including a dynamical vegetation component provide one possible explanation. Because both precipitation and evaporation increased during HSs and their effects nearly cancelled each other, there was a negligible change in moisture supply. Consequently, the resulting climatic response to HSs might have been too weak to noticeably affect the vegetation composition in the study area. Our results also show that the response to HSs in southern tropical Africa neither equals nor mirrors the response to abrupt climate change in northern Africa.

The complexity of millennial-scale variability in southwestern Europe during MIS 11

2016 ◽  
Vol 86 (3) ◽  
pp. 373-387 ◽  
Author(s):  
Dulce Oliveira ◽  
Stephanie Desprat ◽  
Teresa Rodrigues ◽  
Filipa Naughton ◽  
David Hodell ◽  
...  
Keyword(s):  
Climatic Variability ◽  
Oscillation Mode ◽  
Hydrological Regime ◽  
Meridional Overturning Circulation ◽  
Ice Volume ◽  
Sst Variability ◽  
Prolonged Duration ◽  
Meridional Overturning ◽  
Baseline Climate ◽  
Millennial Scale

AbstractClimatic variability of Marine Isotope Stage (MIS) 11 is examined using a new high-resolution direct land—sea comparison from the SW Iberian margin Site U1385. This study, based on pollen and biomarker analyses, documents regional vegetation, terrestrial climate and sea surface temperature (SST) variability. Suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in SW Iberia throughout MIS 11. Only the most severe events on land are coeval with SST decreases, under larger ice volume conditions. Our study shows that the diverse expression (magnitude, character and duration) of the millennial-scale cooling events in SW Europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states. Repeated atmospheric shifts recalling the positive North Atlantic Oscillation mode, inducing dryness in SW Iberia without systematical SST changes, would prevail during low ice volume conditions. In contrast, disruption of the Atlantic meridional overturning circulation (AMOC), related to iceberg discharges, colder SST and increased hydrological regime, would be responsible for the coldest and driest episodes of prolonged duration in SW Europe.

scholarly journals Testing the Tropical Trigger Hypothesis of Abrupt Climate Variability

2021 ◽  
Vol 9 ◽  
Author(s):  
Jack W. Oughton ◽  
Dunia H. Urrego
Keyword(s):  
South America ◽  
Climate Variability ◽  
Driving Forces ◽  
Temperature Fluctuations ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Abrupt Changes ◽  
Meridional Overturning ◽  
Abrupt Shifts

Dansgaard-Oeschger oscillations (DOs) are abrupt shifts in climate, which are dramatic temperature fluctuations observed in Greenland and recorded globally. These abrupt changes are associated with the slowing and shutting down of the Atlantic Meridional Overturning Circulation (AMOC), but despite their importance the driving forces of DOs are not fully understood. Here we assess the role of the AMOC during DOs, the Northern vs Southern Hemisphere control on AMOC, and the possibility of neotropical moisture as a driver for abrupt climate variability. During DOs, South America has recorded a disparity between the degree of warming, and the change in precipitation at different sites. Based on our current understanding, we propose likely oceanic and continental changes in tropical South America that can help disentangle the triggers of these events. With the margins of error associated with dating sources of palaeo-data, the need for an independent chronology with multiple proxies recorded in the same record, could offer the information needed to understand the driving forces of DOs.

The Role of Oceanic Heat Transport and Wind Stress Forcing in Abrupt Millennial-Scale Climate Transitions

2010 ◽  
Vol 23 (9) ◽  
pp. 2233-2256 ◽  
Author(s):  
Olivier Arzel ◽  
Alain Colin de Verdière ◽  
Matthew H. England
Keyword(s):  
Wind Stress ◽  
Mixed Boundary ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Coupled Models ◽  
Atlantic Sector ◽  
Wide Range ◽  
Meridional Overturning ◽  
Ocean Models ◽  
Millennial Scale

Abstract The last glacial period was punctuated by rapid climate shifts, known as Dansgaard–Oeschger events, with strong imprint in the North Atlantic sector suggesting that they were linked with the Atlantic meridional overturning circulation. Here an idealized single-hemisphere three-dimensional ocean–atmosphere–sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the Holocene. Focusing on the physics of noise-free millennial oscillations under steady external (solar) forcing, it was shown that cold climates become unstable, that is, exhibit abrupt millennial-scale transitions, for significantly lower freshwater fluxes than warm climates, in agreement with previous studies making use of zonally averaged coupled models. This fundamental difference is a direct consequence of the weaker stratification of the glacial ocean, mainly caused by upper-ocean cooling. Using a two-hemisphere configuration of a coupled climate model of intermediate complexity, it is shown that this result is robust to the added presence of a bottom water mass of southern origin. The analysis reveals that under particular conditions, a pronounced interdecadal variability develops during warm interstadials. While the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions, these interstadial–interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes. The wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing. In particular, it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of Dansgaard–Oeschger cycles.

scholarly journals Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

2014 ◽  
Vol 10 (1) ◽  
pp. 63-77 ◽  
Author(s):  
L. Menviel ◽  
A. Timmermann ◽  
T. Friedrich ◽  
M. H. England
Keyword(s):  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
External Boundary ◽  
Ice Volume ◽  
Freshwater Fluxes ◽  
Meridional Overturning ◽  
Northern Hemispheric ◽  
The Continuum ◽  
Millennial Scale

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger events is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-D intermediate complexity earth system model covering the period 50 to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing northern hemispheric ice volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our results are consistent with the idea that variations in ice-sheet calving and subsequent changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of glacial millennial-scale variability seen in paleorecords across the globe.

scholarly journals Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

2020 ◽  
Vol 6 (50) ◽  
pp. eabd1654
Author(s):  
J. W. B. Rae ◽  
W. R. Gray ◽  
R. C. J. Wills ◽  
I. Eisenman ◽  
B. Fitzhugh ◽  
...  
Keyword(s):  
North Pacific ◽  
Global Climate ◽  
Regional Climate ◽  
Climatic Conditions ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
The Pacific ◽  
Meridional Overturning ◽  
And Migration

Although the Pacific Ocean is a major reservoir of heat and CO2, and thus an important component of the global climate system, its circulation under different climatic conditions is poorly understood. Here, we present evidence that during the Last Glacial Maximum (LGM), the North Pacific was better ventilated at intermediate depths and had surface waters with lower nutrients, higher salinity, and warmer temperatures compared to today. Modeling shows that this pattern is well explained by enhanced Pacific meridional overturning circulation (PMOC), which brings warm, salty, and nutrient-poor subtropical waters to high latitudes. Enhanced PMOC at the LGM would have lowered atmospheric CO2—in part through synergy with the Southern Ocean—and supported an equable regional climate, which may have aided human habitability in Beringia, and migration from Asia to North America.

The Agulhas Bank Circulation

2020 ◽  
Author(s):  
Ricardo Matano
Keyword(s):  
Coastal Region ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Oceanic Circulation ◽  
Boundary Current ◽  
Benguela Current ◽  
Ocean Region ◽  
Meridional Overturning ◽  
Shelf Region

<p>The southern tip of Africa is the gateway between the Indian and Atlantic oceans, one of the most widely recognized chokepoints of the meridional overturning circulation (MOC). The oceanic circulation in this region connects a western boundary current, the Agulhas Current, to an eastern boundary current, the Benguela Current, a connection not replicated elsewhere and quite important, not only because of its peculiarity, but also because of its role in the MOC. During the last few decades numerous international research programs have collected large amounts of oceanographic data of this region. All these efforts, however, have been largely focused on the deep-ocean, leaving the coastal region practically unattended. In this presentation we will use the results of a suite of process-oriented numerical experiments to discuss the circulation along the Agulhas Bank (AB)—the shelf region sandwiched between the eastern and western margins of the African continent; in particular to illustrate its connections and interactions with the deep-ocean region. As we shall show these shelf/deep-ocean interactions, are not only important to the shelf but also to the Indian/Atlantic interoceanic exchange.</p>

scholarly journals Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice-sheet–climate coupled model

2021 ◽  
Vol 17 (5) ◽  
pp. 2179-2199
Author(s):  
Aurélien Quiquet ◽  
Didier M. Roche ◽  
Christophe Dumas ◽  
Nathaëlle Bouttes ◽  
Fanny Lhardy
Keyword(s):  
Ice Sheet ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Oceanic Circulation ◽  
Last Deglaciation ◽  
Grounding Line ◽  
Meridional Overturning ◽  
Reference Experiment ◽  
The Last Deglaciation

Abstract. The last deglaciation offers an unique opportunity to understand the climate–ice-sheet interactions in a global warming context. In this paper, to tackle this question, we use an Earth system model of intermediate complexity coupled to an ice sheet model covering the Northern Hemisphere to simulate the last deglaciation and the Holocene (26–0 ka). We use a synchronous coupling every year between the ice sheet and the rest of the climate system and we ensure a closed water cycle considering the release of freshwater flux to the ocean due to ice sheet melting. Our reference experiment displays a gradual warming in response to the forcings, with no abrupt changes. In this case, while the amplitude of the freshwater flux to the ocean induced by ice sheet retreat is realistic, it is sufficient to shut down the Atlantic meridional overturning circulation from which the model does not recover within the time period simulated. However, with reduced freshwater flux we are nonetheless able to obtain different oceanic circulation evolutions, including some abrupt transitions between shut-down and active circulation states in the course of the deglaciation. The inclusion of a parameterisation for the sinking of brines around Antarctica also produces an abrupt recovery of the Atlantic meridional overturning circulation, absent in the reference experiment. The fast oceanic circulation recoveries lead to abrupt warming phases in Greenland. Our simulated ice sheet geometry evolution is in overall good agreement with available global reconstructions, even though the abrupt sea level rise at 14.6 ka is underestimated, possibly because the climate model underestimates the millennial-scale temperature variability. In the course of the deglaciation, large-scale grounding line instabilities are simulated both for the Eurasian and North American ice sheets. The first instability occurs in the Barents–Kara seas for the Eurasian ice sheet at 14.5 ka. A second grounding line instability occurs ca. 12 ka in the proglacial lake that formed at the southern margin of the North American ice sheet. With additional asynchronously coupled experiments, we assess the sensitivity of our results to different ice sheet model choices related to surface and sub-shelf mass balance, ice deformation and grounding line representation. While the ice sheet evolutions differ within this ensemble, the global climate trajectory is only weakly affected by these choices. In our experiments, only the abrupt shifts in the oceanic circulation due to freshwater fluxes are able to produce some millennial-scale variability since no self-generating abrupt transitions are simulated without these fluxes.

scholarly journals Abrupt shifts of the Sahara–Sahel boundary during Heinrich stadials

2013 ◽  
Vol 9 (3) ◽  
pp. 1181-1191 ◽  
Author(s):  
J. A. Collins ◽  
A. Govin ◽  
S. Mulitza ◽  
D. Heslop ◽  
M. Zabel ◽  
...  
Keyword(s):  
High Resolution ◽  
West Africa ◽  
Sand Dunes ◽  
Sediment Cores ◽  
Meridional Overturning Circulation ◽  
Saharan Dust ◽  
Major Element Composition ◽  
Meridional Overturning ◽  
Discontinuous Nature ◽  
Abrupt Shifts

Abstract. Relict dune fields that are found as far south as 14° N in the modern-day African Sahel are testament to equatorward expansions of the Sahara desert during the Late Pleistocene. However, the discontinuous nature of dune records means that abrupt millennial-timescale climate events are not always resolved. High-resolution marine core studies have identified Heinrich stadials as the dustiest periods of the last glacial in West Africa although the spatial evolution of dust export on millennial timescales has so far not been investigated. We use the major-element composition of four high-resolution marine sediment cores to reconstruct the spatial extent of Saharan-dust versus river-sediment input to the continental margin from West Africa over the last 60 ka. This allows us to map the position of the sediment composition corresponding to the Sahara–Sahel boundary. Our records indicate that the Sahara–Sahel boundary reached its most southerly position (13° N) during Heinrich stadials and hence suggest that these were the periods when the sand dunes formed at 14° N on the continent. Heinrich stadials are associated with cold North Atlantic sea surface temperatures which appear to have triggered abrupt increases of aridity and wind strength in the Sahel. Our study illustrates the influence of the Atlantic meridional overturning circulation on the position of the Sahara–Sahel boundary and on global atmospheric dust loading.

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