scholarly journals High-latitude obliquity forcing drives the agulhas leakage

2011 ◽  
Vol 7 (3) ◽  
pp. 2193-2215 ◽  
Author(s):  
T. Caley ◽  
J.-H. Kim ◽  
B. Malaizé ◽  
J. Giraudeau ◽  
T. Laepple ◽  
...  
Keyword(s):  
High Latitude ◽  
Decadal Variability ◽  
Global Climate ◽  
Decisive Role ◽  
Meridional Overturning Circulation ◽  
Climate Forcing ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Forcing Mechanisms

Abstract. The Agulhas Current (AC) transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), has a profound role in the decadal variability of the Atlantic meridional overturning circulation (AMOC), which influences global climate. On glacial-interglacial timescales, paleostudies postulate that Agulhas leakage plays a decisive role for AMOC resumption during terminations (glacial-interglacial transitions). However, efforts to elucidate forcing mechanisms connecting Agulhas leakage with glacial-interglacial AMOC variability have been hampered due to a lack of climate records extracted from the area where the AC originates. Here we present 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the AC. These records contain strong obliquity-driven 41-kyr cycles, nearly in phase with changes in annual mean insolation and air temperature at high southern latitudes. In contrast, precession-driven cycles were negligible in our SST records, which is surprising given the low-latitude location of the Agulhas leakage. Together, this suggests that long-term Agulhas leakage dynamics are associated with a high latitude rather than a tropical climate forcing mechanism, probably by varying the position of the Southern Hemisphere subtropical convergence (STC) and its associated westerlies. We argue that during terminations stronger Agulhas leakage was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

scholarly journals High-latitude obliquity as a dominant forcing in the Agulhas current system

2011 ◽  
Vol 7 (4) ◽  
pp. 1285-1296 ◽  
Author(s):  
T. Caley ◽  
J.-H. Kim ◽  
B. Malaizé ◽  
J. Giraudeau ◽  
T. Laepple ◽  
...  
Keyword(s):  
High Latitude ◽  
Global Climate ◽  
Current System ◽  
Meridional Overturning Circulation ◽  
Agulhas Current ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Forcing Mechanisms ◽  
Glacial Periods

Abstract. The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquity-driven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the low-latitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current, associated with a northward shift of the wind system during glacial periods, leads to reduced leakage, in accordance with the theory. We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

A simple model to assess the long term fate of the AMOC

2021 ◽  
Author(s):  
Richard Wood
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Global Climate Models ◽  
Meridional Overturning Circulation ◽  
Weak State ◽  
Meridional Overturning ◽  
Industrial Level ◽  
Industrial Strength ◽  
Insight Into

<p>Studies with global climate models over the past 25 years have shown a range of long-term responses of the Atlantic Meridional Overturning Circulation (AMOC), in response to scenarios in which greenhouse gases are increased then eventually stabilised at some value, possibly after temporarily overshooting that value. AMOC responses include stabilisation at weaker than the pre-industrial level, rapid recovery following a period of quasi-steady weak circulation, overshooting to stronger than pre-industrial strength, or tipping to a quasi-permanent weak state. While many of these studies have gained insight into the mechanisms behind their individual model behaviour, no overarching understanding exists of what determines how a particular model will respond.   </p><p>We present a simple AMOC model suitable to characterise the different possible long-term responses, and use it as a (partially) unifying framework to show how the different behaviours can arise from a competition between thermal and haline feedbacks. The results are relevant to defining ‘safe mitigation pathways’ that avoid or reduce the risk of AMOC tipping or potentially dangerous overshoots.</p>

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

2021 ◽  
Author(s):  
Tomas Jonathan ◽  
Mike Bell ◽  
Helen Johnson ◽  
David Marshall
Keyword(s):  
Global Climate ◽  
Dominant Role ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Relative Role ◽  
Overturning Circulation ◽  
Near Surface ◽  
Boundary Density ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. </p><p>The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.</p>

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

2013 ◽  
Vol 9 (4) ◽  
pp. 4771-4806 ◽  
Author(s):  
L. Menviel ◽  
A. Timmermann ◽  
T. Friedrich ◽  
M. H. England
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
External Boundary ◽  
3 Dimensional ◽  
Ice Volume ◽  
Freshwater Fluxes ◽  
Meridional Overturning ◽  
The Continuum

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger (DO) and Heinrich events (HE) 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-dimensional intermediate complexity Earth system model covering the period 50 ka BP 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 Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our analysis is consistent with the idea that variations in ice sheet calving and related changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of DO and HE variability seen in paleorecords across the globe.

scholarly journals The sloshing and diapycnal meridional overturning circulations in the Indian Ocean

2020 ◽  
Author(s):  
Lei Han
Keyword(s):  
Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Coast ◽  
Ekman Pumping ◽  
Meridional Heat Transport ◽  
Vertical Movements ◽  
Sloshing Mode ◽  
Meridional Overturning ◽  
The Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product, ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally-integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

scholarly journals A 3500-Year Ice Chemistry Record From The Dominion Range, Antarctica: Linkages Between Climatic Variations and Precipitation Chemistry

1990 ◽  
Vol 14 ◽  
pp. 358-358
Author(s):  
Mary Jo Spencer ◽  
Paul A. Mayewski ◽  
W. Berry Lyons ◽  
Mark S. Twickler ◽  
Pieter Grootes
Keyword(s):  
Oxygen Isotope ◽  
Global Climate ◽  
Ice Core ◽  
Precipitation Chemistry ◽  
Climate Forcing ◽  
Climatic Forcing ◽  
Climatic Variations ◽  
Oxygen Isotope Record ◽  
Isotope Record

In 1984 a 200-m ice core was collected from a local accumulation basin in the Dominion Range, Transantarctic Mountains, Antarctica. A complete oxygen isotope record has been obtained and a considerable portion of the core has been analyzed in detail for chloride, nitrate, sulfate, and sodium. About half of the chloride is due to sea salt with the remainder originating as gaseous HCl. Nitrate levels have increased markedly over the last 1000 years whereas the levels of the other constituents have remained fairly constant.The oxygen isotope results suggest that this region of Antarctica is responding to long-term global climate forcing as well as to shorter-term climatic variations. This data will be compared with the anion and sodium records in order to determine the effects of climatic forcing on these other records. In particular, nitrate appears to vary in concert with fluctuations in long-term climate. Additionally, variations in each constituent over the 3500 year period will be examined in detail to determine the influence of other processes which affect their concentrations.

The Atlantic Meridional Overturning Circulation (AMOC) simulated during interglacials and its impact on climate

2021 ◽  
Author(s):  
Zhiyi Jiang ◽  
Chris Brierley ◽  
David Thornalley ◽  
Sophie Sax
Keyword(s):  
Spatial Structure ◽  
Surface Temperature ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Last Interglacial ◽  
Overturning Circulation ◽  
Poleward Heat Transport ◽  
Meridional Overturning ◽  
Proxy Reconstructions

<p>The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of poleward heat transport and an important part of the global climate system. How it responded to past changes inforcing, such as experienced during Quaternary interglacials, is an intriguing and open question. Previous modelling studies suggest an enhanced AMOC in the mid-Holocene compared to the pre-industrial period. In previous simulations from the Palaeoclimate Modelling Intercomparison Project (PMIP), this arose from feedbacks between sea ice and AMOC changes, which also depended on resolution. Here I present aninitial analysis of the recently available PMIP4 simulations. This shows the overall strength of the AMOC does not markedly change between the mid-Holocene and piControl experiments (at least looking at the maximum of the mean meridional mass overturning streamfunction below 500m at 30<sup>o</sup>N and 50<sup>o</sup>N). This is not inconsistent with the proxy reconstructions using sortable silt and Pa/Th for the mid-Holocene. Here we analyse changes in the spatial structure of the meridional overturning circulation, along with their fingerprints on the surface temperature (computed through regression). We then estimate the percentage of the simulated surface temperature changes between the mid-Holocene and pre-industrial period that can be explained by AMOC. Furthermore, the analysis for the changes in the AMOC spatial structure has been extended to see if the same patterns of change hold for the last interglacial. The simulations will be compared to existing proxy reconstructions, as well as new palaeoceanographic reconstructions.</p>

scholarly journals The Iceland-Faroe Slope Jet: a conduit for dense water toward the Faroe Bank Channel overflow

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Stefanie Semper ◽  
Robert S. Pickart ◽  
Kjetil Våge ◽  
Karin Margretha H. Larsen ◽  
Hjálmar Hátún ◽  
...  
Keyword(s):  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Common Source ◽  
Overturning Circulation ◽  
Nordic Seas ◽  
Dense Water ◽  
The North ◽  
Denmark Strait ◽  
Meridional Overturning

Abstract Dense water from the Nordic Seas passes through the Faroe Bank Channel and supplies the lower limb of the Atlantic Meridional Overturning Circulation, a critical component of the climate system. Yet, the upstream pathways of this water are not fully known. Here we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel using high-resolution, synoptic shipboard observations and long-term measurements north of the Faroe Islands. The bulk of the volume transport of the current, named the Iceland-Faroe Slope Jet (IFSJ), is relatively uniform in hydrographic properties, very similar to the North Icelandic Jet flowing westward along the slope north of Iceland toward Denmark Strait. This suggests a common source for the two major overflows across the Greenland-Scotland Ridge. The IFSJ can account for approximately half of the total overflow transport through the Faroe Bank Channel, thus constituting a significant component of the overturning circulation in the Nordic Seas.

scholarly journals A New Index for the Atlantic Meridional Overturning Circulation at 26°N

2014 ◽  
Vol 27 (17) ◽  
pp. 6439-6455 ◽  
Author(s):  
A. Duchez ◽  
J. J.-M. Hirschi ◽  
S. A. Cunningham ◽  
A. T. Blaker ◽  
H. L. Bryden ◽  
...  
Keyword(s):  
Time Scales ◽  
Decadal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Current Estimate ◽  
Overturning Circulation ◽  
Main Hypothesis ◽  
Meridional Overturning ◽  
Sverdrup Transport

Abstract The Atlantic meridional overturning circulation (AMOC) has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26°N made by the Rapid Climate Change (RAPID) array provide the best current estimate of the state of the AMOC. The period 2004–11 when RAPID AMOC is available is too short to assess decadal variability of the AMOC. This modeling study introduces a new AMOC index (called AMOCSV) at 26°N that combines the Florida Straits transport, the Ekman transport, and the southward geostrophic Sverdrup transport. The main hypothesis in this study is that the upper midocean geostrophic transport calculated using the RAPID array is also wind-driven and can be approximated by the geostrophic Sverdrup transport at interannual and longer time scales. This index is expected to reflect variations in the AMOC at interannual to decadal time scales. This estimate of the surface branch of the AMOC can be constructed as long as reliable measurements are available for the Gulf Stream and for wind stress. To test the reliability of the AMOCSV on interannual and longer time scales, two different numerical simulations are used: a forced and a coupled simulation. Using these simulations the AMOCSV captures a substantial fraction of the AMOC variability and is in good agreement with the AMOC transport at 26°N on both interannual and decadal time scales. These results indicate that it might be possible to extend the observation-based AMOC at 26°N back to the 1980s.

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