scholarly journals Response of the AMOC to reduced solar radiation – the modulating role of atmospheric chemistry

2016 ◽  
Vol 7 (4) ◽  
pp. 877-892 ◽  
Author(s):  
Stefan Muthers ◽  
Christoph C. Raible ◽  
Eugene Rozanov ◽  
Thomas F. Stocker
Keyword(s):  
Solar Radiation ◽  
North Atlantic ◽  
Stratospheric Ozone ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Solar Radiation Management ◽  
Solar Forcing ◽  
Model Simulations ◽  
The North ◽  
The North Atlantic

Abstract. The influence of reduced solar forcing (grand solar minimum or geoengineering scenarios like solar radiation management) on the Atlantic Meridional Overturning Circulation (AMOC) is assessed in an ensemble of atmosphere–ocean–chemistry–climate model simulations. Ensemble sensitivity simulations are performed with and without interactive chemistry. In both experiments the AMOC is intensified in the course of the solar radiation reduction, which is attributed to the thermal effect of the solar forcing: reduced sea surface temperatures and enhanced sea ice formation increase the density of the upper ocean in the North Atlantic and intensify the deepwater formation. Furthermore, a second, dynamical effect on the AMOC is identified driven by the stratospheric cooling in response to the reduced solar forcing. The cooling is strongest in the tropics and leads to a weakening of the northern polar vortex. By stratosphere–troposphere interactions, the stratospheric circulation anomalies induce a negative phase of the Arctic Oscillation in the troposphere which is found to weaken the AMOC through wind stress and heat flux anomalies in the North Atlantic. The dynamic mechanism is present in both ensemble experiments. In the experiment with interactive chemistry, however, it is strongly amplified by stratospheric ozone changes. In the coupled system, both effects counteract and weaken the response of the AMOC to the solar forcing reduction. Neglecting chemistry–climate interactions in model simulations may therefore lead to an overestimation of the AMOC response to solar forcing.

scholarly journals Response of the AMOC to reduced solar radiation – the modulating role of atmospheric-chemistry

2016 ◽  
Author(s):  
Stefan Muthers ◽  
Christoph C. Raible ◽  
Thomas F. Stocker
Keyword(s):  
Solar Radiation ◽  
Thermal Effect ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Solar Forcing ◽  
Model Simulations ◽  
The North ◽  
Climate Interactions ◽  
The North Atlantic

Abstract. The influence of reduced solar forcing (grand solar minimum or geoengineering scenarios like solar radiation management) on the Atlantic meridional overturning circulation (AMOC) is assessed in an ensemble of atmosphere-ocean-chemistry-climate model simulations. Ensemble sensitivity simulations are performed with and without interactive chemistry. Without chemistry-climate interaction the AMOC is intensified in the course of the solar radiation reduction (SRR), which is attributed to the thermal effect of the solar forcing: reduced sea surface temperatures and enhanced sea ice formation increase the density of the upper ocean in the North Atlantic and intensify the deepwater formation. In simulations with chemistry-climate interactions a second, dynamical effect on the AMOC is identified which counteracts the thermal effect. This dynamical mechanism is driven by the stratospheric cooling in response to the reduced solar forcing, which is strongest in the tropics and leads to a weakening of the Northern polar vortex. In simulations with interactive chemistry, these stratospheric changes are strongly amplified by the reduction of stratospheric ozone. By stratosphere-troposphere interactions, the stratospheric circulation anomalies induce a negative phase of the Arctic Oscillation in the troposphere, which is found to weaken the AMOC through wind stress and heat flux anomalies in the North Atlantic. Neglecting chemistry-climate interactions in model simulations may therefore lead to an overestimation of the AMOC response to solar forcing.

scholarly journals Change in the North Atlantic circulation associated with the mid-Pleistocene transition

2018 ◽  
Vol 14 (11) ◽  
pp. 1639-1651 ◽  
Author(s):  
Gloria M. Martin-Garcia ◽  
Francisco J. Sierro ◽  
José A. Flores ◽  
Fátima Abrantes
Keyword(s):  
North Atlantic ◽  
Major Change ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
North Atlantic Current ◽  
The North ◽  
Oceanic Fronts ◽  
Surface Circulation ◽  
The North Atlantic

Abstract. The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20–MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37∘34.285′ N, 10∘7.562′ W; 2585 m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55∘ N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south–north, and east–west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.

scholarly journals The Mechanisms of the Atlantic Meridional Overturning Circulation Slowdown Induced by Arctic Sea Ice Decline

2019 ◽  
Vol 32 (4) ◽  
pp. 977-996 ◽  
Author(s):  
Wei Liu ◽  
Alexey Fedorov ◽  
Florian Sévellec
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Deep Convection ◽  
Arctic Sea Ice ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
The North ◽  
Arctic Sea ◽  
Meridional Overturning ◽  
The North Atlantic

We explore the mechanisms by which Arctic sea ice decline affects the Atlantic meridional overturning circulation (AMOC) in a suite of numerical experiments perturbing the Arctic sea ice radiative budget within a fully coupled climate model. The imposed perturbations act to increase the amount of heat available to melt ice, leading to a rapid Arctic sea ice retreat within 5 years after the perturbations are activated. In response, the AMOC gradually weakens over the next ~100 years. The AMOC changes can be explained by the accumulation in the Arctic and subsequent downstream propagation to the North Atlantic of buoyancy anomalies controlled by temperature and salinity. Initially, during the first decade or so, the Arctic sea ice loss results in anomalous positive heat and salinity fluxes in the subpolar North Atlantic, inducing positive temperature and salinity anomalies over the regions of oceanic deep convection. At first, these anomalies largely compensate one another, leading to a minimal change in upper ocean density and deep convection in the North Atlantic. Over the following years, however, more anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over most of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection. The thermal and haline contributions to these buoyancy anomalies, and therefore to the AMOC slowdown during this period, are found to have similar magnitudes. We also find that the related changes in horizontal wind-driven circulation could potentially push freshwater away from the deep convection areas and hence strengthen the AMOC, but this effect is overwhelmed by mean advection.

Polar Climate Instability and Climate Teleconnections from the Arctic to the Midlatitudes and Tropics

2009 ◽  
Vol 22 (13) ◽  
pp. 3513-3539 ◽  
Author(s):  
Guido Vettoretti ◽  
Marc d’Orgeville ◽  
William R. Peltier ◽  
Marek Stastna
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Overturning Circulation ◽  
Polar Climate ◽  
Enso Events ◽  
The North ◽  
The North Atlantic ◽  
The Tropics ◽  
The Impact

Abstract It is generally accepted that the ocean thermohaline circulation plays a key role in polar climate stability and rapid climate change. Recently reported analyses of the impact of anomalous freshwater outflows from the North American continent onto either the North Atlantic or Arctic Oceans demonstrate that, in either case, a clear reduction in the Atlantic meridional overturning circulation, accompanied by an increase in sea ice extent, is predicted. The results also reconcile proxy-inferred Younger Dryas Greenland temperature variations. The aim of the present work is to provide a detailed investigation of the pathways along which the signal associated with overturning circulation anomalies propagates into both the midlatitudes and the tropics and the effect such teleconnections have on the tropical ocean–atmosphere system. The authors consider both the impact of substantial slowing of the overturning circulation due to freshwater forcing of the North Atlantic as well as its recovery after the anomalous forcing has ceased. The changes in tropical climate variability are shown to manifest themselves in shifts of both the typical time scale and intensity of ENSO events in the model. Evidence is presented for mechanisms that involve both atmospheric and oceanic pathways through which such Northern Hemisphere high-latitude events are communicated into both the midlatitudes and the tropics and thereafter transformed into changes in the nature of tropical variability.

Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition

2020 ◽  
Author(s):  
David Hutchinson ◽  
Helen Coxall ◽  
Matt O'Regan ◽  
Johan Nilsson ◽  
Rodrigo Caballero ◽  
...  
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
North Atlantic Ocean ◽  
Late Eocene ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Deep Circulation ◽  
The North ◽  
The North Atlantic ◽  
The North Pacific

<p><strong>The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. Proxies of deep circulation suggest a contemporaneous onset or strengthening of the Atlantic meridional overturning circulation (AMOC). Proxy evidence of gradual salinification of the North Atlantic and tectonically driven isolation of the Arctic suggest that closing the Arctic-Atlantic gateway could have triggered the AMOC at the EOT. We demonstrate this trigger of the AMOC using a new paleoclimate model with late Eocene boundary conditions. The control simulation reproduces Eocene observations of low Arctic salinities. Subsequent closure of the Arctic-Atlantic gateway triggers the AMOC by blocking freshwater inflow from the Arctic. Salt advection feedbacks then lead to cessation of overturning in the North Pacific. These circulation changes imply major warming of the North Atlantic Ocean, and simultaneous cooling of the North Pacific, but no interhemispheric change in temperatures.</strong></p>

scholarly journals Role of the North Atlantic circulation in the mid-Pleistocene transition

2018 ◽  
Author(s):  
Gloria M. Martin-Garcia ◽  
Francisco J. Sierro ◽  
José A. Flores ◽  
Fátima Abrantes
Keyword(s):  
North Atlantic ◽  
Major Change ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
North Atlantic Current ◽  
The North ◽  
Oceanic Fronts ◽  
Surface Circulation ◽  
The North Atlantic

Abstract. The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents, and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20-MIS14) is reconstructed, based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37°34.285' N, 10°7.562' W; 2585 m bsl). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55° N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100-ky cyclicity associated to the Mid-Pleistocene Transition. In surface, this change consisted in the re-distribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF) and the increase in the North Atlantic Deep Water (NADW) formation. During glacials prior to MIS 16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south-north, and east-west, thermal gradients and blockading the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the important increase in the meridional overturning circulation, in combination with the north-westward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheets growth, which worked as a positive feedback to prolong the glacials towards 100-ky cycles.

scholarly journals Sea ice-air interactions amplify multidecadal variability in the North Atlantic and Arctic region

2020 ◽  
Author(s):  
Aiguo Dai ◽  
Jiechun Deng
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Arctic Sea Ice ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Multidecadal Variability ◽  
The North ◽  
Arctic Sea ◽  
The North Atlantic

Abstract Winter surface air temperature (Tas) over the Barents-Kara Seas (BKS) and other Arctic regions has experienced rapid warming since the late 1990 that has been linked to the concurring cooling over Eurasia1-3. However, the cause of this accelerated BKS warming is not well understood, and whether and how internal variability may have contributed to this warming is unclear. Through analyses of observations and model simulations, we show that two-way interactions between sea ice and air amplify multidecadal variability in Arctic sea-ice cover (SIC) and sea surface temperatures (SST) from the North Atlantic to BKS, and produce large multidecadal variations in Tas over the BKS, Greenland-Norwegian Seas and Baffin Bay. Advection of SST anomalies from the North Atlantic to the Arctic causes SIC to change, which produces large anomalies in surface energy fluxes and Tas. However, the sea ice-air interactions also amplify the variations in SIC and SST, and the Atlantic Meridional Overturning Circulation (AMOC) mainly through local surface fluxes. When sea ice is fixed or melts away under increasing CO2, not only Arctic Tas multidecadal variations disappear, but also the SIC, SST and AMOC variations are greatly reduced. The results suggest that sea ice-air interactions are vital for multidecadal climate variability not only in the Arctic but also in the North Atlantic, similar to air-sea interactions for tropical climate. As Arctic sea ice is projected to melt away4,5, these interactions and thus multidecadal variability from the North Atlantic to the Arctic will likely weaken in the coming decades.

scholarly journals Modeling the climatic implications and indicative senses of the Guliya δ<sup>18</sup>O-temperature proxy record to the ocean–atmosphere system during the past 130 ka

2013 ◽  
Vol 9 (2) ◽  
pp. 735-747 ◽  
Author(s):  
D. Xiao ◽  
P. Zhao ◽  
Y. Wang ◽  
X. Zhou
Keyword(s):  
Solar Radiation ◽  
North Atlantic ◽  
The Arctic ◽  
Proxy Record ◽  
The Past ◽  
The North ◽  
Incoming Solar Radiation ◽  
The North Atlantic ◽  
Temperature Proxy ◽  
Anomalous Pattern

Abstract. Using an intermediate-complexity UVic Earth System Climate Model (UVic Model), the geographical and seasonal implications and indicative senses of the Guliya temperature proxy found in the Guliya δ18O ice core record (hereinafter, the Guliya δ18O-temperature proxy record) are investigated under time-dependent orbital and CO2 forcings with an acceleration factor of 50 over the past 130 ka. The results reveal that the simulated August–September Guliya surface air temperature (SAT) reproduces the 21-ka precession and 43-ka obliquity cycles of the Guliya δ18O-temperature proxy record, showing an in-phase variation with the latter. Moreover, the Guliya δ18O-temperature proxy record may be also an indicator of the August–September Northern Hemispheric (NH) SAT. Corresponding to the difference between the extreme warm and cold phases of the precession cycle in the Guliya August–September SAT, there are two anomalous patterns in SAT and sea surface temperature (SST). The first anomalous pattern shows increases of SAT and SST toward the Arctic, which is possibly associated with an increase of the NH incoming solar radiation that is caused by the in-phase superposition between the precession and obliquity cycles. The second anomalous pattern shows increases of SAT and SST toward the equator, which is possibly due to a decrease of incoming solar radiation over the NH polar that results from the anti-phase counteraction between the precession and obliquity cycles. The summer (winter) Guliya and NH temperatures are higher (lower) in the warm phases of the August–September Guliya than in their cold phases. Moreover, in August–September, the Guliya SAT is closely related to the North Atlantic SST, in which the Guliya precipitation might act as a "bridge" linking the Guliya SAT and the North Atlantic SST.

scholarly journals The Observation-Based Application of a Regional Thermohaline Inverse Method to Diagnose the Formation and Transformation of Water Masses North of the OSNAP Array from 2013 to 2015

2020 ◽  
Vol 50 (6) ◽  
pp. 1533-1555
Author(s):  
Neill Mackay ◽  
Chris Wilson ◽  
N. Penny Holliday ◽  
Jan D. Zika
Keyword(s):  
North Atlantic ◽  
Inverse Method ◽  
Sea Water ◽  
Surface Fluxes ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Intermediate Water ◽  
The North ◽  
The North Atlantic

AbstractThe strength of the meridional overturning circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed regional thermohaline inverse method (RTHIM) to a region encompassing the Arctic and part of the North Atlantic subpolar gyre for the years 2013, 2014, and 2015. RTHIM is a novel method that can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv (1 Sv ≡ 106 m3 s−1) during 2014–15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat, and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program (OSNAP) and show good agreement, lending validity to our results.

scholarly journals Radionuclide wiggle-matching reveals a non-synchronous Early Holocene climate oscillation in Greenland and Western Europe around a grand solar minimum

2019 ◽  
Author(s):  
Florian Mekhaldi ◽  
Markus Czymzik ◽  
Florian Adolphi ◽  
Jesper Sjolte ◽  
Svante Björck ◽  
...  
Keyword(s):  
North Atlantic ◽  
Solar Minimum ◽  
Ice Cores ◽  
Meridional Overturning Circulation ◽  
Early Holocene ◽  
Solar Forcing ◽  
Concentration Data ◽  
The North ◽  
The North Atlantic ◽  
Climate Events

Abstract. Several climate events have been reported from the Early Holocene superepoch, the best known of these being the Preboreal oscillation (PBO). It is still unclear how the PBO and the number of climate events observed in Greenland ice cores and European terrestrial records are related to one another. This is mainly due to uncertainties in the chronologies of the records. Here, we present new high resolution 10Be concentration data from the varved Meerfelder Maar sediment record in Germany, spanning the period 11310–11000 years BP. These new data allow us to synchronize this well-studied record as well as Greenland ice-core records to the IntCal13 time-scale via radionuclide wiggle-matching. In doing so, we show that the climate oscillations identified in Greenland and Europe between 11450 and 11000 years BP were not synchronous but terminated and began, respectively, with the onset of a grand solar minimum. A similar spatial anomaly pattern is found in a number of modeling studies on solar forcing of climate in the North Atlantic region. We further postulate that freshwater delivery to the North Atlantic would have had the potential to amplify solar forcing through a slowdown of the Atlantic meridional overturning circulation (AMOC) reinforcing surface air temperature anomalies in the region.

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