scholarly journals Implications of Arctic sea ice changes for North Atlantic deep convection and the meridional overturning circulation in CCSM4-CMIP5 simulations

2013 ◽  
Vol 40 (6) ◽  
pp. 1206-1211 ◽  
Author(s):  
Alexandra Jahn ◽  
Marika M. Holland
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Deep Convection ◽  
Arctic Sea Ice ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Arctic Sea ◽  
Meridional Overturning

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.

scholarly journals Impacts of three types of solar geoengineering on the North Atlantic Meridional Overturning Circulation

2021 ◽  
Author(s):  
Mengdie Xie ◽  
John C. Moore ◽  
Liyun Zhao ◽  
Michael Wolovick ◽  
Helene Muri
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Arctic Sea Ice ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Solar Geoengineering ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. Climate models simulate lower rates of North Atlantic heat transport under greenhouse gas climates than at present due to a reduction in the strength of the North Atlantic meridional overturning circulation (AMOC). Solar geoengineering whereby surface temperatures are cooled by reduction of incoming shortwave radiation may be expected to ameliorate this effect. We investigate this using six Earth System Models running scenarios from GeoMIP (Geoengineering model intercomparison project) in the cases of: i) reduction in the solar constant, mimicking dimming of the sun; ii) sulfate aerosol injection into the lower equatorial stratosphere; and iii) brightening of the ocean regions mimicking enhancing tropospheric cloud amounts. We find that despite across model differences, AMOC decreases are attributable to reduced air-ocean temperature differences, and reduced September Arctic sea ice extent, with no significant impact from changing surface winds or precipitation-evaporation. Reversing the surface freshening of the North Atlantic overturning regions caused by decreased summer sea ice sea helps to promote AMOC. Comparing the geoengineering types after normalizing them for the differences in top of atmosphere radiative forcing, we find that solar dimming is more effective than either marine cloud brightening or stratospheric aerosol injection.

Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic

2021 ◽  
Author(s):  
Claus W. Böning ◽  
Arne Biastoch ◽  
Klaus Getzlaff ◽  
Patrick Wagner ◽  
Siren Rühs ◽  
...  
Keyword(s):  
High Resolution ◽  
North Atlantic ◽  
Deep Convection ◽  
Alternative Interpretation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
First Results ◽  
Meridional Overturning

<p>A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by “sinking” (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.</p>

scholarly journals Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

2020 ◽  
Vol 6 (26) ◽  
pp. eaaz4876 ◽  
Author(s):  
Wei Liu ◽  
Alexey V. Fedorov ◽  
Shang-Ping Xie ◽  
Shineng Hu
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Arctic Sea Ice ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Overturning Circulation ◽  
Meridional Overturning

While the Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down under anthropogenic warming, the exact role of the AMOC in future climate change has not been fully quantified. Here, we present a method to stabilize the AMOC intensity in anthropogenic warming experiments by removing fresh water from the subpolar North Atlantic. This method enables us to isolate the AMOC climatic impacts in experiments with a full-physics climate model. Our results show that a weakened AMOC can explain ocean cooling south of Greenland that resembles the North Atlantic warming hole and a reduced Arctic sea ice loss in all seasons with a delay of about 6 years in the emergence of an ice-free Arctic in boreal summer. In the troposphere, a weakened AMOC causes an anomalous cooling band stretching from the lower levels in high latitudes to the upper levels in the tropics and displaces the Northern Hemisphere midlatitude jets poleward.

The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation

2018 ◽  
Vol 31 (13) ◽  
pp. 5165-5188 ◽  
Author(s):  
He Wang ◽  
Sonya Legg ◽  
Robert Hallberg
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Deep Convection ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate.

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 Persistent Freshening of the Arctic Ocean and Changes in the North Atlantic Salinity Caused by Arctic Sea Ice Decline

2021 ◽  
Author(s):  
Hui Li ◽  
Alexey Fedorov
Keyword(s):  
Sea Ice ◽  
Arctic Ocean ◽  
North Atlantic ◽  
Arctic Sea Ice ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Ice Melting ◽  
Sea Ice Decline ◽  
Arctic Sea

Abstract Arctic sea ice has been declining over past several decades with the largest ice loss occurring in summer. This implies a strengthening of the sea ice seasonal cycle. Here, we examine global ocean salinity response to such changes of Arctic sea ice using simulations wherein we impose a radiative heat imbalance at the sea ice surface, inducing a sea ice decline comparable to the observed. The imposed perturbation leads to enhanced seasonal melting and a rapid retreat of Arctic sea ice within the first 5-10 years. We then observe a gradual freshening of the upper Arctic ocean that continues for about a century. The freshening is most pronounced within the central Arctic, including the Beaufort gyre, and is attributed to excess surface freshwater associated with the stronger seasonal sea ice melting, as well as a greater upper-ocean freshwater storage due to changes in ocean circulation. The freshening of the Nordic Seas can also occur via a distillation-like process in which denser saline waters with increased salinity are exported to the subtropical/tropical North Atlantic by meridional overturning circulation. Thus, enhanced seasonal sea ice melting in a warmer climate can lead to a persistent Arctic freshening with large impacts on the global salinity distribution.

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