The Role of Northern Sea Ice Cover for the Weakening of the Thermohaline Circulation under Global Warming

Abstract An increase in atmospheric CO2 concentration and the resulting global warming are typically associated with a weakening of the thermohaline circulation (THC) in model scenarios. For the models participating in the Coupled Model Intercomparison Project (CMIP), this weakening shows a significant (r = 0.62) dependence on the initial THC strength; it is stronger for initially strong overturning. The authors propose a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMBER-3α. The mechanism is based on the fact that sea ice cover greatly reduces heat loss from the ocean. The extent of sea ice is strongly influenced by the near-surface atmospheric temperature (SAT) in the North Atlantic but also by the strength of the THC itself, which transports heat to the convection sites. Consequently, sea ice tends to extend farther south for weaker THC. Initially larger sea ice cover responds more strongly to atmospheric warming; thus, sea ice retreats more strongly for an initially weaker THC. This sea ice retreat tends to strengthen (i.e., stabilize) the THC because the sea ice retreat allows more oceanic heat loss. This stabilizing effect is stronger for runs with weak initial THC and extensive sea ice cover. Therefore, an initially weak THC weakens less under global warming. In contrast to preindustrial climate, sea ice melting presently plays the role of an external forcing with respect to THC stability.

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Warm Greenland during the last interglacial: the role of regional changes in sea ice cover

Climate of the Past ◽

10.5194/cp-12-2011-2016 ◽

2016 ◽

Vol 12 (10) ◽

pp. 2011-2031 ◽

Cited By ~ 8

Author(s):

Niklaus Merz ◽

Andreas Born ◽

Christoph C. Raible ◽

Thomas F. Stocker

Keyword(s):

Sea Ice ◽

Ice Cover ◽

Labrador Sea ◽

Last Interglacial ◽

Atlantic Sector ◽

Nordic Seas ◽

Proxy Records ◽

Ice Retreat ◽

Sea Ice Retreat

Abstract. The last interglacial, also known as the Eemian, is characterized by warmer than present conditions at high latitudes. This is implied by various Eemian proxy records as well as by climate model simulations, though the models mostly underestimate the warming with respect to proxies. Simulations of Eemian surface air temperatures (SAT) in the Northern Hemisphere extratropics further show large variations between different climate models, and it has been hypothesized that this model spread relates to diverse representations of the Eemian sea ice cover. Here we use versions 3 and 4 of the Community Climate System Model (CCSM3 and CCSM4) to highlight the crucial role of sea ice and sea surface temperatures changes for the Eemian climate, in particular in the North Atlantic sector and in Greenland. A substantial reduction in sea ice cover results in an amplified atmospheric warming and thus a better agreement with Eemian proxy records. Sensitivity experiments with idealized lower boundary conditions reveal that warming over Greenland is mostly due to a sea ice retreat in the Nordic Seas. In contrast, sea ice changes in the Labrador Sea have a limited local impact. Changes in sea ice cover in either region are transferred to the overlying atmosphere through anomalous surface energy fluxes. The large-scale spread of the warming resulting from a Nordic Seas sea ice retreat is mostly explained by anomalous heat advection rather than by radiation or condensation processes. In addition, the sea ice perturbations lead to changes in the hydrological cycle. Our results consequently imply that both temperature and snow accumulation records from Greenland ice cores are sensitive to sea ice changes in the Nordic Seas but insensitive to sea ice changes in the Labrador Sea. Moreover, the simulations suggest that the uncertainty in the Eemian sea ice cover accounts for 1.6 °C of the Eemian warming at the NEEM ice core site. The estimated Eemian warming of 5 °C above present day based on the NEEM δ15N record can be reconstructed by the CCSM4 model for the scenario of a substantial sea ice retreat in the Nordic Seas combined with a reduced Greenland ice sheet.

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Warm Greenland during the last interglacial: the role of regional changes in sea ice cover

10.5194/cp-2016-12 ◽

2016 ◽

Cited By ~ 3

Author(s):

Niklaus Merz ◽

Andreas Born ◽

Christoph C. Raible ◽

Thomas F. Stocker

Keyword(s):

Sea Ice ◽

Ice Cover ◽

Labrador Sea ◽

Last Interglacial ◽

Atlantic Sector ◽

Climate System Model ◽

Nordic Seas ◽

Ice Retreat ◽

Sea Ice Retreat

Abstract. The last interglacial, the Eemian, is characterized by warmer than present conditions at high latitudes and is therefore often considered as a possible analogue for the climate in the near future. Simulations of Eemian surface air temperatures (SAT) in the Northern Hemisphere, however, show large variations between different climate models and it has been hypothesized that this model spread relates to diverse representations of the Eemian sea ice cover. Here we use versions 3 and 4 of the Community Climate System Model (CCSM3 and CCSM4), to highlight the crucial role of sea ice and sea surface temperatures during the Eemian, in particular for SAT in the North Atlantic sector and in Greenland. A substantial reduction in sea ice cover results in an amplified atmospheric warming and, thus, a better agreement with Eemian proxy records. Sensitivity experiments with idealized lower boundary conditions reveal that warming over Greenland is mostly due to a sea ice retreat in the Nordic Seas. In contrast, sea ice changes in the Labrador Sea have a limited local impact. Changes in sea ice cover in either region are transferred to the overlying atmosphere through anomalous surface energy fluxes. The large-scale warming simulated for the sea ice retreat in the Nordic Seas further relates to anomalous heat advection. Diabatic processes play a secondary role, yet distinct changes in the hydrological cycle are possible. Our results imply that temperature and accumulation records from Greenland ice cores are sensitive to sea ice changes in the Nordic Seas but insensitive to sea ice changes in the Labrador Sea. Moreover, our simulations suggest that the uncertainty in the Eemian sea ice cover accounts for 1.6 °C of the Eemian warming at the NEEM ice core site. The estimated Eemian warming of 5 °C above present-day based on the NEEM δ15N record can be reconstructed by the CCSM4 model for the scenario of a substantial sea ice retreat in the Nordic Seas combined with a reduced Greenland ice sheet.

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Consistent Changes in the Sea Ice Seasonal Cycle in Response to Global Warming

Journal of Climate ◽

10.1175/2011jcli4051.1 ◽

2011 ◽

Vol 24 (20) ◽

pp. 5325-5335 ◽

Cited By ~ 31

Author(s):

Ian Eisenman ◽

Tapio Schneider ◽

David S. Battisti ◽

Cecilia M. Bitz

Keyword(s):

Sea Ice ◽

Northern Hemisphere ◽

Surface Albedo ◽

Climate Models ◽

Climate Model ◽

Ice Cover ◽

Climate Forcing ◽

Sea Ice Extent ◽

Ice Retreat ◽

Sea Ice Retreat

Abstract The Northern Hemisphere sea ice cover has diminished rapidly in recent years and is projected to continue to diminish in the future. The year-to-year retreat of Northern Hemisphere sea ice extent is faster in summer than winter, which has been identified as one of the most striking features of satellite observations as well as of state-of-the-art climate model projections. This is typically understood to imply that the sea ice cover is most sensitive to climate forcing in summertime, and previous studies have explained this by calling on factors such as the surface albedo feedback. In the Southern Hemisphere, however, it is the wintertime sea ice extent that retreats fastest in climate model projections. Here, it is shown that the interhemispheric differences in the model projections can be attributed to differences in coastline geometry, which constrain where sea ice can occur. After accounting for coastline geometry, it is found that the sea ice changes simulated in both hemispheres in most climate models are consistent with sea ice retreat being fastest in winter in the absence of landmasses. These results demonstrate that, despite the widely differing rates of ice retreat among climate model projections, the seasonal structure of the sea ice retreat is robust among the models and is uniform in both hemispheres.

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Clouds damp the impacts of Polar sea ice loss

10.5194/tc-2019-283 ◽

2019 ◽

Author(s):

Ramdane Alkama ◽

Alessandro Cescatti ◽

Patrick C. Taylor ◽

Lorea Garcia-San Martin ◽

Herve Douville ◽

...

Keyword(s):

Solar Radiation ◽

Sea Ice ◽

Cloud Cover ◽

Climate Models ◽

Arctic Sea Ice ◽

Ice Melting ◽

Ice Retreat ◽

Sea Ice Retreat ◽

The Impact

Abstract. Clouds plays an important role on the climate system through two main contrasting effects: (1) cooling the Earth by reflecting to space part of incoming solar radiation; (2) warming the surface by reducing the Earth’s loss of thermal energy to space. Recently, scientists have paid more attention to the warming role of clouds because of the acceleration of Arctic sea ice melting and because of recent studies that did not find any response of cloud cover fraction to reduced sea ice in summer. On the contrary, with this work based on satellite CERES data and 32 CMIP5 climate models, we reveal that the cooling role of clouds is dominant. Indeed, cloud dynamic occurring in combination with sea-ice melting plays an important cooling effect by altering the surface energy budget in an apparently contradicting way: years with less sea ice are also those that show an increase of the radiative energy reflected back to space by clouds. An increase in absorbed solar radiation when sea ice retreats (surface albedo change) explains 66 ± 2 % of the observed signal. The remaining 34 ± 1 % are due to the increase in cloud cover/thickness when sea ice retreat and associated reflection to space. This interplay between clouds and sea ice reduces by half the increase of net radiation at the surface that follows the sea-ice retreat, therefore damping the impact of polar sea ice loss. We further highlight how this process is mis-represented in some climate models.

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Sea Ice Trends in Climate Models Only Accurate in Runs with Biased Global Warming

Journal of Climate ◽

10.1175/jcli-d-16-0455.1 ◽

2017 ◽

Vol 30 (16) ◽

pp. 6265-6278 ◽

Cited By ~ 48

Author(s):

Erica Rosenblum ◽

Ian Eisenman

Keyword(s):

Global Warming ◽

Sea Ice ◽

Climate Models ◽

Arctic Sea Ice ◽

The Arctic ◽

Antarctic Sea Ice ◽

Arctic Sea ◽

Ice Retreat ◽

Sea Ice Retreat ◽

The Antarctic

Observations indicate that the Arctic sea ice cover is rapidly retreating while the Antarctic sea ice cover is steadily expanding. State-of-the-art climate models, by contrast, typically simulate a moderate decrease in both the Arctic and Antarctic sea ice covers. However, in each hemisphere there is a small subset of model simulations that have sea ice trends similar to the observations. Based on this, a number of recent studies have suggested that the models are consistent with the observations in each hemisphere when simulated internal climate variability is taken into account. Here sea ice changes during 1979–2013 are examined in simulations from the most recent Coupled Model Intercomparison Project (CMIP5) as well as the Community Earth System Model Large Ensemble (CESM-LE), drawing on previous work that found a close relationship in climate models between global-mean surface temperature and sea ice extent. All of the simulations with 1979–2013 Arctic sea ice retreat as fast as observations are found to have considerably more global warming than observations during this time period. Using two separate methods to estimate the sea ice retreat that would occur under the observed level of global warming in each simulation in both ensembles, it is found that simulated Arctic sea ice retreat as fast as observations would occur less than 1% of the time. This implies that the models are not consistent with the observations. In the Antarctic, simulated sea ice expansion as fast as observations is found to typically correspond with too little global warming, although these results are more equivocal. As a result, the simulations do not capture the observed asymmetry between Arctic and Antarctic sea ice trends. This suggests that the models may be getting the right sea ice trends for the wrong reasons in both polar regions.

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The Response of the Nordic Seas to Wintertime Sea-ice Retreat

Journal of Climate ◽

10.1175/jcli-d-20-0932.1 ◽

2021 ◽

pp. 1-40

Author(s):

Yue Wu ◽

David P. Stevens ◽

Ian A. Renfrew ◽

Xiaoming Zhai

Keyword(s):

Global Warming ◽

Sea Ice ◽

Climate Model ◽

Sea Temperature ◽

Nordic Seas ◽

Ocean Response ◽

Marginal Ice Zone ◽

Ice Retreat ◽

Ice Edge ◽

Sea Ice Retreat

AbstractThe ocean response to wintertime sea-ice retreat is investigated in the coupled climate model HiGEM. We focus on the marginal ice zone and adjacent waters of the Nordic Seas, where the air-sea temperature difference can be large during periods of off-ice winds promoting high heat flux events. Both control and transient climate model ensembles are examined, which allows us to isolate the ocean response due to sea-ice retreat from the response due to climate change. As the wintertime sea-ice edge retreats towards the Greenland coastline, it exposes waters that were previously covered by ice which enhances turbulent heat loss and mechanical mixing, leading to a greater loss of buoyancy and deeper vertical mixing in this location. However, under global warming, the buoyancy loss is inhibited as the atmosphere warms more rapidly than the ocean which reduces the air-sea temperature difference. This occurs most prominently further away from the retreating ice edge, over the Greenland Sea gyre. Over the gyre the upper ocean also warms significantly, resulting in a more stratified water column and, as a consequence, a reduction in the depth of convective mixing. In contrast, closer to the coast the effect of global warming is overshadowed by the effect of the sea-ice retreat, leading to significant changes in ocean temperature and salinity in the vicinity of the marginal ice zone.

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A 14 - 0 ka record of trends and events of sea ice cover, primary production and freshwater discharge in the Beaufort Sea, Arctic Ocean

10.5194/egusphere-egu2020-4699 ◽

2020 ◽

Author(s):

Junjie Wu ◽

Ruediger Stein ◽

Kirsten Fahl ◽

Nicole Syring ◽

Jens Hefter ◽

...

Keyword(s):

Sea Ice ◽

Primary Production ◽

Beaufort Sea ◽

Ice Cover ◽

Sediment Flux ◽

The Arctic ◽

Proxy Records ◽

Ice Retreat ◽

Sea Ice Retreat ◽

Natural Climate

<p>The Arctic is changing rapidly, and one of the main and most obvious features is the drastic sea-ice retreat over the past few decades. Over such time scales, observations are deficient and not long enough for deciphering the processes controlling this accelerated sea-ice retreat. Thus, high-resolution, longer-term proxy records are needed for reconstruction of natural climate variability. In this context, we applied a biomarker approach on the well-dated sediment core ARA04C/37 recovered in the southern Beaufort Sea directly off the Mackenzie River, an area that is characterized by strong seasonal variability in sea-ice cover, primary productivity and terrigenous (riverine) input. Based on our biomarker records, the Beaufort Sea region was nearly ice-free in summer during the late Deglacial to early Holocene (14 to 8 ka). During the mid-late Holocene (8 to 0 ka), a seasonal sea-ice cover developed, coinciding with a drop in both terrigenous sediment flux and primary production. Supported by multiple proxy records, two major flood events characterized by prominent maxima in sediment flux occurred near 13 and 11 ka. The former is coincident with the Younger Dryas Cooling Event probably triggered by a &#160;freshwater outburst from the Lake Agassiz. The origin of the second (younger) one might represent a second Mackenzie flood event, coinciding with meltwater pulse IB/post-glacial flooding of the shelf and related increased coastal erosion. Here, our interpretation remains a little bit speculative, and further research is needed and also in progress.</p>

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Phytoplankton distribution in unusually low sea ice cover over the Pacific Arctic

Biogeosciences ◽

10.5194/bg-9-4835-2012 ◽

2012 ◽

Vol 9 (11) ◽

pp. 4835-4850 ◽

Cited By ~ 52

Author(s):

P. Coupel ◽

H. Y. Jin ◽

M. Joo ◽

R. Horner ◽

H. A. Bouvet ◽

...

Keyword(s):

Sea Ice ◽

Marine Ecosystem ◽

Environmental Parameters ◽

Arctic Basin ◽

Ice Cover ◽

Phytoplankton Distribution ◽

The Pacific ◽

Ice Retreat ◽

Sea Ice Retreat ◽

The Impact

Abstract. A large part of the Pacific Arctic basin experiences ice-free conditions in summer as a result of sea ice cover steadily decreasing over the last decades. To evaluate the impact of sea ice retreat on the marine ecosystem, phytoplankton in situ observations were acquired over the Chukchi shelf and the Canadian basin in 2008, a year of high melting. Pigment analyses and taxonomy enumerations were used to characterise the distribution of main phytoplanktonic groups. Marked spatial variability of the phytoplankton distribution was observed in summer 2008. Comparison of eight phytoplankton functional groups and 3 size-classes (pico-, nano- and micro-phytoplankton) also showed significant differences in abundance, biomass and distribution between summer of low ice cover (2008) and heavy ice summer (1994). Environmental parameters such as freshening, stratification, light and nutrient availability are discussed as possible causes to explain the observed differences in phytoplankton community structure between 1994 and 2008.

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Deep-water formation and Arctic circulation under sea-ice retreat: a comparison between CMIP6 models

10.5194/egusphere-egu21-12155 ◽

2021 ◽

Author(s):

Anais Bretones ◽

Kerim Hestnes Nisancioglu ◽

Mari Fjalstad Jensen

Keyword(s):

Sea Ice ◽

Climate Models ◽

Mixed Layer Depth ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Overturning Circulation ◽

Ice Retreat ◽

Meridional Overturning ◽

Sea Ice Edge ◽

Sea Ice Retreat

<div> <div> <div> <div> <p>While a rapid sea-ice retreat in the Arctic has become ubiquitous, the potential weakening of the Atlantic Meridional Overturning circulation (AMOC), in response to rising greenhouse gases, is still under debate. Although climate models predict a weakening of the AMOC, observations are so far inconclusive. It has been suggested that the strength and vertical extent of the AMOC responds to sea-ice retreat, as deep mixing occurs in open-ocean areas close to the sea-ice edge. Here, we investigate this hypothesis by looking at the Arctic tidional Overturning Circulation (ArMOC) and mixed-layer depth in several CMIP6 models forced with the SSP5- 8.5 scenario. For every models we find a decoupling of the ArMOC with the AMOC: while the AMOC weakens during the 21st century, the ArMOC is enhanced.</p> </div> </div> </div> </div>

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Faster Arctic Sea Ice Retreat in CMIP5 than in CMIP3 due to Volcanoes

Journal of Climate ◽

10.1175/jcli-d-16-0391.1 ◽

2016 ◽

Vol 29 (24) ◽

pp. 9179-9188 ◽

Cited By ~ 18

Author(s):

Erica Rosenblum ◽

Ian Eisenman

Keyword(s):

Sea Ice ◽

Climate Models ◽

Arctic Sea Ice ◽

Sea Ice Extent ◽

Arctic Sea ◽

Volcanic Forcing ◽

Global Mean Surface Temperature ◽

Ice Retreat ◽

Sea Ice Retreat ◽

Global Mean

Abstract The downward trend in Arctic sea ice extent is one of the most dramatic signals of climate change during recent decades. Comprehensive climate models have struggled to reproduce this trend, typically simulating a slower rate of sea ice retreat than has been observed. However, this bias has been widely noted to have decreased in models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) compared with the previous generation of models (CMIP3). Here simulations are examined from both CMIP3 and CMIP5. It is found that simulated historical sea ice trends are influenced by volcanic forcing, which was included in all of the CMIP5 models but in only about half of the CMIP3 models. The volcanic forcing causes temporary simulated cooling in the 1980s and 1990s, which contributes to raising the simulated 1979–2013 global-mean surface temperature trends to values substantially larger than observed. It is shown that this warming bias is accompanied by an enhanced rate of Arctic sea ice retreat and hence a simulated sea ice trend that is closer to the observed value, which is consistent with previous findings of an approximately linear relationship between sea ice extent and global-mean surface temperature. Both generations of climate models are found to simulate Arctic sea ice that is substantially less sensitive to global warming than has been observed. The results imply that much of the difference in Arctic sea ice trends between CMIP3 and CMIP5 occurred because of the inclusion of volcanic forcing, rather than improved sea ice physics or model resolution.

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