Abrupt changes in sea ice and dynamics of Dansgaard-Oeschger events

Changes in sea ice are proposed as an important component in Dansgaard-Oeschger events; the abrupt climate change events that occurred repeatedly during the last ice age. Paleoclimatic reconstructions suggest an expansion of sea ice in the Nordic Seas during the cold stadial periods of the Dansgaard-Oeschger cycles. However, as the present configuration of the Nordic Seas does not allow for an extensive sea-ice cover in this region, the hydrography must have been different during glacial times. In fact, reconstructions show that the Nordic Seas hydrography during cold stadial periods was similar to the stratification of the Arctic Ocean today. However, the dynamic impacts of changing freshwater input and Atlantic water temperature on the Arctic stratification and sea ice are unclear. This study aims to assess the potential for Arctic-like stratification in the Nordic Seas during the last glacial period and the dynamics behind Dansgaard-Oeschger events, using models and theory. The results are presented in three papers. In the first paper, we develop a simple conceptual two-layer ocean model including sea ice representing the Nordic Seas during stadial times. Here, we find that a sea-ice cover is sensitive to changes in freshwater input, subsurface temperature, and the representation of vertical mixing. Abrupt changes in sea ice can occur with small changes to surface freshwater supply or Atlantic water temperatures. In the second paper we apply a three-dimensional eddy resolving numerical model to the same problem and find further support for the conclusions from Paper I; the stability of a sea-ice cover in the Nordic Seas is dependent on the background climate and large changes in stratification and sea ice occur with small changes in forcing. In addition, additional results presented in this dissertation (Sec. 6.2.1) show self-sustained oscillations in sea-ice cover without a change in forcing. From Paper II we learn that an extensive sea-ice cover and an Arctic-like stratification with a fresh surface layer and a halocline can exist in the Nordic Seas without an external freshwater supply. Under sufficient cold conditions, a halocline capped by sea ice emerges spontaneously due to redistribution of freshwater through sea-ice formation and melt. We find that an extensive sea-ice cover slows down the local overturning in the Nordic Seas; decreases the heat import to the basin; warms intermediate waters, and cools deep waters. In Paper III, the importance of background climate is further stressed. In this study, we move away from studying an Arctic-like stratification, and focus on sea-surface temperature variability in the region of the Nordic Seas and North Atlantic. We compile all available planktic foraminifera records from the North Atlantic with a sea-surface temperature reconstruction from the Dansgaard-Oeschger events. These are then combined with fully coupled climate model simulations using a proxy surrogate reconstruction method. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. However, forced runs from glacial times are needed to capture the amplitude of the temperature variability as seen in the proxy records. We suggest that sea-ice changes are important in extending the oceanic temperature signals to land. Combined, the three papers argue for an important role of the Nordic Seas during Dansgaard-Oeschger events, consistent with paleoclimatic reconstructions. Our results are also relevant for understanding potential future changes in Arctic sea-ice cover, and we argue that changes in Atlantic water temperature are of large importance

The interaction between sea ice and salinity-dominated ocean circulation: implications for halocline stability and rapid changes of sea ice cover

Changes in the sea ice cover of the Nordic Seas have been proposed to play a key role for the dramatic temperature excursions associated with the Dansgaard–Oeschger events during the last glacial. In this study, we develop a simple conceptual model to examine how interactions between sea ice and oceanic heat and freshwater transports affect the stability of an upper-ocean halocline in a semi-enclosed basin. The model represents a sea ice covered and salinity stratified Nordic Seas, and consists of a sea ice component and a two-layer ocean. The sea ice thickness depends on the atmospheric energy fluxes as well as the ocean heat flux. We introduce a thickness-dependent sea ice export. Whether sea ice stabilizes or destabilizes against a freshwater perturbation is shown to depend on the representation of the diapycnal flow. In a system where the diapycnal flow increases with density differences, the sea ice acts as a positive feedback on a freshwater perturbation. If the diapycnal flow decreases with density differences, the sea ice acts as a negative feedback. However, both representations lead to a circulation that breaks down when the freshwater input at the surface is small. As a consequence, we get rapid changes in sea ice. In addition to low freshwater forcing, increasing deep-ocean temperatures promote instability and the disappearance of sea ice. Generally, the unstable state is reached before the vertical density difference disappears, and the temperature of the deep ocean do not need to increase as much as previously thought to provoke abrupt changes in sea ice

Large changes in sea ice triggered by small changes in Atlantic water temperature

The sensitivity of sea ice to the temperature of inflowing Atlantic water across the Greenland–Scotland Ridge is investigated using an eddy-resolving configuration of the Massachusetts Institute of Technology General Circulation Model with idealized topography. During the last glacial period, when climate on Greenland is known to have been extremely unstable, sea ice is thought to have covered the Nordic seas. The dramatic excursions in climate during this period, seen as large abrupt warming events on Greenland and known as Dansgaard–Oeschger (DO) events, are proposed to have been caused by a rapid retreat of Nordic seas sea ice. Here, we show that a full sea ice cover and Arctic-like stratification can exist in the Nordic seas given a sufficiently cold Atlantic inflow and corresponding low transport of heat across the Greenland–Scotland Ridge. Once sea ice is established, continued sea ice formation and melt efficiently freshens the surface ocean and makes the deeper layers more saline. This creates a strong salinity stratification in the Nordic seas, similar to today’s Arctic Ocean, with a cold fresh surface layer protecting the overlying sea ice from the warm Atlantic water below. There is a nonlinear response in Nordic seas sea ice to Atlantic water temperature with simulated large abrupt changes in sea ice given small changes in inflowing temperature. This suggests that the DO events were more likely to have occurred during periods of reduced warm Atlantic water inflow to the Nordic seas

Transient Increase in Arctic Deep-Water Formation and Ocean Circulation under Sea Ice Retreat

Abstract 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 global warming is still under debate. As deep mixing occurs in the open ocean close to the sea ice edge, the strength and vertical extent of the AMOC is likely to respond to ongoing and future sea ice retreat. Here, we investigate the link between changes in Arctic sea ice cover and AMOC strength in a long simulation with the EC-Earth–Parallel Ice Sheet Model (PISM) climate model under the emission scenario RCP8.5. The extended duration of the experiment (years 1850–2300) captures the disappearance of summer sea ice in 2060 and the removal of winter sea ice in 2165. By introducing a new metric, the Arctic meridional overturning circulation (ArMOC), we document changes beyond the Greenland–Scotland ridge and into the central Arctic. We find an ArMOC strengthening as the areas of deep mixing move north, following the retreating winter sea ice edge into the Nansen Basin. At the same time, mixing in the Labrador and Greenland Seas reduces and the AMOC weakens. As the winter sea ice edge retreats farther into the regions with high surface freshwater content in the central Arctic Basin, the mixing becomes shallower and the ArMOC weakens. Our results suggest that the location of deep-water formation plays a decisive role in the structure and strength of the ArMOC; however, the intermittent strengthening of the ArMOC and convection north of the Greenland–Scotland ridge cannot compensate for the progressive weakening of the AMOC

Consistent fluctuations in intermediate water temperature off the coast of Greenland and Norway during Dansgaard-Oeschger events

Rapid warmings epitomize the Dansgaard-Oeschger events that are recorded in Greenland ice cores and imprinted in ocean sediment cores. While the abrupt climate changes appear connected to perturbations in sea ice and ocean circulation, it is unclear how the water masses within the Nordic Seas responded and were influenced by the inflowing Atlantic Water in the absence or presence of sea ice. High resolution reconstructions of benthic Mg/Ca, together with stable isotopes of carbon and oxygen (δ13C and δ18O), show a recurring warming (2.5° ± 0.5 °C) occurring consistently at the inflow and outflow of the Nordic Seas at intermediate depths down to 1500 m during Greenland Stadials 9–6. Using idealized numerical simulations with an eddy-resolving ocean model we investigate the impact of an isolating sea ice cover and freshwater lid in the Nordic Seas. With the presence of an extensive sea ice cover, the warm Atlantic Water entering the Nordic Seas in the east, retains its heat as it exits in the west. The depth of the recirculating warm Atlantic Water increases when including an external freshwater source at the surface of the Nordic Seas. These findings support the view that cold stadials are accompanied by pervasive intermediate water warming across the Nordic Seas. Given the current rates of Arctic sea ice loss, these results provide a potential mechanism for water-column destabilization and inception of abrupt climate change

Consistent fluctuations in intermediate water temperature off the coast of Greenland and Norway during Dansgaard-Oeschger events

Rapid warmings epitomize the Dansgaard-Oeschger events that are recorded in Greenland ice cores and imprinted in ocean sediment cores. While the abrupt climate changes appear connected to perturbations in sea ice and ocean circulation, it is unclear how the water masses within the Nordic Seas responded and were influenced by the inflowing Atlantic Water in the absence or presence of sea ice. High resolution reconstructions of benthic Mg/Ca, together with stable isotopes of carbon and oxygen (δ13C and δ18O), show a recurring warming (2.5° ± 0.5 °C) occurring consistently at the inflow and outflow of the Nordic Seas at intermediate depths down to 1500 m during Greenland Stadials 9–6. Using idealized numerical simulations with an eddy-resolving ocean model we investigate the impact of an isolating sea ice cover and freshwater lid in the Nordic Seas. With the presence of an extensive sea ice cover, the warm Atlantic Water entering the Nordic Seas in the east, retains its heat as it exits in the west. The depth of the recirculating warm Atlantic Water increases when including an external freshwater source at the surface of the Nordic Seas. These findings support the view that cold stadials are accompanied by pervasive intermediate water warming across the Nordic Seas. Given the current rates of Arctic sea ice loss, these results provide a potential mechanism for water-column destabilization and inception of abrupt climate change

Consistent fluctuations in intermediate water temperature off the coast of Greenland and Norway during Dansgaard-Oeschger events

Rapid warmings epitomize the Dansgaard-Oeschger events that are recorded in Greenland ice cores and imprinted in ocean sediment cores. While the abrupt climate changes appear connected to perturbations in sea ice and ocean circulation, it is unclear how the water masses within the Nordic Seas responded and were influenced by the inflowing Atlantic Water in the absence or presence of sea ice. High resolution reconstructions of benthic Mg/Ca, together with stable isotopes of carbon and oxygen (δ13C and δ18O), show a recurring warming (2.5° ± 0.5 °C) occurring consistently at the inflow and outflow of the Nordic Seas at intermediate depths down to 1500 m during Greenland Stadials 9–6. Using idealized numerical simulations with an eddy-resolving ocean model we investigate the impact of an isolating sea ice cover and freshwater lid in the Nordic Seas. With the presence of an extensive sea ice cover, the warm Atlantic Water entering the Nordic Seas in the east, retains its heat as it exits in the west. The depth of the recirculating warm Atlantic Water increases when including an external freshwater source at the surface of the Nordic Seas. These findings support the view that cold stadials are accompanied by pervasive intermediate water warming across the Nordic Seas. Given the current rates of Arctic sea ice loss, these results provide a potential mechanism for water-column destabilization and inception of abrupt climate change

Dansgaard-Oeschger and Heinrich event temperature anomalies in the North Atlantic set by sea ice, frontal position and thermocline structure

We use eighteen timescale-synchronised near-surface temperature reconstructions spanning 10–50 thousand years before present to clarify the regional expression of Dansgaard-Oeschger (D-O) and Heinrich (H) events in the North Atlantic. The North Atlantic Drift region shows D-O temperature variations of ca. 2–5° with Greenland-like structure. The Western Iberian Margin region also shows Greenland-like structure, but with more pronounced surface cooling between interstadials and Heinrich stadials (ca. 6–9 °C) than between interstadials and non-Heinrich stadials (ca. 2–3 °C). The southern Nordic Seas show smaller D-O temperature anomalies (ca. 1–2 °C) that appear out of phase with Greenland. These spatial patterns are replicated in a new global climate model simulation that features unforced (D-O-like) and freshwater forced (H-like) abrupt climate changes. The model simulations and observations suggest consistently that the spatial expression and amplitude of D-O and H event temperature anomalies are dominated by coupled changes in the Atlantic Meridional Overturning, sea ice extent, polar front position and thermocline structure

Dansgaard-Oeschger and Heinrich event temperature anomalies in the North Atlantic set by sea ice, frontal position and thermocline structure

We use eighteen timescale-synchronised near-surface temperature reconstructions spanning 10–50 thousand years before present to clarify the regional expression of Dansgaard-Oeschger (D-O) and Heinrich (H) events in the North Atlantic. The North Atlantic Drift region shows D-O temperature variations of ca. 2–5° with Greenland-like structure. The Western Iberian Margin region also shows Greenland-like structure, but with more pronounced surface cooling between interstadials and Heinrich stadials (ca. 6–9 °C) than between interstadials and non-Heinrich stadials (ca. 2–3 °C). The southern Nordic Seas show smaller D-O temperature anomalies (ca. 1–2 °C) that appear out of phase with Greenland. These spatial patterns are replicated in a new global climate model simulation that features unforced (D-O-like) and freshwater forced (H-like) abrupt climate changes. The model simulations and observations suggest consistently that the spatial expression and amplitude of D-O and H event temperature anomalies are dominated by coupled changes in the Atlantic Meridional Overturning, sea ice extent, polar front position and thermocline structure