Category: GEOF338

Recent rapid sea-ice reduction in the Arctic Ocean (AO) has been well documented in model simulations, observations, and reconstructions. The decline in Arctic sea ice in recent decades has been attributed, in large part, to increasing greenhouse warming and we are expected to observe an ice-free summer in upcoming decades. As a result of the above trends in observed Arctic sea ice, it is important to understand the consequences of Arctic sea ice loss for both local and global climate patterns and marine ecosystems. To understand these consequences, we present a review of recent studies that investigate the implications of Arctic sea ice loss. We will in particular focus on sea ice decline in the Barents Sea (20-60°E, 67.5-85°N). Changes to Atlantic Water inflow in this region play a significant role in determining sea ice cover. Through the case study on the Barents Sea, we conclude that the main consequences of Arctic sea ice loss in recent decades include; disturbance of global weather patterns, a threat to coastal communities, and extreme temperatures. We also demonstrate how a decline in sea ice in the AO, will drive further Arctic warming and sea ice loss due to Arctic Amplification. The above consequences provide a rationale for continued monitoring of Arctic sea ice.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to experience a slowdown due to anthropogenic global warming. Concurrently, the Arctic Ocean is undergoing rapid transformations, including retreating sea ice extent, changing freshwater inputs from rivers and melting ice and increased Atlantic Water (AW) heat transport and air-sea heat fluxes. Following the realization that not only the Nordic Seas, but also the Arctic Ocean is a major component in sustaining the AMOC, the Arctic Ocean has emerged as a region of particular interest for further research. In the Arctic Ocean, the transformation of AW yields two key products, related to distinct overturning circulation branches: dense overflow waters, contributing to the thermal overturning branch, and Polar Waters (PW) created by mixing with freshwater from sources like rivers and melting ice, forming the estuarine circulation branch. Here we will discuss an outlook for the Arctic Ocean overturning in a future climate, as well as some of the processes involved in potentially counterbalancing AMOC decline. For instance, recent research contrastingly highlights the potential for a northward shift of deep convection areas, strengthening Arctic overturning, and the potential for the collapse of one of its overturning cells.

The general surface circulation of the Nordic Seas consists of poleward flowing relatively warm waters from the Atlantic and a return flow along the East Greenland Current with colder and fresher waters, along with transformed Atlantic waters sub-surface. The mid-depth circulation of the Nordic Seas, particularly the exchange between the various basins, however, has not been well studied, but is important for understanding the main pathways between dense water formation regions towards the overflows of the Greenland Scotland Ridge. The transformation of the water to denser and colder waters is important for dense water overflowing towards the North Atlantic and supplying the lower limb of the AMOC. Here we use Argo floats drifting at intermediate depths (1000 – 1500 m) to provide an overview of this mid-depth circulation. We found the highest velocities along the boundaries and in the Norwegian Sea along steep topography, while the velocities in the interior basins are lower. Floats generally remain within the basin they were deployed in unless they are entrained into the boundary current. This occurs primarily when the interior gyre circulation is close to the boundary, or where steep topography can cause instabilities. The exchanges across submarine ridges show certain preferred inter-basin pathways, except for the crossings between the Greenland and Lofoten Seas. This circulation shows similar features to the surface boundary current, but interior pathways are distinctly different and reveal the main pathways connecting the basins.

Warm water from the Atlantic is flowing into the Arctic through the Fram strait, after following the Norwegian coast. After a full circulation in the Arctic Sea, it exits through the Fram strait again, only now as a denser water mass in the East Greenland Current (EGC). The Atlantic Current that circumnavigates in the Arctic is an extension of the Atlantic Water Boundary Current (AWBC) and flows eastwards along the northern continental slope of Eurasia and North America. During this circulation the AWBC is gradually being modified to a denser water mass. During its circulation the AWBC interacts with freshwater input from continental rivers, the sea-ice cover, different types of bathymetries, the atmosphere and other different water masses in the Arctic. Over the most recent decades a retreating sea ice cover has increased the amount of interaction between AWBC and the atmosphere, implying an increase in water modification due to heat loss to the atmosphere. This poster will elaborate the modification throughout.

The Antarctic Slope Current (ASC) circumnavigates the Antarctic continent following the continental slope, separating the waters on the continental shelf from the deeper offshore Southern Ocean and regulates the flow towards the Antarctic coastline. We can classify the current’s frontal structure in 3 ranges: warm shelf, fresh shelf and dense shelf. It is difficult to reveal its spatial and sub-annual variability, as direct velocity measurements are sparse. Spatial variability of the ASC has also been characterized as three flow regimes: the surface-intensified ASC, the bottom-intensified ASC, and the reversed ASC. The surface-intensified ASC regime is stronger in the winter months. Seasonality of the winds may be the relevant driver for the seasonality along stretches of the coastline with a surface-intensified ASC. The reversed ASC has an inverted seasonality, meaning it is weaker in the winter months. The surface-intensified ASC occurs at sections of the continental slope with a fresh shelf, the bottom-intensified ASC occurs at dense shelves, and the reversed ASC occurs at warm shelves. Seasonality of the ASC is influenced not only by the mechanical forcing provided by the winds and sea ice at the ocean surface, but also by a geostrophic adjustment to changes in the cross-slope density gradient via freshwater input from basal melting and via surface water mass transformation.

The East Greenland Current (EGC) transports fresh and cold Polar Water (PW) southwards from the central Arctic Ocean. Observations of strength and properties of the EGC have been monitored through moored instruments in Fram Strait since 1997. Freshwater transport since 2015 has decreased, with an exception for 2017. The general decrease is caused by a velocity reduction of the EGC and PW. Results points to an “Atlantification” of the western Fram Strait section. Mechanisms controlling the export of freshwater from the EGC, both in liquid and solid forms, are explored using an idealized numerical model and scaling theory. Climate models predict an increase of offshore winds and decrease in Arctic export of sea-ice for the coming future. This leads to a decrease in offshore sea-ice transport. Uncertainties occur for onshore freshwater transport, as it may both increase or decrease depending on the changing effects of Ekman transport and offshore eddy fluxes.

Ice shelves are thick floating platforms of ice that connect land and ocean. Since 2016, sustained warming has led to significant and increasing Antarctic basal melting - melting of the underside of ice an shelf by the ocean. The rate of basal melting is primarily influenced by the characteristics of the sub-ice shelf–ocean boundary layer, a turbulent region responsible for the exchange of heat and salt between the ocean and the ice. This turbulent layer is usually driven by convection or shear-driven processes. As ice shelves thin due to basal melting, their ability to support the ice upstream decreases, leading to an increase in the ice flow from land to sea and subsequent sea level rise. Most of the heat responsible for melting ice shelves is transported by ocean currents flowing into the cavities beneath the ice shelves. Recent observations from ApRES (Autonomous phase-sensitive Radio-Echo Sounder) at the Fimbul Ice Shelf in East Antarctica have detected a long-term mean ablation rate of up to 1.0 ± 0.4 meters per year beneath the central region of the ice shelf. Further observations from 2017 to 2021 have shown sub-weekly to monthly variations in basal melting rates attributed to shear-driven turbulent heat transfer processes along with seasonal variations driven by ocean warming during the austral summer.

Due to increasing temperatures, the sea ice is thinning and weakening in the Arctic. This makes sea ice more prone to changes from atmospheric forcings such as wind and waves. In the Marginal Ice Zone (MIZ) there are many processes occurring, and one of them are waves-in-ice dynamics. Researchers are trying to implement these processes in coupled ice-ocean models, but due to large differences in types of ice and ice thickness it is hard to find a good model accounting for every physical process.
The poster will review how ice thickness and floe size influences waves when the waves are dampened by the ice. It is hard to directly measure the specific influence of ice thickness and floe size in the MIZ due to the large differences in ice characteristics over small areas, difficult access and rapid changes. Therefore most data are collected in land-fast ice, however some research has been done directly in the MIZ based upon visual estimates. It is clear that increasing ice thickness and floe size attenuate and dampen waves to a greater extent than thinner ice. This could have impacts on the Arctic ecosystems, specifically due to an increasing MIZ when the sea ice is thinner, so waves can travel further and open areas which have been closed and protected by sea ice.

The density of water depends on temperature, salinity and pressure. When sea ice forms brine rejection takes place which leads to an increase in the density of the underlying water. Depending on the ocean stratification, this can lead to convection where the denser water sinks. This type of convection is called type I convection and occurs regularly during winter in polar seas. It slowly deepens the mixed layer as the pycnocline gets eroded. However, there is also a type II convection that can only take place under specific circumstances, and which can lead to ventilation of the entire water column. This type of convection relies on thermobaricity: the effect of pressure on water’s density. Colder, fresh waters get more compressed (and thus dense) under pressure than warm, saline waters. This means that a seemingly stable ocean profile might be unstable when a water parcel reaches a depth where thermobaricity becomes important. This can then result in type II convection. However, in order to get a type II convection, the background stratification needs to be overcome. The normalised strength of thermobaricity indicates the likeliness of this to happen. Still, there is another important factor that determines whether type II convection can occur: the thermobaric barrier. This poster dives into the processes controlling thermobaricity and examines a case study over Maud Rise near Antarctica. Lastly, it provides an overview of stratification types in the World oceans.

Convection and deep-water formation in the Greenland Sea play crucial roles in the Atlantic Meridional Overturning Circulation (AMOC). Much of this convection occurs along the sea-ice edge, driven by strong temperature gradients between the atmosphere and the exposed ocean. This process leads to cooling of the surface layer, resulting in increased density and subsequent sinking of surface waters. As the climate changes, due to global warming, the sea-ice edge is retreating, causing a re-organization of water mass transformation. Understanding this re-organization is essential, as the AMOC significantly impacts global climate and temperatures, as well as regional climate here in Bergen. This poster investigates the relationship between sea-ice edge retreat and convection in the Greenland Sea. It highlights a significant retreat of the sea-ice edge along the east coast of Greenland over recent decades, particularly noticeable during the winter months when convection is most active. Additionally, the poster explores how the re-organization of convection could imply both a weakening or even give resilience to the AMOC. By examining these dynamics, the poster shed light on relationships between the sea-ice edge, convection, and broader implications for global climate.

Polynyas are primarily open water sea ice free areas surrounded by sea ice, formed by either strong winds or oceanic currents that drives away the sea ice. There are two types of polynyas: coastal polynyas (very thin ice present) and open water polynyas (no sea ice). For both types of polynyas, it is the direct contact between the atmosphere and the water that leads to the formation of deep water. Indeed, the water will cool much faster in polynyas thanks to the absence of sea ice or the presence of very thin sea ice, thus the water will be denser and even more so with the salinity released from the surrounding sea ice. The two main areas for deep water production are the North Atlantic (North Atlantic Deep Water (NADW)) and the Southern Ocean (Antarctic Bottom Water (AADW)).