Arctic Sea Ice Heated from Below

Australia: The Land Where Time Began

Arctic Sea Ice Heated from Below

The water beneath the fresh, cold Arctic surface water the water originates in the Atlantic Ocean and is more saline and warmer. Pan-Arctic measurements of turbulent mixing suggest that in some areas tidal mixing is bringing substantial amounts of heat to the surface from beneath.

Rapid transformation has been occurring in the Arctic Ocean over the past few decades. The most striking feature of this changing environment is the rapidly shrinkage of the sea ice cover. Lique suggests the main driver of the decline of sea ice in the Arctic is atmospheric warming (Perovich & Richter-Menge, 2009). And a large amount of that heat is present in the layer of Atlantic water that is found at medium depths in the Arctic Ocean that could melt the Arctic sea ice within a few years if it is brought to the surface. According to Rippeth et al. (Rippeth et al., 2015) the tide generated turbulent mixing can be large enough locally to bring significant amounts of heat up to the surface layer of the Arctic Ocean.

Salty water at relatively warm temperatures of 2-3^oC is transported to the Arctic Ocean from the North Atlantic Ocean via Fram Strait and the Barents Sea. A large amount of heat is brought to the Arctic Ocean in this water mass. This warm water mass descends beneath the fresh surface layer, the halocline, of the Arctic Ocean, which is largely the result of river runoff. Though it is colder than the warm water mass, as the result of its much lower salt content than the warm waters it is less dense than the warmer water. This insulates the sea ice of the surface layer from the heat of the deeper Atlantic water. Within the Eurasian and Canadian Basins the Atlantic water circulates anticlockwise, and follows the slope of the sea floor. It is eventually exported back in to the Atlantic Ocean as cold water near the freezing point.

The Atlantic water must undergo substantial cooling along its transit within the Arctic Basin, based on simple heat budget considerations. This heat loss has been found to occur mainly by upward transfer. Knowledge of the upward heat flux from the layer of Atlantic water, as well as the mechanisms that control it, is still incomplete, though it is crucial for understanding the role of the ocean in the stability of the sea-ice cover.

Rippeth et al. (Rippeth et al., 2015) have used the most extensive survey of microstructure measurements of turbulent mixing that has been carried out so far to address this question. In some locations they infer large vertical heat fluxes of up to 50 W/m². Their estimates if turbulent vertical mixing vary widely in space: it has been found that vertical mixing tends to intensify over regions with steeper bathymetry, though it appears to be insensitive to the absence of sea ice. Based on these findings it is suggested by Rippeth et al. the spatial variations of vertical mixing are mostly driven by variations of tidal dissipation, which can be important locally.

Understanding of the mechanisms that can transfer heat from warm Atlantic water to the overlying surface layer has been improved by these results. Internal waves at density interfaces within the ocean can be generated by tidal flow over topography, as well as wind blowing at the surface of the ocean. These internal waves are the main source of vertical mixing in the ocean as they propagate and eventually break. Injection of wind energy to the surface of the ocean in the Arctic is impeded by the cover of sea ice, which forms a barrier between the atmosphere and the ocean. Also, most of the Arctic Basin is located poleward of the critical latitude that is the boundary beyond which the free generation of internal waves that are tide-generated is inhibited. It is therefore expected that the amount of mixing at a distance from strong tidal generation will be limited, which is in line with the findings of Rippeth et al. Therefore, the Arctic basin is remarkably quiet compared to the other oceans of the world, as has been shown by earlier direct observations of turbulent mixing (Rainville & Winsor, 2008).

The amount of wind forcing energy that is being put into the Arctic Ocean has been increasing over the past few decades as the sea-ice pack has been retreating further and for longer periods each year. Seasonal internal wave –mixing may have resulted from this effect, the possible consequence of which is the destabilisation of the water column (Rainville, Lee & Woodgate, 2011). It is possible that competing factors may also have worked in favour of a more stable water column: It has been found that the Arctic hydrological cycle has intensified, and river dischargers have increased, as well as the melting of Greenland ice and increases of precipitation over the Arctic Basin, all of which have been adding increasing amounts of fresh water to the Arctic Ocean. These trends, combined with the warming ocean surface, enhance the stratification within the top layers of the ocean, and consequently are expected to reduce the amount of vertical mixing (Guthrie, Morison & Fer, 2013).

Levels of vertical mixing, that have intensified regionally, have been observed by Rippeth et al. have resulted in large upward heat fluxes from the Atlantic layer. The entrance of the Arctic Ocean to the north of Fram Strait is where the largest rates of dissipation are found, the point at which the Atlantic water is at its warmest and can be in direct contact with the atmosphere in years when the ice cover is at its lowest extent. In this region Rippeth et al.(Rippeth et al., 2015) infer vertical heat flux of up to 50 W/m². This heat loss represents a very large fraction of loss of heat by the Atlantic water mass during its entire transit through the Arctic basin, though this heat loss also occurs over a small region. The remaining heat loss occurs over a large area of the interior of the Arctic basin by very small vertical fluxes(Timmermans, Toole, Krishfield & Winsor, 2008), and this has a very limited effect on the sea-ice pack.

The important role of tides compared with winds and sea ice has been identified by this study by Rippeth et al., in the control of vertical mixing and the associated heat fluxes in the Arctic Basin. Though if a full understanding of the influence of the ocean on current and future loss of sea ice is to be had, there are also other processes such as the absorption of solar radiation in the surface layers of the Arctic Ocean that will also need to be considered.

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